military standard gage inspection - elsmar cove standards/mil-std-120.pdf · mil-std-i20 12...

MIL-STD-I2012 December 1950

W/CHG. NOTICE #1

9 S E P T 1 9 6 3

MILITARY STANDARD

GAGE INSPECTION

UNITED STATES

GOVERNMENT PRINTING OFFICE

WASHINGTON : 1951

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington 25, D. C. - Price $1.00

MIL-STD-12012 December 1950

MUNITIONS BOARD STANDARD AGENCY

DEPARTMENT OF DEFENSEWASHINGTON, D. C.

12 DECEMBER 1950Gage InspectionMIL-STD-120

1. This standard has been approved by the Departments of the Army, theNavy, and the Air Force for the purpose of establishing uniform practicesthroughout the Military Services for the care and use of inspection gagesand special measuring devices used in connection with the dimensional controlof all products and equipment procured by the U. S. Armed Forces.

2. The Munitions Board Standards Agency approved this standard forprinting and inclusion in the MIL series of standards on 12 December 1950.

3. In accordance with established procedure, the Munitions Board Stand-ards Agency has designated the Ordnance Corps, the Bureau of Ordnance,and the Air Force, respectively, as joint custodians of this standard.

4. When repeated deviations from this standard are required by a depart-ment or technical service, a report shall be submitted to the Munitions BoardStandards Agency by the activity concerned explaining the reason for thedeviation.

II


CONTENTSParagraph

11.11.1.11.1.21.1.3

1.1.3.11.1.3.21.1.3.31.1.3.41.1.3.5

1.1.3.6

1.1.3.71.1.3.8

2

2.12.22.2.12.2.22.2.32.2.42.2.52.2.62.2.72.2.82.2.92.32.3.12.3.22.3.32.3.3.12.3.3.22.3.42.3.5

2.3.63

3.13.2

PURPOSE AND SCOPE--------General----------------------Scope---- - - - - - - - - - - - - - - - - - - - -Purpose----- - - - - - - - - - - - - - - - -Classification of gages and

measuring equipment--------Length standards-----------------Master gages----------------------Inspection gages------------------Manufacturer’s gages-----------Nonprecision measuring equip-

ment - - - - - - - - - - - - - - - - - - - - -Precision measuring equip-

ment - - - - - - - - - - - - - - - - - - - - -Comparators------- - - - - - - - -Optical comparators and

gages---- - - - - - - - - - - - - - - - - - -DIMENSIONS AND TOL-

ERANCES------ - - - - - - - - - -Genera l - - - - - - - - - - - - - - - - - - - -Definitions---------------------Dimension----------------------Nominal size-----------------------Basic size---------------------------Design size-------------------------Actual size-------------------------Limits of size-----------------------F i t - - - - - - - - - - - - - - - - - - - - - - - - -To lerance - - - - - - - - - - - - - - - - - -Allowance----------------------Tolerances-----------------------Unilateral tolerances------------Bilateral tolerance---------------Gage tolerances------------------Tolerances on plain gages-------Tolerances for thread gages-------Gage wear allowance-------------Relation of gage limits to part

limits-----------------------Disputed rejections--------------NONPRECISION MEASUR-

ING EQUIPMENT-----------General----------------------Description------ - - - - - - - - - - -

Page

1111

11111

1

11

1

2222222222222222344

45

666

Paragraph

3.2.13.2.23.2.33.2.43.2.53.2.6

4

4.14.24.2.14.2.24.2.34.2.44.2.54.2.6

4.34.3.14.3.1.1

4.3.24.3.34.3.44.3.54.3.6

4.44.4.14.4.1.1

4.4.1.2

4.4.2

4.4.34.4.44.4.54.54.5.14.5.24.64.6.1

Steel rule---------------------------Hook rule---------------------------Steel rule with holder------------Rule depth gage-------------------Adjustable steel square---------Combination set and combina-

tion square----------------------PRECISION MEASURING

EQUIPMENT--------------G e n e r a l - - - - - - - - - - - - - - - - - -Length standards-----------------Line-graduated standards------Precision gage blocks-----------End measuring rods-------------Master disks-----------------------Wires and rolls--------------------Solid angle standards and pre-

cisions squares-----------------Scale and vernier----------------Vernier caliper--------------------Directions for reading the ver-

nier caliper----------------------Vernier depth gage---------------Vernier height gage-------------Gear tooth vernier caliper-----Vernier bevel protractor--------Directions for reading the ver-

nier protractor-----------------Micrometers---------------Micrometer caliper---------------Directions for reading a mi-

crometer caliper---------------Directions for reading the ver-

nier micrometer caliper------Screw thread micrometer cal-

iper--------------------------Inside micrometers--------------Micrometer depth gage---------Supermicrometer---------------Measuring machines------------Universal measuring machine----Special measuring machines-----Comparators----------------------Vertical comparators-----------

Page

66677

7

999999

1010

101111

1112121213

131616

18

19

192020212222232323

III

MlL-STD-12012 December 1950

Paragraph

4.6.24.7

4.7.14.7.24.7.34.7.44.84.8.14.8.24.8.34.8.44.8.54.8.655.15.1.15.1.1.15.1.1.25.1.25.1.35.1.45.1.55.1.65.25.2.15.2.1.15.2.1.2

5.2.1.3

5.2.1.4

5.2.1.5

5.2.1.6

5.2.25.2.35.2.4

5.2.55.2.65.2.6.15.2.6.25.2.75.2.85.2.95.2.10

IV

Electric comparators---------------Optical instruments and com-

parators----- - - - - - - - - - - - - -Optical flats--------------------------Optical projection machines----Tool maker’s microscope-----Optical dividing head--------------Auxiliary equipment--------------Sine bars and plates---------------Dial indicators-----------------------Surface laces-------------------------Tool maker’s flat--------------------Hardened steel square------------Pipe thread checking block------TYPES OF GAGE---------------General-----------------------Scope-------------------------------Standard gage blanks-------------Standard screw thread gages---“Go” gages-----------------------------“Not Go” gages----------------------Acceptance check gages-----------Wear limit check gages-----------Master setting gages--------------Plug gages-----------------------------Plain cylindrical plug gages-----Standard designs-------------------Cylindrical plug gage, single-

end, solid----------------------------Cylindrical plug gage, single-

end, progressive------------------Cylindrical plug gage, double-

end---- - - - - - - - - - - - - - - - - - - - - -Cylindrical plug gage, replace-

able--------------------------------Cylindrical plug gage, revers-

ible-----------------------------Plain taper plug gages-----------Thread plug gages-----------------Taper threaded pipe plug

gages----------------------------Taper plain pipe plug gages-----Spline plug gages------------------Involute spline plug gages------Straight-sided spline plug gage-Alinement plug gages-------------Graduated plug gages------------Flat plug gages----------------------Miscellaneous plug gages---------

Page

24

25252527272727273030313233333333333333333333333333

34

34

34

34

343535

353536363636373737

Paragraph

5.35.3.15.3.25.3.35.3.45.3.55.3.65.3.75.3.85.45.4.15.4.25.55.5.15.5.25.5.3

5.5.45.5.4.15.5.4.25.5.4.3

5.5.4.4

5.6

5.6.15.6.25.6.35.6.45.6.55.6.6 5.6.75.75.7.15.7.25.7.35.85.8.15.8.25.8.35.8.46

6.16.26.2.16.2.26.2.3

Ring gages-----------------------------Plain ring gages---------------------Twin ring gages---------------------Progressive ring gages------------Taper ring gages--------------------Thread ring gages------------------Taper threaded pipe ring gages-Taper plain pipe ring gages----Spline ring gages-------------------Triroll gages---------------------------Threaded pipe triroll gages-----Taper plain pipe triroll gages----Snap gages----------------------------Snap gage construction------------Adjustable length gages----------Combination ring and snap

gages-----------------------------Thread snap gages-----------------Roll thread snap gages------------Flat-anvil thread snap gages--Single-point thread snap

gages----------------------------Segment roll thread snap

gages-------------------------Miscellaneous and special

gages----------------------------Flush pin gages----------------------Spanner gages-----------------------Fixture gages-------------------------Concentricity gages----------------Template or profile gages-------Functional gages--------------------Chamber gages----------------------Air gages-------------------------------Pressure type air gage----------Flow type air gage------------------Air snap gage-----------------------Bore inspection devices-----------Star gazes-----------------------------Dial indicator bore gages-------Horoscopes-------------------------Mirror and lamp devices-------CARE, USE, AND MAIN-

TENANCE-------------------------General----------------------------Surveillance of gages--------------Identification marking-----------Loan of gages to contractors---Cleaning---------------------------

Page

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40404041

42

42

4343434344444445464646474747474949

505050505050


Paragraph

6.2.46.2.5

6.2.5.16.2.5.26.2.5.3

6.2.6

6.2.6.16.2.6.26.2.6.36.2.6.46.36.3.16.3.2

6.3.3

6.3.4

6.3.4.16.3.4.26.3.4.36.3.4.46.3.4.56.3.4.66.3.4.7

6.3.4.8

6.3.4.9

6.3.4.106.3.4.116.3.4.126.3.4.136.3.4.146.3.4.15

6.3.4.16

6.3.4.17

7

7.17.1.17.1.27.1.3

Preservation procedure-----------Maintenance and control pro-

cedures-------------------------Gage records--------------------------Time limit checking system------Use of checker master setting

gages-------------------------Damage control and correc-

tion---- - - - - - - - - - - - - - - - - - - - -Removal of nicks and burrs----Control of wear-----------------------Seizure and galling-----------------Inspection of dropped gages---Application procedures------------Forcing of gages---------------------Method of applying snap

gages-----------------------Method of applying small

plug gages--------------------------Methods of applying thread

gages-------------------------Function of thread gages---------“Go” thread plug gages----------“Not Go” thread plug gages------“Go” thread ring gages------------“Not Go” thread ring gages---Roll thread snap gages------Standard taper pipe thread

g a g e s - - - - - - - - - - - - - - - - - - - -Aeronautical type taper

threaded pipe ANPT gages---L1 and L3 taper threaded pipe

plug gages-------------------------Taper plain pipe plug gage----Threaded pipe triroll gage------Taper plain triroll gages-------Threaded pipe ring gages--------Taper plain ring gages-----------Special threaded pipe gaging

conditions--------------------------Check or master setting thread

plug gages--------------------------Check or setting taper

threaded pipe plug gages-----SURFACE ROUGHNESS

MEASUREMENT---------------G e n e r a l - - - - - - - - - - - - - - - - - -Definition---------------------Standardizat ion- - - - - - - - - -Application to gages---------------

Page

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515151515151515151

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52

52525353535454

55

56

565758596162

62

62

63

6565656565

Paragraph

7.2

7.2.17.2.27.2.2.17.2.2.27.2.2.37.2.2.47.2.2.5

7.2.2.67.2.2.77.2.37.2.3.17.2.3.27.2.3.37.2.3.47.2.3.57.2.3.6

7.2.3.77.2.3.87.2.3.9

7.2.4

8

8.18.1.18.1.28.1.38.28.2.18.2.28.2.38.2.48.2.5

8.3

8.3.18.3.28.3.38.3.48.3.5

8.3.68.3.6.18.3.6.2

Surface roughness measure-ment----- - - - - - - - - - - - - - - - - - -

Tracer type instruments--------Profilometer-----------------------Tracer unit----------------------------Microinch meter---------------------Mototrace----------------------------Glass roughness specimen------Interpretation of meter read-

ings--------------------------------Using the profilometer------------Profilometer rotary pilotor-----Brush surface analyzer-----------Pick-up arm---------------------------Surface plate--------------------------Drive head-----------------------------Amplifier---------------------------Direct inking oscillograph--------Interpreting the oscillograph

c h a r t - - - - - - - - - - - - - - - - - - - -Root mean square meter-------Glass calibration standard-------Using the brush surface ana-

l y z e r - - - - - - - - - - - - - - - - -Surface roughness comparator

blocks-------------------------G A G E I N S P E C T I O N

METHODS---------------------General-----------------------------Purpose--------------------------Scope--------------------------------Arrangement--------------------Laboratory conditions------------Laboratory temperatures--------Atmospheric conditions-----------Cleanliness--------------------------Lighting-----------------------------Location of precision instru-

merits--------------------------General inspection instruc-

tions----------------------------Drawings-----------------------Marking------------------------Gage records cards------------------Precautions------------------------Selecting the inspection meth-

od-----------------------------Steps in the inspection process--Cleaning------------------------Hardness measurement----------

Page

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676869697070707071

717272

73

73

75757575757575757575

75

7676767676

77777777

v


Paragraph

8.3.6.38.3.6.48.3.6.5

8.3.6.68.3.6.7

8.3.6.88.3.78.3.7.18.3.7.28.4

8.4.18.4.28.4.2.18.4.2.28.4.2.38.4.38.4.3.18.4.3.28.4.3.38.4.3.4

8.4.48.4.58.4.5.18.4.5.2

8.4.5.38.4.5.48.4.5.58.4.5.68.4.68.4.78.4.88.4.98.5

8.5.18.5.28.5.38.5.4

8.5.58.5.5.18.5.5.28.5.68.5.6.18.5.6.2

VI

Demagnetizing---------------------Visual impaction--------------------Checking loosely toleranced di-

mensions-------------------------Checking for function-------------Checking closely tolerance di-

mensions-------------------------Sealing-----------------------------Finishes------------------------------Requirements--------------------Inspection----------------------------Care and use of inspection

tools--------------------------------General----------------------------Micrometer calipers---------------Outside micrometers--------------Inside micrometers----------------Micrometer depth gages----------Vernier gages------------------------Vernier calipers---------------------Vernier depth gages--------------Vernier height gages-------------Vernier height gage with test

indicator-------------------------Dial indicator gages--------------Surface plate and accessories--Surface plate-------------------------Angle iron and tool maker’s

knee------------------------------Parallels----------------------------Precision straight edge-----------Universal precision square----Planer gage---------------------------Roll holder----------------------------Hold-down fixture------------------Channel-bar fixture---------------Locating plate-----------------------Care and use of precision

measuring instruments-------General----------------------------Gage blocks and accessories---Sine bars and sine plates -----Light wave measuring equip-

ment------------------------------Optical instruments---------------Contour projector------------------Tool maker’s microscope---------Direct measuring equipment--Supermicrometer-----------------Universal measuring machine-

Page

7878

7878

787 9797979

797 97979808081818181

82848585

858687878787888889

89899092

94979799

100100102

Paragraph

8.5.7

8.5.7.18.5.7.28.5.7.38.5.7.48.5.7.58.5.7.68.5.7.78.5.88.5.8.18.5.8.2

8.5.98.6

8.6.1

8.6.2

8.6.3

8.78.7.1

8.7.2

8.7.3

8.7.4

8.7.5

8.8

8.98.9.18.9.1.18.9.1.28.9.1.38.9.1.48.9.1.58.9.1.68.9.28.9.2.1

8.9.2.2

Mechanical and electrical com-parators-------------------------

Metron gages-------------------------Electolimit gages-------------------Visual gages--------------------------Supersensitive comparator-----Dial comparator---------------------Dial bore gage---------------------Using comparators-----------------Indexing equipment--------------Precision indexing head--------Bench center with sine bar

indexing face plate-------------Hardness tester---------------------Gage inspection of plain plug

gages---------------------------Measurement with a bench

micrometer-----------------------Measurement with a mechan-

ical comparator------------------Measurement with a precision

measuring machine-----------Inspection of tapered plugs---Measuring straightness and

angle of taper--------------------Checking gages when front face

is not square----------------------Checking diameters of taper

plugs with rolls------------------Measuring the diameters of

tapered plugs with roll andsine bar-----------------------------

Checking the diameter oftapered plugs with masterrings-----------------------------

Alinement and concentricityplugs--------------------------

Ring and receiver gages---------Plain ring gages---------------------Check plugs--------------------------Internal calipers--------------------Dial bore gages----------------------Casts--------------------------------Internal measuring machine Gage blocks---------------------------Receiver gages----------------------Check plug and blueing (angle

check) - - - - - - - - - - - - - - - - - - - -Pencil lines and check plug

(angle check)----------------------

Page

102102102103105105105106107107

107107

108

108

109

110110

110

113

114

115

116

117122122122122122122122123123

123

123


Paragraph

8.9.2.3

8.9.2.4

8.9.2.5

8.9.2.6

8.108.10.18.10.28.10.38.118.11.18.11.2

8.11.3

8.11.4

8.11.58.11.68.11.78.11.88.11.98.12

8.12.18.12.28.12.38.12.4

8.12.58.13

8.13.18.13.28.13.38.13.48.13.58.14

8.15

8.16

8.16.1

Sine bar and angle plate (anglecheck)--------------------------

Precision balls (angle and di-ameter check)-------------------

Tapered check plugs (di -ameter check)------------------

Calibrated tapered plugs (di-ameter check)------------------

Inspection of snap gages-------Plain and built-up snap gages---Adjustable snap gages-----------Large snap gages (all types)----Thread plug gages----------------Lead-----------------------------Checking angle with a test

t o o l - - - - - - - - - - - - - - - - - - - -Checking angle with an optical

comparator-----------------Checking angle with a tool

makers’ microscope-----------Root-------------------------------Pitch diameter---------------------Major diameter-------------------Concentricity----------------------Finish----------------------------Inspection of thread ring

gages - - - - - - - - - - - - - - - - - - - - -Thread form------------------------Lead--------------------------------Pitch diameter---------------------Concentricity of pitch and

minor diameters---------------Minor diameter-------------------Inspection of tapered thread

plug gages-----------------------Angle and relief-------------------Lead-------------------------------Pitch diameter and taper-------Major diameter-------------------Convolution----------------------Inspection of tapered thread

ring gages-----------------------Inspection of thread snap

gages-----------------------Inspection of qualifying thread

gages----- - - - - - - - - - - - - - - - -Checking qualifying thread

gages with a contour projec-tor---------------------------

Page

123

123

125

125126126126127127127

128

129

132132133136136136

136136137138

139139

139139139140144145

145

146

146

147

Paragraph

8.16.2

8.17

8.18

8.19

8.208.20.18.20.2

8.20.3

8.20.48.20.5

8.20.68.20.78.218.21.1

8.21.1.1

8.21.1.2

8.21.1.3

8.21.1.4

8.21.2

8.21.38.21.3.1

8.21.3.2

8.21.3.3

8.21.3.4

8.228.22.18.22.28.23

8.23.18.23.2

Checking qualifying threadswith a thread template------

Inspection of segmental threadg a g e s - - - - - - - - - - - - - - - - - -

Inspection of Acme thread gages-----------------------

Inspection of modified squarethread gages--------------------

Inspection of flush pin gages---General---- - - - - - - - - - - - - - - - - -Depth measuring flush pin

gage------------------------Length measuring flush pin

gages------------------------Post type flush pin gages-------Flush pin gages for location of

holes---- - - - - - - - - - - - - - - - - - - -Tapered flush pin gages---------Gooseneck flush pin gages-----Inspection of profile gages------Template gages for profile of

radius----------------------Checking radii where great ac-

curacy is not required-----------Checking radii where accuracy

is required-----------------------Checking radii with gage

blocks, tool maker’s buttonsand parallels--------------------

Checking template gages withspecial indicator fixture-----

Template gages for profile ofangle-----------------------

Profile plug gages-----------------Measuring plug with plain

radius----------------------Measuring profile ogive check

plugs with single radius---------Measuring profile ogive check

plugs with double radius-----Measuring plugs with convex

and concave radii--------------Inspection of fixture gages------Flush pin type fixture gages----Indicator type fixture gages----Inspection of miscellaneous

gages - - - - - - - - - - - - - - - - - - - -Dial indicator gages--------------Special indicator snap gages---

Page

148

150

151

151152152

152

153154

155156156157

157

158

158

159

165

166169

169

169

171

172172172174

175175176

VII


Paragraph

8.23.38.23.48.23.58.23.68.23.78.23.88.23.98.248.24.18.24.28.24.38.24.48.24.58.24.68.24.78.24.88.24.98.25

Spanner gages----------------------Dovetail gages----------------------Spline gages-------------------------Spline ring gages------------------Tapered spline gages-------------Large gages--------------------------Cams------------------------------------Checking gages for bear---------Plain plug gages--------------------Plain ring gages--------------------Thread plug gages-----------------Thread ring gages-----------------Thread concentricity gages-----Snap gages----------------------------Indicator gages---------------------Flush pin gages---------------------Fixture gages------------------------Calibrating thread measuring

wires---------------------------

Figure 1.Figure 2.Figure 3.Figure 4.Figure 5.Figure 6.Figure 7.Figure 8.Figure 9.Figure 10.Figure 11.Figure 12.Figure 13.Figure 14.Figure 15.Figure 16.Figure 17.Figure 18.Figure 19.Figure 20.Figure 21.Figure 22.Figure 23.Figure 24.Figure 25.Figure 26.Figure 27.

Page

176177182184185185186187187188188188189189189189189

190

Paragraph

8.268.26.1

8.26.28.26.3

8.278.27.1

8.27.2

8.27.3

8.27.4

8.27.5

8.27.6

LIST OF FIGURES

Unilateral tolerances.Bilateral tolerances.Unequal bilateral tolerances.Plug gage tolerance.Built-up snap gage tolerances.Ring gage tolerances.Steel rule.Hook rule.Steel rule with holder.Rule depth gage.Adjustable steel square.Combination set.Precision gage block set.Gear and thread measuring wires.Cylindrical square.Vernier caliper.Vernier scale.Vernier depth gage.Vernier height gage.Gear tooth vernier caliper.Vernier bevel protractor.Vernier on bevel protractor.Micrometer caliper.Micrometer barrel and thimble scales.Vernier micrometer scales.Examples of micrometer vernier readings.Screw thread micrometer caliper.

Calibrating gage blocks---------Suggestions in reinspecting

gage blocks-------------------------“A’’ precision gage blocks------“B” precision gage blocks or

used gage blocks---------------Helix angles--------------------------Tables for American national

coarse and fine thread series-Chart for determining helix

angles----------------------------Thread flank angle correction

factors----- - - - - - - - - - - - - - - -Tables for thread lead check-

ing---------------------------------Tables of equivalent hardness

numbers-----------------------Key to table of equivalent

hardness numbers------------

Page

190

190193

193194

194

194

194

197

197

197

VIII


Figure 28.Figure 29.Figure 30.Figure 31.Figure 32.Figure 33.Figure 34.Figure 35.Figure 36.Figure 37.Figure 38.Figure 39.Figure 40.Figure 41.Figure 42.Figure 43.Figure 44.Figure 45.Figure 46.Figure 47.Figure 48.Figure 49.Figure 50.Figure 51.Figure 52.Figure 53.Figure 54.Figure 55.Figure 56.Figure 57.Figure 58.Figure 59.Figure 60.Figure 61.Figure 62.Figure 63.Figure 64.Figure 65.Figure 66.Figure 67.Figure 68.Figure 69.Figure 70.Figure 71.Figure 72.Figure 73.Figure 74.Figure 75.Figure 76.Figure 77.

Inside micrometer (caliper-type).Inside micrometer (rod-type).Micrometer depth gage.Supermicrometer.Universal measuring machine.Electrolimit comparator.Visual gage.Dial comparator.Optical flats.Optical comparator.Contour measuring projector.Tool maker’s microscope.Optical dividing head.Sine bar.Sine bar set upon gage blocks.Sine plate.Sine plate with base plate.Dial indicator (gear train type).Dial test indicator.Surface plate.Tool maker’s flat.Hardened steel square.Pipe thread gage checking block.Cylindrical plug gage, single-end solid.Cylindrical plug gage, single-end progressive.Cylindrical plug gage, double-end.Cylindrical plug gage, replaceable.Cylindrical plug gages, reversible.Plain taper plug gages.Thread plug gages.Taper threaded pipe plug gages.Taper plain pipe plug gages.Involute spline plug gage.Straight-sided spline plug gage.Alinement plug gages.Graduated plug gage.Flat plug gage.Miscellaneous plug gages.Plain ring gages.Twin ring gages.Progressive ring gages.Thread ring gages.Taper threaded pipe ring gage.Taper plain pipe ring gage.Spline ring gages.Threaded pipe triroll gage.Taper plain pipe triroll gage.Adjustable snap gages.Adjustable length gage.Combination ring and snap gage.

IX

MIL-STD-12012 December

Figure 78.Figure 79.Figure 80.Figure 81.Figure 82.Figure 83.Figure 84.Figure 85.Figure 86.Figure 87.Figure 88.Figure 89.Figure 90.Figure 91.Figure 92.Figure 93.Figure 94.

1950

Roll thread snap gage.Flat-anvil thread snap gage.Single-point thread snap gage.Segment roll thread snap gage.Flush pin gage.Spanner gage. Fixture gage. Concentricity gage.Template or profile gage.Functional gage.Chamber gage.Air gage.Star gage.Dial indicator bore gage.Proper method of inserting “Go” or “Not Go” thread plug gage.Improper method of inserting “Go” or “Not Go” thread plug gage.Aline bolt with “Go” rolls and lower into gage. Black line on screw shows orientation

of gage.Figure 95. “Not Go” rolls stop bolt when it passes through “Go” rolls.Figure 96. Bolt is turned ¼ turn at “Not Go” rolls to check for out-of-roundness. Note the new

position of the black line on bolt.Figure 97. Raise bolt back through “Go” rolls and check further for out-of-roundness.Figure 98. Assembled threaded pipe and coupling at handtight engagement, showing dimensions

L1, L2, and L3.Figure 99. L1 gage screwed into product. The gage’s basic notch has stopped at fitting’s face.Figure 100. Pipe fitting shown on figure 99 is next checked with L3 gage, and here stops with

basic notch within ½ turn (thread) from the product face.Figure 101. Taper plain plug gage is used next on product shown on figures 99 and 100. Here

it has stopped with basic notch at face of product. Fitting is acceptable.Figure 102.Figure 103.Figure 104.


X

Excessive shake or play of taper plain pipe plug gage is cause for rejection of product.Two fittings cross-sectioned for inspection, Center object is proof cast.Fitting screwed into threaded pipe triroll gage. Small end of fitting is flush with

basic step.Taper plain triroll gage with fitting inserted. Flush pin shows fitting to be basic.Setting plug gage having full and truncated threads on each member.Taper pipe thread setting plug gage.Threaded pipe triroll gage set correctly.Taper plain setting pipe threaded plug gage.Taper plain triroll gage.Profilometer with mototrace.Tracer unit with interchangeable skid mounts.Microinch meter and scale-selector switch.Glass roughness specimen.Profilometer rotary pilotor.Brush surface analyzer.Pick-up arm.Amplifier.Direct inking oscillograph.Root mean square meter.Glass calibration standard.


Figure 122.Figure 123.Figure 124.Figure 125.Figure 126.Figure 127.Figure 128.Figure 129.Figure 130.Figure 131.Figure 132.Figure 133.Figure 134.Figure 135.Figure 136.Figure 137.Figure 138.Figure 139.Figure 140.Figure 141.Figure 142.Figure 143.Figure 144.Figure 145.Figure 146.Figure 147.Figure 148.Figure 149.Figure 150.Figure 151.Figure 152.Figure 153.


Surface roughness comparator blocks.Adjusting micrometer depth gage.Use of vernier height gage.Use of vernier height gage with test indicator.Error in test indicator.Method of placing heavy object on a surface plate.Tool maker’s knee—universal angle iron.Checking tool maker’s knee for squareness.Checking squareness by means of tool maker’s buttons.Universal precision square.Planer gage. Reversible roll holder.Conventional roll holder.Hold-down fixtures.Channel bar fixture.Locating plate.Checking profile gage on locating plate.Snap gage made of gage blocks.Sine plate with centers.Use of a sine plate.Measurement by means of light waves.Computing error in flatness.Interference pattern. pleasuring height with an optical flat.0–2" light wave micrometer with roll anvil, etc.Aligning fixture for projector.Positioning fixture for tool maker’s microscope.Setting up bench micrometer for measurement over rolls.Metron comparator.Electrolimit comparator.Electrolimit height gage.Pressure regulator for use with visual gage thread checking attachment (cover re-

moved to show construction).Universal external visual gage.Supersensitive comparator. Dial comparator.Dial bore gage.Precision indexing head.Bench center with sine bar indexing face plate.Rockwell hardness tester.Measuring small plug gages.Measuring large plug gages.Checking front face of plug for perpendicularity.Checking taper on sine plate.Measuring taper on special sine plate fixture.Measuring taper and diameter of tapered plug.Set-up for measuring taper.Tapered plug-drawing specifications.Computing tapered plug diameter from measurements over rolls.Finding location of datum diameter.

XI


Figure 171.Figure 172.Figure 173.Figure 174.Figure 175.Figure 176.Figure 177.Figure 178.Figure 179.Figure 180.Figure 181.Figure 182.Figure 183.Figure 184.Figure 185.Figure 186.Figure 187.Figure 188.Figure 189.Figure 190.Figure 191.Figure 192.Figure 193.Figure 194.Figure 195.Figure 196.Figure 197.Figure 198.Figure 199.Figure 200.Figure 201.Figure 202.Figure 203.

Checking diameter with roll and sine bar.Method of clamping gage over sine bar.Roll for checking taper plug.Length of block for odd size roll.Checking tapered plug with check rings.Computing diameter of tapered plug.Computing location of intersection (small taper over large taper).Computing location of intersection (large taper over small taper).Computing distances from back face to intersections of tapers.Special fixture for checking taper and diameter of tapered plugs.Checking taper with special fixture.Checking length to datum diameter with special fixture.Checking tapered ring on sine bar.Set-up for measuring the angle and the diameter of an internal conical surface.Computing internal angle from measurements over two balls.Computing diameter at large end of taper from measurement over ball.Measuring length to datum diameter by means of calibrated plug.Checking parallelism of anvils on large snap gages.Checking thread angle by means of a test tool.Test tools and correction spacers for checking thread angle.Relationship between axial plane and plane of projection.Computation of helix angle.Correction factor for thread angle.Common visual errors found in thread gages.Root inspection of thread gages.Three-wire method of measuring pitch diameter of thread plug gages.Pitch line. Best size wire.Height of wire above vertex.Height of pitch line above vertex.Method of making cast of thread ring gage.Making lead cast of thread ring.American gage design standard thread ring locking device. 1, Locking screw; 2,

Sleeve; 3, Adjusting screw; 4, Body; 5, Adjusting slots; 6, Adjusting slot terminal hole; 7,Locking slot.

Figure 204. Measurement of pitch diameter of taper thread gages by the two-wire method.Figure 205.Figure 206.Figure 207.Figure 208.Figure 209.Figure 210.Figure 211.Figure 212.Figure 213.Figure 214.Figure 215.Figure 216.Figure 217.Figure 218.

Locating height of fixed measuring wire.Computing pitch diameter and taper of taper thread plug.Specification-¾-14 N. P. T. thread plug gage.Set-up for measuring pitch diameter with taper fixture.Calibrating taper thread fixture.Taper thread checking fixture.Hold-down wire for taper thread checking fixture.Measuring major diameter over rolls.Measuring major diameter pipe threads by roll and gage block method.Thread snap gages.Setting thread snap gage.Set-up for checking qualified threads on a projector.Measuring length to reference surface.Thread checking template.

XII


Figure 219.Figure 220.Figure 221.Figure 222.Figure 223.Figure 224.Figure 225.Figure 226.Figure 227.Figure 228.Figure 229.Figure 230.Figure 231.Figure 232.Figure 233.Figure 234.Figure 235.Figure 236.Figure 237.Figure 238,Figure 239.Figure 240.Figure 241.Figure 242.Figure 243.Figure 244.Figure 245.Figure 246.Figure 247.Figure 248.Figure 249.Figure 250.Figure 251.Figure 252.Figure 253.Figure 254.Figure 255.Figure 256.Figure 257.Figure 258.Figure 259.Figure 260.Figure 261.Figure 262.Figure 263.Figure 264.Figure 265.Figure 266.Figure 267.Figure 268.

Special angle iron for supporting thread template.Set-up for checking qualifying thread gage (top view).Measuring major diameter of segmental thread gage.Set-up for checking depth measuring flush pin gage.Set-up for checking length measuring flush pin gage—pin method.Checking length measuring flush pin gage—pin method.Set-up for checking length measuring flush pin gage—micrometer method.Checking length measuring flush pin gage—micrometer method.Post type flush pin gage.Conditions commonly found on tapered pins.Flush pin for measuring location of holes.Set-up for checking small conical flush pin gage.Computing length to apex on tapered flush pin gage.Measuring radius with projector.Set-up for checking radius with indicator.Checking radius with fixed button and gage blocks.Checking uniformity of radius.Checking inside radius (with no precision reference surface).Checking inside radius (where radius is tangent to straights surface).Checking inside radius (where radius is not tangent to straight surface).Computing distance from origin to straight surface.Computing vertical distance from origin to center of roll.Computing distance from parallel to roll.Setting template gage in position.Checking radius on template gage.Checking outside radius (where radius is tangent to straight surface).Checking outside radius (where radius is not tangent to straight surface).Set-up for checking a large radius with a special indicator fixture.Checking a large radius with a special indicator fixture.Set-up for checking a small radius with a special indicator fixture.Checking a small radius with a special indicator fixture.Checking template gage for location of intersection.Measuring distance to apex on template gage.Checking template gage for accuracy of angles.Checking outside radius (where there is no precision reference surface).Measuring profile ogive check gage.Measuring profile check plug (single radius).Measuring second radius of profile check plug.Computing point of tangency of double radius.Computing variable angle in double radius measurements.Computing position of roll in double radius measurement.Computing measurement over rolls in double radius measurement,Measuring outside radius of profile plug with outside and inside radii.Measuring inside radius of profile plug with outside and inside radii.Checking flush pins and steps on fixture gage.Checking angle of dovetail gage.Checking female dovetail gage.Checking male dovetail gage.Measuring width of tapered dovetail on an indexing plate.Computing relationship of angles of tapered dovetail gage.

XIII


Figure 269.Figure 270.Figure 271.Figure 272.Figure 273.Figure 274.Figure 275.Figure 276.Figure 277.Figure 278.Figure 279.Figure 280.Figure 281.

Checking taper dovetail on sine plate.Measuring distance between apexes of tapered dovetail-ball method.Measuring angles, of sides of tapered dovetail-ball method.Computing distance between apexes—ball method of measurement.Checking spline ring gage.Checking large locating ring gage.Checking a cam.Set-up for checking parallelism of gage blocks.Horizontal check for gage block parallelism.Vertical check for gage block parallelism.Rough check of difference in length of two gage blocks.Check of difference in length of two gage blocks.Chart for determining helix angles of screw threads.

Page 3 Table IPage 194 Table IIPage 194 Table IIIPage 195 Table IVPage 195 Table VPage 197 Table VIPage 197 Table VII

XIV

LIST OF TABLES

Tolerances for plain cylindrical plug and ring gages.American national coarse thread seriesAmerican national fine thread series60 degree included thread angle29 degree included thread angleThread lead checkingEquivalent hardness numbers

MIL-STD-120CHANGE NOTICE 19 SEPTEMBER 1963

MILITARY STANDARD

GAGE INSPECTIONTO ALL ACTIVITIES :

1. THE FOLLOWING PAGES OF MIL-STD-120 HAVE BEENSEDE THE PAGES LISTED:

SUPERSEDEDNEW PAGES DATE PAGE

89 9 SEPTEMBER 1963 8990 " " " 90

169 " " " 169

REVISED AND SUPER-

DATE

12 DECEMBER 1950 " " " " " "

2. RETAIN THIS NOTICE AND INSERT THE TABLE OF CONTENTS.


1

PURPOSE AND SCOPE

1.1 GENERAL

1.1.1 Scope. This standard is to providecorrelated technical information applicable tothe inspection of gages, special tools, andmeasuring devices. The principal subjects cov-ered are nomenclature, tolerances and fits,measuring tools and equipment, gages, andmethods of measurement and inspection. De-tails of gage design are not included.

1.1.2 Purpose. The purpose of gaging isto determine compliance of the componentparts of a mechanical product with the dimen-sional requirements of contracts, drawings, andspecifications, whether such product is madeby a single manufacturer or whether parts tobe assembled are made by several manufac-turers variously located. Gaging thereby as-sures the proper functioning and interchange-ability of parts, that is, one part will fit inthe same place as an-y similar part and performthe same function, whether the part is for theoriginal assembly or replacement in service.

1.1.3 Classification of gages and measur-ing equipment. Gages and measuring equip-ment are classified as follows:

1.1.3.1 Length standards. Standards oflength and angle from which all measurementsof gages are derived consist of precision gageblocks, end measuring rods, line-graduatedstandards, master disks, calibrated wires androlls, precision squares , graduated circles, andsimilar items.

1.1.3.2 Master gages. Master gages aremade to their basic dimensions as accuratelyas possible and are used for reference, such asfor checking or setting inspection or manufac-turer’s gages.

1.1.3.3 Inspection gages. Inspection gagesare used by the representative of the purchas-ing agency to inspect products for acceptance.

These gages are made in accordance with estab-lished design requirements. Inasmuch as in-spection gages are subjected to continuous use, agage maker’s tolerance will always be appliedand a wear allowance, where applicable, maybe included in the design of these gages. Tol-erances of inspection gages are prescribed byspecified drawing limits.

1.1.3.4 Manufacturer’s gages. Manufac-turer’s gages are used by the manufacturer orcontractor for inspection of parts during pro-duction. In order that the product will bewithin the limits of the inspection gages, man-ufacturer’s or working gages should havedimensional limits, resulting from gage toler-ances and wear allowances, slightly fartherfrom the specified limits of the parts inspected.

1.1.3.5 Nonprecision measuring equip-ment. Nonprecision measuring equipment aresimple tools such as rules and plain protractorsused to measure by means of line graduationssuch as those on a scale.

1.1.3.6 Precision measuring equipment.Precision measuring equipment are tools usedto measure in thousandths of an inch or finerand usually employ a mechanical, electrical oroptical means of magnification to facilitatereading.

1.1.3.7 Comparators. Comparators are pre-cision measuring equipment used for compara-tive measurements between the work and a con-tact standard, such as a gage or gage blocks.

1.1.3.8 Optical comparators and gages.Optical comparators and gages are those whichapply optical methods of magnification ex-clusively. Examples are optical flats, toolmaker's microscopes, and projection compara-tors. Projection comparators usually providefor absolute measurements as well as for com-parative measurements.

1


2DIMENSIONS AND TOLERANCES

2.1 GENERAL.initions relating to

This section presents def-dimensions and tolerances

and standard practice relative to gage toler-ances and wear allowances. This subject isdealt with further in MIL-STD-8.

2.2 DEFINITIONS2.2.1 Dimension. A dimension is a speci-

fied size of a geometrical characteristic such asa diameter, length, angle, circumference, orcenter distance.

2.2.2 Nominal size. The nominal size isthe designation which is used for the purposeof general identification.

2.2.3 Basic size. The basic size of a dimen-sion is the theoretical size from which the limitsof size for that dimension are derived by theapplication of the allowance and tolerances.

2.2.4 Design size. The design size of a di-mension is the size in relation to which thelimits of tolerance for that dimension are as-signed.

2.2.5 Actual size. The actual size of a di-mension is the measured size of that dimensionon an individual part.

2.2.6 Limits of size. These limits are themaximum and minimum sizes permissible fora specific dimension.

2.2.7 Fit. The fit between two mating partsis the relationship existing between them withrespect to the amount of clearance or inter-ference which is present when they are as-sembled.

2.2.8 Tolerance. The tolerance on a di-mension is the total permissible variation inits size. The tolerance is the difference betweenthe limits of size.

2.2.9 Allowance. An allowance is an inten-tional difference in correlated dimensions ofmating parts. It is the minimum clearance(positive allowance) or maximum interference(negative allowance) between such parts.

2.3 TOLERANCES. Variation in size canbe restricted but not avoided. However, a cer-tain amount of variation can be tolerated with-

out impairing the functioning of a part. Thepurpose of tolerances is to confine such varia-tion within definite limits.

2.3.1 Unilateral tolerances. Unilateraltolerances are those which are applied to thebasic or design size in one direction only. Theunilateral system of tolerances will generallybe used on all drawings. The application ofunilateral tolerances is as shown on figure 1.

Figure 1. Unilateral tolerances.

2.3.2 Bilateral tolerances. Bilateral tol-erances are those which are applied to the basicor design size in both directions. The basicor design size may or may not be the mean size,as the plus and minus tolerances are not neces-sarily equal. The application of bilateraltolerances is as shown on figures 2 and 3.

Figure 2. Bilateral tolerances.

2.3.3 Gage tolerances. Gage tolerances areapplied to the gaging dimensions of gages in

2


Figure 3. Unequal bilateral tolerances.

order to limit variations in size during theirmanufacture. In general practice, gage tol-erances should not exceed 10 percent of thetolerance of the part to be gaged.

2.3.3.1 Tolerances on plain gages. Toler-ances for plain cylindrical plug and ring gageshave been standardized in four classes accord-ing to nominal size and the degree of accuracynecessary to cover most gaging operations, asshown in table I.

TABLE I. TOLERANCES FOR PLAIN CYLINDRICAL PLUG AND RING GAGES

922173 0 -51-2 3

MIL-STD-l2012 December 1950

2.3.3.1.1 Class XX (plug gages only).Class XX gages are precision lapped to thesmallest practicable tolerances and are com-monly applied to master gages and set-upstandards.

2.3.3.1.2 Class X (plug and ring gages).Class X gages are precision lapped to close tol-erances for many types of masters and the high-est quality working and inspection gages.

2.3.3.1.3 Class Y (plug and ring gages).Class Y gages have a good lapped finish withslightly larger tolerances and are commonlyapplied to working gages. NO T E: X and Yclassification and tolerances for plain cylindri-cal plug and ring gages should not be confusedwith X and Y classification and tolerances forthread gages.

2.3.3.1.4 Class Z (plug and ring gages).Class Z gages have a commercial finish, groundand polished, but not fully lapped, and are usedfor working gages where part tolerances arelarge.

2.3.3.2 Tolerance for thread g a g es .Standard tolerances for thread plug and ringgages and threaded setting plugs are tabulatedin the current issue of National Bureau ofStandards Handbook H28, Screw ThreadStandards for Federal Services, and are asfollows :

2.3.3.2.1 W thread gages. W thread gagetolerances represent the highest commercialgrade of accuracy or workmanship. They areespecially applicable to truncated setting plugsfor thread ring gages and to gages used forthreaded products made to special closetolerances.

2.3.3.2.2 X thread gages. X thread gagetolerances are larger than W tolerances and aresuitable for inspection and setting gages ex-cept where W tolerances may be more desir-able.

2.3.3.2.3 Y thread gages. Y thread gagetolerances are larger than X thread gage tol-erances and in addition provide standard wearallowances. Y gages are considered suitablefor inspection and working gages for classes 1and 2 threads, ¼ inch diameter and larger.They may also be used as working gages forclasses 2 and 3.

23.4 Gage wear allowance. A wear allow-

4

ance on a “go” gage is an intentional differencebetween the nominal or specified size of the gageand the basic or design size of the part. Itspurpose is to provide for a moderate amount ofwear to occur before the gage is worn beyondthe product limit. Wear allowance, when re-quired, is applied on all “go” gages where thepart rubs or presents a wearing action to thegage. When part tolerances are very small, itis not feasible to provide a wear allowance be-cause the wear allowance, plus the requiredgage tolerance, would deprive the part manu-facturer of too much of his tolerance. otherexceptions are dial indicator gage assembliesand adjustable snap gages which may be resetas soon as they wear materially, and flush pingages which may have the step reground (seefigs. 4,5, and 6).

Figure 4. Plug gage tolerance.

2.3.5 Relation of gage limits to partlimits. The limits of the part, as specified onthe drawings, shall not be exceeded as a resulteither of tolerance or wear of the gages. There-fore, the extreme sizes for all gages shall notexceed the extreme limits of the part to be


gaged. All variations in the gage, whatevertheir cause or purpose, shall bring the gagewithin these extreme limits. Thus a gage thatchecks a minimum dimension may be larger, butnever smaller, than the minimum size specifiedfor the part to be gaged. Likewise, the gagethat checks the maximum dimension may besmaller, but never larger, than the maximumsize specified for the part to be gaged.

Figure 5. Built-up snap gage tolerances.

Figure 6. Ring gage tolerances.

2.3.6 Disputed rejections. Any part whichis so close to either rejection limit as to be im-properly rejected, either as a result of toleranceor wear of the inspection gages, will be rein-spected, using a gage as close to the productlimit as is practicable, or by use of precisionmeasuring instruments. Any observationalerrors in the reinspection must, however, be inthe direction of safety rather than in the direc-tion of danger of acceptance of improper parts.

5


3

NONPRECISION MEASURING EQUIPMENT

3.1 GENERAL This section describesvarious commercially available nonprecisionmeasuring equipment. Such equipment nor-mally are tools used in the inspection of partswhen quantities are small and the tolerances aresufficiently large to permit satisfactory appli-cation. For example, line-graduated handmeasuring tool, such as a rule, does not controlaccuracy of measurement beyond the smallestgraduation on its scale. The scale is usuallyread to its nearest graduation.

3.2 DESCRIPTION3.2.1 Steel rule. The steel rule (see fig.

7) is a line-graduated measuring instrumentwhich is read by the comparison of its scalewith the edge or surface to be measured. Steelrules are made in various shapes or sizes andwith various attachments and refinements. The6-inch steel rule is most frequently used in theshop. Rules in the inch system are graduated in binary fractions down to inch, in decimal

spring steel allow measurements to be takenover curved or warped surfaces. Rules havingend scales facilitate measurements in restrictedspace.

Fillet rules which have one end cornertrimmed at an angle allow measurements to bemade across corner fills and fillets.

Figure 7. Steel rule.

3.2.2 Hook rule. The hook rule (see fig. 8)is a standard rule with a fixed shoulder attachedto one end. This rule facilitates accurate meas-urements from an end surface on a part. It isespecially useful in positions where the end ofthe rule is hidden or out of reach and the scalegraduations cannot be accurately alined withthe edge of the part. It is frequently used forsetting inside calipers and dividers.

Figure 8. Hook rule.

fractions down to one-hundredth of an inch.They are also available in the metric system.A large number of combinations of scales isavailable. A rule commonly has four scales, acoarse and a fine scale on each side. The scaleson one side normally run in reverse to those onthe other. Flexible rules in thin tempered

3.2.3 Steel rule with holder. The steelrule with holder (see fig. 9) consists of a set ofspecial short rules and a holder which is slottedat one end to allow insertion of the rules.Usually there is a set of five rules of varyinglength ( ¼ to 1 inch) with each holder. Therules are graduated on both sides.

6


in sets of various widths, lengths, and configu-rations.

3.2.6 Combination set and combinationsquare. The combination set consists of aheavy graduated blade, a square, a center head,and a protractor (see fig. 12). The combina-tion square is the same as the combination setbut without the protractor. The combination

Figure 9. Steel rule with holder.

set is practically a universal measuring instru-ment for nonprecision line measurement andlayout. The square of the combination can bemoved along the blade and clamped in any

Figure 10. Rule depth gage.

3.2.4 Rule depth gage. The rule depthgage (see. fig. 10) is an ordinary rule equippedwith a sliding shoulder or head so that it can beused to check the depth of holes or slots. Therule may be in the form of a rod. A modifica-tion of the rule depth gage has the rule equippedwith a fixed shoulder to facilitate the measure-ment of holes which extend completely througha piece.

3.2.5 Adjustable steel square. The ad-justable steel square (see fig. 11) is a type ofbeam square, having a sliding blade insteadof a fixed blade. The blade fits into a groovein the base of the beam and can be locked inplace by a thumb screw in the head of the beam.Blades for the adjustable square usually come

position by means of a set screw. When it isclamped in position, one side of the squaremakes a 90 degree angle with the blade and theother side makes a 45 degree angle. It usually

Figure 11. Adjustable steel square.

7


incorporates a spirit level set at right angles tothe blade. The center head forms a centersquare when clamped to the steel rule. It hastwo flat measuring surfaces intersecting at 90degrees. One of these surfaces is slotted sothat when the head is placed on the blade, theblade bisects the angle of the head. The pro-tractor of the combination set is provided witha swivel or turret to which the steel rule clamps.The revolving turret is graduated from O to 90degrees in either direction. The protractorhead may be either single with the shoulder

extending from only one side of the blade, ordouble with the shoulder extending from bothsides. Most protractor heads are equippedwith a spirit level. Note: The type of pro-tractor shown in figure 12 is called a plain pro-tractor by some tool makers, and a bevel pro-tractor by others. The terms plain protractorand reversible protractor are also used in con-nection with this instrument to denote singleand double type heads. This tool should not beconfused with the vernier type protractor whichis a precision measuring instrument.

Figure 12. Combination set.

8


4

PRECISION MEASURING EQUIPMENT

4.1 GENERAL. Precision measurement isa process of accurately deriving the numericalvalue or size of a physical dimension in stand-ard units of measurement such as inches, centi-meters, degrees, minutes and seconds, by com-

paring the dimension with a precision standardof length or angle.This section describes vari-ous commercially available precision measuringequipment and the length or angle standardswith which they are associated. Such tools andstandards are used primarily for the measure-ment of gages, but they are also used, to a con-siderable extent, in the inspection of manufac-tured parts.

4.2 LENGTH STANDARDS4.2.1 Line-graduated standards. Line-

graduated standards used in gage inspectioncommonly have the form of accurately ruledscales and dials embodied in various types ofprecision measuring instruments.

4.2.2 Precision gage blocks. Precisiongage blocks (see fig. 13) are square or rectangu-lar pieces of hardened alloy steel or sinteredcarbide, having lapped, flat and parallel meas-uring surfaces on opposite ends. This permitscombining them, by wringing, to produce anydesired size. They are usually measured bycomparison with standard blocks, which havebeen calibrated and certified by the NationalBureau of Standards, using comparator read-ings in millionths of an inch. original or abso-lute measurements of gage blocks are derivedby comparison with standard wave lengths oflight by means of an interferometer. Gageblocks are most commonly used as length stand-ards in gage inspection and are usually suppliedin sets varying from 28 to 103 blocks of differ-ent lengths. The most generally used set ofthe inch system has 81 blocks with possibly twoadditional carbide wear blocks. Gage blocksare made in several degrees of accuracy, ofwhich the most generally used is ± 0.000004 inchand designated as “A” quality The highest

9

or laboratory grade, designated as “AA,” has aguaranteed accuracy of ± 0.000002 inch for allblocks up to 1 inch in length and ± 0.000002inch per inch of length for larger blocks. Otherlower grades are also available. Variations inthe lengths of blocks have a random distribu-tion between plus and minus. The total errorin a block combination, therefore, seldom ex-ceeds twice the error in a single block, andoften is actually less than that of a singleblock. If the error were permitted to be inone direction only, the accumulated error in acombination might be considerable. Althoughprecision gage blocks are most widely used asmasters for checking other gages, their use inthe shop for setting of tools and in layout workis increasing. Chromium plated and carbideblocks are used where blocks are subject toconsiderable wear.

Figure 13. Precision gage block set.

4.2.3 End measuring rods. End measur-ing rods normally are made of tool steel,hardened on the ends, drawn and seasoned, andsometimes tipped with a suitable wear-re-


sisting material. They are commercially avail-able in lengths from 1 to 23 inches and aresuitable as standards for measurements to±0.0001 inch. They are cylindrical in shapeand have either flat and parallel end surfaces orspherical end surfaces of radius equal to orslightly less than one-half the length of therod. The body is equipped with suitable in-sulating grips to permit handling the rod withminimum heat absorption.

4.2.4 Master disks. Master disks are cus-tomarily made of tool steel in a range of sizesfrom 0.105 to and including 8.010 inches indiameter, and are accurate within ± 0.00001inch per inch of diameter. They are given thesame hardening, seasoning, and lapping processas that for precision gage blocks, and are usedas standards for comparators, supermicrom-eters, and similar instruments. (See 5.1.6 andfig. 31.)

4.2.5 Wires and rolls. Wires and rolls ofhardened steel or sintered carbide are used be-tween measuring contracts and the work whena dimension on the work to be measured is notdirectly accessible to flat measuring contacts.They are used, for example, for measuring thepitch diameter of thread plug gages, or formeasuring a dimension at a designated pointon a sloping surface. They are available in

binary fractional inch sizes and in certain deci-mal inch sizes suitable for the measurement ofscrew threads and gears. Thread wires arefurnished in sets of three wires of a givendiameter; gear wire sets contain two wires; thediameters of these wires being the “best size”for a given pitch (see fig. 14). They are re-quired to be within 0.0001 inch of the nominalsizes and the wires of a set must be equal insize, round, and straight within 0.00002 inch.

4.2.6 Solid angle standards and precisionsquares. Solid angle standards have a varietyof forms such as angle gage blocks, universalangle gages, graduated cone points, and pre-cision squares. Angle gage blocks function inthe same manner as gage blocks for lengthmeasurements, and may be combined by wring-ing. Blocks may be assembled to serve as thecomplement of the angle being gaged. Thus,a set of 14 or 16 blocks suffices to form anyangle to 1 second of arc from 1 second to 360degrees. Cone points consist of hardened steel,short, cylindrical rods, each having a coaxialconical point at one end. A set of such pointsin increments of 5 minutes is particularly usefulin the measurement of flank angles of largethread gages. The cylindrical square (see fig.15) is a master square against which squaresused in inspection are checked.

Figure 14. Gear and thread measuring wires.

.

Figure 15. Cylindrical square.

It is an alloy steel cylinder, ground and lappedto be an accurately true cylinder and to havethe end planes at 90 degrees to the longitudinalaxis. Either end of the cylinder may be usedas the base. The end surfaces are in relief so

10

that the base plane consists of a narrow peri-pheral notched ring so dust particles can beworked out by rotating the square when restingit on a flat surface.

4.3 SCALE AND VERNIER. Precisionmeasuring equipment which embody line-grad-uated standards, scales, or graduated circles,equipped with verniers for subdividing scaledivisions, are grouped into the following types:

4.3.1 Vernier caliper. The vernier caliper(see fig. 16) consists of a solid L-shaped frameand a sliding jaw which can be moved alongthe long arms of the L-shaped frame. Thesliding jaw is in two sections joined by a hori-zontal screw. The larger section carries themeasuring jaw and vernier scale; the smallersection or binding clasp has an adjusting nutacting on the horizontal screw. Both sectionscan be clamped independently to the long armof the L-shaped frame. Thus the sliding jawcan be moved into an approximate position, theclasp clamped, and the larger section and meas-uring jaw moved to the exact position by move-ment of the adjusting nut. The blade of thesolid L-shaped frame usually is graduated onboth sides, one side for outside measurementsand the other for inside measurements. Somecalipers have only one scale, and it is necessaryto add the width of the gaging jaws to the read-ing when an inside measurement is taken.


The main scale is divided into 1.0-, 0.1-, and0.025-inch intervals, but only the 1.0 and 0.1 in-tervals are numbered. The vernier scale has25 divisions. A point is placed on the slidinghead and another one of the frames so thatdividers can be set to transfer distances. Forsome applications the vernier caliper has defi-nite advantages over the micrometer caliper (see4.4. 1) because of its wide range and because itcan be used for measuring both inside and out-side dimensions. It does not, however, havethe accuracy of a micrometer caliper. A ver-nier caliper should show any l-inch interval oflength within an accuracy of ±0.001 inch. Inany 12 inches its accuracy should be within±0.002 inch, and the error should not increaseby more than about ±0.001 inch for every 12inches thereafter .

4.3.1.1 Directions for reading the verniercaliper. The vernier scale is a short auxiliaryscale which either slides along the main scaleor is stationary while the main scale moves.It allows readings to be made between the grad-uations on the main scale. The principle of thevernier is applied to many types of measuringinstruments. A vernier exhibits an ordinarilyimperceptible difference in length or movement.The vernier scale usually has one or more grad-uations in the same length as the main scale.If the whole vernier scale contains one more

Figure 16. Vernier caliper.

11


division that the main scale over an equal length,each division on the vernier scale is propor-tionally smaller than a corresponding divisionon the main scale. On figure 17, the 25 divi-sions on the vernier are equal to 24 divisions onthe main scale; therefore, each division on thevernier scale is of a division smaller than adivision on the main scale. Since each mainscale division is equal to or 0.025 inch andeach vernier scale division is smaller thana main scale division, each vernier scale divisionis equal to of 0.025 or 0.001 inch. Readingacross the vernier scale from zero, 0.001 inch isadded to the reading obtained from the mainscale just to the left of the zero on the vernierscale for each vernier scale division, as follows:The vernier scale with the zero line directly inline with a graduation on the main scale isshown by “A” on figure 17. In this case the

Figure 17. Vernier scale.

12

vernier scale is not necessary as there is no frac-tional part of a division to determine. Thereading is 2.350. The vernier scale with the zeroline beyond the 2.35 division on the main scaleindicates that the dimension is 2.35 plus a partof a main scale division, as shown by “B” on fig-ure 17. In order to determine how many thou-sandths are to be added to the 2.35 reading, it isnecessary to find the division on the vernierscale that exactly coincides with a divisionon the main scale, in this case the 18th, so thatthe total reading is 2.350 plus 0.018=2.368.

4.3.2 Vernier depth gage. The vernierdepth gage (see fig. 18) consists of a blade hav-ing the main scale and a shoulder which slideson the blade and contains the vernier scale.The vernier depth gage is almost as accurate asthe micrometer depth gage, and a wide range ofdepth can be measured without changing rods.

4.3.3 Vernier height gage. The vernierheight gage (see fig. 19) is a caliper with a spe-cial foot block which adapts it for use on a sur-face plate. The mechanism of the movablejaw and the reading of the vernier scale areidentical with those of the vernier caliper. Themost commonly used vernier height gages meas-ure 10,18, 24, or 40 inches above a surface plate.The 10-inch size is sometimes furnished withtwo scales, one for use as a height gage and theother for use as an outside vernier caliper.

4.3.4 Gear tooth vernier caliper. T h egear tooth vernier caliper (see fig. 20) is aspecial form of caliper having two scales atright angles to each other. By the use ofverniers, the scales can be read in thousandthsof an inch. The horizontal scale is for meas-uring the thickness of a tooth at the pitch line,or chordal thickness by the setting of the jaws.The vertical scale is for measuring the distancefrom the top of the tooth to the chord at thepitch line by means of a tongue which movesup and down behind the jaws. When cor-rectly set, this tongue keeps the jaws in theright position to measure the thickness of thetooth at the pitch circle. The gear tooth ver-nier caliper is made commercially in two sizes:20 to 2 diametral pitch and 10 to 1 diametralpitch. Although widely used, the gear toothvernier caliper has certain disadvantages, ofwhich the most undesirable is the measurement


Figure 18. Vernier depth gage.

taken by the corners of the jaws, causing rapidwear of the tool.

4.3.5 Vernier bevel protractor. The ver-nier bevel protractor (see fig. 21 ) also calleda universal bevel protractor, is an angle meas-uring instrument designed for the layout andchecking of angles to an accuracy greater than1 degree. It consists of two members, a baseand a blade, which can be set in any angularrelationship to each other. The position ofone member relative to the other is indicated ona circular dial which has four O- to 90-degreescales. By means of a vernier, the scale can beread to the nearest 5 minutes or degree. Theblade is adjustable and, by means of a clamp-ing mechanism and thumb nut, can be lockedto the central connecting arm at any positionalong its axis. The ends of the blades areusually beveled to allow the protractor to beused in measuring from square to undercutshoulders where a square end blade would notenter. The blade assembly (blade and centralarm) is locked to the scale and base by a largecentral locking nut. Either a separate or in-tegral means of fine adjustment is usually pro-vided to allow controlled final movement ofthe blade through a small arc. The instrumentshown in figure 21 has an adjustable arm oracute angle blade for the purpose of measuringsmall angles on pieces so shaped that they

13

would be difficult to measure from the fixedbase. The back of the vernier bevel protractoris flat so that the tool may be placed flat ona surface plate or on the work being checked.

4.3.6 Directions for reading the vernierprotractor. The vernier indicates every 5minutes or degree. When the zero on thevernier exactly coincides with a graduation onthe scale, the reading is in exact degrees, as inthe upper scale shown on figure 22, in whichthe reading is 17 degrees O minutes. When thezero graduation of the vernier does not coincideexactly with a graduation on the scale, thenumber of degree or 5 minute intervals tobe added to the whole degree reading is deter-mined by counting the number of divisionsfrom the zero of the vernier to the first line onthe vernier that coincides with a line on thescale. As each division on the vernier repre-sents 5 minutes, the number of these divisionsmultiplied by 5 will be the number to be addedto the whole number of degrees. For example,the lower scale shown on figure 22 shows thezero on the vernier between 12 and 13 on thescale. The zero on the vernier has, therefore,moved 12 whole degrees to the right from zeroon the scale. In the same direction, the tenthline of the vernier, representing 50 minutes isthe line which exactly coincides with a line onthe scale. The reading, then, is 12 degrees 50


Figure 19. Vernier height gage.

14


Figure 20. Gear tooth vernier caliper.

15


Figure 21. Vernier bevel protractor.

Figure 22. Vernier on bevel protractor.

minutes. Since the divisions, both on the scaleand on the vernier, are numbered both to rightand left from zero, any size of angle can bemeasured. The readings on the scale and onthe vernier are taken either to the right or left,according to the direction in which the zeroon the vernier is moved.

4.4 MICROMETERS4.4.1 Micrometer caliper. The micrometer

caliper (usually called a micrometer or “mike”)is second only to the steel rule in the extent ofits use by an inspector. It utilizes the relation-ship between the axial and circular motions ofa screw as a means of precision measurement.The micrometer caliper consists essentially ofa U-shaped frame and a screw (see fig. 23).One end of the U-shape holds the anvil, whichis a hardened steel button pressed or screwedinto the frame, and the other end of the U-shapecarries the hollow steel barrel in which thescrew turns. The threads of the screw engage

16


Figure 23. Micrometer caliper.

the threads of a nut supported in the outer endof the barrel. The inner end of the screw isunthreaded and constitutes the spindle whichprovides the second and movable reference sur-face. The spindle rides in a plain bearingwhich is part of the frame. The thimble,which is attached to the screw, is a graduated,

hollow sleeve fitting over the barrel. Thethimble is fixed in relation to the circular posi-tion of the screw and carries graduations onits front by means of which the amount of rota-tion of the screw is determined in relation toa longitudilnal reference line on the barrel. Thereference line on the barrel is the base of the

17


micrometer scale which shows the axial move-ment of the screw. The thimble graduations,therefore, subdivide the relatively larger divi-sions of the barrel scale. Some micrometercalipers have a ratchet on the top of the thimblecap connected to the thimble and screw throughan over-riding clutch held together by a spring.The diameter of the nut which engages thescrew can be adjusted to compensate for wearby tightening the ring nut. Some micrometercalipers have a clamp ring or lock nut set in theU-shaped frame at the spindle bearing. Thisdevice acts as a caroming roll, which operatesagainst a split nut to bind the spindle bearing.Turning the clamping ring until it is just tighteffectively locks the spindle in position. Themost common size micrometer caliper measuresfrom 0 to 1 inch. The larger size micrometercalipers also measure within a range of 1 inch.Thus, for example, 1 inch must be added to thebarrel reading of a 2-inch micrometer caliperto give the correct distance between anvil andspindle. Most micrometer calipers are gradu-ated to read to thousandths of an inch and maybe read to ten-thousandths by estimating tenthsof the thimble graduations. If they areequipped with a vernier scale, they can be readdirectly to a ten-thousandth of an inch. Thisis approximately the limit of accuracywhich is built into an ordinary commercialmicrometer.

4.4.1.1 Directions for reading a microme-ter caliper. The distance between the anviland the unthreaded part of the screw, calledthe spindle, is determined by the number ofwhole turns and fractions of a turn of the screw.The screw of a micrometer caliper reading inthe inch system has 40 threads per inch, thuseach revolution of the screw opens the microm-eter caliper or 0.025 inch. The barrel isgraduated to show the revolutions of the screw.There are 40 graduations on the barrel of amicrometer caliper having a range of 1 inch.To simplify the reading, every fourth gradua-tion representing tenths of an inch, is numbered.The thimble is graduated so that each of aturn, or 0.001 inch) is indicated byone graduation on the thimble. The thimblethus has 25 graduations, every fifth graduation

being numbered for convenience in reading asshown on figure 24. A measurement in thou-sandths on the micrometer caliper is to be readas follows:

a. Note the number of graduations on thebarrel uncovered by the thimble travel and mul-tiply this number by 0.025.

b. Note the thimble graduation which coin-cides most closely with thebarrel.

BARREL

reference line on the

SCALE

Figure 24. Micrometer barrel and thimble scales.

c. Add this reading in thousandths of aninch to the barrel reading. This method is ap-plied to the example shown on figure 24, asfollows :

Take care in reading a micrometer caliper whenthe thimble is approaching the completion ofa revolution. If the thimble zero has not passedthe index line on the barrel, do not read the

18


barrel for an extra 0.025 inch. The next gradu- thimble and to add that vernier graduationation may begin to show but has not actually number in ten-thousands to the regular microm-become a portion of the barrel reading. That eter reading. The numbers on the coincidingportion of the graduation that starts to show graduation of the thimble scale are to be com-near the end of the revolution is accounted foras the thimble reading. The accuracy of meas-urements made with the micrometer dependson the contact load (or in common parlance,measuring pressure) used. It is necessary todevelop a sense of “feel” in adjusting the mi-crometer to the work. This can best be doneby taking measurements of precision gageblocks or other accurately known standards.Also, a sensitive feel is best developed by grasp-ing the smooth portion of the thimble rather

pletely disregarded in the use of the vernierscale. Never use the number of a thimble scalegraduation that coincides with a vernier gradu-ation to obtain the vernier reading. Examplesof micrometer vernier readings are shown onfigure 26. In example A, the barrel reading is0.45. The thimble reading lies between 0.019and 0.020. The seventh graduation of thevernier scale coincides with a graduation of thethimble scale, showing that the exact. readingof the thimble scale is 0.0197. The micrometer

than the knurled portion. reading for example A is, therefore, 0.4697.4.4.1.2 Directions for reading the vernier In example B, the barrel reading is 0.45. The

micrometer caliper. The vernier micrometer thimble reading is exactly 0.019. This is veri-caliper is a standard micrometer equipped with fied by the fact that only the zero graduationsa vernier scale. The vernier micrometer scale of the vernier coincide with thimble scale(see fig. 25) is etched on the barrel in line withand contiguous to the thimble scale. It con-sists of 11 equally spaced lines, the first 10 linesnumbered from O to 9, the last graduationmarked with a second O. The 10 divisions ofthe vernier scale occupy the same space as 9divisions on the thimble. One vernier divi-sion therefore is inch. Thedifference between the thimble and vernierdivision are thusallowing a reading of inch. Thus, whena thimble graduation does not coincide with theindex line on the barrel, it is necessary onlyto determine which vernier graduation on thebarrel coincides with a graduation on the

graduations. The micrometer reading for ex-ample B is therefore 0.4690. The vernier scaledoes not enter into this reading except to con-firm the apparent position of the thimble scaleon the index line.

Figure 26. Examples of micrometer vernier readings.

Figure 25. Vernier micrometer scales.

4.4.2 Screw thread micrometer caliper.The screw thread micrometer caliper (see fig.27) is a micrometer caliper adapted for meas-uring pitch diameters of threads. The spindlehas a cone-shaped tip which is truncated so thatit does not contact the root of the thread. Theanvil is swiveled and has a truncated V-grooveto fit a thread. The contacts occur at the sidesof the thread. The point on the spindle couldbe used for any pitch but the V-shaped anvil islimited in its range. For this reason, six differ-ent micrometer calipers are necessary to cover

922179 0 -51-3 19

M I L - S T D - 1 2 01 2 D e c e m b e r 1 9 5 0

a range of 4½ to 64 threads per inch in a givendiameter range. when the micrometer caliperis set to zero, the pitch lines ofthe spindleand anvil coincide. When open, the readinggives the pitch diameter. Thread micrometersare convenient to use but are subject to certainlimitations. When the micrometer is set toread zero with the thimble and anvil together,as is usually done, the reading over the threadsis always slightly distorted because, the threadsurfaces being warped, contact of the anvil onthe thread is not in the same axial plane whichthe spindle cone contacts. The readings arenot distorted when thread micrometers are setto a standard thread plug gage of known pitchdiameter and used for measuring threads of thesame p0itch and diameter as the plug. When soset, however, the tool will not read exactly zerowhen spindle and anvil are brought together.

Figure 28. Inside micrometer, caliper-type.

4.4.3.2 Rod-type. The rod type inside mi-cromer (see fig. 29) is used to measure insidedimensions larger than those covered by thecaliper type inside micrometer. It consists ofa micrometer head and a series of interchange-able extension rods of varying lengths. Anespecially short inside micrometer with a rangeof from 1 to 2 inches may be obtained, and bymeans of extension rods, a length up to 100inches or more may be measured. The rangeof the micrometer screw itself is short. Thesmallest size micrometer has ¼ inch length ofscrew and the largest only a 1 inch length ofscrew. some sets contain a collar by which theinch steps between rods may be split up whenthe screw is ½ inch. The collar extends thelength of any rod ½ inch.

Figure 27. Screw thread micrometer caliper.

4.4.3 Inside micrometers. Inside microm-eters are of two types, the caliper-type and therod-type. Both types read in thousands of ani n c h . 4.4.3.1 Caliper type. The caliper type in-side micrometer is designed for measuring smallinside dimensions. It is made commercially inranges of from 0.2 to 1 inch, ½ to 1½ inchesand 1 to 2 inches. The minimum dimension isdetermined by the width of the measuring jaws.These are gound with a small radius to insureline contact in a hole. Some caliper-type insidemicrometers have the barrel scale on the spindle,which does not turn (see fig. 28). They areread in the same way as an ordinary outsidemicrometer caliper. Others have the barrelscale on the barrel in the same position as on anoutside micrometer caliper but the scale readsfrom the right to left instead of left to right.

Figure 29. Inside micrometer, rod-type.

4.4.4 Micrometer depth gage. The mi-crometer depth gage (see fig. 30) consists of aflat shoe or shoulder attached to the barrel ofa micrometer head. The base of the shoulderis hardened and ground in order to be at rightangles to the axis of the spindle. The spindle,which protrudes from the shoulder, may bea flat strip or round rod and may be eithergraduated or ungraduated. The range ofmeasurement of the type of gage shown onfigure 30 can be increased by the use of inter-changeable extension rods of different lengths.

2 0


scale on the depth micrometer reads fromright to left. 4.4.2 Supermicrometer. The supermi-crometer (see fig. 31) is a bench-type microm-eter with controlled contact load. It bridgesthe gap between the ordinary hand micrometerand the very accurate measuring machine. Itreads in ten-thousandsof an inch and is ac-curate to a ten thousandth, whereas the ordi-nary vernier micrometer, although it can beread in ten-thousandths, will usually have er-rors of from 1 to 3 ten-thousandths inch. Thesupermicrometer consists of a heavy cylindricalbed on which is mounted a headstock containingthe micrometer screw, and a tailstock contain-ing both the mechanism for maintaining con-stant measuring load and the anvil. In theheadstock, a large graduated drum divides thegraduations of 0.05 inch on the spindle into0.001 inch increments, this arrangement being

Figure 30. Micrometer depth gage.

These rods, however, are not interchangeablein different tools. By removal of the thimblecap, the desired rod may be inserted easily andquickly in the gage through a hole in the mi-crometer screw. When the thimble cap is re-placed, it holds the rod firmly in position. The

Figure 31. Supermicrometer.

21


similar to that found on micrometer calipers.Alongside the graduated drum is a vernier scaleby which readings to 0.0001 inch can be made.The micrometer screw of the supermicrometerhas a range of from 0 to 1 inch. but the instru-ment itself covers a range of 8 inches in 1-inchsteps by the use of l-inch master disks as shownon figure 31. The tailstock can be moved toany position along the bed. The pressuremechanism can be adjusted by means of knurledknob for a contact load of 1 or 2½ pounds.These are the two loads recommended by theNational Bureau of Standards for measuringthreads by the three-wire method.

4.5 MEASURING MACHINES

4.5.1 Universal measuring machine. Theuniversal measuring machine (see fig. 32) gen-erally used in the United States is a bench typemeasuring machine consisting of a heavy basecontaining accurate ways upon which a mi-crometer headstock with microscope can bepositioned in relation to a master bar graduatedat each l-inch interval. A pressure tailstock,which is also adjustable on the ways, providesmeans of adjusting and controlling electricallythe measuring load from 1 to 2½ pounds, in

increments of 8 ounces. It is similar to thesupermicrometer in appearance and operation,but the accuracy is greater as direct measure-ments can be taken to 0.00001-inch. The ma-chine is furnished in various sizes having capac-ities from 0 to 12, 24, 26, 48, 80, or 120 inches.The master bar of this type of measuring ma-chine is a line-graduated standard which runsthe length of the machine at the rear. It ismade of seasoned steel and bears microscopicreference lines on highly polished plugs. Thesegraduations are placed 1 inch apart and areused for setting the measuring head at eveninch positions. The measuring head includesa microscope, with a double cross-hair, to seton the inch graduations of the master bar. Thepressure tailstock makes possible the duplica-tion of exact measuring load at all times.Changes in load are made by turning the knobat the rear of the tailstock until the desiredload is reached, as indicated on a graduatedscale which is calibrated in 8-ounce divisionsfrom 1 to 2½ pounds. A milliammeter,located.on the measuring head, shows when the cor-rect measuring load has been reached. By ad-vancing the anvil in the measuring head, thework is pressed against the tailstock anvil until

Figure 32. Universal measuring machine.

22

the pointer of the milliammeter reaches thecenter of the scale. The size is the sum of thenumber of the inch graduation of the masterbar to which the microscope is set and thereading of the micrometer head.

4.5.2 Special measuring machines. Meas-uring machines of special design for specificpurposes are available commercially. Amongthese are lead measuring machines for deter-mining errors in pitch of screw thread gagesand internal measuring machines for measur-ing the diameters of ring gages.


4.6 COMPARATORS. Comparators areused for determining dimensions by measur-ing the difference in length between a cent actlength standard, such as a master disk or gageblock combination, and the work.

4.6.1 Vertical comparators. Most com-parators are of the vertical bench type, consist-ing of a ribbed cast iron base upon which ismounted an anvil for supporting the work anda vertical hollow steel shaft or column whichsupports the measuring head. The measuringhead contains the measuring spindle and the

Figure 33. Electrolimit comparator.

23


amplifying mechanism. The column permitsswiveling of the head and is equipped with arapid traverse elevating mechanism for raisingor lowering the measuring head. Measuringspindles may have a diamond or carbide tipwhich is either flat or hemispheric]. Anvilsmay be had in various sizes and shapes. withor without carbide inserts. Backstops are alsoavailable.

4.6.2 Electric comparators. In electriccomparators, such as the “electrolimit” compar-ator (see fig. 33), amplification is obtained bymeans of a balanced induction bridge circuitwhich is unbalanced by displacement of themeasuring spindle. The amount of currentwhich is thus caused to flow through the bridgeis proportional to the linear displacement of thespindle, and this amount is read by means ofa meter which may be mounted on the measur-

ing head or in a separate meter box. Magnifi-cations available range from 750 to 22,500 times,the corresponding meter scale graduations be-ing one division equals 0.0002 inch and one divi-sion equals 0.0000025 inch.

4.6.2.1 Visual gage. The visual gage (seefig. 34 ) employs a combination of mechanicaland optical levers as the means of magnifica-tion, using electricity to produce a beam oflight. Magnifications up to 20,000 times areavailable.

4.6.2.2. Dial comparator. The dial com-parator ( see fig. 35) is a type of vertical com-parator in which the amplification is mechani-al and the magnifying means is a dial indi-cator ( see 4.8.2)

Figure 34. Visual gage.

24

Figure 35. Dial comparator.


4.7 OPTICAL INSTRUMENTSCOMPARATORS. For certain typesspection. instruments and comparators

ANDof in-based

on optical principles are more advantageouslyapplicable than mechanical means. suchequipment includes optical flats, optical com-parators, tool maker’s microscopes, and opticaldividing heads.

4.7.1 Optical flats. Optical flats (see fig.36) are transparent disks of glass, quartz, orsemiquartz. One side of the disk is an opticallyflat surface, being within 1 or 2 millionths inchof a true plane. Interference fringes, or lightand dark bands, are produced when an opticalflat is superimposed on another flat surface insuch a way that there is a very small angle be-tween the surfaces and the surfaces are suitablyilluminated with approximately monochro-matic light. The straightness of the fringesis a measure of the flatness of the surface beingtested, one fringe corresponding approximatelyto 0.00001 inch in great light and 0.000012 inchin yellow light. In addition to checking flat-ness, optical flats can be used for comparingthe length and parallelism of nominally equalprecision gage blocks.

Figure 36. Optical flats.

4.7.2 Optical projection machines. Theoptical comparator ( see fig. 37 ) and contourmeasuring projector ( see fig. 38) are both opti-cal projection machines for measuring or com-paring objects by means of a magnified shadowimage. A concentrated, parallel beam of lightis projected across the object to be measuredand by a system of optical magnification thecontour of the part is projected onto a screen.

25

Figure 37. Optical comparator.

The shadow is compared with lines on a chartcorresponding to the limits of size of the con-tour of the part being checked. The magnifi-cation of the comparator is the ratio of the sizeof the shadow on the screen to the size of theobject. The magnification is dependent pri-marily upon the projection lens system and onthe screen distance. Interchangeable lens sys-tems are available, with magnifications of 10,20, 25, 3l¼, 50, 62½, and 100 times. The pro-tractor screen consists of a circular disk of finely


Figure 38. Contour measuring projector.

26


ground glass mounted in a ring which is grad-uated and provided with a vernier which per-mits the measurement of all angles to a mini-mum reading of 1 minute of arc. The groundglass of the screen is provided with fine brokenreference lines, etched in the glass. These linesare filled with black pigment and can bematched with the image when taking measure-ments. The work table has a longitudinal andtransverse travel and is equipped with microm-eters having a minimum reading of 0.0001 inch.

4.7.3 Tool maker’s microscope. The toolmaker’s microscope, (see fig. 39) is an opticalinstrument for measuring linear and angulardimensions. It consists of a microscope with aprotractor head, mounted on a support columnwith adjustable tilt, and a mechanical stagepermitting longitudinal, transverse, and an-gular measurement. The object being meas-ured is seen through the microscope enlarged30 or more times, depending on the objectivelens which is used. The protractor head givesangular readings by vernier to 1 minute, andhas two rotatable interrupted cross lines. Themicrometer stage is operated by micrometerscrews which have drums graduated to read0.0001 inch per division. The full range of thestage is 4 inches of longitudinal motion and 2inches of transverse. Of the longitudinal mo-tion, 2 inches are by screws and 2 additionalinches by insertion of gage blocks.

4.7.4 Optical dividing head. The opticaldividing head (see fig. 40) is a final inspectioninstrument designed to give the greatest ac-curacy possible in circular measurement ofangular spacing. The complete unit consistsof the dividing head, surface plate, and tail-stock. The measuring elements consist of aprecision graduated circle, which is read bymeans of a microscope, and an auxiliary gradu-ated drum which permits reading to 6 secondsor 1 second of arc. Accuracy within 2 secondsof arc may be obtained with this instrument.The dividing head may be equipped with astandard driver designed for work of varioussizes and characteristics, a chuck having in-dividually adjustable jaws, or a precision faceplate. The surface plate is made of fine-grain

way for maintaining accurate alinement of tail-stock to the dividing head by means of a keywhich fits into the keyway in the surface plate.

4.8 AUXILIARY EQUIPMENT. Someof the more important items of equipment,which are auxiliary to precision measuring in-struments and comparators, are:

4.8.1 Sine bars and plates. —The sine barand sine plate are tools designed for measuringangles accurately or for locating work to agiven angle within very close limits. The sinebar (see figs. 41 and 42) consists of an accuratestraightedge in which are set two hardened andground plugs or buttons. These plugs or but-tons must be of the same diameter, and theircenter distance is commonly an even 5, 10, or20 inches to facilitate calculations. All ref-erence surfaces must be parallel with eachother and with the centerline of the plugs orbuttons. The sine plate (see figs. 43 and 44)must meet the same requirements as the sinebar and differs only in that the reference sur-face is in the form of a plate. Some are fur-nished with a base plate and are hinged.

4.8.2 Dial indicators. A dial indicator,also called a dial gage, is a mechanical leversystem or gear train used for amplifying smalldisplacement and measuring it by means of apointer which traverses a graduated dial. Itis not a complete gage unless it is mounted onan arm extending from the frame of a ma-chine, or on some type of pedestal or gagingfixture. The dial indicator. when so mounted,can then measure or indicate the differencebetween the piece being checked and the masteror standard by which it was set.Indicators are of two types, the dial indicator(gear train type) (see fig. 45) and the dialtest indicator (see fig. 46). A sharp distinctionbetween dial indicators and test indicators can-not be made. The dial indicator, however, willoperate through a longer range, and trans-forms a small linear motion of a spindle, bymeans of a rack and pinion and a gear train,into a greatly magnified circular motion of ahand on a dial, while the test indicator em-ploys a lever or combination of levers insteadof a rack and pinion for magnification. Stand-

cast iron alloy and mounted on a three point ard dial indicators are made in sizes rangingsupport. It is provided with a precision key- from to 3¾ inches in diameter. The

27


Figure 39. Tool maker's microscope.

28

Figure 40. Optical dividing head.


Figure 41. Sine bar.

Figure 42. Sine bar set upon gage blocks.

29


Figure 43. Sine plate.

length of the scale or range of measurementvaries from 0.002 to 1.000 inch. The value ofthe individual graduations varies from 0.00005to 0.001 inch. Regardless of the scale gradu-ations, indicators are always classed either asthousandths indicators or as ten-thousandthsindicators, that is, the scale graduations readeither 0.001 or 0.0001” inch. The thousandthindicator is the one most commonly used oninspection fixtures. The ten-thousandth isused for checking dimensions to an accuracybeyond the range of the thousandth indicator,and the length of the spindle travel is muchsmaller.

4.8.3 Surface plates. A surface plate (seefig. 47) is a granite block, or a metal block orcasting, whose working surface is finished to arequired degree of accuracy. It provides anaccurate flat surface which will serve as a ref-

Figure 44. Sine plate with base plate.

30

Figure 45. Dial indicator (gear-train type).

Figure 46. Dial test indicator.

erence plane for inspection and layout work.The working surface of the plate is usuallyproduced by methods which eliminate any tend-ency of gage blocks or measuring instrumentsto wring or stick. Metal surface plates aremade as rigid as possible by heavy constructionand reinforcing ribs under the entire plate, asshown on figure 47, to minimize deflection whena load is applied.

4.8.4 Tool maker’s flat. The tool maker’sflat (see fig. 48) is a very accurate surface platefor the finest and most exacting measurements.


The size of the flat is 5 inches inthickness of inch. Thus, the

Figure 47.

diameter with aflat is limited to

small work. It is recommended for use withgage blocks as they will wring to either surfaceof the flat. Tool maker’s flats are made of steel,hardened, ground, and lapped, top and bottomto a tolerance of 0.000010 inch on flatness andparallelism.

Figure 48. Tool maker’s flat.

31

Surface plate.

4.8.5 Hardened steel square. The hard-ened steel square (see fig. 49) sometimes calleda beam square, consists of a thin blade mounted

Figure 49. Hardened steel square.


at right angles to a beam several times as thickas the blade. The mounting is rigid, and boththe inside and outside angles formed are rightangles. Beam and blade are ground so thatthe sides of each are parallel and straight. Thesteel square is sometimes furnished with a scaleon the blade and sometimes with a beveled bladethat gives a better sight line. Such an edge,however, is liable to wear and damage. The in-side corner of tile intersection between the beamand the blade should be recessed so that burrs onthe edge of the part being checked will not af-fect the accuracy of the square.

4.8.6 Pipe thread gage checking block.The pipe thread gage checking block (see fig.50) is a fixture designed to facilitate the check-ing of pitch and major diameters of AmericanNational taper pipe thread plug gages. Theblock is so constructed that the pitch diameterof a taper pipe thread plug gage can be checkingby the three wire method with tile same ease asif it were straight thread. The major diameter

Figure 50. Pipe thread gage checking block.

of taper pipe thread plug gages or plain taperplug gages can be measured directly without theuse of wires.

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5

TYPES OF GAGES

5.1 GENERAL5.1.1 Scope. This section presents general

descriptions of designs of various commonlyused types of gages,including some designswhich have been standardized by mutual agree-ment among makers and users. Gages madein accordance with recognized standards shouldbe used whenever practicable, as the quantityproduction of gages, thereby made possible,yields considerable economies in their manu-facture and procurement.

5.1.1.1 S t a n d a r d gage blanks. Gageblanks are gages in their unfinished state, thatis, prior to grinding and lapping their gaging(Dimensions to specified sizes. “American GageDesign Standard” gage blanks are those madeto the design specifications promulgated by theAmerican Gage Design Committee in the cur-rent issue of Commercial Standard CS8, GageBlanks, obtainable from the Superintendent ofDocuments.

5.1.1.2 Standard screw thread gages .Standard limits of size of screw thread plugand ring gages and setting plugs for standardsizes, pitches, and classes of screw threads, aretabulated in the current issue of the NationalBureau of Standards Handbook H28, ScrewThread Standards for Federal Services, obtain-able from the Superintendent of Documents.

5.1.2 “Go” gages. “Go” gages control theextent of the tolerance in the direction of thelimit of maximum metal, and represent themaximum limit of the internal member (i. e.,shaft ) and the minimum limit of the externalmember ( i. e., hole). “Go” gages should bedesigned to check simultaneously as many di-mensions as practicable. For example, inchecking a rectangular hole, a rectangular plug“Go” gage would correctly gage both widths ofthe hole at one pass of the gage.

5.1.3 “Not Go” gages. “Not Go” gagescontrol the extent of the tolerance in the direc-

33

tion of the limit of minimum metal, and repre-sent the minimum limit of the internal mem-ber ( i. e., Shaft) and the maximum limit of theexternal member ( i. e.. hole). To be acceptable,parts must not enter or be entered by proper“Not Go” gages. “Not Go” gages should bedesigned to check only a single dimension. Inchecking a rectangular hole in order to insurethat neither width is too large, it is necessaryto check each width seperately with its own“Not Go” gage.

5.1.4 Acceptance check gages. A checkgage is usually a gage to which other gages arefitted in order to determine their size during thegage-making process. They are also used bythe laboratory to inspect the gages submittedfor calibration.

5.1.5 Wear limit check gages. This cate-gory includes gages for the surveillance of.gages in service, such as limit checks for twinrings, limit checks for snap gages on diametersat noses of fuzes, shells, etc.

5.1.6 Master setting gages. These gagesare normally used for properly setting, adjust-ing, or checking inspection gages.

5.2 PLUG GAGES 5.2.1 Plain cylindrical plug gages. A plaincylindrical plug gage is considered to be themost elementary form of a gage. This gageis a cylinder, the cylindrical surface of whichis true to the required degree of accuracy asto size, roundness, and finish and the sectionsof which, at right angles to the axis, are asnearly perfect circles as practicable. Theaxial length of the gage is sufficient to preventthe plug from being canted in the hole and todistribute wear over a large surface. 5.2.1.1 Standard designs. Four seperatedesigns have been adopted for plain cylindri-cal plug gages; the wire type design for ranges0.030 to and including 0.510 inches; the taperlock design for ranges from 0.059 to and in-


eluding 1.510 inches; the reversible ordesign with reversible gaging membersrange from 1.510 to and including 8.010

trilockfor theinches;

and the annular design for the range 8.010 toand including 12.010 inches. For sizes 0.240inch to and including 2.510 inches, both separ-ate “Go” and “Not Go” and progressive gagingmembers are provided. Reference: Commer-cial Standard C58, Gage Blanks (latest edi-tion).

5.2.1.2 Cylindrical plug gage, single-end,solid. This gage (see fig. 51) is merely a cylin-der with a suitable handle integral with thegage. This form is capable of testing only onesize of hole and can make only one test, either“Go” or “Not Go,” but not both.

Figure 51. Cylindrical plug gage, single-end, solid.

5.2.1.3 Cylindrical plug gage, single-end,progressive. This gage (see fig. 52) has twoadjacent cylinders with a common axis, one the“Go” gage and the other the “Not Go” gage.

Figure 52. Cylindrical plug gage, single-end, progressive.

5.2.1.4 Cylindrical plug gage, double-end.This gage (see fig. 53) has the “Go” cylinderon one end of the handle and the “Not Go”on the other, either integral with the handleor with the gaging members and handle asseparate parts.

Figure 53. Cylindrical plug gage, double-end.

5.2.1.5 Cylindrical plug gage, replaceable.The replaceable type (see figs. 52, 53, and 54)is one in which the gaging element or cylindermay be separated from the handle, when worn,and replaced by a new one. The plug propermay have a tapered shank on one end, whichassembles into a tapered hole in the handle,or may be the straight wire type which is se-cured in the handle by means of a chuck orcollet type locking device.

Figure 54. Cylindrical plug gage, replaceable.

5.2.1.6 Cylindrical plug gage, reversible.In the reversible type (see fig. 55) the gagingmember may be turned end for end and reas-sembled to the handle so as to present a sub-

34

stantially unworn surface for the work ofgaging.

Figure 55. Cylindrical plug gages, reversible.

5.2.2 Plain taper plug gages. A plaintaper plug gage (see fig. 56) is an internal gage,for the size control of conical holes, which hasa tapered gaging surface but otherwise is sim-ilar to a plain cylindrical plug gage.

Figure 56. Plain taper plug gages.

5.2.3 Thread plug gages. The thread pluggage (see fig. 57) is the means generally usedto check the thread in a tapped hole. There


is no satisfactory method for directly measur-ing an internal thread, and the thread plug isthe indirect means for checking product andring gages alike. This gage is similar to theplain plug, except that the gaging surface hasthe form of a thread. The taper lock, revers-ible or trilock, and annular designs have beenadopted for thread plug gage blanks and followthe plain cylindrical plug gage sizes. How-ever, the length of the thread gaging memberis slightly different in some instances.

5.2.4 Taper threaded pipe plug gages.The taper threaded pipe plug gage (see fig. 58)differs from the straight thread plug gage inthat the threads are tapered and of a specifiedlength. Separate “Go” and “Not Go” gagesare not used for inspecting tapered threads.The limits of size of taper pipe threads are in-corporated in the gage in the form of stepsor gaging notches representing basic, maxi-mum, and minimum sizes.

5.2.5 Taper plain pipe plug gages. Ataper plain pipe plug gage (see fig. 59) is usedto check the minor diameter of the tapped holefor size, and permissible truncation of thethreads at the minor diameter. The permis-sible variations of the minor diameter, orthread truncation of the product, are trans-posed into longitudinal travel of the gage intothe product and are shown by steps or gagingnotches on the plug. There are six steps in all,arranged in pairs. One pair covers the allow-able variation in truncation for a basic sizethread; one pair is for a maximum size thread,and one pair is for a minimum size thread.Reference Specification AN–P-363.

Figure 57. Thread plug gages.

922173 O - 51 - 4 35


Figure 58. Taper threaded pipe plug gages.

Minimum Basic MaximumLimits Limits L i m i t s

Figure 59. Taper plain pipe plug gages.

5.2.6 Spline plug gages. Spline pluggages are used to check splined holes in partsthat operate on splined shafts. Spline gagesare special as they are usually fabricated forthe inspection of some particular part.

5.2.6.1 Involute spline plug gages. Theinvolute spline plug gage (see fig. 60) containsall of the geometric elements of gears and, de-pending upon their design, may check theseelements individually or as a composite.

Figure 60. Involute spline plug gage.

5.2.6.2 Spanner gages. The straight -sidedspline plug gage (see fig. 61) checks the ele-ments of straight-sided splines in fittings, forstraightness, spacing, tooth thickness or spacewidth, concentricity, alinement, and parallelismas well as the composite of all elements.

Figure 61. Straight-sided spline plug gage.

5.2.7 Alinement plug gages. Alinementplug gages (See fig. 62) resemble in appearancethe cylindrical or progressive plug gage. How-ever, they are usually longer and are used tocheck the alinement of two concentric holes ofthe same or different sizes.

Figure 62. Alinement plug gages.

36


5.2.8 Graduated plug gages. Graduatedplug gages (see fig. 63) are generally of thepiloted type with the larger diameter graduated,and are used principally for checking microm-eter scales on fire control instruments.

Figure 63. Graduated plug gages.

5.2.9 Flat plug gages. Flat plug gages(see fig. 64) have cylindrical gaging surfaces,and are usually made from flat stock, the thick-ness of which may be about one-fourth of thecylindrical portion.

5.3 RING GAGES5.3.1 Plain ring gages. Plain ring gages

(see fig. 66) are external gages of cylindricalform, employed for size control of externaldiameters. The use of the solid ring gage de-sign for external size control has been fairlywell established. In the smaller sizes of plainring gages, a hardened bushing ma-y be pressedinto a soft gage body in place of the one-piecering gage. This design is optional in the rangeabove 0.059 to and including 0.510 inch. How-ever, the single-piece gage may be employed inthis range, and is standard in all cases above0.510 inch. Gages in sizes above 1.510 inchesare flanged, in order to eliminate unnecessaryweight and to facilitate handling. Blank di-mensions of “Go” and “Not Go” rings of identi-cal size range are the same, but an annulargroove is provided in the periphery of the “NotGo” blank as a means of identification. Gagesin sizes above 5.510 inches are provided withball handles.

Figure 64. Flat plug gages.

5.2.10 Miscellaneous plug gages. Thesegages (see fig. 65) are made to meet specificconditions and have various cross sections. Thecross section designates the type of plug gage,such as keyed, grooved, square, rectangular,triangular, oval, etc. Figure 66. Plain ring gages.

5.3.2 Twin ring gages. Twin ring gages(see fig. 67) consist of a flat blank or gage bodyof unhardened steel, bored out to accommodate“Go” and “Not Go” ring gage bushings of hard-ened tool steel. These gages are convenient forthe rapid inspection of certain types of smallprecision parts.

5.3.3 Progressive ring gages. Progressivering gages (see fig. 68) are a variety of the plainring gages, similar to progressive plug gages inthat they have two diameters for checking ‘Go”and “Not Go” dimensions. They are used forcylindrical work of rather short lengths havingsuch close tolerances that a “Not Go” snap gage

Figure 65. Miscellaneous plug gages. is undesirable. They are also used to advantage

37


on soft or thin walled material which might bedistorted by snap gages.

Figure 67. Twin ring gages.

c. A thick flanged type with two adjustingslots for all “Go” coarse pitch gages, ½ to andincluding 5 ½ inches.

d. A thin flat type provided with ball handlesand with a plurality of adjusting slots for allfine pitch “Go” gages and all “Not Go” gagesin the range 5.510 to and including 12.260inches.

e. A thick flanged type provided with ballhandles and a plurality of adjusting slots forall coarse pitch “Go” gages in the range 5.510to and including 12.260 inches.

Figure 68. Progressive ring gages. .

5.3.4 Taper ring gages. Taper ring gagesare variations of plain ring gages except thatthe shafts gaged are conical rather than cylin-drical.

5.3.5 Thread ring gages. The thread ringgage (see fig. 69) is used to inspect external ormale screw threads. The following types ofthread ring gage blanks for straight threadshave been standardized :

a. A thin flat disk type with one adjustingslot ( two slots optional) for all diameters andpitches, both “Go” and “Not Go,” Nos. 0 to 6,inclusive.

b. A thin flat disk type with two adjustingslots for the following:

Figure 69. Thread ring gages.(1) All diameters and pitches, “Go” and

“Not Go” No. 6 to and including ½ inch. 5.3.6 Taper threaded pipe ring gages.(2) Fine pitches “Go” and “Not Go” The taper threaded pipe ring gages (see fig. 70)

only ½ to and including 5 ½ inches. differ from the straight thread ring gages in(3) Coarse pitches, “Not Go” only ½ that the threads are tapered and of a specified

to and including 5 ½ inches. length.

38


Figure 70. Taper threaded pipe ring gage.

. 5.3.7 Taper plain pipe ring gages. Taperplain pipe ring gages (see fig. 71) are the soliddisk type and are used to check the major diam-eter of the external pipe thread for size and per-missible truncation of the thread at the majordiameter. The permissible variation of themajor diameter of thread truncation of theproduct is transposed into longitudinal travelof the gage on the product and are shown bysteps or gaging notches on the ring. There aresix steps in all, arranged in pairs. One pair isfor the maximum size thread; one pair is for aminimum size thread, and one pair is for a basicsize thread. Reference Specification AN-P-363.

5.3.8 Spline ring gages. Spline ring gages(see fig. 72) are used to check splined shafts,either straight-sided or involute, and controlthe same elements as the spline plug gage.These gages are considered special, as they areusually designed and made for the inspectionof some particular part.

Figure 72. Spline ring gages.

5.4 TRIROLL GAGES5.4.1 Threaded pipe triroll gages. T h e

threaded pipe triroll gage ( see fig. 73) is usedto provide a functional check on pitch diameter,taper, angle, lead, and thread form of eitherright- or left-hand taper pipe threads. This

Figure 71. Taper plain pipe ring gage.

39

Figure 73, Threaded pipe triroll gage.


gage checks the full thread length and has theadvantage of permitting thread form, lead)angle, or off-taper conditions to be checkedvisually when the part is in the gage.

5.4.2 Taper plain pipe triroll gages.Taper plain pipe triroll gages (see fig. 74) aredesigned for use in conjunction with the pipethread triroll gage and check the major dianm-eter of external pipe threads for size and thepermissible truncation of the thread crest inrelation to the thread size or pitch diameter.Off-taper conditions are clearly visible whenthe part is in the gage.

Figure 74. Taper plain pipe triroll gage.

5.5 SNAP GAGES5.5.1 Snap gage construction. A snap

gage (see fig. 75) consists fundamentally of twoparallel measuring surfaces (points or anvils)supported in such a manner that the distancebetween their measuring surfaces represents thedistance to be checked. The snap gage may beof a single piece, built up, or adjustable con-struction. It may also be single-end, progres-sive, or double-end.

5.5.2 Adjustable length gages. The ad-justable length gage (see fig. 76) employs forgaging members, and for adjusting and lockingmeans, the same fittings that are used in adjust-able snap gages. The gage heads are designedeither with two pairs of gaging members on thesame side of the spacing bar, or with the “Go”and “Not Go” members on opposite sides of thespacing bar. These gages may be used to covera very wide range, as the spacing bar may beconstructed in any length desired.

5.5.3 Combination ring and snap gages.Snap gages are sometimes made in combina-tion with ring gages (see fig. 77) with the ringas the “Go” and the snap as the “Not Go” gage.

5.5.4 Thread snap gages. There are sev-eral types of thread snap gages, as follows:

5.5.4.1 Roll thread snap gages. R o l lthread snap gages (see fig. 78) may be substi-

Figure 75. Adjustable snap gages.

40


Figure 76. Adjustable length gage.

Figure 77. Combination ring and snap gage.

tuted for standard thread ring gages when au-thorized, as they have the advantages of speedin gaging and long wear life. The setting orresetting of roll thread snap gages should beperformed by the gage laboratory wheneverposs ib l e .

5.5.4.2 Flat-anvil thread snap gages.Flat-anvil thread snap gages (see fig. 79) useflat anvils with a series of grooves representing

Figure 78. Roll thread snap gage.

the number of threads per inch. It is welladapted for parts intended for selective as-sembly.

41


Figure 79. Flat-anvil thread snap gage.

5.5.4.3 Single-point thread snap gages. (see fig. 81) comprises two nearly semicircu-Single-point thread snap gages (see fig. 80) lar gaging members similar to two half nuts.are particularly adaptable to “Not Go” check- These are mounted on pivots to permit theming, but are not recommended for use as “Go” to open and close as the work is passed

Figure 80. Single-point thread snap gage.

gages. One anvil of the gage is a cone and theother a “V”.

5.5.4.4 Segment roll thread snap gages.Another very satisfactory thread snap gage

42

through. The principal advantage of the gageis that it inspects the entire circumference ofthe threaded part instead of merely a singlediameter, as do other thread snap gages.


Figure 81. Segment roll thread snap gage.

5.6 MISCELLANEOUS AND SPECIALGAGES

5.6.1 Flush pin gages. The flush pin gage(see fig, 82) is essentially sleeve containinga pin or plunger, which moves freely, but not

Figure 82. Flush pin gage.

5.6.2 Spanner gages. The spanner gage(see fig. 83) is generally a rectangular holdercontaining two or more pins for gaging dis-tance between holes, such as spanner holes.

5.6.3 Fixture gages. Occasionally a prob-lem will arise in which the test to be made isnot a simple distance, size, or shape, but per-haps a combination of any or all of these. Inthese cases, the dimensions are usually located

Figure 83. Spanner gage.

loosely, therein. The top of the sleeve is ac- from a specific point or center, and a fixturecurately finished and lapped with a step repre- gage (see fig. 84) is required to determine thesenting the tolerance on the component, and accuracy of their relationship. These gagesboth surfaces coming in contact with the work should be designed so that the work is accu-are also lapped. This type is for gaging the rately supported at the point from which thedepth of holes, height of bosses, location of dimensions are taken, with careful considera-holes, etc. tion being given to the manner in which the

4 3


part will function or assemble. Holes may be template or profile gage (see fig. 86) is gen-checked by plugs or flush pins which are an erally used. These gages are usually a nega-integral part of the fixture and are guided tive of the shape of the work to be gaged, sowith proper accuracy to the locating points. that when the part and the gage are placed

Figure 84. Fixture gage.

tour line of5.6.4 Concentricity gages. In order to together the

verify the concentricity of elements having acommon axis, or to limit the eccentricity to apermissible amount. a gage of special designis required to meet the specific conditions.These gages (see fig. 85) may be simple, as forchecking a hole and counterbore, or compli-cated, as for checking simultaneously the rela-tion of several integral holes or shaft sectionsof various diameters.

5.6.5 Template or profile gages. In orderto check the contour or outline of a piece, a

light is shut out between the con-the part and the gage, when the

part is correctly made.5.6.6 Functional gages. Functional gages

(see fig. 87) determine whether or not a partor assembly of parts will assemble and func-tion with some other part or assembly. Oneof the best examples of this type of gage is theprofile alinement gage or the chamber gage,used to check a complete round of ammunitionafter assembly of the cartridge case and theprojectile. This gage simulates the chamber

44


Figure 85. Concentricity gage.

Figure 86. Template or profile gage.

of the gun into which the round must fit.Other types of functional gages include splineand concentricity gages.

5.6.7 Chamber gages. Usually a chambergage (see fig. 88) only assures that the pieceor assembly will fit into the space allotted forit. It does not necessarily show whether ornot the component or assembly is correct. Infact, the various elements of the work may not

45

be acceptable but would be passed by the cham-ber gage. Therefore, it is evident that variouselements of the work must also be gaged in-dividually. The larger chamber gages aremade of several rings, held in alinement andspacing by sleeves, each ring representing a“Go” gage for a portion of the component.A thorough knowledge of the requirements thatthe assembly must meet is necessary in order


Figure 87. Functional gage.

Figure 88. Chamber gage.

that the gage will pass a large number of as-semblies and still have a maximum wear life.

5.7 AIR GAGES. By measuring the pres-sure of air in, or the flow of air through, aclearance space between a gaging spindle of agiven size and the work being gaged. it ispracticable to gage parts to a high degree ofaccuracy. Air gages (see fig. 89) are primar-ily for checking internal diameters. However,they can be used for gaging external diametersand for gaging the distance between both in-ternal and external parallel flat surfaces, es-pecially in instances where it is not desirableto use gaging pressure, and where these dis-tances must be checked over considerable

lengths as in long strips of sheet metal, or longbores.

5.7.1 Pressure type air gage. The pres-sure type air gage operates on the principle ofvarying air pressures. In using this type ofgage, it is necessary to wait for the pressure inthe air column to equalize; the longer the col-umn, the longer the time to wait for equaliza-tion.

5.7.2 Flow type air gage. This gage oper-ates on the principle of varying air flow at con-stant pressure. With this gage, the gagingspindle and the indicating instrument may bewidely separated. The spindle may be an in-tegral part of the gage or at the end of a long

46

Figure 89. Air gage.

length of flexible tubing. Response is prac-tically instantaneous in either case. This gagechecks internal diameters, bell mouth, out-of-roundness, and the average diameter of thinwall cylinders.

5.7.3 Air snap gage. Air snap gages areused for checking external dimensions for di-


ameter, taper, out-of-roundness, and wall thick-ness. Thin walled cylinders can be checkedwithout danger or distortion. Highly finishedor soft plated parts can be checked with aminimum possibility of marring or scratching.

5.8 BORE INSPECTION DEVICES

5.8.1 Star gages. Star gages (see fig. 90)are instruments used to measure accuratelybore diameters. They are used chiefly to meas-ure the normal or abnormal wear and erosionof bores and chambers of cannon to determinetheir remaining accurate life and their fitnessfor use; and to measure pits, burrs, etc., whichoccasionally. form in the bores of cannon. Stargages are used during the manufacture of can-non to determine the interior diameters of mat-ing components such as tubes, jackets, hoops,liners, etc. They may also be used to measureaccurately the inside diameters of recoil cylin-ders, recuperator cylinders, and other objectsof circular bore.

5.8.2 Dial indicator bore gages. Dial in-dicator bore gages (see fig. 91) are spring actu-ated devices for measuring variations in borediameters. All dial indicator gages for stargaging cannon of 37-mm caliber and higher are

Figure 90. Star gage.

47


Figure 91. Dial Indicator bore gage.

48


similar in construction and operation. Theycomprise a head tube, indicator housing tube,and indicator housing assembly.

5.8.3 Horoscopes. Horoscopes are devicesused for visual examination of cannon or sim-ilar tubes. Several types are manufacturedunder different trade names. Each horoscopeincludes an illuminating head which containsa small mirror or a small prism and one or morelamps; a cable for connecting the lamps to the

source of electric power directly, or to a trans-former; one or more tubes, each containing atleast one condensing lens; and an ocular at-tachment.

5.8.4 Mirror and lamp devices. The mir-ror and lamp devices utilize a convex lens ora concave mirror for magnifying the portionof the bore being examined. These devices areused precisely as horoscopes provided withheads having mirrors.

49

MIL-STD-12O12 December 1950

6

CARE, USE, AND MAINTENANCE

6.1 GENERAL. This section deals withthe care, preservation, maintenance, and useof gages.

6.2 SURVEILLANCE OF GAGES6.2.1 Identification marking. It is im-

portant that proper identification markings andsizes be stamped on all gages. Such identifi-cation markings should, in all cases, agree withdrawings and gage record cards.

6.2.2 Loan of gages to contractors. In-spectors will not loan gages to employees ofcontractors unless properly authorized. Gagesshall not be left where they can be used, ortampered with, by unauthorized persons.

6.2.3 Cleaning. Prior to use, and againafter being used and prior to storage, gagesshall be cleaned in Stoddard solvent or othercleansing fluid, so that all shop coatings, dirt,metal chips, finger prints, and other foreign sub-stances are completely removed. Cleaning isbest accomplished by dipping in an agitatedsolvent. When this is not practicable, the itemsshall be cleaned by using a soft brush or cleancloth soaked in solvent. Care shall be taken toprevent trapping of solvent in holes or crevices.

6.2.4 Preservation procedure. Gages forstorage, for shipment between facilities, or forshipment from a facility to a gage laboratory,shall be coated with a corrosion preventive assoon as possible after cleaning operations. Thecorrosion preventive to be used will depend onthe type of gage being stored or shipped. Anapproved rust-preventive compound, light ormedium, should be used on unit gages and fix-ture gages, except in the case of fixture gageshaving internal, close-tolerance gaging surfaces.These should be coated with a light lubricatingoil to which an approved preservative has beenadded. Fixture gages incorporating dial in-dicators should have the indicators removedbefore preservation.

6.2.5 Maintenance and control proce-dures. Standard procedures for the mainte-

nance and control of gages are for the purposeof assuring the supply of dimensionally cor-rect gages at all times. They include the de-tection of damaged gages; assuring that dimen-sionally correct gages are on hand to meet allrequirements; and assuring that all gages inuse are at all times in accordance with appli-cable drawings and specifications, and the latestrevisions thereof. Gages in use must bechecked periodically to insure that they havenot worn to the extent that they accept defectivemateriel or reject acceptable materiel. Varioussystems have been devised, based on usage andwear life that can be expected from the varioustypes of gages. Periodic inspect ion of gagesshould be carried on in all manufacturing plantssupplying materiel to the services. The follow-ing are examples of systems which have beenused with satisfactory results.

6.2.5.1 Gage records. A gage record cardis filled out for each gage at the time it is placedin use. The information on this form shouldinclude the probable number of applicationsof the gage, or the length of time the gageshould remain in use, before rechecking, aswell as gaging dimensions. The card is thenfiled under the indicated date for reinspection.The person responsible for the rechecking ofthe gages need only remove from the file eachmorning the cards that are dated for that dayand check the gages listed. In order to main-tain a complete history of the gage, the recordof actual gage usage can be recorded on thereverse side of the gage record card. The gageusage record is for. the protection and conven-ience of theracy of themine whendimensionalon the gageinto service,experience.

persons responsible for the accu-gages. It enables them to deter-gages should be submitted for arecheck. Gage recheck schedulesrecord card, for new gages goingare estimations based on previous

50


6.2.5.2 Time limit checking system. An-other system of periodic gage inspection is thetime limit checking system wherein recheck-ing is on a time basis. Gages are divided intoclasses; those requiring checking every 30, 60,or 90 days, respectively. In determining theclassification, tolerances and frequency of usemust be considered. This period may bechanged as experience dictates; gages assignedto a 30-day class may be found to belong in the60-day class, or vice versa. In cases of ex-tremely high production, where gages are sub-ject to severe wear, it may be necessary to sub-divide certain classes into groups for more fre-quent rechecking.

6.2.5.3 Use of check or master settinggages. Check or master setting gages will notbe applied to product. They are to be used ongages only. Inspectors should have access tothese gages and make frequent use of them toinsure the proper functioning of his inspectiongages. Extreme care should be exercised whenusing check or master setting gages to avoid anypossible injury to their gaging surfaces.

6.2.6 Damage control and correction.6.2.6.1 Removal of nicks and burrs. The

inspector should check gages for burrs result-ing from machining and have them removedprior to application. Rough or careless han-dling of gages often will produce nicks andscratches, thereby damaging them. Gagesshould be handled with care and never piledon a bench or in a box. All burrs, nicks, andscratches should be removed only by competentpersonnel. Only a hard “Arkansas” stone willbe used to remove them.

6.2.6.2 Control of wear. The principalcause of gage wear is the abrasive action be-tween the gage and the part. Inspectors shouldnot apply gages to parts from which dirt,machine chips, loose scale, or abrasive have notbeen removed. Failure to clean parts will dam-age gages and result in inaccurate inspection.Inspectors should become familiar with wearrates and characteristics. Application of suchinformation will result in much greater usefullife of gages. When doubt exists as to the ex-tent of wear on a gage, it should be forwardedto the appropriate gage checking facility for a

complete inspection. A gage wears most rap-idly at the entering end. Wear at this pointmay be disregarded, or the gage can be correctedby removal of the worn portion provided thatthe remaining portion of the gage will stillfunction satisfactorily.

6.2.6.3 Seizure and galling. Occasionallya gage will seize or gall in or on a component.This usually results from forcing the gage whenapplying it to a part. Care should be exer-cised to aline the gage with the part beinginspected. Seizure is sometimes unavoidablewhen the component dimensions are close to thegage dimensions. The inspector should useextreme caution when this condition exists.When a gage is seized in a component, it shouldbe removed by gently tapping with a soft objectsuch as a brass rod or piece of wood. In caseof extreme seizure, it may be necessary to applyheat locally to the gage or component to releaseby expansion. The amount of heat should notraise the temperature of either above 150° F.,as higher temperatures may draw the temper ofthe gage and may cause warpage.

6.2.6.4 Inspection of dropped gages.Gages which have been dropped should bethoroughly inspected before using. If there isany doubt as to its condition, the gage shouldbe sent to the appropriate gage checking facil-ity for inspection and approval for use. Agage dropped on a concrete floor will be dam-aged. If possible, gaging operations should bemade in a location where there is a woodenfloor, wooden platform, or a linoleum floor cov-ering. Use extreme care to avoid dropping ofgages.

6.3 APPLICATION PROCEDURES. Pro-cedures to be followed in the application ofspecific types of gages to product are specifiedin this section.

6.3.1 Forcing of gages. It is extremelyimportant that the force or torque applied to“Go” gages should not be greater than thatwhich can be easily exerted on the gage handle.Excessive force applied to “Go” gages will re-sult in the acceptance of oversize productswhich should be rejected. Thread gages areprecision equipment and should be used accord-ingly. Hand torque is adequate to insure prop-er inspection. Under no circumstances should

922173 0 - 51 - 5 51


a wrench or lever be used to force a threadgage in or on a product. It is well to rememberthat all thread snap gages and pipe thread tri-roll gages are “comparators”; that is, they com-pare the dimensions of a setting plug gage withthe dimensions of the product being inspected.The force applied to these gages when inspect-ing parts should in no case exceed, and pref-erably be less than, the force applied whensetting the gages.

6.3.2 Method of applying snap gages.Snap gages will be applied with the gaging sur-faces square with the surface to be gaged. Aslow steady motion should be used when ap

plying the snap gages to the component. Smallcylindrical pieces should be rolled between thegaging surfaces of the snap gage. When largecylindrical pieces are gaged, the larger gagingsurface should be placed in contact with theupper surface of the part and the lower gagingsurface gently rocked over the lower surface.

6.3.3 Method of applying small pluggages. Small plug gages should be used withcare, as rough and careless methods of appli-cation will result in broken gages.

6.3.4 Methods of applying thread gages.6.3.4.1 Function of thread gages. T h e

proper use of thread gages will ensure the in-

Figure 92. Proper method of inserting “Go” and “Not Go” thread plug gage.

52


terchangeability and assembly of matingthreaded parts of any given size, pitch, andclass. The use of both “Go” and “Not Go”thread gages is the most general, satisfactorymeans of assuring a product that will be withinthe specified limits of size.

6.3.4.2 “Go” thread plug gages. “ G o ”thread plug gages should check simultaneouslyas many elements as possible, therefore, the gageis designed to have a full form thread and tocheck pitch diameter, major diameter, threadangle, and lead. “Go” thread plug gages con-trol the extent of the product tolerance in thedirection of the limit of maximum-metal, andrepresent the minimum limit of internalthreads. The gages should not be allowed to beused when worn beyond the product limit. Thegage should enter the full threaded length ofthe product freely, with no more force than canbe applied by the fingers (see fig. 92) ; forceshould never be applied as shown on figure 93.A “Go” thread plug gage should never be forcedinto a “Go” thread ring gage, and an adjustable“Go” thread ring or snap gage should never beset with either a working or inspection “Go”

thread plug gage but with a proper settingplug.

6.3.4.3 “Not Go” thread plug gages. “NotGo” thread plug gages are made as nearly aspracticable to check pitch diameter only. Thisis accomplished by reducing both the length ofthe thread flank and the length of the thread.The thread flank is shortened by removing aportion of the thread crest and by a greaterwidth of relief at the thread root. “Not Go”thread plug gages control the extent of theproduct tolerance in the direction of the limitof minimum-metal and represent the maxi-mum limit of internal threads. In use, “NotGo” gages should be applied as shown on figure92 and should not enter the product in excess ofthe limitations contained in applicable threadspecifications.

6.3.4.4 “Go” thread ring gages. “ G o ”thread ring gages, like “Go” thread plug gages,are designed to check as many elements as pos-sible. They are used to check pitch diameter,minor diameter, thread angle and lead, controland extent of the product tolerance in the di-rection of the limit of maximum-metal, and

Figure 93. Improper method of inserting “Go” and “Not Go” thread plug gage.

53


represent the maximum limit of externalthreads. In use, “Go” thread ring gages shouldpass over the entire threaded portion of theproduct and with no more force than can nor-mally be applied with the fingers.

6.3.4.5 “Not Go” thread ring gages. “NotGo” thread ring gage, like the “Not Go” threadplug gage, is made to check pitch diameter only.The length of the thread flank is reduced byincreasing the minor diameter and the widthof the thread relief at the major diameter. The“Not Go” thread ring gage controls the extentof the product tolerance in the direction ofminimum-metal, and represents the minimumlimit of external threads. In use, the productshould not enter the gage in excess of the limita-tions contained in applicable thread specifica-tions.

6.3.4.6 Roll thread snap gages. The rollthread snap gage (see fig. 78) affords a visualmeans of inspection which is both fast and ac-curate. Maximum and minimum limits arechecked in one operation, thus reducing thenumber of gages that would otherwise be re-quired. There is no interference of any conse-quence with the helix of a thread because theribs on the rolls are annular and not helical.Thus a single gage can be used for either right-or left-hand threads, regardless of threadlength. The roll thread snap gage may alsobe adjusted to any class of thread, that is, forclass 1, 1A, 2, 2A, 3, or 3A and still maintainits accuracy. The “Go” rolls on the roll threadsnap gage check lead, maximum pitch diameter,minor diameter, thread angle, and roundness.The ‘(Not Go” rolls on the roll thread snap gagecheck minimum pitch diameter and uniformityof diameter from end to end. The rolls becauseof their design, are not generally subject to theaccumulation of foreign matter. The entryand passage of the screw through the “Go” rollsare shown on figures 94 through 97. By hold-ing the gage and screw as shown, the thumb canbe used to aline the screw with the “Go” rolls.Proper alinement is necessary to produce ac-curate results. Accumulative lead errors inthe length of engagement are checked by themultiple ribs on the “Go” rolls. The maxi-mum pitch diameter is checked by the settingof the “Go” rolls at that measurement, and the

Figure 94. Aline bolt with “Go” rolls and Iower into gag..

Block Iine on screw shows orientation of gage.

passing of the product through the full-thread-form ribs of the “Go” rolls shows up errors inthread depth, form, and angle. The productis then moved to the “Not Go” rolls which haveonly two adjacent ribs, thus avoiding all pos-sible interference from lead errors of the partbeing inspected. The “Not Go” rolls are set tothe minimum pitch diameter limit and checkminimum pitch diameter and uniformity of di-ameter because the rolls are truncated at the

major diameter and are cleared at the minor

Figure 95. “Not Go” rolls stop bolt when it passes through

“Go” rolls.

54


Figure 96. Bolt is turned ¼ turn at “Not Go” rolls to check

for out-of-roundness. Note the new position of the black

line on bolt.

diameter, leaving a narrow flank to contact orgage the product thread at the pitch diameter.Consequently, most of the effects of threadangle errors in the product are avoided. Theproduct thread should not pass through the“Not Go” rolls; parts which do pass throughshould be gaged in accordance with standard“Not Go” gaging practices.

Figure 97. Raise bolt back through “Go*’ rolls and check

further for out-of-roundness.

The product is next given a quarter turn tocheck out-of-roundness, and if the pitch diam-eter is undersize at any point, the part will passthrough the rolls and be rejected. Assumingthat the screw is not found to be undersize atany point, it is passed back between the “Go”rolls for a further check of out-of-roundness.Should the diameters of the screw thus pre-sented to the “Go” rolls be greater than the per-missible maximum pitch diameter, the screwwill not pass between the “Go” rolls, and is notacceptable. Screw threads are acceptable whenthe “Go” rolls pass over the thread with a forcenot in excess of the weight of the roll threadsnap gage.

6.3.4.7 Standard taper threaded pipegages. A standard set of taper threaded pipegages (see figs. 58 and 70) consists of a taper-threaded plug gage and a taper-threaded ringgage. The plug gage has a gaging notch lo-cated a distance L1 from the small end. Thethickness of the ring gage is equal to L1, so thatwhen fitted to the plug gage both the gagingnotch and the small end are flush with the facesof the ring. In checking an external thread,the working ring gage is screwed on the producthandtight. The thread is within the permis-sible limits when the gaging face of the ringgage is not more than one turn, large or small,from being flush with the small end of the prod-uct thread. In checking internal threads, theworking plug gage is screwed into the producthand-tight and is within the permissible limitswhen the gaging notch is not more than one turnlarge or small from being flush with the largeend of the product thread. A tolerance of plusor minus 1 ½ turns is permitted when inspectiongages are used.

MEANING OF TERMS, Ll, L2, L3.

Figure 98. Assembled threaded pipe and coupling at handtight

engagement, showing dimensions L1, L2, and L3.

55


6.3.4.8 Aeronautical type taper threadedpipe ANPT gages. When it is necessary tohold the individual pipe thread elements moreclosely than is possible with the standard taperpipe thread gages, a gaging system involving acombination of specially designed gages com-prised of the following can be used: Two taperthreaded pipe plug gages, L1, and L3, (see fig.58) and one taper plain pipe plug gage (seefig. 59) are used to check internal threads(tapped holes). One threaded pipe triroll gage(see fig. 73) and one taper plain pipe trirollgage (see fig. 74) are used to check externalthreads (pipe). Two taper threaded pipe ringgages and one taper plain ring gage may be sub-stituted for the triroll gages when desired.

6.3.4.9 L1, and L3, taper threaded pipe pluggages. The L1, and L3, taper threaded pipeplug gages check the normal and effectivelength of internal pipe threads and check forthe tolerance of plus or minus one turn (or

thread) from a basic thread size. Three gag-ing steps are provided on the L1 gage, basic,maximum, and minimum. The L1 gaging member checks the normal length of engage-ment which would occur between mating partswhen put together by hand.

The L3, gaging member, the length of whichis equal to L1 plus three threads, and which hasonly four threads at the small end, checks theeffective or usable length of the internal pipethread. The L1 gage is screwed into the inter-nal thread of l the product handtight (see fig.99) and the position of the gaging notch withrelation to the face or reference point of theproduct is noted. The operation is repeatedwith the L3 pipe thread plug gage (see fig. 100).The position of the gaging notch on the L3 gageshall not vary more than ½ turn (or thread)from the position that was noted when the L1

gage was used. For example: if the basicnotch of the L1 gage is ½ turn above the face

Figure 99. L1 gage screwed into product. The gage’s basic notch has stopped at the fitting's face.

56


Figure 100. Pipe fitting shown on figure 99 is next checked with the L3gage, and here stops with basic notch within ½ turn

(thread) from the product face.

or reference point of the product, then the basicnotch of the L3 gage should also be ½ turnabove the face or reference point of the productif the internal thread is correct. If the inter-nal thread is not correct, the L3 gage shall bewithin the permissible limits of ½ turn fromthis position, that is, ½ turn above or below theposition previously noted with the L1 gage.

N O T E: With respect to the use of the gage, the radiallocations of the respective gaging notches are not im-portant ; only the variation from the face of the prod-uct is considered, as mentioned above. However,when the basic notch is produced on the gage, thecircumferential location should be such that the planeof the notch intersects the following thread flank ator near the pitch line.

These gages reveal taper errors, insufficientlength of thread, and excessive truncation at themajor diameter. Figure 98 shows a standardinternal pipe thread. Note the position of thebasic gaging notch of the L1 plug gage in rela-

tion to the face of the product. Next note theposition of the basic gaging notch on the L3

plug gage which has been screwed into thesame thread. The basic gaging notch of the L1

gage is flush with the face of the product. Thebasic gaging notch of the L3 gage is within ½turn of the product face, thereby indicatingthat the product thread is acceptable.

6.3.4.10 Taper plain pipe plug gage. Afterthe L1 and L3 taper threaded pipe plug gagesare used, as specified, the taper plain pipe pluggage must be used to check the truncation of thethreads at the minor diameter (see fig. 101).The permissible variations on the minor diam-eter, or thread truncation, of the product are in-dicated by notches or stepson the plug, namely,one turn either side of basic. There are sixsteps in all, arranged in pairs. one pair coversthe allowable variation in truncation for a basicthread, one pair is for a minimum thread, and

5 7


Figure 101. Taper plain plug gage is used next on product shown on flgures 99 and 100. Here it has stopped with basic notch

at face of product. The fitting is acceptable.

one pair is for a maximum thread. In thismanner, the minor diameter or truncation of theproduct is checked in relation to the thread sizeor pitch diameter as determined by the L1 andL3 plug gages. If the thread size of a producthas been found to be basic by the L1 and L3

plug gages, then the face or reference point ofthe product must be between the pair of stepsmarked “basic” on the taper plain plug gage.Similarly if a thread size is maximum or mini-mum or at any intermediate position or threadsize, which may be estimated, the appropriatesteps are used. Errors in taper or roundnessare indicated by excessive shake of the gagein the threaded hole and are cause for rejection(see fig. 102). The form of external pipethreads may be easily checked as they are read-ily accessible. Further inspection of internalpipe threads can be made by cutting a cross sec-tion from a sample thread, or by inspection ofa cast of the thread (see fig. 103). Externallythreaded parts or cross sections, and proof casts

58

of internally threaded parts, may be checkedwith an approved type tool maker’s microscopeor optical projector.

6.3.4.11 Threaded pipe triroll gage. Thet breaded pipe triroll gage checks the effectivelength of the external pipe thread for round-ness, lead, size, angle, minor diameter, form ofthread, and taper. These gages are used simi-larly to threaded pipe ring gages in that thethreaded product is screwed into the gage undernormal finger torque, until a perceptible re-sistance is encountered. DO NOT FORCETHE WORK INTO THE GAGE. Thethread is then ready for gaging (see fig. 104).The tolerance of plus or minus one turn is de-termined by observing the position of the endof the thread in relation to the stepson the gage.The lowest in height of these steps representsthe minimum acceptable thread size. Themiddle step is the basic or standard size of athreaded part. The highest step represents themaximum acceptable thread size of the product.


Figure 102. Excessive shake or play of taper plain pipe plug gage is cause for rejection of product.

Figure 103. Two fittings cross-sectioned for inspection. Center object is proof cast.

The end of the threaded product should in no 6.3.4.12 Taper plain triroll gage. Aftercase extend beyond the lowest or highest gaging a part is checked with the threaded pipe trirollstep. Thread form and taper are inspected gage, as specified, the taper plain triroll gagevisually by observing the contact made between is used to check the major diameter of externalthe product and the rolls. pipe threads for permissible truncation of the

59

MIL-STD-l 2012 December 1950

Figure 104. Fitting screwed into threaded pipe triroll

thread crest, by checking the relation of thecrest to the thread size or pitch diameter. Thegage is slipped over the product until a per-ceptible resistance is encountered between therolls of the gage and the major diameter of theproduct. DO NOT FORCE OR ROTATE.Simultaneous contact is made between the endof the product and the flush pin contact flangeof the gage (see fig. 105). The variation of

60

gage. Small end of fitting is flush with basic stop.

the major diameter, or thread crest truncationof the product, is transposed into longitudinaltravel of the flush pin. The position of thegaging reference points located on the end ofthe flush pin, in relation to the steps on thehub at the rear of the taper plain triroll gage,indicates the amount of truncation. The threesteps on the hub of the gage represent one turneach, and are identified as basic, maximum, and


minimum. The step on the end of the flush pinrepresents the allowable variation on the majordiameter, or thread crest truncation. If thethreaded pipe triroll gage indicates that theproduct’s thread is basic, when measured as de-scribed, the basic step on the hub of the taperplain triroll gage is used to determine the sizeof the major diameter. The major diameter iswithin the specified tolerances for truncationwhen the basic step on the hub is either flushwith or within the step of the flush pin. Simi-larly the maximum or minimum step is usedwhen the thread is maximum or minimum. Atany intermediate size it is necessary to estimatethe proper position. Error in taper is indi-cated by excessive shake of the gage. For allpractical purposes the error in taper may be

measured by inserting thickness gages betweenthe gage rolls and the major diameter of theproduct at the point of extreme gap. Thick-ness gages of the same size should be insertedat each roll to avoid canting.

6.3.4.13 Threaded pipe ring gages. Whenthreaded pipe ring gages are used, it is recom-mended that two types be used. The L2 ringgage should have the threads removed suf-ficiently to clear the major diameter of theproduct for a distance of L1 minus pitch fromthe small end of the gage. The remainingthreads should have a full profile and be trun-cated at their minor diameter an amount equalto the maximum specified truncation of theproduct, The L1 ring gage, which has thethreads truncated at their minor diameter an

Figure 105. Taper plain triroll gage with fitting inserted. Flush pin shows fitting to be basic.

61


amount equal to 0.15 pitch should also be used.In checking the product, the L2 ring gage isscrewed tightly onto the product by hand. DONOT FORCE. The position of the face at thesmall or cleared end of the ring, in relation tothe end of the product, is noted. If the L2

ring gage shows the product to be within thepermissible tolerance of plus or minus one turnfrom the end or reference point of the product,the operation is repeated with the L1 ring gage.The relative position of the reference faces ofthe two gages must not vary more than ½ turnfrom the end of the product. Size and off-taper conditions are readily checked in thismanner and such defects as are produced byexcessive chamfer in the throat of the dies orchasers, worn die and chaser threads, and threadangle errors will be revealed within the limits ofcombination pipe thread ring gaging.

6.3.4.14 Taper plain ring gages. Whenusing taper plain ring gages to check the majordiameter. over the effective length L2 of the ex-ternal pipe threads, for size and permissibletruncation of the thread crest, in relation to thethread size or pitch diameter, it is recommendedthat the ring gage incorporate six gagingnotches or steps arranged in pairs (see fig. 71).One pair is to cover the allowable variation intruncation for a basic thread; one pair for aminimum thread, and one pair for a maximumthread. When the threaded pipe triroll gage orthreaded pipe ring gages indicate the threadsize to be basic, then only the basic gagingnotches or steps on the plain taper ring gageshall be used in determining the allowablevariation in major diameter. The small end

of the threadedtwo steps whenslipped tightly

product shall be within thesetile plain taper ring gage is

over the product. DO NOTFORCE. Similarly the appropriate steps shallbe used when the thread size is maximum orminimum, or at any intermediate position orsize which may be estimated. Errors in taperand roundness are indicated by excessive shakeor play.

6.3.4.15 Special threaded pipe gaging con-ditions. Should drawings and other data fur-nished by the procuring agency require externaland internal pipe threads to be chamfered orcountersunk in excess of the dimensions shownin specifications, the thread size shall be deter-mined by using the end of the chamfer or bot-tom of the countersink as the reference point,which shall be considered as being the inter-section of the chamfer cone and the pitch coneof the thread, instead of from the end of thepipe, fitting or coupling.

6.3.4.16 Check or master setting threadplug gages. These are thread plug gageswhich are used to set adjustable thread ringgages, thread snap gages, and other thread com-parators. They are of two standard designsdesignated as basic-form setting plugs and trun-cated setting plugs. The basic form settingplug (see fig. 106) has a major diameter cor-responding to the basic or maximum majordiameter of the screw. The truncated settingplug has the crest of the thread truncated forapproximately ½ its length. CommercialStandard CS8 (latest edition) shows a settingplug which has approximately one-half of thethreads truncated and the other half full form.

Figure 106. Setting plug gage having full and truncated threads on each member.

62


The truncated threads control the pitch diam-eter of the ring gage, and the full form threadsthe thread form and clearance at the majordiameter. Ring gages should be adjusted to fitthe basic form portion of the setting plug andthen tried on the truncated portion. Thereshould be only a slight difference in the fit.The presence of shake-or play on the truncatedportion indicates inadequate clearance at theroot of the ring thread. Basic form settingplugs are required only for setting roll threadsnap gages.

6.3.4.17 Check or setting taper threadedpipe plug gages. These are taper-threadedplug and taper plain plug gages. The taperpipe thread setting plug gages (see fig. 107)have a thread length of L1 and have one gagingnotch located a distance of L1 from the smallend of the gage. They are used for checkingpipe thread ring gages for size and the settingof threaded pipe triroll gages. When the L1

ring gage is assembled handtight, the gagingface should be flush with the small end of theplug and the opposite face flush with the gag-ing notch on the plug. The L2 r ing gageshould assemble handtight with the gaging faceat the small end of the ring flush with the smallend of the plug. A threaded pipe triroll gageis set correctly when the small end of the plugis flush with the basic step on the triroll framewhen assembled handtight (see fig. 108). The

Figure 108. Threaded pipe triroll gage set correctly.

taper plain setting pipe thread gages (see fig.109) have no gaging notches, the face at thesmall end being the gaging surface. They areused for checking taper plain pipe thread ringgages and setting taper plain pipe thread trirollgages. When the plug and ring are assembledhandtight, the small end of the plug shouldbe flush with the basic gaging step) on the ringgage. The taper plain triroll gage is set cor-rectly when the gaging end of the flush pin isflush with the basic step on the hub of the gageframe, when assembled handtight with thesetting plug gage (see fig. 110).

Figure 107. Taper pipe thread seetting plug gage. Figure 109. Taper plain setting pipe threaded plug gage.

63


Figure 110. Taper plain triroll gage.

64


7

SURFACE ROUGHNESS MEASUREMENT

7.1 GENERAL This section describes in-struments and methods applicable to the meas-urement of the roughness or surface finish ofsolid surfaces.

7.1.1 Definition. Surface roughness is therelatively finely spaced irregularities on thesurface of an object. On surfaces produced bymachining and abrasive operations, the irregu-larities produced by the cutting action of tooledges and abrasive grains are roughness. Sur-face roughness should not be confused withwaviness, as surface roughness is the finer ofthe two and may be considered as being super-posed on a wavy surface.

7.1.2 Standardization. Military Stand-ard, MIL–STD–10, Surface Roughness Wavi-ness and Lay, establishes a uniform method forindicating on drawings the surface roughness,waviness, and lay of surfaces. It is in agree-ment with the American Standard ASAB46.1—1947 and other National standards.

7.1.3 Application to gages. The surfaceroughness on gaging surfaces affects the wearlife of a gage. The rougher the gaging surfaces,the faster the wear on a gage in service. Thesurface roughness of all gaging surfaces mustbe controlled if the usual gage ‘life expectancyis to be obtained.

7.2 SURFACE ROUGHNESS MEAS-UREMENT

7.2.1 Tracer type instruments. Numer-ous methods have been devised for measuringsurface roughness. For practical purposes, in-struments of the tracer type which employ apoint or stylus to explore the irregularities andrecord them greatly magnified are generallyused. The two such instruments most exten-sively used are the profilometer and the brushsurface analyzer.

7.2.2 Profilometer. The profilometer withmototrace (see fig. 111) has a tracer unit which

is drawn over the surface and the movement ofthe tracer point perpendicular to the surface ismagnified electrically by an amplifier, Theroughness value is read in microinches of rootmean square average roughness on a metermounted on the instrument panel.

7.2.2.1 Tracer unit. The tracer unit (seefig. 112) is small enough to be held between thefingers and moved over a surface by hand.However, provision has been made for attach-ing a stiff arm which is a handle extendingfrom the rear of the tracer to facilitate handoperation. A linkarm connects with the moto-trace when automatic tracing is used. Thetracer contains two hard polished metal skidswhich support the tracer on the surface beingtraced and establish a reference plane fromwhich the deviations are measured. The tracerpoint consists of a small shaft with a diamondtipped point (0.0005-inch radius) which ex-tends below the two skids sufficiently to contactthe surface of the work with a very light pres-sure. No adjustment is needed on the latestmodel profilometer but the proper skids must beused. Older model tracers are equipped with amicrometer adjustment knob by which thetracer - point can be raised or lowered, therebyregulating the position of the diamond pointwith respect to the skids and also the pressurebetween the diamond and the working surface.

To permit the measurement of various con-tours, three interchangeable skid mounts (seefig. 112) are furnished differing in size, shape,and center distance. The largest skid mount isfor surfaces ¾ inch outside diameter to flat,and the smallest mount for surfacesinch outside diameter only.

7.2.2.2 Microinch meter. Surface rough-ness measurements are read directly from themicroinch meter (see fig, 113) in units of mico-inches (0.000001 inch), average roughness(r. m. s.). Because of the several stages of

6 5

MIL-STD-12012 Docombor 1950

amplification with which the

Figure 111. Profilometer with mototrace.

profilometer is 2, and 3; if the scale-selector is set at 30, theequipped, as indicated by the scale-selector values on the top meter scale are 10, 20, and 30;switch, the microinch meter is provided with and if the scale-selector is set at 300, the valuestwo scales, the upper scale reading 0 to 30 and on the top meter scale are 100, 200, and 300.the lower scale reading from 0 to 10. The If the scale-selector is set at 10, the values onmeter scale used, and the way in which it is read, the lower meter scale are 2, 4, 6, 8, and 10; ifis determined by the setting of the scale-selector the scale-selector is set at 100, the values on theswitch. The setting of the scale-selector corre- lower meter scale are 20, 40, 60, 80, and 100;spends to the maximum reading of the scale on and if the scale-selector is set at 1000, the valuesthe meter. When the scale-selector is set to 3, on the lower meter scale are 200, 400, 600, 800,30, or 300 the top scale only on the meter is read; and 1,000.when the scale-selector is set to 10, 100 or 1000 7.2.2.3 Mototrace. The mototrace is athe bottom scale only on the meter is read. For mechanical means of moving the profilometerexample, if the scale-selector is set at 3, the top tracer over a selected distance at a uniform ratescale on the meter is read and the values are 1, of travel. It permits more accurate and con-

66


Figure 112. Tracer unit with interchangeable skid mounts.

sistent measurements on very smooth surfacesand surfaces where only a very short stroke canbe made. The mototrace enables greater effi-ciency when large numbers of similar parts areto be measured. The stroke of the mototraceis adjustable from inches by the move-ment of two thumbscrews on top of the case.

7.2.2.4 Glass roughness specimen. Aglass roughness specimen of a known roughnessis furnished with each profilometer (see fig.114). This is used to test the condition of thetracer and profilometer before use. When thetracer is moved back and forth over the glass,the meter should indicate the value that ismarked on the back of the glass specimen.

7.2.2.5 Interpretation of meter readings.The reading of the profilometer meter is inmicroinches of average roughness (r. m. s.).If the specifications for a part call for a finishnot rougher than 20 microinches (r. m. s.) andthe reading obtained is 25, the part is not ac-ceptable. Likewise, if a minimum limit of 10microinches (r. m. s.) has been specified, areading of 7 indicates that the part is toosmooth. In most cases it will be found that

Figure 113. Microinch meter and scale-selector switch.

the roughness of a part varies from point topoint along the surface. Generally when anappreciable portion of the surface is rougherthan specified, the part is rejected. This va-riation is generally caused by variations in thefinishing process and it is the custom to con-sider the extreme readings for purposes of ac-ceptance or rejection. However, high readingsof short duration are generally ignored andonly sustained values considered. When usingthe mototrace on short strokes, the meter willbe observed to “kick” occasionally at the pointof stroke reversal. These kicks do not occurat every reversal, but are merely transient phe-nomena l and should be ignored. By disregard-ing these irregular fluctuations of the meter, itwill be seen that the meter follows, for the mostpart, a regular pattern which repeats betweeneach reversal of the stroke. The reading indi-cated by this pattern should be noted. The

Figure 114. Glass roughness specimen.

922173 O - 51 - 6 67


Figure 115. Proniometer rotary pilotor.

tracer should always be moved across the sur-face pattern of the part in the direction thatproduces the roughest or highest reading.

7.2.2.6 Using the profilometer. To use theprofilometer it is only necessary to plug into apower line of appropriate voltage and fre-quency, turn on the switch and allow about 5minutes for warm up, mount the proper skidmount for the surface to be traced, check on

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the glass roughness specimen, and proceed totake measurements. However, to obtain themost accurate results; the following pointsshould be observed:7.2.2.6.1 Set-up. The profilometer should

be set up on a sturdy work bench and locatedas far as possible from vibrating or reciprocat-ing machinery. Connection should not bemade to power lines supplying heavy induction


Figure 116. Brush surface analyzer.

equipment, such as induction furnaces and in-duction motors which are frequently startedand stopped. For particularly accurate meas-urement of very smooth surfaces, it will benecessary to provide a surface plate supportedwith rubber mounting on the work bench todampen any vibration that might be present.The profilometer can be placed immediatelybehind the surface plate and the plate usedonly for holding the work.

7.2.2.6.2 Condition of the work. The sur-face to be traced should be clean and free fromforeign particles such as dust, metal shavings,and abrasive compounds. Excessive oil orgrease should be wiped from the surface andgreat care taken that oil does not penetrate thetracer and gum up the moving parts. A slightfilm of oil on the part, however, is not objec-tionable and may cause the tracer to move moresmoothly.

7.2.2.6.3 Holding the work. Appropriatemeans for holding the work steady, and main-taining the surface to be measured parallel tothe top of the bench or surface plate should beavailable. The average part is heavy enoughto stay in position while the tracer is being

moved across the surface. On lighter or roundparts where there is a tendency to move or roll,small dabs of modeling clay may be used tohold the work on the surface plate. A smalltool maker’s vise is ideal for holding small ir-regularly shaped parts.

7.2.2.7 Profilometer rotary pilotor. Theprofilometer rotary pilotor (see fig. 115) is apower driven instrument designed to providethe tracing motion for measuring the surfaceroughness of circular and cylindrical surfaces.It is used in conjunction with a standard pro-filometer and a suitable tracer. By appropri-ate mounting of the tracer on a rotatable ad-justable arm either inside or outside diameterscan be traced. It is adapted particularly fortracing the arc of an inner or outer ball-bearingrace groove, and for tracing the surface of aball. It is readily set up, however, for use ona great variety of surfaces which have a cylin-drical or circular shape and require a rotarytracing motion for surface roughness measure-ments.

7.2.3 Brush surface analyzer. The brushsurf ace analyzer (see fig. 116) is an instrumentdesigned for the exploration and recording of

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Figure 117. Pick-up arm.

surface contours of metals, glass, wire, plastics,paper, plated and painted surfaces, and toolcutting edges. It is similar in principle tothe profilometer, inasmuch as it also employsthe tracer method. It consists of four prin-cipal parts: The motor driven pick-up arm, thecalibrating amplifier, the direct inking oscillo-graph, and the root mean square meter forvisual indication in microinches of averageroughness.

7.2.3.1 Pick-up arm. The pick-up arm (seefig. 117) contains a piezoelectric crystal or mag-netic element, housed in the outer end which isconnected through a lever system to a diamondpoint stylus located at the extreme tip of thepick-up arm. Directly behind the stylus is anadjustable positioning shoe. This shoe is ofsuch design and size that it rests upon a rela-tively large surface, bearing the entire freeweight of the arm and establishing a zero refer-ence level from which the stylus measures de-partures. The pick-up arm cannot be operatedby hand; it must be moved by the drive head.The trace of the diamond stylus is alwaysparallel to the path of motion of the recipro-

cating drive shaft. This permits the explora-tion of recessed or curved surfaces that areotherwise impractical to analyze. The pick-uparm normally supplied has a diamond pointwith a radius of 0.0005 inch which is suitablefor normal work having finishes from 1 to 100microinches (r. m. s.). Other pick-ups forspecial applications are available. There isalso a drive head and pick-up arm available formeasuring rougher surfaces to 3,000 micro-inches.

7.2.3.2 Surface plate. In order to providea working surface isolated from extraneous vi-brations, a rubber shock mounted surface plateis provided, upon which the drive head standand specimen under test may be placed. Thesurface plate is 10 by 14 inches and weighs 35pounds. The rubber mounted feet are capableof supporting specimens up to about 30 pounds.

7.2.3.3 Drive head. The drive head con-sists of a 110 volt, 60 cycle motor and geartrain enclosed in an aluminum housing. Themotor drives a cam, which in turning, causesa spring loaded vertical shaft extendingthrough the bottom of the housing to execute areciprocal motion of inch in a horizontalplane. To this shaft. is fastened a yoke whichholds the pick-up arm. One complete cycle ofmotion requires 10 seconds. corresponding toa tracing speed of 0.0125 inches per second.The position of the pick-up at any instant rela-tive to its front and back positions is indicatedby a dial on the front of the housing. Thedrive unit is mounted on a tripod stand andplated shaft. The head can be swung hori-zontally, while a rack and pinion assembly per- .mits vertical adjustments. A knurled knobpermits clamping of the drive unit in any posi-tion.

7.2.3.4 Amplifier. The amplifier (see fig.118) greatly magnifies the small electrical im-pulses generated by the piezoelectric crystalcontained in the pick-up and delivers them toeither the oscillograph pen rotor or the r. m. s.meter. It has a six-step attenuator or sensitiv-ity control for different degrees of roughnessof the analyzed surface, while the “gain control”provides a fine adjustment of the amplification.The calibrating voltmeter, in conjunction with

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erly adjusted to 10 millimeters overall. The

Figure 118. Amplifier.

the calibration voltage control, provides ameans for accurately correlating a given oscillo-graph pen deflection with the correspondinginput voltage.

7.2.3.5 Direct inking oscillograph. Thedirect inking oscillograph (see fig. 119), re-cords the amplifier output on a moving stripchart, making a graphic record of the profileof surface irregularities of the specimen undertest. It is made up of a crystal or magneticpen motor with a 3-inch nickel pen and ink-well, and a chart drive mechanism. The latteris a motor driven arrangement for moving thechart paper under the point of the pen. A se-lective gear train gives a choice of three paperspeeds of 5, 25, and 125 millimeters per second,corersponding approximately to 1, and 5inches per second. Choice of these speeds ismade by a small knob at the right of the oscillo-graph. The knob all the way in gives slowspeed, halfway out gives medium speed, and allthe way out gives high speed. The oscillo-graph case is provided with a hinged coverwindow which may be opened to allow the userto make notations on the chart.

7.2.3.6 Interpreting the oscillographchart. The dark band in the lower portion ofthe chart is the trace resulting from the cali-bration procedure and shows the width prop-

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remainder of the chart represents the trace ofthe surface of a ground specimen, with the at-tenuator set on the 0.01 tap, and the chart speedon intermediate setting. With the instrumentproperly calibrated, each millimeter (small di-vision) of the chart across the width of thepaper represents a multiple of 10 microinches,as indicated in the following table:

With the attenuator Each small divisionset on tap of chart paper repre-

sents0.001 1 microinch

(0.000001 inch)0.01 10 microinches



(0.001000 inch)

In the example given, it will be noted that thelargest deflection to the right of the centerlineis about 10 millimeters while the largest peakat the left is 7.5 or 17.5 millimeters overall. Thiscorresponds to a maximum roughness of 10 by17.5 or 175 microinches (0.000175 inch) since theattenuator was on the tap where 1 millimeterequals 10 microinches. Viewing the chartpaper at the end of the oscillograph from whichit emerges, deflections to the left of the center-

Figure 119. Direct inking oscillograph.


line indicate peaks above the mean surface; de-viations to the right represent valleys. Inaddition to the depths of surface irregularities,the chart shows other microscopic surface fea-tures such as width, spacing, and contour.Spacing or width of irregularities can be cal-culated from the chart speed. At low speedeach longitudinal chart division represents ap-proximately 0.0125 inch; at second or inter-mediate speed each division represents 0.0025inch: at high speed 0.0005 inch. These are sev-eral possible ways of evaluating a surface bymeans of a chart trace. One is the use of actualmaximum heights or depths of peaks andvalleys, as read directly from the chart. Formany applications, a figure representing theaverage surface roughness is desirable. Thiscan be obtained by use of the r. m. s. meter or itmay be computed from a large number, such as100, equally spaced readings on the chart.

7.2.3.7 Root mean square meter. The rootmean square meter (see fig. 120) is a device foruse in conjunction with the surface analyzerfor giving a quick, visual indication of averagesurface roughness. It may be quickly insertedinto the standard set-up and may be used eitheralong with the chart recording, or separately.Located on top of the meter chassis are two con-trols, one marked “meter setting” which is alsothe “off-on” switch, and a button marked “pressfor 3 scale”, which increases the sensitivity ofthe meter 3 times. To place the meter in op-eration after the proper connections have beenmade and the analyzer calibrated, the same as ifthe chart only were to be used, turn the meteron, press the “calibrate” button on the ampli-fier, and adjust the “meter setting” control ontop of the meter chassis until the meter needleindicates 3.5 (red line on face). The meter isthen calibrated. Adjust the attenuator settingto give a good readable deflection as the pick-uparm moves over the surface being analyzed.With the attenuator on the 0.001 tap, the upperscale on the meter reads directly in microinchesof average roughness (r. m. s.), a full scaledeflection corresponding to 10 microinches(r. m. s.). For the 0.01 tap, multiply the upperscale readings by 10, for the 0.1 tap multiplyby 100, and for the 1.0 tap multiply by 1000.By pressing the button marked “press for 3

scale” the bottom (green) scale is read. In thiscase, a full scale meter deflection correspondsto: 3 microinches (r. m. s.) with attenuator seton 0.001 tap, 30 microinches (r. m. s.) with at-tenuator set on 0.01 tap, 300 mimoinches(r. m. s.) with attenuator set on 0.1 tap, and3000 microinches (r. m. s.) with attenuator seton 1.0 tap.

7.2.3.8 Glass calibration standard. Grad-ual changes in amplifier or pick-up arm char-acteristics and general aging may make neces-sary from time to time a slight revision of thesupplied calibration voltage (marked on theinside cover of the pick-up arm box). Thiscalibration may be corrected by means of theglass calibration standard furnished with theinstrument (see fig. 121. ) This glass standardhas two precision scratches on its surface, lo-cated approximately as indicated by two whitelines on a black background. There is one deepand one shallow scratch (approximately 100and 10 microinches, respectively), the correctdepths of which are indicated opposite the re-spective background lines. With the instru-ment set up and calibrated properly, the traceris allowed to traverse the deep scratch severaltimes at right angles. The total deflection onthe chart should equal the given depth of thescratch. If there is a difference between theactual and recorded scratch depths, proper ad-

Figure 120. Root mean square meter.

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Figure 121. Glass calibration standard.

justments should be made until the actual depthis recorded. Occasionally the tracer should berun over the shallow scratch to check the con-dition of the diamond point. The depth of theshallow scratch should not differ more than 2microinches from the depth specified on thestandard when the tip of the diamond is in goodcondition. Do not calibrate the instrumentwith the shallow scratch. Each calibrationstandard is designed for use with a specifiedradius of tracer point and is so marked. Donot use for any other radius.

7.2.3.9 Using the brush surface analyzer.The operator should follow in detail the com-plete operating instructions furnished witheach analyzer in order to obtain best results.In general, the “set-up” and condition of thework are the same as specified for the profilom-

eter (see 7.2.2.6) but the adjustment for con-tact pressure is entirely different. However,the following points should be observed:

a. Never allow the pick-up arm to be heatedabove 120° F. Temperatures above this willpermanently injure a crystal element.

b. When handling the pick-up arm or leav-ing it set for use, be careful not to bump thediamond point against a hard surface or leaveit where it may swing against any object.

c. When leaving the analyzer set up for use,turn the attenuator to 1, 10, or 100 to preventlarge disturbances of the pick-up arm fromoverloading the oscillograph or amplifier.

d. Never check hot surfaces, as this maysoften the cement holding the diamond pointin the pick-up arm.

7.2.4 Surface roughness comparatorblocks. While surface roughness may be meas-ured directly by the brush surface analyzer andprofilometer, a quick, practical method ofchecking the rougher, noncritical surfaceswhere visual and tactual comparison is suffi-cient, is by the use of surface roughness com-parator blocks (see fig. 122). These blocks arefurnished in sets of varying numbers and eachsmall steel block, or replica, represents a par-ticular type of surface finish such as grind,turn, shape, mill, lap, and polish. The degreeof roughness is marked on each block in micro-inches (r. m. s.). In use, the piece to be in-spected is compared by feel and sight with theappropriate block. Emphasis should be placedon feel rather than sight, as visual inspectionis not reliable because of differences in material,contour, and manner of machining.

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Figure 122. Surface roughness comparator blocks.


8

GAGE INSPECTION METHODS

8.1 GENERAL

8.1.1 Purpose. This section is intended forthe use of gage inspectors in the gage labora-tories and other gage inspection establishmentsof the Armed Services.

8.1.2 Scope. This section relates largely totile application to precision measuring equip-ment (see sec. 4) to the inspection of gages.Included within its scope are instructions withregard to laboratory conditions, general gageinspection, care and usc of inspection tools, andcare and use of precision measuring instru-ments. Detailed methods are specified for in-specting various types of gages and calibrat-ing precision measuring equipment.

8.1.3 Arrangement. The section has beenarranged in ascending order of difficulty. Thefirst port ions are largely elementary and areintended primarily for the use of comparativelyinexperienced gage inspectors. However, theinformation in the first portion should not beneglected by experienced inspectors. Thelater portions contain formulas and methodswhich should be of value to experienced aswell as inexperienced inspectors. It is impor-tant that gage inspectors study the completetext on methods of inspecting any one type ofgage instead of merely glancing at illustrationsand formulas. Errors often result when in-spectors use a method or formula without thor-oughly understanding the manner in which itshould be applied.

8.2 LABORATORY CONDITIONS

8.2.1 Laboratory temperatures. When-ever precision measurements are to be made, thetemperature should constantly be kept as nearto 68° F. as possible. Since most gages andmeasuring instruments are usually made ofsteel, they have practically the same coefficientof expansion, and, therefore, the requirementthat the temperature remain constant is more

75

important than the actual temperature. Vari-ations in temperature can cause considerableerror in precision measurements, especiallywhen the complete inspection operation takesconsiderable time. Gages should be stored ina constant temperature room several hours be-fore they are inspected in order to make certainthat all portions of the gages attain the sametemperature as the room. The amount of time

required for this storage depends on the accu-racy of the measurement to be made and uponthe size of the gage or object to be measured.

8.2.2 Atmospheric conditions. The rela-tive humidity of the atmosphere in a gage lab-oratory should preferably be kept under 50percent in order to minimize the possibility ofcorrosion. In general, the air conditioningsystem of a laboratory should remain in opera-tion during weekends, hollidays, or such othertimes as inspection operations are not in prog-ress. This is both because the change in hu-midity may cause corrosion and because thereare delays while instruments and gages reachthe proper temperature on the day when thelaboratory is reopened.

8.2.3 Cleanliness. Cleanliness is an im-portant requirement for a good gage labora-tory. Small particles of dirt cause serious er-rors in precision measurement and bring aboutexcessive wear of precision instruments.

8.2.4 Lighting. Good lighting facilities arevaluable, not only because they help gage in-spectors make more accurate measurements, butalso because they reduce eye strain and generalfatigue.

8.2.5 Location of precision instruments.Precision instruments should not be placedwhere the direct rays of the sun can fall onthem. The heat of the sun will cause expan-sion of instruments and subsequent contractionwill set in when the instruments are no longerexposed to the rays.


8.3 G E N E R A L INSPECTION IN-STRUCTIONS

8.3.1 Drawings. Gage inspectors should beprovided with the applicable revisions of thegage drawings for the gages they are to inspect.When necessary or advisable, the drawing ofthe part for which the gage is intended shouldalso be supplied.

8.3.2 Markings. After gages are inspectedthey must be marked for identification. Wherethere is any possibility that stamping will im-pair the accuracy of a gage, markings shouldalways be put on with an electric pencil or bymeans of an etching or engraving process. Asan example, the base of a locating gage shouldnot be stamped because it has been found thatthe stresses set up in stamping tend to shift theposition of the gaging members.

8.3.3 Gage records cards. In order to lo-cate gages in storage and provide a method ofrecording historical data pertaining to eachgage, it is necessary that some form of per-manent record be prepared and maintained.Usually this is done by means of a gage recordcard. The format of this record, and the man-ner in which it is prepared, varies among theservices. In general, however, the followinginformation should be recorded:

a. Name of installation or establishmentwhere the inspection and initial acceptance wasconducted.

b. Identification number of the gage, includ-ing the letters used by the accepting laboratory.

c. Gage drawing and revision date or letterto which the gage was made.

d. Name of the item to which the gage is toapply.

e. Number and date of the drawing showingthe item.

f. Any identifying letter, number or symbolfor a particular part, shown on the item draw-ing, to which the gage applies.

g. Value of the gage as represented by thecost price.

h. Type of the gage.i. Function of the gage.j. Dimensions to be checked by the gage, as

represented on the item drawing.

k. Actual gage dimensions as determined bythe gage inspector.

1. Exact information as to the storage loca-tion of the gage.

m. Name of the gage inspector and the dateupon which the gage was checked.

n. Any additional remarks deemed neces-sary.

8.3.4 Precautions.8.3.4.1 In making precision measurements,

inspectors should be careful not to handle gagesand instruments any more than necessary, be-cause the heat of the hands will cause expan-sion. All gages, tools, and instruments, as wellas the inspector’s hands should be clean at alltimes in order to prevent minute particles ofdirt from interfering with proper measure-ment. Another reason for keeping the handsclean is that the natural acid content of thesecretions of human hands tends to corrodeprecision surfaces.

8.3.4.2 When making computations, thenumber of figures carried should always be onegreater than the number of figures desired inthe answer. For example, in measuring ataper plug less than one-tenth in diameter, itis preferable to calculate to hundred thous-andths, to use five place trigonometric tables,and to carry five decimal places in all numbersexcept the final answer. When a square rootis to be extracted, it should first be decided howmany significant figures are required in theanswer. Then the quantity from which thesquare root is to be extracted must contain thesame number of significant figures. Zeros im-mediately after the decimal place do not con-stitute significant figures. For example, thenumber 0.0036 contains only two significantfigures.

8.3.4.3 When the dimensions of gages arevery close to the specified limits, or when cer-tain gages are needed very urgently, the gageinspector will often be required to exercise con-siderable judgment in deciding whether or notthe gage should be accepted. An importantfactor to be considered in this connection is thedirection of the tolerances given on the gagedrawing. Generally no great harm will be dormif the gage exceeds its limits in the directionof the product tolerance, but if the gage exceeds

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the limits in the direction away from the toler- percent of the gage tolerance should be usedante, the error is serious. For example, a plain whenever such an instrument is available pro-“Go” plug gage is given a plus tolerance. If vialed that its use does not involve an excessivethe plus tolerance should be a little greater than expenditure of time. When the acceptabilitythat specified, the plug gage will tend to reject of a gage is questionable because it is near thea few more components than a properly made tolerance limit, the gage may be reinspected byplug gage. On the other hand if a plain “Go” more accurate instruments.plug gage should have a diameter less thannominal-in other words, if it is undersize, thegage could pass unsatisfactory components.For this reason, a “Go” plug gage slightly abovethe specified limits is sometimes accepted, buta “Go” plug gage below the specified limitsshould never be accepted. Gage inspectorsshould not accept gages which do not complyin all respects with the specifications of thedrawing, unless approved by the supervisor.

8.3.5 Selecting the inspection method.8.3.5.1 In selecting an inspection method,

the actual use to which the gage is to be put,or the function of the dimension which is mostimportant, should always be borne in mind.For example, if it is known that only the frontportion of a plain plug gage is to engage acomponent, or if a plain plug gage is beingchecked for wear, the instrument used shouldhave a rounded contact. This readily permitsaccurate measurements to be taken at a pointvery close to the front face of the gage. Onthe other hand, if the roundness of the plainplug gage is of primary importance, the gageshould be checked by means of an instrumentwhich has flat gaging members. This will per-mit the gage to be rotated between the measur-ing members at the same time that the measure-ment is being taken, thus giving an indicationof the diameter of the gage at all points aboutits circumference.

8.3.5.2 The measuring instruments usedshould always be suitable for the tolerancespecified on the gage drawings. The use ofgaging instruments which are too inaccuratewill result in errors or loss of time through thenecessity of remeasurement. The use of gaginginstruments which are too accurate will result

8.3.6 Steps in the inspection process. Thefollowing steps must be taken in the inspectionof all types of gages. Although these stepswill not be mentioned in the instructions forinspecting each type of gage, it is very impor-tant that they be followed at all times:

8.3.6.1 Cleaning. Local safety regulationswill govern the selection of cleaning solvents.If the gage has not recently been cleaned, itshould be immersed and brushed in a solventand dried. Carbon tetrachloride, kept in smallcovered containers when not in use, is preferred.Benzol and gasoline are sometimes used, butare not desirable because of inflammable andtoxic properties. Stoddard solvent is not al-ways satisfactory for final cleaning, becauseit may not remove all grease film. Alcohol isalso used to some extent. Stoddard solvent oran equivalent is a suitable agent for removingheavy grease by means of a hot bath. Regard-less of whether or not the gage has recentlybeen cleaned, precision surfaces should alwaysbe carefully wiped with lintless cloth immedi-ately before measurements are taken. Un-bleached muslin with no sizing, or rayon aresuitable cloths for this purpose.

8.3.6.2 Hardness measurement. Thehardness of gages should be determined by asuitable hardness tester (see 8.5.9). The hard-ness of all gaging surfaces should be specifiedon the gage drawing and will govern the ac-ceptance in all instances. If hardness has notbeen adequately specified on the gage drawing,the following practices will serve as a guide:A hardness of 63 to 66 on the Rockwell “C”scale or the equivalent is a reasonable require-ment for tool steel gaging members. Thread

in unnecessary expense and in loss of time. In gages should have a minimum hardness of C60.general, the accuracy of the measuring instru- Gages which have small diameters or sharp,ment should be less than 20 percent of the tol- narrow surfaces may have a hardness as low aserance on the gage being inspected. A measur- C56. Hardness measurements should be takening instrument which has an accuracy of 10 as near the gaging surfaces as possible.

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8.3.6.3 Demagnetizing. When steel is mag-netized, it not only has a different feel in meas-urement, but tends to collect tiny steel particleswhich interfere with measurements. Before in-spection, it is advisable to check all gages forresidual magnetism by placing a compass ora small piece of iron with low permeabilityvery close to the gage and noting whether thereis attraction. A strip of iron from a core of thewindings in an electric transformer is wellsuited for this purpose. The best way to usethis is to suspend it with a thread, and holdthe gaging surface of any questionable gagenear the iron. If there is residual magnetismpresent, the iron will swing toward the gage.Residual magnetism can be removed by meansof a demagnetizing device. Be sure to removethe gage before the current in the demagnetiz-ing device is turned off as otherwise the lasthalf cycle may magnetize the gage again. Takethe gage off the demagnetizing device slowly.

8.3.6.4 Visual inspection. See that themarking on the gage is clear and agrees withthe marking on the drawing, and that all sur-faces on the gage are of the specified quality.Check lapped surfaces for scratches and unfin-ished portions. Check ground surfaces forscratches, chatter marks, burns, and unfinishedportions. Check machined surfaces for cracksand excessive scale. Check all surfaces forburrs. The places where ground or lapped sur-faces form an edge or a corner should be stonedsufficiently to prevent an operator from cuttinghis fingers. Generally speaking, this edgeshould not be rounded to a radius greater than

of an inch. In instances where the edgeis used in gaging operations, as in the case ofthe flush pin gage, the sharp corners should notbe broken. Burrs on such surfaces can be re-moved by stoning parallel to the gaging sur-faces. All types of gaging surfaces where gageblocks are to be placed should be stoned. Ahard “Arkansas” stone should be used for re-moving burrs and touching up gaging surfaces.

8.3.6.5 Checking loosely tolerance di-mensions. Unless otherwise specified, all di-mensions given on the gage drawing in terms offractions of an inch should be accurate to plusor minus inch. Dimensions expressed indecimals should be accurate to plus or minus

78

0.01 inch, unless the drawing specifically indi-cates a different tolerance. All dimensions ofcommercial standard gages not explicitly given

- in the drawings should conform to AmericanGage Design Standards. (See the current is-sue of Commercial Standard, CS8, GageBlanks, available from the Superintendent ofDocuments, Government Printing Office, Wash-ington 25, D. C.)

8.3.6.6 Checking for function. If the itemdrawing is available, it should be studied tosee if the gage is constructed to permit properfunctioning. Points to consider in this con-nection are whether the gage handle is smallenough so that it will not touch the component,whether all necessary air grooves are present,whether the gage appears to be strong enoughto maintain continuous gaging accuracy in use,etc. Other points to consider in the check foroperation are whether the workmanship is sat-isfactory, whether or not exactly tolerance;for example, whet her all slide fits functionfreely with no shake or side play; whether alldowel pins are tight, etc. Indicators are to bechecked in accordance with 8.4.4. one of thebest ways of checking for operation is to trya component in or on the gage, if available.The laboratory should obtain the correspond-ing components for all types of gages whichare frequently checked. The gage inspector isexpected to point out to his supervisor or tothe engineering department any errors of anysort which he may notice in the gage design.However, this check of the gage design is in-tended primarily to detect functional faultssuch as those specified in the first part of thisparagraph, other faults which are readilynoticed by studying the gage itself, and by try-ing a component in the gage. The primary re-sponsibility of the inspector is limited to theoperational check. He is not expected to checkall gage design computations in detail. How-ever, if measurements taken over various partsof the gage do not appear to be consistent, orif there is some other special reason for him tosuspect there may be an error in the gage designcomputations, he is expected to check thecomputations.

8.3.6.7 Checking closely tolerance di-mensions. This is to be done in accordance

with the following instructions and those con-tained in 8.5. In general, the exact values ofclosely tolerance dimensions are to be measuredand entered on the gage record card. This ap-plies where actual measurements can be takenwithout taking up extra time, when dimensionsdo not fall within the gage drawing limits, whencheck or setting g-ages are being inspected, andwhen gages are to be sent out for salvage.However, when gages are within the drawinglimits, and when calibration would requireextra work such as combining different gageblocks, the gage may be accepted as beingwithin drawing limits without spending thetime to measure the exact dimensions. In in-stances where a gage appears to be only slightlyoutside the gaging limits, it is desirable to re-inspect by other methods of measurement tomake sure that there has been no error in in-spection. Both methods should give identicalresults. If not, the source of error should bedetermined. Gaging surfaces should be touchedby hand as little as possible during inspection.Immediately after inspection is completed, orwhen the gage is left to stand overnight, thegaging surfaces should be wiped with a clothwhich has been diped in a neutralizing and tem-porary preservative oil.

8.3.6.8 Sealing. After the gage has beenapproved, all adjusting and lock screws whichcould change a dimension of the gage should besealed with wax. It has been found that re-setting is facilitated if a small disk of felt,paper, or similar material is placed over theheads of adjusting screws before sealing. Thefelt keeps the wax from entering the head of thescrew. Without this device, considerable timeis frequently spent in cleaning wax out of screwheads, particularly in the case of adjustablesnap gages.

8.3.7 Finishes.8.3.7.1 Requirements. The finishes on all

gages must conform to the requirements of theapplicable gage drawings. Generally, gagingand functioning surfaces of precision gages re-quire lapping or must, at least, be finely ground.Ordinary machine finishes are usually adequatefor other surfaces of gages.

8.3.7.2 Inspection. The degree of smooth-ness is ascertained in accordance with the provi-


sions of Standard MIL-STD-10. Inspectionmay be conducted with the aid of a suitablesurface finish measuring instrument or visually,depending upon the accuracy required. In ad-dition to smoothness measurement, highly fin-ished surfaces should be visually inspected forchatter marks and burns. Both of the latterdefects are undesirable as chatter marks de-crease the gaging area of the members, thusdecreasing the wear life, and burn marks indi-cate overheating which affects the gage hardnessand causes cracks.

8.4 CARE AND USE OF INSPECTIONT O O L S

8.4.1 General. Since no measurement canbe more accurate than the instrument used ininspecting, the care of gage inspection tools isan important phase of the gage checker’s work.In general, the checker should avoid droppingor striking tools in order to avoid the possi-bility of causing burrs, springing frames, ordestroying adjustment; he should check toolsfrequently for wear and for proper adjust merit.,and he should be careful to wipe tools withcleaning fluid, dry them and then wipe with aslightly oily chamois or cloth after use, in orderto prevent corrosion. The following sugges-tions apply to particular tools:

8.4.2 Micrometer calipers.8.4.2.1 Outside micrometers. The outside

micrometer is the instrument most commonlyused for measuring distances between externalsurfaces where extreme accuracy is not re-quired. Experienced gage makers can dupli-cate measurements with a micrometer within0.0001 inch. However, it generally is not de-sirable to assume that the accuracy of a l-inchmicrometer is better than plus or minus 0.0002inch. Larger size micrometers are somewhatless accurate. A micrometer can be used tobest advantage where the tolerance of the gagepart being measured is 0.0005 inch or greater.Inexperienced inspectors should check theirmicrometer readings against gage blocks andprecision rolls until they have developed theproper feel. The adjustment of the microm-eter used for gage work should be checkedfrequently by means of gage blocks or precisionrolls, depending upon whether the micrometeris most often used on flat work or rounded sur-

79


faces. The widely accepted practice of check-ing the setting of a micrometer by bringinganvils together and observing the zero readingis not a good method because there is nothingbetween the anvils to simulate the feel of thecomponent. Gage blocks or rolls used for set-ting should be approximately the size as thecomponent most frequently checked, or if themicrometer is used for all around measure-ment, a size at the middle of the micrometerscale is best. For example, when it is desiredto make a quick check of a l-inch micrometerused for general purposes, ½-inch gage blockshould be used. The purpose of this is to mini-mize the effect of lead error. Occasionally, athorough check of the micrometer should bemade by using four or five gage blocks of equalsize intervals distributed over the total range.A check for periodic lead error can be made bytrying three different combinations of gageblocks, each 0.008 inch different from theother in size. Wear on micrometer anvils canbe detected by measuring a precision ball be-tween various points on the anvil surface, par-ticularly near the edges. In general, themaximum variation in reading should not ex-ceed 0.0002 inch. Wear on the micrometerlead screw can be detected by pushing the thim-ble to and fro in the direction of the lead screwaxis. If there is any shake, the lead screw ad-justment should be tightened. However, thisadjustment is seldom needed. Care should betaken not to tighten the adjustment so that thelead screw binds. The thimble of the microm-eter should turn easily at all positions. Theparallelism of micrometer anvils can be checkedby measuring two precision balls in variouspositions. The size of the two balls shouldbe such that the position of the thimble will bealtered by half a turn in changing from oneball to another. In other words, the size ofthe balls should differ by about 0.012 inch, ora multiple of 0.025 plus 0.012 inch. If thelock nut on a micrometer is used, be sure totighten the nut without excessive force and tocheck the measurement again after the lock nuthas been tightened. Tightening the lock nutsometimes shifts the position of the spindle onsome micrometers. If the ratchet is used fortightening the micrometer, care should be taken

to tighten the spindle against the work gently.Otherwise momentum of the rotating thimblemay make the micrometer much tighter thanthe checker realizes.

8.4.2.2 Inside micrometers. The insidemicrometer generally cannot be depended uponfor a gaging accuracy less than ±0.0003 inch.It can best be used to measure internal surfaceswhich have tolerances greater than 0.001 inch.In using the inside micrometer to measure holes,one anvil should be kept stationary and the otheranvil should be moved back and forth, and upand down, while the micrometer is being ex-panded. When the free anvil no longer movesfreely the operator can be assured that he hasplaced the micrometer across exactly oppositepoints on the diameter. It is good practice totake two or more readings in order to makesure that the minimum diameter has been meas-ured. An inside micrometer should have radiion its anvils which are smaller than the radiusof the hole being measured. The inside mi-crometer wears on the radius of the anvils,where a flat is formed. This can be detectedby studying the surface of the anvils visuallyor by placing the anvils in a contour projector.Where it is desired to make an internal meas-urement to an accuracy greater than that of theinside micrometer, measurement can be madeby means of the transfer method. This con-sists of obtaining the proper fit of the internalmicrometer in the hole, and then locking thethimble of the micrometer in position. Themicrometer is then removed from the hole andthe distance between the anvils is measuredwith an outside micrometer, using a very lightfeel, or by gage blocks with internal accessories.Whenever a different anvil or special extensionrods are used, it is important to recheck thesetting of tile internal micrometer.

8.4.2.3 Micrometer depth gages. The mi-crometer depth gage is generally accurate toabout ± 0.0003 inch and is used to measure thedepth of internal surfaces where the toleranceof the surfaces is 0.001 inch or greater. Theimportant point to remember in using the mi-crometer depth gage is that a comparativelylight pressure in advancing the thimble canraise the anvil away from the surface uponwhich it is resting, thereby creating an error

80

Figure 123. Adjusting micrometer depth gage.

in the reading. For this reason, the contactstem should be lowered very carefully with alight touch on the thimble, and should be with-drawn slightly and brought to bear on the com-ponent two or three times in order to make sureit has not over run the proper reading (see fig.123). The correction for a micrometer depthgage is determined by reading over two equalgage block combinations placed on a tool mak-er’s flat. The correction should be determinedeach time a different extension is used.

8.4.3 Vernier gages.8.4.3.1 Vernier calipers. Vernier calipers

can be used to measure internal or externaldimensions. They are generally accurate toplus or minus 0.001 inch and are used when thetolerance of the work to be measured is greaterthan 0.005 inch. The reading can be taken moreaccurately if a magnifying glass is held in frontof the scale. When calipers are used to measuresmall external surfaces, they should be manipu-lated to obtain the minimum reading. Calipersused to measure internal surfaces should be ma-nipulated to obtain the maximum reading. Ver-


nier calipers are sometimes used to make moreaccurate measurements, particularly in the caseof internal cylindrical surfaces, by means ofthe transfer method. When this is done, theverniers are adjusted until they fit the surfacesbeing measured and are then locked in posi-tion. After this, the distance between the gag-ing surfaces of the calipers is measured directlyby means of micrometers. This method is oftenused in measuring surfaces on which externalor internal micrometers cannot easily be fitted.The adjustment of the calipers is made by plac-ing gage blocks against the jaw when the ex-ternal measuring function is checked, and byplacing the jaws directly against gage blockswith internal setting accessories when the in-ternal measuring function is checked. Wearon the jaws can best be checked by visual meansand by measuring rolls or rings of known di-mensions. Another point in connection withwear is to make sure that the sliding head isnot loose.

8.4.3.2 Vernier depth gage. The accuracyof this gage corresponds to the vernier caliper.Its only advantage over the micrometer depthgage is that it is considerably faster when di-mensions to be checked vary over a considerablerange. This gage can best. be used where it isnecessary to measure the length or depth ofseveral surfaces whose tolerance is 0.005 inchor greater.

8.4.3.3 Vernier height gage. After a mod-erate amount of use, the accuracy of this gageis generally slightly less than that of the ordi-nary vernier calipers. The gage can be usedto measure height directly by bringing the gag-ing surface of the sliding head in contact withthe object whose height is to be measured.However, greater accuracy can be obtained byindicating over the object with a dial indicatorand then bringing the height gage up underthe indicator until the indicator reads the sameas it did when passed over the object (see fig.124). The reading on the vernier scale indi-cates the height. The scale of the height gagecan be read somewhat more accurately if a mag-nifying glass is used. The only advantage inusing a vernier height gage in this way is thatit is very fast. When height must be estab-lished accurately, of course, comparative read-

81


to save time. However, if the indicator read-

Figure 124. Use of vernier height gage.

ings must be taken over block combinations asspecified in 8.4.3.4.

8.4.3.4 Vernier height gage with test indi-cator. A satisfactory method of measuringheights accurately is to use a vernier heightgage with an indicator attached, employingthe height gage merely as an adjustable supportfor the indicator and disregarding the scaleon the height gage (see fig. 125). The accu-racy of the measurements made with this ar-rangement will depend on the accuracy of thetest indicator and the rigidity of the support-ing means. The gage is used by indicatingover the surface whose height is being meas-ured and adjusting the indicator to give a zeroreading. Then the height gage is drawn backand a gage block combination built up underthe indicator until the reading is zero. Thedesired height measurement is obtained by add-ing the lengths of the gage blocks. A trialstack of blocks equal to the nominal height tobe measured can be used as a first step in order

82

ing obtained over them is not very nearly zero,a new combination of blocks. estimated to bringthe indicator to zero, should be tried. Thedifference between the actual height and thenominal dimensions can be estimated by read-ing the graduations of the indicator, when. aslightly lower accuracy is acceptable. Whenreadings are taken on an indicator height gage,the base of the height gage should be tappedvery lightly in order to get the indicator prop-erly settled in position. A precaution to takewhen making extremely accurate measurementsis to draw the ball point over surfaces fromthe back side in order to avoid the jar whichis caused when the ball point is thrust forwardonto an object. Greater accuracy will be main-tained in comparative readings if the indicatoris always moved from lower to higher surfaces.For example, indicate from the low step tothe high step in indicating over the steps ofa flush pin gage. Another precaution whichmust be taken if accuracy is to be maintainedis to make sure that the test indicator has beensecurely fastened to the support and that thesupport is rigid and strong. Unsuitable sup-porting means are frequently the cause of errorsin measurement. Always test the support bybringing the indicator point in contact withan object and noting the variations in the indi-cator reading as pressure is exerted on top ofthe indicator. When this gage is used to meas-ure the height of flat surfaces, the indicator ballpoint should be run over the entire length ofthe surface in order to make certain that nopart of the surface is outside of the prescribedlimits. When measuring the height. of a pinthe ball should be run over the entire lengthof the pin for the same purpose. When estab-lishing the center of a pin by measuring overthe top of the pin with a height gage, it isimportant to subtract from the reading themeasured radius of the pin instead of the nom-inal radius. When a test indicator is used inconjunction with a height gage, the indicatorshould be adjusted so that the contact arm isas nearly horizontal as possible. When the armmakes an angle with the horizontal the indi-cator readings do not record variations in theheight of the work directly; they must be mul-


Figure 125. Use of vernier height gage with test indicator.

tiplied by the cosine of the angle which thearm makes with horizontal. This can be under-stood by studying figure 126, in which “a” isthe angle made with horizontal, “x” is the leverlength to which the indicator dial was cali-brated, and “y” is the effective arm length ofthe stem for any given angle.

Since the cosine of 15 degrees is 0.966, it iscommon practice to permit the arm to form anangle up to plus or minus 15 degrees with hori-zontal, on the assumption that indicator read-ings will still represent the actual work varia-tions to the nearest 0.0001 inch, provided thedifference between the gage block stack and thework height does not exceed 0.001 inch. The

922173 O - 51 - 7

angular position of the indicator itself will notaffect the accuracy. In instances where thenature of the work surface makes it necessaryto adjust, the stem to an angle greater than 15degrees, the combination of blocks should bealtered until the indicator reading is exactlyzero over both the blocks and the work. Sincean inspector must depend on his test indicatorvery frequently for accurate measurements, itshould be kept in the best possible condition.The test indicator should be checked often, giv-ing consideration to the following points:

a. Move the contact point with the fingersand note whether the indicator shows any tend-ency to stick.

83


Figure 126. Error in test indicator.

b. Check the calibration by trying differentcombinations of gage blocks under the indicatorwhile it is held rigidly and noting whether thevariations in the indicator readings correspondto the variations in the size of the blocks used.Due allowance should be made for errors foundby such a calibration when using the indicator.

c. There should be no backlash. Check forthis point by proceeding from larger to smallercombinations of blocks in addition to proceed-ing from smaller to larger blocks when check-ing the calibration.

d. Try to move the contact point from sideto side. There should be no play which reg-isters on the dial.

e. Check to see whether the contact pressureis the same with the reversing stud up as it iswith the reversing stud down.

f. Be sure that the contact points used arethose designed for the particular model of testindicator in question, and that they are securelypositioned. Large errors are sometimes an in-dication that the wrong contact point has beenu s e d .

g. Make sure that the shift lever (reversingstud) is all the way down or all the way up be-fore checking or using the indicator.

h. Test indicators are not dustproof and pre-cautions should be taken to see that the instru-ment is protected from dust at all times. Theinstrument should be disassembled and cleanedoccasionally by a person familiar with the con-struction of test indicators.

i. Do not tighten the lock screw that holds thecase in place any more than is necessary tomaintain proper posi t ion.

j. The accuracy of the test indicator dependsupon proper alinement of the bearings. Forthis reason, it is important not to do anythingwhich could bend or twist the case.

8.4.4 Dial indicator gages. Dial indicatorsare available in a variety of styles, magnifica-tions, and ranges. Some are graduated in in-crements as fine as 0.00005 inch and ranges canbe obtained commercially up to 1 inch. In se-lecting a suitable indicator it should be remem-bered that the range of an indicator decreasesas the accuracy increases. In the gage labora-tory, dial and test indicators are used princi-pally to compare heights as described in sec-tion 8.4.3.4, and to check parallelism concen-tricity and out-of-roundness.

8.4.4.1 Parallelism is determined by adjust-ing the position of one surface until the indi-cator does not vary as it is moved over the sur-face. The indicator is then passed over anyother surfaces which are supposed to be parallelto the first surface. Any variations in the indi-cator reading will denote errors in parallelism.

8.4.4.2 Out-of-roundness and concentricityis checked by placing the gage between centersor on a “V” block with the indicator in contactwith the surface to be checked and rotating thegage. Variations in the indicator reading willindicate runout or twice the eccentricity.Where the drawing specifies “permissibleeccentricity 0.001 inch,” the variation in read-

ing observed on the indicator may be 0.002 inch.8.4.4.3 The indicator is also used as part of

a mechanical comparator in which the indica-tor is adjustably mounted over a stationarybase and a fixed anvil. This arrangement iscommonly employed to measure a number ofparts of the same size whose tolerance is 0.0005inch or more. When a dial indicator is used inthis manner the setting of the indicator shouldbe checked after all pieces have been measured

84


as well as before, in order to make sure the indi-cator setting has not been disturbed duringmeasurement.

8.4.4.4 Methods of checking and maintain-ing dial indicators are specified in 8.4.3.4.

8.4.5 Surface plate and accessories.8.4.5.1 Surface plate. Surface plates are

used when several portions of a gage are meas-ured from a common base when heights on twoor more gages are compared and when a specialset-up of angle irons and clamps is constructedto hold a gage in a special position. Cast ironsurf ace plates used for gage inspection shouldbe flat within 0.0003 inch between any twopoints not greater than 18 inches apart. Theflatness of the surface plate can be checked bymeans of a knife-edge straightedge, or by meansof blueing against plates of known flatness.More accurate measurement can be made withan autocollimating telescope. The irregularityof a surface plate, or the difference betweenadjacent high and low spots, should not exceed0.0002 inch. The irregularity can be deter-mined by mounting a dial indicator on an ad-justable movable support and setting it up tobring the indicator point into contact with thesurface plate. The indicator then is movedabout on the plate and the irregularity as shownon the indicator is noted. The scraping on acast iron surface plate should be such that thereare at least 15 high points per square inch.Granite surface plates should be accurate to0.0001 inch over the total area. It is importantto recheck surface plates periodically for wornspots, because cast iron surface plates can wearmore rapidly than most gage checkers realize.Cast iron plates should be protected by settingall pieces on the plate gently and by making surethat all burrs are removed from items whichare put on the plate. The proper way to set

Figure 127. Method of placing a heavy object on a

surface plate.

85

heavy objects on the plate is to place themgently on one edge of the plate and then slidethem into position (see fig. 127). Do not placeballs or small cylinders directly on a surfaceplate because these objects have a small area ofcontact which may not rest on enough highpoints to give accurate positioning. These ob-jects are also more apt to wear a surface platedue to the high unit pressure which they exert.Some inspectors place their surface plates ontwo large pieces of wood which are bolted to-gether at the center with a single bolt. Thesurface plate is placed on the upper woodenblock, thereby providing a means for rotatingthe surface plate. This makes it possible forthe checker to reach all portions of the set-upeasily without going around his bench.

8.4.5.2 Angle iron and tool maker’s knee.Angle irons and tool maker’s knees differ fromeach other in construction as shown on figure128. Tool maker’s knees generally are smallerand more accurate than angle irons. Ex-perienced checkers often make themselves sev-eral plates of convenient sizes with precisionsurfaces at right angles and with tapped holesfor fastening the work. These are sometimescalled “angle plates”. The faces of tool mak-er’s knees used for gage inspection should besquare within 0.0001 inch for each 6 inches oflength, and opposite sides should be parallelwithin 0.0001 inch for each 6 inches of length.Manufacturers of angle irons generally specifyan accuracy of 0.0002 inch for the relationshipof the precision surfaces. However, it is pref-erable to use angle irons accurate to 0.0001 inchfor gage inspection. These can be obtained byreworking irons or by selecting the best fromnew commercial irons and reserving them foruse on work requiring high accuracy. Knownerrors in angle irons or other devices should betaken in account for very precise measure-ments. For example, if the edges of an angleiron are known to be 0.0002 inch out of square,this fact must be taken into account when theiron is turned on one of its edges to check thesquareness of two surfaces on a gage which hasbeen clamped on the iron. If the left edge ofthe iron is high, then the 0.0002 inch should besubtracted from the indicator reading when theindicator is at the left side of the gage. If the


Figure 128. Tool maker’s knee—universal angle iron.

precision surface on the gage is not so longas the edge of the angle iron, the indicator read-ing will not vary by 0.0002 inch but by anamount equal to the proportion of the lengthsto 0.0002 inch. The squareness of the faces ofangle irons and tool maker’s knees can bechecked by means of a master cylindrical squareand to gage blocks, one block 0.0001 inch largerthan the other. They are set up as shown onfigure 129 with the smaller block at one end ofthe square. It should not be possible to insertthe larger block between the square and thetool maker’s knees at any point. The processis repeated with the small block at the other endof the square. Parallelism can be checked bydirect measurement, or by indicating over onesurface when the opposite surface is resting ona surface plate. A convenient and accurateway to check for squareness when a mastercylindrical 1 square is not available is shown onfigure 130. To check an angle iron in this man-ner, attach two tool maker’s buttons of thesame size to a plate, one directly above theother. They need not be located accurately;they need only be securely fastened. Place theangle iron near one side of the rolls as shown infigure 130 and aline it so that its face is parallelto the axis of the buttons. Then measure thedistance between the angle iron and each of thebuttons by means of gage blocks. Place theangle iron on the opposite side of the buttonsand repeat the process. The difference in thedistance to the buttons in each of the two in-stances should be equal and opposite if the

angle iron is square. If they are not, theamount of the discrepancy will indicate theerror in squareness of the angle iron for a dis-tance equal to the distance between the centersof the buttons.

Tool maker’s knees and angle irons, like par-allels and surface plates should be stoned when-ever there is any indication of burrs.

8.4.5.3 Parallels. Parallels used for gageinspection should be parallel within 0.0001 inchin 6 inches of length and should be accuratefor size within 0.0002 inch. Each pair of par-allels should be kept together. Parallels fromdifferent sets may not be of equal size, eventhough they are satisfactory when matchedwith the proper parallel. Parallels should behandled carefully to avoid injury to themselvesand the surface plates upon which they areplaced. Roll heavy parallels on to the platein the same manner as other heavy objects (seefig. 127) . Burrs should be removed with anArkansas stone. Most parallels tend to beslightly warped. The effects of this condition

Figure 129. Checking tool maker’s knee for squareness.

Figure 130. Checking squareness by means of tool maker’s

buttons.

86

will be minimized by placing the concave sideof the parallels on the surface plate. The dif-ference between the concave and convex sidescan be detected by twirling the parallel on asurface plate. The convex side will tend torotate about the center of the parallel, whereasthe concave side will tend to drag at the ends.Wear on parallels can be detected by directmeasurement of thickness or by indicating alongthe surfaces.

8.4.5.4 Precision straightedge. There aretwo types of precision straightedges; those withbevelled edges and those with knife edges.Those with bevelled edges generally are usedfor positioning. The straightedge is clampedto a plate and gage blocks of appropriate di-mensions are built up between the straightedgeand the sections of the gage which are to bepositioned. The gage is then pressed againstthe blocks and clamped on the plate. The work-ing surfaces of bevel-edge straightedges shouldbe flat within 0.0001 inch for each 6 inches oflength. Knife-edge straightedges are to be usedto check the flatness of precision surfaces. Thestraightedge is placed against a surface andviewed with a strong 1ight in back of thestraightedge. Light which passes between thetwo indicates errors in flatness of the surface.A knife-edge straightedge may be checked forstraightness, by placing it on a tool maker’s flatin front of a strong light. If the straightedgeis in good condition, no 1ight will pass betweenthe edge and the flat.

8.4.5.5 Universal precision square. Uni-versal precision squares (see fig. 131 ) usuallyare employed to check the perpendicularity oftwo surfaces. The square is placed against thesurfaces in front of a strong light. Errors insquareness are clearly visible under such condi-tions. If the light test shows the surfaces arenot perpendicular, the error can be estimatedby placing a gage block between the squareand the surface at one end of the square.Blocks of varying dimensions are then triedat the other end of the square until a block isfound which exactly fits the space between thesquare and the surface.not to force too large aposition. The error in asented by the difference

Care must be takengage block into thissurface will be repre-in size of the blocks

87


Figure 131. Universal precision square.

at each end of the square. The gaging surfacesof precision squares should be perpendicularand flat within 0.0001 inch for each 6 inches oflength. Precision squares are checked forsquareness by means of a cylindrical square andgage blocks, the method being similar to thatpreviously described for checking angle irons.The straightness of precision squares can bechecked against a tool maker’s flat.

8.4.5.6 Planer gage. Planer gage (see fig.132) is used in conjunction with a height gageand gage blocks to measure heights. Its chiefadvantage is that it can be adjusted to a givenheight in far less time than is required to builda stack of gage blocks. It also has an advan-tage in that it is somewhat more stable thanvery long combinations of gage blocks. TO usethe planer gage in checking the difference inheight of two surfaces, an indicator is adjustedto read zero when passed over the lower sur-face. The adjustable member of the planergage then is brought up under the indicatoruntil it again reads zero. A stack of gageblocks equal to the specified difference inheight is wrung to the planer gage and thelocation of the higher surface is checked byindicating over the top of the gage blocks andthe higher surface. After a planer gage hasbeen tightened into position an indicator shouldbe moved across the adjustable member to checkparallelism.

8.4.6 Roll holder. Roll holders are used tohold precision measuring rolls in position while


Figure 132. Planer gage.

a measurement is taken. These tools are par-ticularly useful in locating the intersection oftwo precision surfaces by means of an indicatorheight gage and gage blocks in cases wheremeasurements cannot be made directly.

8.4.6.1 Roll holders are of types, the re-versible (see fig. 133 ) and the conventionaltype (see fig. 134) . On the holder shown onfigure 133, the lock screw and clamp can be

used at either end, thereby making it possibleto use the holder with a greater range of rollsizes.

8.4.6.2 Where a gage has a finish ground orlapped surface perpendicular to the two sur-faces being measured, the roll holder can bewrung to this surface and slid along until theroll is in contact with both the surfaces whichare being measured.

8.4.6.3 When a gage does not have conveni-ent precision surfaces, the roll holder can some-times be wrung to the plate on which the gagehas been clamped or to gage blocks which inturn have been wrung to the plate.

8.4.7 ‘Hold-down fixture. This device hasproved very useful in the measurement of tap-ered plugs and plugs which have radii. Whenthe entering end of the plug is square, thissurface is placed directly on the hold-downfixture (see fig. 135) and the plug clampedin position by tightening the screw at the topof the fixture. Gage blocks then can be wrungto the base of the fixture for supporting rollsand the entire unit can be placed between theanvils of a bench micrometer or a precisionmeasuring machine.

8.4.8 Channel-bar fixture. The channel-bar fixture (see fig. 136) is used principallyto hold precision measuring rolls in positionwhen measurement is not made directly overthe rolls. The rolls are placed on the base ofthe fixture and pressed against the channel side.A gage part with a radius or a taper then isplaced on the rolls and the diameter of thegage at the point where it contacts the rolls

Figure 133. Reversible roll holder.

Figure 134. Conventional roll holder.

88

M I L - S T D - 1 2 09 SEPTEMBER 1963

Figure 186. Hold-down f ixtures.

is determined by measuring the height of thegage.

8.4.8.1 The sides of the channel-bar fixtureshown in figure 136 are adjustable and can bespaced any desired distance apart by placinggage blocks between the bars and then lockingthem in place against the gage blocks.

8.4.8.2 Another variation of this idea is anonadjustable channel bar fixture made in onepiece. Varying widths are obtained by placinggage blocks between the precision measuringrolls and the channel bars as spacers.

8.4.8.3 The adjustable type of fixture has agreater range and versatility than the nonad-justable, but the nonadjustable type is moredependable and accurate, and may be set upmore quickly.

8.4.9 Locating plate. The special plateshown in figure 137 has proven very valuablefor a wide variety of measurements.

8.4.9.1 Small profile gages may be locatedby inserting gage blocks between two preci-sion surfaces on the gage and two sides of theplate. The gage then is clamped in positionand other dimensions can be checked by in-

serting gage blocks and buttons where neces-sary between the edges of the plate and theother surfaces.

8.4.9.2 The buttons shown on figure 137can be located in any position on the plateby means of gage blocks, and can then belocked in position. The plate is used principal-ly for the measurement of profile gages whichdo not have any reference surfaces fromwhich the gage can be located. Such a gageis shown on figure 138. The top surface ofthis gage is not precision ground and is notperpendicular to the centerline of the gage.

8.4.9.3 In use the distance between theradii are computed at two different points,and the rolls locked in positions calculated tocontact a gage of exactly nominal dimensions.A gage is then placed between the rolls. Ifthere is any shake between the gage andeither pair of rolls, the error in the gage canbe determined by repositioning the rolls untilthey fit, and by locating the new position ofthe rolls with gage blocks.

8.5 CARE AND USE OF PRECISIONMEASURING INSTRUMENTS

8.5.1 General. Space does not permit a de-

89SUPERSEDES PAGE 89 OF 12 DECEMBER 1950

MIL-STD-l209 September 1963

tailed discussion of the methods of operatingprecision measuring instruments. For this in-formation the reader is referred to the cata-logs and instruction books published by themanufacturers. This section is intended topoint out the uses for which these instru-ments are best suited, and to point out factsrelating to the care and use of the instru-ments.

8.5.2 Gage blocks and accessories. Gageblocks should be used only for the adjustmentof precision instruments or as a means of es-tablishing dimensions by reference, (For ex-ample, use for establishing heights in con-junction with an indicator height gage.)

8.5.2.1 It is sometimes necessary to checkthe distance between two surfaces by insert-ing gage blocks between them, and it alsomay sometimes be necessary to use gageblocks to position gages or gage parts. Whenthis is done, the blocks should never be forcedinto position. Forcing not only injures the

blocks; it also results in a false measurement.

8.5.2.2 Gage blocks should never be placedin contact with surfaces which are not finishground, lapped, or scraped; Before placingblocks on any surface, remove burrs or anyother irregularities from the surface. Gageblocks shouldn't be placed directly on a sur-face plate when it can be avoided. Preferablythey should be wrung to a piece of steel withlapped, parallel sides, which in turn is placedon the surface plate. Direct contact should beavoided because scraped surface plates do notalways provide adequate bearing area forblocks and tend to scratch them. Excessivewear on commonly used blocks can be avoidedby the use, wherever possible, of less com-monly used blocks. For example, if a combi-nation equal to 1.200 inch is desired, use a0.900- and a 0.300-inch block or a 0.800- anda 0.400-inch block instead of the commonlyused 1.000- and 0.200-inch blocks.

8.5.2.3 Gage blocks should be thoroughly

Figure 136. Channel-bar f ixture.

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SUPERSEDES PAGE 90 OF 12 DECEMBER 1950


Figure 137. Locating plate.

cleaned with rayon, unbleached muslin or otherlintless cloth before use. If a chamois is usedit should be thoroughly clean. Always shakea cloth or chamois before wiping blocks. Ifblocks are too dry or worn to wring readily,they may be wiped across the palm of the handbefore wringing. The hand will add enoughoil to the surfaces to make a tighter wring.Blocks are wrung together by sliding one ontop of another starting with a light pressure.Blocks which do not wring tightly should betaken apart and cleaned again. Any burrsshould be removed with a hard Arkansas stonehaving a very flat surface. After the blockshave been found to fit smoothly they are pressedtogether firmly starting at an edge or cornerand wrung to each other. In the case of Hoke,or square type, blocks wringing is accomplishedbetter by rotating the blocks as they are pressedtogether. In rectangular blocks, the blocksshould be started about inch off center and

wrung by sliding into position such that one isdirectly above the other. In combining Hoketype blocks the tie rod, or fastening screwwhich passes through the hole in the center of

Figure 138. Checking profile gage on locating plate.

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the blocks, should never be tightened sufficientlyto bind on the blocks. The only purpose of thistie rod is to keep the blocks or accessories fromfalling if they should become loose from the restof the stack. The tie rod should not be usedin wringing the blocks together or in holdingthem in an exact position as tightening the tierod will alter the size of the stack. When thisdevice is used, it should be inserted after theblocks are wrung together in the usual fashion,using a very light pressure in tightening thelock nut.

8.5.2.4 Burrs on gage blocks can be detectedby use of an optical flat.

8.5.2.5 Gage blocks should be handled bytheir edges to reduce the possibility of corro-sion and to minimize the effect of the heat ofthe hand. Blocks should always be wiped offwith a clean chamois or cloth after use.

8.5.2.6 Gage blocks that are in use should becalibrated periodically for size, flatness, andparallelism. (For methods of calibration see8.26. ) The latest calibrated values of theblock sizes, as given on the card inclosed in thecase with the blocks, should always be usedrather than the nominal sizes when gages madeto close tolerance limits are being checked.

8.5.2.7 When the construction of a gage issuch that it is impossible to place close toleranceparts between the anvils of a bench micrometeror a precision mechanical comparator, the di-mension can sometimes be checked by buildinga stack of blocks to form a snap gage as shownon figure 139. When this is done it is advisableto use chromium plated or carbide surfacedblocks for the outer members in order to mini-mize the effects of wear.

8.5.2.8 Carbide or chrome plated blocks canalso be used to advantage at the ends of ordi-nary gage block combinations particularlywhere the combination is used on surfaces hav-ing a poor finish.

Figure 139. Snap gage made of gage blocks.

8.5.2.9 The importance of cleanliness as anaid to accurate measurement with gage blockscannot be overemphasized. When blocks areused for setting purposes, be sure the anvils,gaging members or other surfaces with whichthe blocks come in contact, as well as the blocks,are thoroughly clean. The checker shouldpractice using blocks until he is able to tell thedifference in feel between a good fit and a poorfit caused by a film of foreign matter.

8.5.3 Sine bars and sine plates. The sinebar or sine plate is the device most commonlyused to establish angles. Its name is derivedfrom the fact that measurements with the baryield the sine of the desired angle.

8.5.3.1 The sine bar differs from the sineplate in that the sine bar generally is not morethan about 1 inch wide. The sine plate (seefig. 43) is much wider and usually has a heeland tapped holes for fastening work in place.The sine plate sometimes is referred to as a sineblock. The information relative to the geo-metric properties of the sine plate also apply tothe sine bar.

8.5.3.2 Sine plates are known as 5-inch, 10-inch, 20-inch, etc., depending on the distancebetween the two rolls on which they rest.This distance between the rolls constitutes thehypotenuse of the triangle formed by the sineplate, the surface plate, and the block combi-nation used to elevate one end of the sine plate.The angle between the sine plate and the sur-face plate is the desired angle and the height ofthe gage blocks equals the length of the sideopposite the angle.

8.5.3.3 The proper height of gage blocks fora desired angle using a 5-inch sine plate is thesine of the angle multiplied by 5 inches. Sincemultiplying by five is the same as multiplyingby ten and dividing by two, this computationcan be made easier by finding the sine of theangle in a table of natural functions, movingthe decimal point one place to the right, anddividing by two. The height of the block com-bination for a 10-inch sine bar can be found byfinding the sine and moving the decimal pointone place to the right, while the proper heightfor the 20-inch sine bar is found by finding thesine, moving the decimal point one place to theright and multiplying by two.

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Figure 140. Sine plate with centers.

8.5.3.4 The accuracy of a sine plate is pro-gressively less with larger angles. With largeangles a small change in the height of the blockscorresponds to a comparatively large change inangle. A sine plate is less likely to remain se-curely in position at large angles. Sine barsor plates should preferably not be used for thedirect measurement of angles larger than 45degrees.

8.5.3.5 Gages which have more than oneangle should preferably be set up so that it isnot necessary to unclamp the gage and reposi-tion it between the measurement of the angles.Extra time is required and observational errorsare increased when an angle is checked againstanother angular surface rather than againsta basic reference surfuce.

8.5.3.6 Where great accuracy is desired, gageblocks should be used under both rolls of thesine bar because the roll makes line contactonly with the surface plate, and it may not con-

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tact enough high points on the plate to be ac-curately positioned. The accuracy of a sineplate may be determined by placing it directlyon a surface plate and measuring the parallel-ism over the top of the bar with an indicatingdevice. If the surfaces are not parallel the rollshave not been properly positioned on the sineplate or the rolls have worn excessively.

8.5.3.7 Greater stability can be obtained, ifneeded, by placing the side of the lower rollagainst a parallel which has been clamped inplace on the surface plate (see fig. 140).Hoke, or square type, blocks also providegreater stability. If these are not available,two equal stacks of USA type blocks may beused in the case of large sine plates. The blocksshould be placed so that their long sides areparallel with the long sides of the angle plate.If a sine plate is used for a steep angle and thework is very high, an inverted “V” block or aheavy object may be placed at the high end of


the sine plate in order to counterbalance theweight of the gage and the angle iron. 8.5.3.8 A serious mistake sometimes made bybeginners is placing a rough surface directlyon the sine plate. It nearly always is necessaryto clamp the gage to an angle plate and in turnplace the angle plate on the sine plate. Thebasic precision surface or centerline from whichthe angles are dimensioned is then checked forparallelism by means of an indicator, and theposition of the gage on the angle plate adjusteduntil the reference surface is parallel with theprecision surface upon which the rolls of thesine plate rest. One end of the sine plate thenis placed on a stack of gage blocks. The surfacewhich formerly was tilted through the givenangle will now be horizontal, provided the gageis correct. This is checked by indicating overthe surface (see figure 141 ). If the surfacebeing checked is not horizontal, the blocks arechanged until the surface becomes horizontal.The actual angle of the surface then is deter-mined by noting the height of the gage blockstack and computing the corresponding angle.

8.5.3.9 The inspection of gages with centerholes can be greatly facilitated by the use ofsine plates which have centers. When this isdone the checker should adjust the centers ofthe plate snugly in the center holes of the gage.The gage is then checked for concentricityby rotating it under an indicator. A specialsine plate with centers is shown on figure 140.A special feature of this plate is the overchang-ing stocks which hold the centers thereby re-ducing the weight of the instrument. Anotheradvantage of this plate is that the rolls are heldat the ends only and are locked in position byset screws. This makes it possible to reduce theeffects of wear on the rolls by loosening a newposition. The rolls should be checked for par-allelism after an adjustment. Another specialtype of sine plate incorporates a chuck for hold-ing the work. This type of sine plate is usefulbecause it can be used for holding gages ofunusual shapes which cannot readily beclamped to a tool maker’s knee.

8.5.4 Light wave measuring equipment.It is known that in certain respects light be-haves as if it traveled in waves. When thewave crests in one tiny beam of light are ex-

actly alongside wave troughs of another tinybeam of light, the light waves neutralize eachother and no light can be seen. The dark areasare known as interference bands.

8.5.4.1 This principle is utilized to measureerrors in flatness, parallelism, and size of pre-cision lapped surfaces.

8.5.4.2 In order to make such measurements,an optical flat is placed on the surface andviewed under light having one color or onepredominant color. Such a light has only onewave length. The light from the helium tubegenerally used for this purpose has a wavelength of 23.2 millionths of an inch. When oneoptical flat is placed over the test surface, twosets of light rays meet the eye, one set of raysbeing reflected from the bottom of the opticalflat and the other set being reflected from themetal surface. The top flat is manipulated sothat a wedge of air is present between the twosurfaces. The distance between the bottom ofthe optical flat and the top of the surface in-creases continuously from the point of contactto the open end of the wedge of air. At thepoint where this distance equals one half of onewave length of light, the ray of light reflectedfrom the blocks travels a distance of one wavelength greater than the light reflected from thebottom of the flat. One might expect the wavesfrom two sources to coincide, thereby buildingup a bright band of light. However, the lightray reflected from the block is reversed fromthe direction of its previous wave motion, sothat the two sources of light are of oppositephase. In other words, the wave crests of oneset of waves fall at the same point as the wavetroughs oft he other set of rays, thus cancelingeach other to form a dark area, or interferenceband (see fig. 142). Whenever the distance be-tween the two surfaces increase by an amountequal to one half the wave length of the light,the reflected light ray from the metal surfacetravels a distance which has been increased bythe wave length of the light, and the wave crestsand the troughs coincide so that whenever thedistance between the optical flat and the surfacebeing inspected changes by an amount equal tohalf the wave length of the light, an interfer-ence band is observed. Knowing the wavelength of light emitted by the light source, we

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Figure 141.

Figure 142. Measurement by means of light waves.

95

Use of sine plates.

can use this principle to measure the differencein the height of two objects or to test the degreeof flatness of a lapped surface. Optical flatsare the most frequently used in gage laborato-ries for the measurement of flatness of very pre-cise surfaces. For this purpose, the optical flatis placed on the surface and tilted enough toobtain about eight interference bands per inchof length. The curvature of the bands indi-cates the error in planeness of the surface. Theerror is computed by multiplying the displace-ment of the bands, in terms of bands by 11.6millionths of an inch.

8.5.4.3 In the example shown on figure 143,the displacement amounts to about 1½ bandsdisregarding about inch of surface nearest


Figure 143. Computing error in flatness.

the edges. This portion of the surface is usuallyneglected in measuring flatness. Therefore,if the flat has been positioned to make contactat the front, or lower portion of the surface,the outer edges of the surface are about 17millionths of an inch lower than the center. Ifthe fiat has been pressed down at the back, ortop, the pattern indicates that the edges ofthe block are about 17 millionths of an inchhigher than the center. Figure 144 is an actualphotograph showing the pattern obtained whenthere is wear at the center of a precision sur-face. In this case the displacement is equalto nearly two bands. Therefore, if the thinpart of the air wedge is at the left side, thecenter of this surface has been worn about 23millionths of an inch lower than the outere d g e s .

8.5.4.4 The optical flat is sometimes used tomeasure height by building a bridge similarto that shown on figure 145 in which a steelball is being used to measure the diameter ofan internal surface having a fairly small taper.The optical flat is particularly useful when ameasurement under a light contact pressure isd e s i r e d .

8.5.4.5 The simplest method of measurementis to vary the gage block combination until theleast number of interference bands is found.The height of the point being measured is thenthe same as the height of the gage block stackto the nearest 0.00001 inch. Experts can usethe interference method for measurements toan accuracy of plus or minus 10 millionths ofan inch. To compare the over-all height ofthe steel ball and gage with a gage block com-

bination as shown on figure 145 they are placedside by side on an optical flat and another opti-cal flat placed over them. If the gage blockcombination is very nearly the same size asthe height of the ball, interference fringes arethen seen over the surface of the block whenviewed normally in monochromatic light. Forconvenience in computing, the distance fromthe center of the ball to the near end of thegage block should be adjusted to equal thelength of the face of the top gage block. Thesize of the gage block combination is prefer-ably selected so that 4 to 10 fringes appear overthe surfice of the top block. If the gage blockcombination is larger than the ball and gagethe difference in height is equal to the numberof fringes on the block and if the gage blockcombination is smaller than the ball and gagethe difference in height is equal to twice thenumber of fringes. Whether the blocks arelarger or smaller than the ball and gage is de-termined by applying a light pressure directlyover the gage block. If there is little changein the number of fringes the gage blocks aresmaller than the ball and gage and if the num-ber of fringes decreases considerably the gageblocks are larger. On figure 145, for example,

Figure 144. Interference pattern.

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Figure 145. Measuring height with an optical flat.

there are 3¾ dark bands visible. If the flatis bearing on the ball and the near end of thegage block, the ball is 44 millionths lower thanthe gage blocks. If the flat is bearing on theball and the far end of the block, the ball is 88millionths higher than the gage blocks. Thismethod is not recommended for beginners be-cause it cannot be used successfully unless theinspector has had considerable experience withoptical flats.

8.5.4.6 Optical flats generally are plane ononly one side. This side is indicated by mark-ing an arrow on the edge of the flat (see figure145). Optical flats must always be placed sothat the arrow points toward the workingsurface.

8.5.4.7 Bands should be viewed from a posi-tion as nearly as possible directly above theflat, since viewing the bands at an angle intro-duce errors.

8.5.4.8 The light wave micrometer is a benchmicrometer which uses the interference prin-ciple to obtain measurements at a constant pres-

sure. It can be used at any pressure from 0to 1½ pounds ( see fig. 146). The accuracy ofthis instrument is about ± 0.00003 inch, pro-vided the lead screw calibration chart is used.It is used principally to measure small preci-sion plugs and thread pleasuring wires.

Best results are with a light wave micrometerif it is placed on a flat surface, such as a surfaceplate.

8.5.5 Optical instruments.8.5.5.1 Contour projector

parator (see figs. 37 and 38)).working conditions, projectorsan accuracy of about ± 0.0002

(optical com-Under normal

can be used toinch, when the

perpendicular distance between two parallelsurfaces is measured. It should be remem-bered that the error can be greater than thiswhen measurements are taken between variouspoints on angular or curved surfaces. Angularreadings can be made to an accuracy of about± 2 minutes, although the accuracy of angularmeasurements is dependent upon the length ofmeasured surface as well as the accuracy of

97


Figure 146. 0—2 inch light wave micrometer with roll anvil for calibrating measuring wires, 60 degrees “V” block for testingroundness, precision balls for checking flatness and parallelism of micrometer anvils, and cylindrical standards for calibrating

micrometor screw.

the projector and the condition of the work. Ingeneral, the accuracy of a projector will de-crease as the thickness of the work increases.The projector is most frequently used to meas-ure dimensions between surfaces which are noteasily accessible to micrometer anvils. It isalso used to good advantage in checking ir-regular curved surfaces. Irregular curvedsurfaces are generally measured by drawing theoutline of the gage accurately to scale on a pieceof transparent material and placing this out-line over the screen of the projector. Theshadow of the gage under inspection is thencompared with the outline of the screen. It isimperative that layouts for the projector be

accurate. The transparent material used for aprojector layout should be a specially preparedmaterial which will not change in size whensubjected to the heat of the projector.

Distances are measured with a projector bythe use of tile micrometers which are incor-porated in the supporting stage. The imageof one of the surfaces or intesections betweenwhich measurement is to be made is located ona line of the projection screen. The position Ofthe screen must be adjusted so that this line isperpendicular to the distance to be measured.The supporting stage then is moved by advanc-ing the micrometer until the image of the othersurface between which the measurement is to

98

be taken falls on the line. The difference be-tween micrometer readings before and afterthis operation will indicate tile desired length.Contour projectors, like micrometers, should bechecked regularly against known standards.The checker should measure a plug of knowndiameter, using the built-in micrometer. Inthis way the most accurate alinement of theimage with a line on the screen can be deter-mined. Comparison with known standards isparticularly worth while after adjustments ofthe lamp house lenses and shutters. When theprojector is used for especially accurate meas-urements, the best procedure is to aline one ofthe surfaces with the line at the center of thescreen. Then, instead of using the table trav-erse micrometer, place a gage block combinationequal to the dimension being measured betweenthe micrometer anvil and the lateral anvil spac-ing rod. Aline the other surface on the screenby adjusting the micrometer, and note the dif-ference in the micrometer readings to obtainthe deviation of the gage from basic size. Thismethod eliminates errors due to any possibleimperfections in the lenses and the micrometerlead screw. Do not change the magnificationor the adjustment of the light source duringmeasurement as this may introduce seriouserrors. Move the cross-slide table gently. Donot allow table mechanism to hit its stopssharply. “Bouncing” these parts not only ishard 011 the instruments but also causes inac-curacy in the measurements. When there is ex-cessive reflection from surfaces of the piecebeing inspected, sharper definition can be ob-tained by reducing the size of the aperture infront of the lamp. Move the cross slide tablefrom back to front and from left to right whenusing the micrometer attachments in order toeliminate the effects of backlash in the microm-eter screw. A gage should not be left in theprojector longer than necessary because the heatof the lamp will warm the gage causing it toexpand. When a supplementary glass plate isplaced over the screen of a projector, meansother than tape should preferably be used forholding it in position. Tape tends to loosenand the plate may fall after it has become warmor after it has stood for a time. Mercury vaportype lamps should be left burning all day, re-


gardless of whether the projector is used con-stantly or not because the lamp will last muchlonger if it is not turned on and off frequently,and because measurements will be more accu-rate if the projector maintains a constant tem-perature throughout the day. when a pro-jector is used frequently, it will pay to constructa curtain somewhat like a shower curtainaround the front of the projector and tile placewhere the operator stands. This will reduceoutside light and improve the image on thescreen. The roller bearings on which the tablemoves should not be oiled as oil tends to collectdirt, which interferes with the proper function-ing of the bearings. Tile bearings will operatesatisfactorily and will not rust if they are leftdry. when the surface illuminator is beingused for measurement from a recess to someother point on the gage, the definition of theedge of the recess will be made much sharperby filling the recess with common salt. If thecontour of the recess at any given height isrequired, it can be obtained by filling the recesswith salt to the appropriate level. After meas-urement has been completed, the salt must becompletely removed, because it induces corro-sion. The special fixture shown on the pro-jector table in figure 147 has proven very usefulin maintaining a fixed angular position whenobjects are moved to various positions on thetable. It is also used as a rapid means of squar-ing gages with the centerlines on the screen.

8.5.5.2 Tool maker’s microscope. The ac-curacy and the use of a tool maker’s microscope(see fig. 39), corresponds quite closely to thoseof the contour projector. The microscope issometimes assumed to be more accurate thanthe projector because it creates a sharper image.However, the sharpness is due mainly to thefact that the microscope usually has a lowerorder of magnification than the projector. Itis particularly important to be sure the micro-scope proper is locked securely in place on thesupport column after each adjustment, or other-wise, it may be badly damaged by falling on thework. The fixture shown on figure 148 is avery useful accessory for a tool maker’s micro-scope. It is used chiefly to position pieceswhere a number of the same size are being in-spected. The fixture is clamped on the micro-

99922173 O - 51 - 8


Figure 147. Alining fixture for projector.

scope just in front of the point where the gagewould normally rest. If the gage has rectangu-lar sides, gage blocks can be placed in the slotsat the top of the fixture; and once proper ad-justment has been made, additional gages canbe rapidly alined for measurement merely bypressing them against the top of the fixture andthe sides of the gage blocks. Round gages canbe alined by means of the “V” slots.

8.5.6 Direct measuring equipment.8.5.6.1 Supermicrometer. The supermi-

crometer (see fig. 31) can be used to an accuracyof about *0.00005 inch for average sizes ofwork. For extremely long dimensions, the ac-

Figure 148. Positioning fixture for tool maker’s microscope.

curacy of the measurement is limited by the ac-curacy of the disks used for setting the instru-ment. For small gages where the length is lessthan 2 inches, the instrument is suitable forchecking gages having tolerances from: 0.0002to 0.0015 inch. For greater lengths, of course,suitable tolerances are proportionately greater.The following precautions should be observedeach time the supermicrometer is used:

a. Check measuring pressure.b. Clean the anvils by drawing a piece of

hard finish paper between them under pressure.Back off the anvil before removing the paper toprevent pieces of lint from being left on theanvils.

c. If the checker is not certain of the condi-tion of the anvils, he should place a precisionball between them and measure it in severalpositions to make sure the anvils are flat andparallel. When very small plugs are meas-ured, or when measurements are made overthread measuring wires, it is best to make this

100


check with a small wire. If the anvils areworn, they should be lapped with the lap andlapping solution provided by the manufacturer.If the supermicrometer is used constantly, theanvils should be lapped at least once a day.Bench micrometers and other precision meas-uring instruments should preferably be set bymeans of reference gages which simulate theform of the work to be inspected. For meas-urements over flat surfaces, the proper settingsof the instrument is established by means ofgage blocks. For measurement over cylindri-cal surfaces, the setting of the instrument isestablished by means of master disks. For ex-tremely accurate measurements the machineshould preferably be set with gage blocks ordisks approximately the same size as the gagein order to reduce the effect of lead errors inthe micrometer screw. In setting an instru-ment by means of gage blocks. be sure that boththe anvils and the blocks are absolutely clean.Slide the blocks between the anvils enough toget a wringing fit but do not slide the blocksaround any more than necessary. The prac-tice of setting a micrometer by bringing theanvils together is unsatisfactory because it doesnot simulate a gage between the anvils, and be-cause it is not possible to detect the presence ofdirt. A useful trick method of setting a ma-chine when measurement is to be made over rollsis as follows:

Place a gage block (or master disk if a plugis to be measured) and two rolls between theanvils as shown in figure 149. Without regardto the actual size of the rolls, assume that thediameters are exactly nominal and set the ma-chine to give a reading equal to the sum of thelength of the block and the nominal diameters

Figure 149. Setting up bench micrometer for measurement

over rolls.

101

of the two rolls. Measurements are then madeover the rolls in the customary fashion, usingthe nominal diameter of the rolls in makingcomputations. Any difference between the ac-tual diameter of the rolls and their nominalsize will be exactly compensated by the originalsetting of the machine. For example, if eachof the rolls happened to be 0.0001 inch undersize, the machine would have been set to read0.0002 inch more than the true value of the ac-tual measurement. This will exactly compen-sate the error of 0.0002 inch in the actual meas-urement. This method is useful for measure-ments over straight surfaces and slight tapers,but it will not be accurate. for measurementsover steep tapers. The advantages of this sys-tem of setting the instrument are that the con-ditions more nearly corresponding to those ofactual measurement, and errors in calibratingthe rolls will not be introduced into the meas-urement. A supermicrometer with which theoperator is not familiar, or on which elevatingtable has been moved, should be checked forsquareness of the table with the faces of themeasuring anvils. Squareness may be checkedby measuring cylindrical square while it isstanding upright on the elevating table repeat-ing the measurement when it is lying on theelevating table in a position to aline itself withthe anvils. The difference in the two measure-ments, if any, will indicate an error in the ele-vating table. Inspect the gage to be measuredand remove all burrs or other imperfectionswhich might interfere with the measurement ordamage the anvils of the supermicrometer.Place the work between the anvils and advancethe screw until the tailstock indicator is on thezero point. If the piece is light enough to besupported by the pressure of the anvils, theelevating table should be lowered from thework to permit proper alinement between theanvils. If the gage is too heavy a small piece ofdrill rod or some other round object may be in-serted between the gage and the elevating tableto permit alinement (see fig. 162). Shift thegage about between the anvils and note whetherthe reading stays constant. This will eliminateerrors caused by dirt or by small imperfectionson the surface of the gage. Record the ob-served reading. Generally, for gages which


have external gaging surfaces, the recordedmeasurement should be the maximum obtainedat any point on the surface since this is the pointwhich will make contact with the component.For example, if a plain plug gage is out-of-round, the largest diameter will bear on thecomponent and determine, for all practical pur-poses, the size of the component.

8.5.6.2 Universal measuring m a c h i n e.The universal measuring machine (see fig. 32)can be used to an accuracy of about ± 0.00002inch. It is used to measure large dimensions incases where a relatively high accuracy is re-quired and to measure small dimensions wherethe tolerance is less than 0.0002 inch. Whensmall dimensions are to be measured to a highaccuracy, it is best to set the instrument withgage blocks or disks equal to the basic value ofthe required dimension in order to eliminatelead screw errors. For very precise measure-ments the gage should be allowed to stand inthe machine to come to temperature. Thisneed not be done unless great accuracy is re-

quired. This instrument will accommodatework of various lengths by moving the head-stock. The headstock can be located accuratelywithout the use of gage blocks by alinementwith reference lines on a master bar with theattached microscope. This method is particu-larly useful when large dimensions are to bemeasured. The position of the tailstock canbe checked by locating the headstock at the 1-inch mark, placing a l-inch gage block betweenthe anvils, and adjusting the position of thetailstock until the milliammeter reads zero.The index for the hand wheel should be set at 1-inch. The elevating table should be used withcaution for the measurement of tapered gagesor other operations in which parallelism of thetable is important. If the elevating table isnot properly alined, the desired set-up oftencan be obtained by placing the gage on parallelsplaced directly on the bed of the machine. Inother respects the measuring machine is usedlike a bench micrometer.

8.5.7 Mechanical and electrical com-parators. This group comprises a large vari-ety of instruments which do not record meas-urements directly but which indicate the varia-tion from a given dimension as established by

102

gage blocks or other setting gages. Theirprincipal use is checking lengths and diameterswhere several gages of the same size are to beinspected. However, they can also be used tocheck individual dimensions when direct meas-uring equipment is not available, or in the caseof the best comparators, where extreme ac-curacy is desired. The following are the prin-cipal types of comparators:

8.5.7.1 Metron gages. This type (see fig.150) is available with magnifications varyingfrom 100 times to 100,000 times. The scalesare graduated in units of 0.001 to 0.000001 inch.Each comparator wil1 operate at four differentorders of magnification depending on the posi-tion of switches. Other features of this instru-ment are a pressure regulator. permittingselect ion of any measuring pressure between2 ounces and 2 pounds, means for tilting themeasuring head to any angle, and means forswivelling the measuring head about the sup-port column.

8.5.7.2 Electrolimit gages. Various mod-els of the electrolimit gage ( see fig. 151) pro-

Figure 150. Metron comparator.


Figure 151. Electrolimit comparator.

vide magnifications between 750 times and22,500 times. The corresponding scale gradua-tions are 0.0002 and 0.000005 inch. The gag-ing head of this instrument can be swivelled.A large variety of special gaging heads andanvils are available for special jobs. A gagefor tile measurement of close tolerance internaldiameters of ¼ inch or more is also manu-factured. A special form of this gage is theelectrolimit height gage (see fig. 152). Thisgage, or a gage of equivalent accuracy, is thepreferred means for checking locations wherethe tolerance is 0.0003 inch or less. Voltage orfrequency regulation of the supply current mayaffect measurements with these comparators.Electrical input characteristics unfavorable tothe instrument can be detected by fluctuationsin setting after temperature equilibrium hasbeen attained.

8.5.7.3 Visual gages. The visual gage (seefig. 34) is available with magnifications varyingfrom 500 times to 20,000 times. The corre-sponding graduations are 0.0001 and 0.000002inch. The gaging heads can be swivelled. Aspecial model of this comparator embodies apressure regulator (see fig. 153 ) used with the

Figure 152. Electrolimit height gage.

Figure 153. Pressure regulator for use with visual gage thread

checking attachment (cover removed to show construction).

103


Figure 154. Universal external visual gage.

104


Figure 155. Supersensitive comparator.

special spindle attachment in the rapid inspec-tion of thread plug gages. Some models ofthis gage are constructed with special types ofgaging members to permit the measurement ofclose tolerance internal diameters. Othermodels having horizontal gaging members formeasuring plain and taper plugs with precisionrolls are also available (see fig. 154).

8.5.7.4 Supersensitive comparator. Thisgage (see fig. 155 ) is a very accurate indicatingunit. The scale is graduated in units of 0.0001inch. Similar units are available having adual or variable magnification.

8.5.7.5 Dial comparator. A dial indicator-type comparator (see fig. 156) is commonlyknown as a dial comparator. The dial is grad-uated in units of 0.0001 inch. This instrumentincorporates an adjustable swiveling gaginghead and an adjustable back stop. The tablesupporting the anvil can be swung to permitside entry of the work.

8.5.7.6 Dial bore gage. This is an indi-cator-type comparator ( see fig. 157) for check-ing internal cylindrical surfaces. The dial isgraduated in units of 0.0001 inch. These gagesare available in sizes suitable for measuring allinternal diameters between 0.122 to 12½ inches.The insert in the upper left of figure 157 showshow the gage is set by means of a ring gage.The gage can also be set with gage blocks andinternal jaws. The instrument and the blockscan be alined more easily if both the end ofthe instrument and the sides of the blocks arerested on a flat surface when this adjustmentis made. The insert on the lower right of fig-ure 157 shows a series of extensions used toadapt each gage for various sizes of work. Thisgage should be checked occasionally to make

Figure 156. Dial comparator.

105


sure that no flat spots have been worn on thecontacts .

8.5.7.7 Using comparators. The followingsteps should be taken in using mechanical andelectrical comparators:

a. When rounded surfaces are inspected, orwhen measurements are taken over wires, aspindle with a flat gaging surface should beused and when flat surf aces are inspected thespindle with a spherical gaging surface shouldbe used. The stationary anvil should be wideenough to prevent any rocking of the work.

b. Clean the anvil and spindle with hardsurface paper or with a chamois. It may oc-casionally be necessary to wash them withsolvent.

c. The anvil should occasionally be checkedfor wear with an optical flat. The error in theflatness of the anvil should be considerably lessthan the accuracy of the instrument.

d. Set the instrument with size blocks, pre-cision roll, or setting gage.

e. Pass the work between the anvils. Thehighest reading of the scale indicates the size.

Figure 157. Dial bore gage.

The work should not be allowed to tip or bemoved too rapidly as this may result in a dam-aged spindle or false readings.

f. When a large number of gages of the samesize are inspected the instrument should bechecked frequently for proper setting. It alsoshould be checked for proper setting after thejob has been completed in order to be sure nodefective gages are accepted.

Figure 158. Precision indexing head.

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Figure 159. Bench center with sine bar indexing face plate.

8.5.8 Indexing equipment.8.5.8.1 Precision indexing head. The pre-

cision indexing head (see fig. 158) is a very ac-curate device for measuring angles. The headcan be set at any given angle to an accuracyof 2 seconds of arc. The instrument can also beused to particular advantage when it is neces-sary to measure large angles and when there areseveral different angles to be checked on thesame gage.

8.5.8.2 Bench center with sine bar index-ing face plate. This device (see fig. 159) in-corporates a square, rotating face plate onwhich are mounted four precision rolls locatedequidistant from each other on a 3 inch radius.To use the instrument, the centerline of theheadstock is determined by clamping the baseprovided with the instrument or a parallel barto the bed and inserting gage blocks betweenthe base and rolls until two diagonally oppositerolls are equidistant from the base. The de-sired angle then can be constructed by multiply-ing the sine of the angle by three, and addingthis amount to the height of the gage blockswhich stand level with the centerline. The faceplate then is rotated until the appropriate rollis in firm contact with the stack of blocks. Theaccuracy of this device is somewhat less thanthat of a 3-inch sine bar for angles less than45 degrees, and somewhat greater than the 3-inch sine bar for angles greater than 45 degrees.This instrument can be used to particular ad-vantage when it is necessary to establish large

angles and when there are several differentangles to be checked on the same gage.

8.5.9 Hardness tester. Gages may bechecked for hardness on a Rockwell hardnesstester (see fig. 160), or equivalent, using the“N Brale” penetrator. A 15-kilogram majorload should be used on lapped surfaces, and a30-kilogram major load should be used onground surfaces. Rough ground surfaces andcoarser surfaces should be checked on a Rock-well hardness tester, or equivalent, using a 90-kilogram major load. If this instrument is notavailable, the superficial tester may be used witha 45-kilogram major load. For case hardenedgages, the superficial hardness tester should beused with a 15-kilogram major load.

8.5.9.1 The following steps should be takenin using the hardness tester:

a. Turn the crank handle (6) forward(counterclockwise) as far as it will go, therebylifting the weights.

b. Be sure the surface of the gage which is torest on the anvil of the tester is free from alldirt, scale, and burrs.

co Place the gage securely upon the anvil ( 1).d. Elevate the gage and table by rotating the

capstan (2) until it comes in contact with thepenetrator, and then further until the smallpointer (8) of the indicating gage is nearlyvertical and slightly to the right of the dot.

e. Turn the zero adjuster (3) until the“set” arrow on the dial is exactly back of thepointer (5).

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Figure 160. Rockwell hardness tester.

f. Push down on the presser bar (4) to applymajor load.

g. Watch the crank handle (6) until it comesto rest. This indicates the major load has beenfully applied.

h. Pull crank handle (6) forward, liftingmajor load but leaving the minor load stillapplied.

i. Read Rockwell hardness number ondial (7).

8.5.9.2 Since hardness specifications on gagedrawings generally are expressed in terms ofthe Rockwell “C” scale, readings obtained on

the “N” scales must be converted to the equiva-lent value on the “C” scale. A table of equiva-lent value is given in 8.27.5.

8.5.9.3 Hardness measurements should bemade on the gaging surface when practical andas near the gaging surface as possible in othercases. However, when the metal at the gagingsurface is thin enough to flex under the appliedload (as in the instance of screw threads) thispractice should not be followed because a falsereading would be obtained. In such cases tilecheck should be made on a surface adjacent toand as near the gaging surface as possible.

8.5.9.4 A false reading is sometimes obtainedon cylinders inch or less in diameter becausethe “V” in the anvil and the diamond brale arenot in line, permitting the work to roll whenpressure is applied. It is essential that thebrale and the “V” are properly alined.

8.6 GAGE INSPECTION OF PLAINPLUG GAGES

8.6.1 Measurement with a bench microm-eter. If the tolerance is between 0.0002 and0.001 inch, inspection of a plug gage can bestbe made with a bench micrometer (for example,the supermicrometer). The proper steps areas follows:

a. Set the pressure adjustment for l-poundpressure unless the plug is quite heavy (3 inchesor more in diameter). In the latter case, use2½-pound pressure. If the plug has been cali-brated at some specific pressure, use the pres-sure of the previous calibration.

b. Clean the anvils and check the adjustmentof the instrument in accordance with 8.5.6.1.

c. Check the gage for visual defects and re-move all burrs which might interfere withmeasurement. Be sure the contact surface isabsolutely clean.

d. Place the plug on the elevating stage,bring the anvils in contact with the front endof the plug and move the plug about a little tobe sure that no dirt or uneven spots interferewith the measurement.

e. If the pressure of the anvils will supportthe gage, lower the elevating table so that theplug is held only by the anvils (see fig. 161).If the plug is too heavy to be supported in thismanner, it can be placed horizontally on the ele-vating table so that it rolls freely under the

108


Figure 161. Measuring small plug gages.

pressure of the anvils permitting self alinement(see fig. 31). If the gage is extremely large, acylindrical piece about ¼ inch in diametershould be placed between the plug and the ele-vating table to permit alinement of the plug(see fig. 162).

f. Rotate the plug 90 degrees and take an-other measurement. In general, pieces shouldnot be moved between the anvils of precisionmeasuring equipment without removing thepressure. However, it may be done by a benchmicrometer if the gage has a good finish andthe anvils of the micrometer are relapped fre-quently enough to keep their faces flat. Gen-erally it takes less time to relap the anvils occa-sionally than it does to back off the anvils eachtime a reading is taken.

g. Measure the diameter at the center of theplug and at the back of the plug to check fortaper. The diameter of the plug should not ex-ceed the specified limits at any point along theaxis. If the permissible variation in taper is

given on the gage drawing or if there is somefunctional characteristic which requires thatthe taper be held more closely than would beindicated by diametral limits, these consider-ations should govern the acceptability of thegage.

h. The maximum reading obtained in mak-ing all the measurements described aboveshould be recorded as the diameter of the gage.

i. A method of checking plug gages for wearis described in 8.24.

8.6.2 Measurement with a mechanicalcomparator. If the gage tolerance is between0.0002 and 0.001 inch and several gages of thesame size are to be checked, the inspection canconveniently be made with a mechanical com-parator which reads directly to 10 thousandthsof an inch and on which hundred thousandthsof an inch can be estimated. This method isparticularly suitable where several gages of thesame size are to be measured. The steps in thisoperation are as follows:

109


Figure 162. Measuring Iarge plug gage.

a. Clean the gaging members by wiping themwith hard finish paper or chamois.

b. Set the comparator with gage disks.c. Place the plug on the anvil and roll it

under the spindle. Rotate the plug and rollit under the spindle again in a position to ob-tain a measurement about 90 degrees from thefirst measurement. Great care should be takennot to tip the gage on the anvil. The anvilshould preferably be as wide as the gage is longexcept for gages over 4 inches long.

d. The diameter should be measured at thefront, center, and back to determine taper. Thediameter of the plug should not exceed thespecified limits at any point along the axis, aspreviously explained.

e. As previously stated the diameter of thegage should be the maximum reading obtainedin all of the measurements described above.

f. The setting of the comparator should berechecked with a gage disk at frequent inter-vals, If an appreciableoccurs all gages checked

change in settingsubsequent to the

previous settingreinspected.

of the instrument should be

g. The anvil of the comparator should bechecked occasionally for flatness by means ofan optical flat or by measuring the height of aroll placed in several different positions on theanvil.

8.6.3 Measurement with a precision meas-uring machine. Gages with a tolerance of lessthan 0.0002 inch or which have unusually large(dimensions should be inspected in a precisionmeasuring machine which reads directly to0.0000l inch. The methods of alining gages inthis machine are similar to those employed witha bench micrometer.

8 . 7 I N S P E C T I O N O F T A P E R E DPLUGS

8.7.1 Measuring straightness and angle oftaper. Where the tolerance for the includedangle of a gage is greater than 20 minutes, itmay be checked by placing the plug betweenthe centers in an optical comparator (contourprojector), adjusting the screen so that one ofthe lines is parallel to the side of the plug, and

110

Figure 163. Checking front face of plug for perpendicularity.

reading the vernier angle scale at tile side ofthe screen.

8.7.1.1 More accurate results can be obtainedwith a sine bar or plate but for small lots moretime is required than when the optical com-parator is used. In general, the sine bar issatisfactory for all taper gages having a tol-erance on diameter of 0.0002 inch or more. Thesine bar is of particular value when a largenumber of gages is to be checked, due to thesaving in time. When measurements are madeby this method. it is essential that the axis of thegage be parallel to the long sides of the sinebar or sine plate. When a sine plate is usedthis can be accomplished by alining an end ofthe gage with the heel or flange of the plateprovided the surface used is perpendicular tothe axis. (Use a method similar to that shownon fig. 163 for this cheek. ) If the back face isnot perpendicular, it may be possible to devise asupport such as parallels clamped to an angleplate to keep a finished cylindrical portion ofthe gage in line with the sides of the sine plate.

8.7.1.2 A set-up for measuring a taper bymeans of a sine plate is shown in figure 164.The sine plate is set to the included angle of


taper and the gage placed in the position shown.The gage must be constantly pressed against theroll and the flange at the top of the plate bymeans of the hand.

8.7.1.3 An indicator is used to measure theheight from one end of the gage to the other.Irregular variations in the indicator readingindicate errors in the uniformity of taper andthe difference between the readings near theend is a measure of the error in taper. The in -dicator should contact the high point of thegage whenever a reading is taken. This methodis best suited for plugs whose included angleis less than 45 degrees. The special fixtureshown on figure 165 will effect considerablesaving in time when several tapered gages ofthe same size are to be checked. The support-ing means make it possible to aline the axis ofthe plug with the center of the sine plate moreaccurately and dependably than would other-wise be possible.

8.7.1.4 Another way of measuring the taperof plugs is by measurements over rolls as shownon figure 166. This method is precise and de-pendable and should be used when the toleranceon taper is very small. However, the rollsmake point contact with the gage and minorsurface irregularities may lead to erroneous re-

Figure 164. Checking taper on sine plate.

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Figure 165. Measuring taper on special sine plate fixture.

suits unless the checker observes the precautionof rotating the plug small amounts and notingany variation in size. If there is no variationin size the checker will know that the measure-ment is true.

8.7.1.5 The first step in using this methodis to mount the gage between centers and rotateit with the ball point of an indicator againstthe face in order to check the squareness of thefront face with the axis of the gage (see fig.163). At the same time the inspector shouldcheck the tapered surface for concentricity withother precision surfaces on the gage.

8.7.1.6 If the face of the gage is not perpen-dicular, an error of 0.010 inch per inch of di-ameter in total indicator variation causes anerror in diameter of approximately 0.00005inch. This relationship is true for all moderatetapers but not for tapers where the includedangle is greater than 30 degrees. Proportionatevariations in indicator readings give propor-tionate errors. For example, if the indicatorvariation is 0.016 inch when the indicator con-tact point is held near the outside diameter of

a plug 1 inch in diameter, the error in com-puting diameter will amount to 0.00008 inch.A diameter half as large would have twice theerror and a diameter twice as large would haveonly one half the error.

8.7.1.7 In general, it is not necessary to con-sider errors in squareness of the end unless theresultant error in the diameter, as computedby the method specified in 8.7.1.6, is greaterthan 10 percent of the gage maker’s tolerance,In borderline cases where a dimension ap-proaches the gage maker’s limits the errorshould be taken into account except where itis less than 50 percent of the possible error inthe measurement .

8.7.1.8 Error in squareness of the face of thegage may also be determined by measuring over the gage with one large and one small roll and

Figure 166. Measuring taper and diameter of tapered plug.

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interchanging the rolls for a second measure-ment. Any difference in the two measurementsindicates an error in squareness of the face.

8.7.1.9 If the end of the gage is found to beperpendicular to the axis it can be placeddirectly on the base of a hold-down fixture ormeasuring machine and the measurements takenas shown on figure 166.

8.7.2 Checking gages when front face isnot square. When the error in squareness islarge enough to affect materially the accuracyof the diameter measurements, it is sometimespossible to mount the gage upright betweencenters for the measurements. The centers canbe part of a small fixture which is adjusted untilthe gage is vertical within the requirements ofthe measurements. The accuracy of the verti-cal placement can be checked by indicating overthe back face of the gage or the horizontal sur-face from which the datum diameter or apexhas been dimensioned.

8.7.2.1 If the gage cannot be mounted oncenters, it is sometimes possible to hold it ina taper shank holder.

8.7.2.2 In selecting rolls it should be remem-bered that the larger the roll, the less will bethe errors due to surface irregularities and de-formation at the contact between the roll andthe gage. Nominal diameters of 0.100, 0.200,0.300, and 0.400 inch are commonly used How-ever, some gages with very short lengths of taperwill require the use of rolls smaller than 0.100inch in order to obtain readings over a reason-ably large portion of the tapered surface. Onthe other hand, rolls up to 1 inch in diameter aresometimes used on very long tapered plugs.

8.7.2.3 The use of extremely small rolls canusually be avoided by placing the front face ofthe gage on parallels and using comparativelylarge rolls which contact the lower portion ofthe taper when resting on the base of the hold-down fixture. Straight and tapered plugs areoften rounded near the ends. Consequently,readings should never be taken closer than ap-proximately 0.050 inch from the ends and pref-erably not closer than 0.200 inch from eithere n d .

8.7.2.4 After the gage has been properly setup and the rolls placed in position for measur-

ing the diameter near the base, measurementsare made at various points around the circum-ference of the gage until the largest diameteris found. This value is recorded as the meas-urement at the base of the gage. The measure-ments at other points along the length of thegage are taken with the gage in the same angu-lar position. This is done because only the highpoints on a gage which is out-of-round makecontact with the component and determine theposition of the gage. The rolls should bepushed lightly with the fingers before eachmeasurement to assure firm contact with thegage, blocks, and micrometer anvils.

8.7.2.5 As an example let us consider themeasurement of taper on the gage shown infigure 167. Rolls 0.4000 inch in diameter areconvenient in this case. A practical value forA, the vertical distance between points of meas-urement, is 0.9000 inch. The exact value of Ais not important as long as the rolls are a rea-sonable distance (generally between 0.050 and0.200 inch) from the end of the plug at thetop position. For the first measurement, thetwo rolls are placed directly on the hold-downfixture and the plug rotated to find the maxi-mum diameter as previously described. Themeasurement in the second position is madewith the gage in the same position and each rollplaced on a 0.9000 inch gage block combination.The height of the blocks for the third position

Figure 167. Set-up for measuring taper.

113


is 1.8000 inches and for the fourth position2.7000 inches. Assume that the measurementsover the rolls are as follows:

1.90000 inch 1st position2.07999 inch 2nd position2.26002 inch 3rd position2.44003 inch 4th position

By subtracting each reading from the follow-ing one, we get the following differences:

0.17999 inch 2nd—lst 0.18003 inch 3rd—2nd0.18001 inch 4th—3rd

Since the greatest difference between these fig-ures is 0.00004 inch, the uniformity of taperis satisfactory.

8.7.2.6 In the preceding example, the actualtaper is found by subtracting 1.90000 inch from2.44003 inch and dividing by 2.70000 inch.This gives 0.2000 inch included taper per inch.The tangent of the half angle is

By reference to trigonometric tables we findthat the angle whose tangent is 0.1000 is 5 de-grees, 42 minutes, 40 seconds. The includedangle is twice this, or 11 degrees, 25 minutes, 20seconds. The included angle cannot be com-puted by assuming that its tangent is 0.2000.

8.7.2.7 Any surface irregularities on theplug which were not detected by the previousmeasurements can be located by a moisture filmcheck made over the entire length of the taper.This is done by drawing the hand across a flatlapped surface creating a very thin film ofmoisture. The contact surface of the gage isthen placed on the surface and slid along per-pendicular to the grain of the moisture film.The track left by the plug will indicate theuniformity of taper. The plug should leavesome mark at both ends of its length and wipeaway three-fourths of the film along its length.Great care should be taken to prevent any see-sawing of the plug in this operation. U n e v e npressure on the plug should also be avoided asthis leads to a false indication whereby a plugwith convex elements appears to have straightelements. This test is very accurate because

114

the moisture film is only a few millionths of aninch thick. The same test can be performedmore easily but less accurately with a thin filmof blueing.

8.7.3 Checking diameters of taper plugswith rolls. For taper plugs having includedangles of less than 90 degrees the most accu-rate method of measuring diameters is by meansof measurements over rolls with the same set-up used to check taper.

8.7.3.1 Before the diameter can be computed,the taper must be determined. The time re-quired to determine diameters on a taperedplug is shortened by using the readings takenin measuring the taper, as described in the pre-ceding paragraphs.

8.7.3.2 The most satisfactory method of set-ting of the machine for this measurement is tomeasure over blocks and rolls, using the nom-inal diameter of the rolls instead of their exactsize as specified in 8.5.6.1. Since this methodeliminates the effect of small departures fromnominal roll size, it is only essential that therolls be as nearly the same size as possible.

8.7.3.3 The necessary computations for de-termining diameters by this method are asfollows:

Find the length from the back face of the gage

Figure 168. Tapered plug drawing specifications.

to the point along the taper where the diameterequals exactly 1.3750 (see fig. 170).

The location of the datum diameter, there-fore, is 4.2830 inches from the back face of theplug, which is within the limits specified on thegage drawing.

8.73.4 Datum diameter system. The datumdiameter is not a dimension to be measured di-

Figure 169. Computing tapered plug diameter from

measurements over rolls.


rectly but is a basic figure used in computingaxial lengths. Variations in the diameter ofthe plug are limited by applying the toleranceto the axial distance from some given point tothe datum diameter. The object of this sys-tem is to eliminate the errors and misinterpre-tations which arise in other systems whereinthe actual position of the tapered surface is af-fected by the tolerances on more than one di-mension (variations in the diameter of the plugas well as variations in the length to the sharpcorner) which introduce the effects of cumu-lative tolerances. An advantage to the gagemaker of the datum system is that it permitshim to control the effective diameter of the plugby grinding or lapping metal off the referencesurface. Such a surface has large tolerancescompared with equivalent tolerances on thediameter. On plugs having slight tapers, con-siderable change in the length to the taperedsection has no more effect on the size of the gagethan very slight changes in diameter.

8.7.4 Measuring the diameters of taperedplugs with roll and sine bar. This method isbest adapted to the measurement of a largenumber of gages of the same size because it isa convenient and rapid method once the initialset-up has been made. However, it is not pre-ferred when only a few gages of the same sizeare to be measured because of the expense ofmaking the special roll or when extreme ac-curacy is required because of “the difficulty inplacing the gage in exactly the right positionwith respect to the sine bar. The geometry ofthe method is such that cumulative toleranceswill be introduced in all cases except those inwhich the front face of the gage is used as thereference plane when the gage is applied to thecomponent.

8.7.4.1 This measurement is accomplished byplacing the tapered plug on a sine bar in a set-up similar to that used for checking the taperwith a sine bar and indicator.

8.7.4.2 A roll is placed on the bar in frontof the plug as shown in figure 171. The di-ameter of the roll is such that its height abovethe surface plate is equal to that of a gage withexactly nominal dimensions. Errors in the di-ameter of the gage are determined by the differ-

922173 O - 51 - 9 115


Figure 170. Finding location of datum diameter.

ence in the height of the top of the roll and thetop of the plug.

8.7.4.3 The gage is held in position by meansof a clamp (see fig. 172). Before positioningthe gage it should be. mounted on centers todetermine the parallelism of the axis of thegage and ground cylindrical surface which isto bear against the angle plate. In setting thegage in a position for measurement the axis ofthe gage must be parallel to the sides of the sinebar and the lower side of the tapered surfacemust be absolutely snug against the surface ofthe sine bar along the full length of the gagingsurface. This latter condition can be checkedby sighting between the gage and the sine bar.

8.7.4.4 The formula for computing the ra-dius of the measuring roll is as shown in figure173. If a roll of the right size is not available,a smaller roll can be placed on gage blocks. Theformula for finding the length of blocks neces-sary to bring the top of the roll level with thetop of the gage is shown on figure 174.

Figure 171. Checking diameter with roll and sine bar.

8.7.5 Checking the diameter of taperedplugs with master rings. When a large num-ber of tapered plugs of the same size are to beinspected, much time will be saved by makingtaper ring check gages to inspect the plug gage.An additional advantage of check rings is that

Figure 172. Method of damping gage over sine bar.

Figure 173. Roll for checking taper plug.

116


Figure 174. Length of block for odd size roll.

they check the entire circumference of the pluggage for maximum diameter.

8.7.5.1 Check rings should be inch thick.They are made in pairs, one ring being as closeas possible to the basic diameter for the frontof the plug and the other being as close aspossible to the basic diameter for the back ofthe plug. The dimensioning of check rings isshown on figure 175.

8.7.5.2 The rings are fitted to master plugs.Great care must be taken to make them veryaccurate, as otherwise they will be of no value.

8.7.5.3 The check rings are used by clampingthe plug in a vertical position and placing therings over it. Each ring should be horizontal.Check by indicating. The rings should be ad-justed in position with only moderate hand pres-sure. approximately the same as that used tofit the rings on the master plug. If the outersurfaces of the rings are exactly flush with thefront and back faces of the gage, the dimen-sions of the plug are as specified on the draw-ing. If the rings are not flush, the diametersof the plug can be determined by measuring, h,

Figure 175. Checking taper plug with check rings.

Figure 176. Computing diameter of tapered plug.

the distance between the face of the plug andthe outside face of the ring with a height gage(see fig. 176). The diameter of the front end isdetermined by multiplying the distance h by theincluded taper per inch. This value is sub-tracted from the diameter of the ring if the faceof the plug is above the face of the ring andadded to the diameter of the ring if the face ofthe plug is below the face of the ring. The re-verse is true at the back of the plug. For exam-ple, let us assume that the front face of the gageshown in figure 175 projects 0.0008 inch fromthe outer face of the check ring. The diameterat the front of the plug is found as follows:

Given:A= 0.5000T. P. I.= 0.2500h= 0.0008x= A–h x T. P. I.x = 0.5000 – 0.0008 x 0.2500=0.4998

Therefore, the diameter at the front of the plugis within the specified tolerance.

8.8 ALINEMENT AND CONCENTRIC-ITY PLUGS

8.8.1 The diameters and tapers of the indi-vidual sections of these plugs are checked ac-cording to the methods specified in 8.6 and 8.7.

8.8.2 The location of the intersection of twotapered surfaces and the diameter at this pointis sometimes required on these gages. This isdone as follows:

8.8.2.1 Stand the plug upright in a hold-down fixture and find the taper of each sectionby measuring over rolls in the same manner ason ordinary taper plug gages.

8.8.2.2 Using the readings taken for thetaper check, compute the diameter d1 at a dis-tance H1 from the front face of the plug (see

117


Figure 177. Computing location of intersection (small taper

over large taper).

fig. 177). If this face has been placed on thehold-down fixture, H1 will equal the height ofthe gage blocks used to make the measurementat this point. The diameter is computed by themethod specified in 8.7. Determine the diam-

Figure 178. Computing location of intersection (large taper

over small taper).

eter at one point on the adjacent tapered sec-tion by the same method.

8.8.2.3 Using the method shown on figure177, compute the distance x from the point ofmeasurement to the intersection.

8.8.2.4 Use the same methods to locate otherintersect ions. If the upper taper is steeperthan the lower, the method shown on figure 178can be followed.

8.8.2.5 Use the method shown on figure 179to find the distance from the intersections to thereference surfaces.

8.8.2.6 The diameters at the intersections canbe computed from the second line of equationsshown on figures 177 and 178.

8.8.3 The special fixture shown on figure 180will reduce the time for the inspection of aline-ment plugs to a small fraction of that normallyrequired. It also can be used to advantage onordinary tapered plugs.

8.8.3.1 Slow tapers are checked by means oftwo adjustable plates at right. angles to oneanother which support the plug in much thesame manner as a “V” block. These plates areadjusted to the half angle of the taper by plac-ing the fixture on a sine plate which has beenset for this angle and adjusting their position

Figure 179. Computing distances from back face to

intersections of tapers.

118

Figure 180. Special fixture for checking taper and diameter of tapered plugs.

119



Figure 181. Checking taper with special fixture.

until the surfaces which support the plug arelevel as indicated by a precision height gage.Sets of feet at right angles are provided so thateach plate can be placed in a vertical positionover the sine block for effecting this adjust-

8.8.3.2 The actual measurement of the taperis made by standing the fixture on the sineplate so that it rests on a third set of feet asshown on figure 181. Any variations in theheight as determined with a precision heightgage must be multiplied by .83 to find the trueerror in the taper. For example, if the differ-ence in the readings at the two ends of the ta-pered section is 0.001 inch, then the actual errorin taper over the length of the section is 0.00083inch. This correction applies to all tapers

which do not exceed 0.1 inch per inch, andwhere the error in taper is not greater than0.010 inch per inch.

8.8.3.3 The diameter of slightly tapered sec-tions can be measured with this set-up whena master gage or a gage which has been ac-curately calibrated is used for setting the heightgage. The master gage is positioned axially bythrusting the steep taper against the measuringrolls as shown on figure 181 or by thrustingsome other precision surface against the fixtureand then comparing the height of the plug withthe master. AS before, differences in heightgage measurements must be multiplied by .83 todetermine the error in taper.

8.8.3.4 In order to check the distance to thedatum diameter of steep tapers, the fixture is

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set up on one end so that the plug is vertical asshown on figure 182. The members of the fix-ture which hold the rolls in position are ad-justable so that the rolls can be spaced anyrequired distance apart. These members aremounted on a common base which is free tomove at right angles to the axis of the rolls topermit proper alinement of the rolls with theaxis of the plug. Two methods can be used tomeasure the length to the datum diameter. Theeasiest method is to place a master plug or acalibrated plug in the fixture and establish theheight of the back face by means of a precisionheight gage. The plugs to be measured arethen inserted in the fixture and the heights ofthe back faces checked by means of the precisionheight gage. The difference in these heights


indicates the amount by which the distances tothe datum diameters of the plugs differ fromthe distance to the datum diameter on the masterplug. For this set-up the rolls are spaced anyconvenient distance apart which will permitcontact with the master near the center of thesteeply tapered section. The other way to meas-ure the length to the datum diameter is to ad-just the fixture so the rolls are some specificdistance from each other. (Use gage blocks toestablish this distance. ) The basic distancefrom the datum diameter of the gage to the backface then is computed by means of the formulaeshown on figures 169 and 170.

8.8.3.5 The angles of steeply tapered sectionscan be measured by altering the distances be-tween the rolls by convenient, but accurately

Figure 182. Checking length to datum diameter with special fixture.

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determined, distances, and remeasuring theheight of the back face. The differences in thedistance between the rolls, divided by twice thechange in the height of the gage gives the tan-gent of the half angle of the taper.

8.8.4 Alinement and concentricity plugsmust also be checked for concentricity. Thiscan be done by mounting the plug between cen-ters and indicating over each of the precisiondiameters while the plug is rotated.

8.8.4.1 If there are no centers on the gageor if the surfaces are not concentric with thecenters, the check for concentricity can be madeby placing one section of the plug in a “V”block. An indicator then is brought into con-tact with the other sections as the plug isrotated.

8.8.4.2 When a drawing specifies eccen-tricity (or concentricity) within a givenamount, the total permissible indicator varia-tion is twice this amount. For example, if adrawing reads “max. permissible eccentricity:0.0002 inch,” the maximum permissible varia-tion in indicator readings is 0.0004 inch.When there are no specifications for concen-tricity it is generally assumed that no part ofa gaging surface exceeds the diametral limitsspecified in the drawing. When the gage draw-ing merely states that certain surfaces must be“concentric” it is assumed that a total indicatorvariation of 0.0001 inch or less is satisfactory.

8.9 RING AND RECEIVER GAGES8.9.1 Plain ring gages. The following

means are used to check the diameters of ringgages:

8.9.1.1 Check plugs. It is common practiceto furnish check plugs with gages less than ¼inch in diameter. These check plugs should fitthe ring with no shake whatsoever. The checkplug should be tried at both ends of the ring tobe certain there is no bellmouthed condition.Check plugs must be absolutely clean and dry.If a check plug 0.0001 inch larger than thefirst is available, it should be tried in the ring.If such a plug is not an extremely tight fit, thegage is unsatisfactory.

8.9.12 Internal calipers. Internal diam-eters with tolerances of 0.0005 inch or greatercan be measured by means of internal calipersor internal micrometers by the transfer method.

This applies to diameters of about 1 inch; themethod becomes less accurate for larger diam-eters. The micrometer is settled in positionby moving it horizontally or vertically with re-spect to the ring until the proper fit is found.This assures alinement of the contacts on adiameter. The micrometer or the caliper thenis locked in position, removed and measuredwith external measuring equipment, using thesame feel employed to establish the internaldiameter. It is important that the radii ofthe caliper or micrometer jaws be less thanthe radius of the gage.

8.9.1.3 Dial bore gages. These gages maybe used to check internal diameters with toler-ances of 0.0002 inch or greater. They are madein sizes suitable for diameters ranging from to 12½ inches. Dial bore gages are particu-larly useful when several ring gages of thesame size are to be checked.

8.9.1.4 Casts. Although the measurementof close tolerance internal diameters by meansof casts made with low melting point metalshas not yet attained widespread recognition,the method has considerable value in some in-

stances. The attainable accuracy is at leastas good as that of the bench micrometer. Thismethod is used only on ring gages less than ½inch in diameter, or less than ¼ inch in diam-eter in case an internal comparator is avail-able. Base metals with a low melting pointwhich expand upon solidifying appear to bewell suited for this work. One such metal issold under the trade name “Cerrobend”. Thering gage to be inspected is cleaned carefullyand placed on a flat plate. Molten metal thenis poured into the ring until it is full and thework allowed to stand overnight. The castsare driven from the rings with punches whichare only sightly smaller in diameter than therings. The casts should be handled only withtweezers, and should be measured between flatanvils within ½ hour after being removed. Theburr on the cast should be filed off before thismeasurement. A second cast, driven out fromthe large end of the gage, should be made ifthere is any indication that the ring is bell-mouthed or tapered.

8.9.1.5 Internal measuring machine. Thisinstrument is suited for all diameters greater

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than ¼ inch and can be read directly to 0.00002inch. It is used to measure the internal diam-eters of gages or gage parts with diametraltolerances of less than 0.0005 inch.

8.9.1.6 Gage blocks. If an internal meas-uring instrument or dial bore gages are notavailable, internal diameters with tolerancesless than 0.0005 inch can be measured by theuse of gage blocks with internal measuring ac-cessories or with rolls. Gage block combina-tions are tried with the rolls or with the acces-sories until a combination is found which willjust barely slide into position with a moderatepressure.

8.9.1.7 Common defects in ring gages arebellmouth, large center, taper, and out-of-roundconditions. They can all be detected by properexploration of the bore of the ring gage.

8.9.1.8 All burrs should be removed fromthe openings before a ring is measured.

8.9.1.9 Another point to watch for is wearon the anvils of the measuring instruments.Anvils with flats should be reconditioned. Thedepth of the wear can be determined by theuse of a contour projector in the case of calipersand micrometers, and by means of direct meas-urement over the anvils in the case of precisionaccessories.

8.9.2 Receiver gages. When a check plugis provided for the receiver, check plug is in-serted in the receiver and checked for shake.No shake whatsoever is permissible. When re-receivers are made of several sections, each sectionshould be disassembled and tried individuallywith the plug to be sure that the correspondingsection of the receiver is not too large. If checkgages are not provided for receiver gages, theymust be checked by the methods used for plainand tapered ring gages. Tapered internal sur-faces may be checked by the following means:

8.9.2.1 Check plug and blueing (anglecheck). Check plugs are usually provided fortapered sections of receiver gages. To inspectthe angle with a check gage, the surface of thecheck is coated with a thin, uniform film ofPrussian blue, cardinal red, or other oil pig-ment. This film is rubbed on the gaging sur-face with the fingers. Extreme care should betaken to apply the film uniformly. The filmshould be thin enough to appear much lighter

in color than when first pressed out of the tube.The thinner the film, the more exacting the testwill be. The check is then placed in a receiver,given one-eighth of a turn, and turned back tothe starting position. The removal of blueingshould show a bearing at each end and on 75percent of the tapered surface.

8.9.2.2 Pencil lines and check plug (anglecheck). A series of longitudinal pencil linesare drawn on the surface of the plug. Thecheck is inserted into the receiver and turned adistance less than the distance between the pen-cil lines, and returned to starting position. Thecheck then is removed and inspected as before.Lines of this sort can also be made with blueing.These are easier to see, but they are a less criti-cal check.

8.9.2.3 Sine bar and angle plate (anglecheck). Another way to check an internal cy-lindrical taper is to bolt the gage to an angleplate and mount the assembly on a sine bar orsine plate as shown on figure 183. This methodis useful because it does not require extensivecomputations and makes possible the measure-ment of the error over the entire tapered sur-face. One precaution to be taken in using thismethod is to be sure that the face’ of a gage isperpendicular to its axis.

8.9.2.4 Precision balls (angle and diam-eter check). The ball method should be used

Figure 183. Checking tapered ring on sine bar.

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only when the included angle of taper at thepoint where the ball rests in the gage is fairlylarge, say 20 degrees or greater. It can be usedto find both the angle and the diameter of thetapered section. The method is particularlyuseful for determining the depth of irregulardepressions such as worn spots, and for meas-uring small curves or radii. The method hasthe followings disadvantages: the accuracy iseffected by minor irregularities on the surfaceof the gage, accurate balls are required andit is sometimes difficult to obtain proper seat-ing without exerting excessive pressure. Thefollowing precautions are to be observed in us-ing this method:

a. Always check the surface being inspectedfor straightness and uniformity. For example,when an internal conical surface is being in-spected, use a check plug and blueing or makea cast in order to be sure that there are noirregular depressions or humps on the surface.

b. Be sure that the internal surface has agood uniform finish (only lapped or finishground surfaces can be inspected).

c. The ball must be measured for size androundness within the accuracy desired for themeasurement. Seat the ball carefully androtate it a bit to obtain a good contact. Use alight pressure in measuring over the ball. Usethe measured value of angle in computingthe diameter. The only measurements re-quired to determine the taper and diameter ofan internal cylindrical surface by means ofballs is to ascertain the relative heights of twodifferent size balls with relation to a referenceplane on the ring. This is usually done by slid-ing the set-up under a precision comparator or

Figure 185. Computing internal angle from measurements

over two balls.

by use of optical flat as shown on figure 184.The taper is measured by comparing the heightsof two precision balls, one of which rests nearthe top of the tapered surface and the othernear the bottom. The uniformity of taper isdetermined by using other sizes of balls andrecomputing the angle. Figure 185 explainsthe mathematical steps for computing the angle.In this figure, h represents the difference of theheights of the tops of the balls, r1 is the radiusof the larger ball, r2 is the radius of the smallerball and A is the half angle of the tapered sur-face. As an illustration of the angle computa-tion, assume that two balls 1.30008 and 2.19993inches in diameter, respectively, are placed suc-cessively in a receiver gage. Assume that thedifference in the heights of the tops of the ballsis 1.12347 inches. The angle is determined asfollows :

Figure 184. Set up for measuring the angle and the diameter

of an internal conical surface,

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By reference to trigonometric tables it will benoted that the angle which corresponds to asine of 0.66801 is 41 degrees 54 minutes 49 sec-onds. Therefore, this is the value of the halfangle of the taper. (In most cases the valueentered on the gage record card for this angleshould be 41 degrees 55 minutes in order not tomisrepresent the accuracy of the measurement.In most instances of this type the angular meas-urement will be accurate only to the nearest 20seconds). The computations necessary to de-termine the diameter at the upper end of atapered surface are shown on figure 186. Inthis figure, h is the distance from the top of theball to the top of the tapered surface; r is theradius of the ball; A is the half angle of thetaper, and x is the diameter of the tapered sec-tion at the top. Suppose that it is desired tomeasure the diameter of the receiver gage forwhich the half angle of taper was computed inthe previous example. It is always necessaryto compute the angle of taper before computingdiameters in order to use the measured valueof the taper in the computation for the diameter.Assume that the diameter of the ball is 2.19993inches and that the distance from the top of theball to the upper surface of the gage is 1.48654inches. In this case the diameter is computedas follows:

Given: A= 41°54´49´´1= 1.48654r= 1.09997

x=2 (1.09997 x 0.66801 + 1.09997 –1.48654) X 0.89768+2 x 1.09997 x 0.74415=2.33353= diameter

8.9.2.5 Tapered check plugs (diametercheck). The diameters of tapered surfaces ofreceiver gages can also be measured by meansof tapered check plugs. These plugs are de-signed so that they are flush with the top ofthe receiver gages if the diameter of the taperedsection of the receiver is normal. A test indi-cator is passed over the top of the plug and thetop of the receiver to measure any difference inheight. If the plug is higher than the receiverthe diameter of the receiver will be less thannormal by an amount equal to the difference inheight multiplied by the included taper per

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Figure 186. Computing diameter at large end of taper

from measurement over ball.

inch. If the plug stands below the receiver thediameter of the tapered section of the receiverwill be greater than normal by this amount.A similar method is shown on figure 176 theonly difference being that the diameter of theplug is given and the diameter of the ringcomputed.

8.9.2.6 Calibrated tapered plugs (diam-eter check). Lengths to datum diameters oninternal tapered surfaces can be computed evenwhen a check plug designed for the receiver isnot available. All that is required is a plugwith a taper the same as that. of the receiver.Assume that the lengths to the datum on theplug is found by measurement to be A. As-semble the plug with the receiver and measurethe height from the reference surface on theplug to the reference surface on the receiver(these are the surfaces from which the datumdiameters were dimensioned on the drawing).


Figure 187. Measuring Iength to datum diameter by means

of calibrated plug.

The difference between these two dimensionsrepresents the actual distance to the datum ofthe receiver (see fig. 187).

8.10 INSPECTION OF SNAP GAGES8.10.1 Plain and built-up snap gages. It

is essential that all screws be tight and that allburrs have been removed from the anvils beforea snap gage is inspected.

8.10.1.1 Snap gages are checked for size andparallelism of anvils by the use of a block androll combination. The blocks are wrung to thelarge anvil and the roll is moved about underthe small anvils. The roll should be tried inpositions at right angles to the gage as well asin positions parallel to the gage. The size ofthe gage (distance between the anvils as speci-fied on the gage drawing) is determined at thepoint where the rolls fit the tightest. Thelength of the gage block combination whichpermits the roll to slide between this point withonly moderate finger pressure, plus the diam-eter of the roll used is the size of the gage.Errors in parallelism are determined by tryingblock combinations at other points. Rolls usedin measuring such gages should be examinedfor flat spots at regular intervals.

8.10.1.2 On small sizes (less than 1 inch) theuse of the roll is not necessary; inspection canbe made by the use of gage blocks alone. Priorto inspection, surfaces of the gage anvils andthe gage blocks should be thoroughly cleaned,otherwise the “feel” of the blocks will beaffected and the setting will be inaccurate.

8.10.1.3 The test is made by resting the gageblocks on the large anvil and sliding it underthe smaller anvils. If the dimension of thegage is the same as the length of the gageblocks a slight amount of extra resistance willbe felt just at the edge of the block combina-tion under the edge of the small anvil. If theblocks do not fit properly under the step, othersizes of blocks are tried until the proper fit isfound. After the proper combination ofblocks has been found, the results should bechecked by trying blocks 0.0001 inch smallerthan the blocks which appear to represent theproper fit. The smaller combination should bea free fit. This method of rechecking is par-ticularly useful when the checker has had littleexperience in checking snap gages.

8.10.1.4 Parallelism can be checked by try-ing the gage blocks in different positions nearthe edges of the anvils and by trying larger orsmaller blocks where the “feel” appears to belooser or tighter than normal. The anvils ofsmall gages should preferably be parallelwithin 0.0001 inch. However, strict interpre-tation of the drawing indicates that the errorin parallelism may be any amount as long asthe distance between opposite anvils fallswithin the gage maker’s tolerance limits at anypoint on the surfaces.

8.10.1.5 When snap gages are used in quan-tity, savings in time and greater accuracy willbe obtained by procuring precision setting rollsfor the common sizes of snap gages.

8.10.1.6 The flatness of the anvils can be in-spected with an optical fiat or a knife-edgestraightedge.

8.10.2 Adjustable snap gages. Beforesetting, these gages should be checked for rangeby moving the anvils to their extreme limitsand measuring with a rule.

8.10.2.1 The size, parallelism, and flatness ofadjustable snap gages are checked by the samemethods used for plain and built-up snap gages.

8.10.2.2 On adjustable snap gages, the “NotGo” anvil should be set first with the “Go” lockscrew tightened. Setting the “Not Go” mem-ber after the “Go” will disturb the adjustmentof the “Go.”

8.10.2.3 In setting the anvils to an exact size,place blocks or rolls of the desired size between

126


the anvils, make the lock screws barely snugenough to permit movement of the anvils, andtighten the anvils by turning the adjustingscrews. When a good slide fit has been ob-tained, tighten the locking screw.

8.10.2.4 After this has been done, tap theanvils to remove any backlash, back off the ad-justing screws slightly and then check the sizewith blocks or rolls again. At the same timethe parallelism should be checked. The objectin backing the adjusting screws is to relievestress which otherwise would cause the anvilsto “creep” over a period of time.

8.10.2.5 When the gage is not set to exactdimension, the anvils should be checked at bothlimits of the range for parallelism. The errorin parallelism preferably should not exceed0.0003 inch. However, it is sometimes neces-sary to accept gages with a greater error thanthis, and lap the anvils parallel when they areset to some particular dimension.

8.10.3 Large snap gages (all types).Large snap gages should always be clampedupright for inspection. The clamps should befastened to the boss of the gage frame, or in itsabsence fastened in a position which will pre-vent any springing of the frame. The clampshould be no tighter than necessary to hold thegage in position.

8.10.3.1 The position of the gage is adjusteduntil the large anvil is level. Parallelism isthen checked by indicating over the other twoanvils.

8.10.3.2 The distances between the anvils ischecked either by gage block combinations orby measuring the distance between the gagingsurfaces with an indicator height gage and gageblocks. When blocks are used on large sizes,it is convenient to wring the blocks to the largeanvil, and to slide only the top block of thestack in checking the smaller anvils.

8.10.3.3 Another way to check the size andparallelism of large snap gages is by clampingthe large anvil on an angle iron as shown onfigure 188. This method is much faster thanthe one specified in 8.10.3.2, but is not quite soaccurate. Parallelism is judged by indicatingover the two smaller anvils, and size is deter-mined by measuring between the smaller anvils

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Figure 188. Checking parallelism of anvils on Iarge

snap gages.

and the top of the iron with an indicator heightgage and gage blocks.

8.10.3.4 The flatness of large snap gage an-vils can be judged by indicating, by sightingover the surface with a knife-edge straightedge,and in the case of very large anvils, by blueinga tool maker’s flat and rubbing the anvil overthe blueing.

8.11 THREAD PLUG GAGES. Beforeattempting to check thread gages, the inspectorshould study Handbook H28 “Screw ThreadStandards for Federal Services” published bythe National Bureau of Standards which isavailable from the Superintendent of Docu-ments, Washington, D. C. A study of this bookis necessary to acquire a knowledge of the defi-nitions, standards and gaging principles relatedto American National and Unified Threads.

8.11.1 Lead. Accurate measurement of thelead of thread gages is more important than themeasurement of all other elements, includingpitch diameter. Each 0.0001 inch of lead errorin a W-degree thread causes an inaccuracyin fit equivalent to a pitch diameter error of


0.00017 inch. If the lead is not strictly withinthe tolerance, values for the pitch diameter willnot have much significance.

8.11.1.1 In the checking of setting plugs, itis important to measure lead accurately and tocalibrate for the effect of errors in the pitchdiameter and the flank angles. Consistent andaccurate setting of thread rings cannot be ob-tained unless the cumulative effects of theseerrors in the setting plug are considered.

8.11.1.2 The lead error constitutes the errorat the point where there is the greatest plusdeviation, added to the error at the point wherethere is the greatest minus deviation.

8.11.1.3 Permissible lead errors of threadgages are sometimes as small as 0.0002 inch,it is desirable therefore to measure lead with aprecision lead testing machine which has ameasuring error of less than 0.00004 inch.Readings for lead error are usually taken aboutsix or seven places in each inch of threadlength.

8.11.1.4 For example, consider the leadmeasurement of a 10-pitch thread. Assumethat the lead testing machine is set at zeroon the first thread, and that every second threadis measured, giving the following readings:

R e a d i n g E r r o r0 00.20010 + 0.000100.40014 + 0.00014 greatest +0.59994 – 0.000060.79984 – 0.00016 greatest –1.00006 + 0.00006

The greatest plus error in these measurementsis 0.00014, and the greatest minus error is0.00016. Therefore, the maximum lead error is0.00030. This value is within the limits of agage whose permissible lead error is specifiedas 30.0003 between any two threads.

8.11.1.5 Although the lead, pitch diameter,and angle errors of thread gages are intendedto be interdependent in theory (permitting lesserror in one element if there is greater error inanother ), it is generally not the practice totake this factor into account except in border-line cases. If the lead error of a plug is nearthe maximum tolerance limit, the plug shouldbe accepted if the pitch diameter and angle

errors are well within their tolerance, but notif the pitch diameter is near its maximum limit,or if the angle is near either limit.

8.11.1.6 If the lead measuring instrumentdetermines the lead by registering variation inthe distance between thread flanks in a lineparallel to the axis of the threads, it is seldompossible to obtain true alinement between theaxis of the thread and the line of motion fol-lowed by the measuring head in moving fromone thread to another. For this reason, it isnecessary to take two sets of readings 180 de-grees apart on the circumference of the gageand to average the lead errors found in thetwo mearurements. Using the average of leadreadings taken on two elements 180 degreesapart eliminates the effect of the misalinementof the axis of the gage with the axis of themeasuring means. When the precision com-parator on the lead testing machine registersvariations in a line parallel to the axis of thegage, instead of variations in a line perpendicu-lar to the axis, it will not usually be necessaryto take two sets of readings 180 degrees apart,to correct for misalinement.

8.11.2 Checking angle with a test tool.One way to check for angle errors on threadgages is to use a precision ground test tool orcone point and light box. The thread plug isplaced between the centers of the box and ro-tated until the thread root is about opposite thepoint of the test tool. The test tool then ispushed forward into the space until there isvery little clearance between the point of thetool and the root of the thread. The tool andthe thread are viewed through a jeweler’s glassof about 7 power, or if the device is equippedwith the microscopic attachment, the micro-scope is used (see fig. 189).

8.11.2.1 Each half angle of the thread is .checked individually by rotating the plugenough to bring each thread flank against theedge of the tool. If any light shows betweenthe tool and the thread flank after they havebeen brought together with slight pressure, testtools with other angles or special spacers forthe basic test tool should be used until a per-fect match is obtained. The error in the angleis known from the dimensions of the tools or

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Figure 189. Checking thread

spacers required to bring about the proper fit(see fig. 190) .

8.11.2.2 The following precautions must beobserved in this inspection:

a. The light box must be constructed so thatthe gaging edge of the test tool will be at exactlythe same height as the axis of the plug, andperpendicular to it.

b. Do not use pressure greater than can beapplied with the fingertips.

c. The test tool must be flat on the box andshould not be pushed out of its position ofperpendicularity with the thread axis.

d. Both the gage and the test tool must beabsolutely clean.

e. All burrs must be stoned off the side of thet o o l .

angle by means of a test tool.

f. Care must be taken not to nick the sidesof the test tool because a badly nicked tool isuseless.

8.11.2.3 This method is very accurate pro-vided that all proper precautions are taken.When the helix angle of the thread is greaterthan 6 degrees, it is a satisfactory way of check-ing the thread angle. Another accurate methodof checking the thread angle is the use of atool maker’s microscope equipped with knife-edge contacting members, using the same prin-ciple as the test tools.

8.11.3 Checking angle with an opticalcomparator. To inspect the angles of a threadplug gage in an optical comparator, mountthe plug between the centers of the comparatorand rotate the table through an angle equal to

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Figure 190. Test tools and correction spacers for checking thread angle.

the helix angle of the thread. The error of theflank angle is determined by rotating theground glass projection plate until one of the30 degree lines is parallel to the image of thethread flank. The error in the angle is de-termined by the reading of the vernier scale atthe side of the screen. It is very importantthat the centers of the projector be horizontal.This can be checked by setting the angle meas-uring scale at zero and noting whether theimage of the major diameter of the threads isparallel to the horizontal line on the projectionscreen. If it is not, rotate the eccentric centeruntil parallelism has been attained. If themajor diameter is not straight, use a long plainplug which is known to have no taper. Thismeasurement can best be made by moving thetable of the machine until the shadow almostfalls upon the 30 degree line, leaving a verythin sliver of light between the shadow and theline. The plug must be turned through thehelix angle in order to obtain a clear imageof the thread profile. The practice of focusingthe projector so that one side of the threadappears to be clear, measuring the flank inquestion, and then focusing the projector so asto clarify the image of the other flank, is notcorrect because the lead of the thread causesparts of the gage in front and in back of theaxial plane to interfere with the light rays,thereby causing a distorted image. This pointcan be understood by studying figure 191.

Tables of the helix angles of commonly usedAmerican National thread sizes are given in8.27.1. If it is not necessary to make an ex-tremely accurate measurement of the threadangle, the helix angle can be found experi-mentally by rotating the table until both threadflanks are sharply defined on the screen.

8.11.3.1 The method of computing the helixangle can be understood by studying figure 192,which represents the upper half of a singlethread turn spread out as if it were unrolled.Therefore, its length is equal to its cricumfer-ence. Accordingly, the length is equal to the pitch diameter of the thread in question.The amount which the thread advances in oneturn is known to be L, the lead of the thread.Therefore, the tangent of the helix angle isequal to the lead divided by the circumferenceor, expressed mathematically:

N equals the number of thread turns per inch.

Figure 191. Relationship between axial plane and plane

of projection.

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Figure 192. Computation of helix angle.

8.11.3.2 It will be noted that a screw threadactually has several circumferences, the largestat the major diameter and the smallest at theminor diameter. This, in turn, means thatevery t bread has several helix angles, depend-ing upon which point on the thread flank isused for computing the angle. The pitch diam-eter, which is in effect an average diameter ofthe thread, is customarily used in computingthe helix angle. The term “helix angle” al-ways means the angle computed by this methodunless some other method is specifically stated.

8.11.3.3 The flank angle of the thread as ob-served in an optical comparator is not the trueangle because the included angle of AmericanNational threads is defined as 60 degrees in theaxial plane, whereas the projector projects animage of the thread in a plane normal to thehelix. This point is shown on figure 193, show-ing a top and a cross-sectioned view of a singleturn of a thread space. It is necessary to em-ploy a correction factor when measuring theangle of a precision thread by means of an op-tical comparator or by means of a tool maker’smicroscope which does not have knife-edgeangle checking members.

8.11.3.4 In figure 193 let h equal the threaddepth, p equal the pitch, s equal the helix angle,a equal the half angle of thread in the axialplane, and a’ equal the half angle of thread inthe plane of projection. Then:

tan a´=tan a cos s

922173 0 - 51 - 10 131

Tables giving the values of a versus a´ forvarious helix angles are given in 8.27.3. Asa specific example, consider the computation ofthe correction factor for a thread plug for ¼inch 20 NC–2 thread. By reference to tablesin 8.27.1 we find that the helix angle of thisthread is 4 degrees 11 minutes. By referenceto the. table of flank angle correction factors,given in 8.27.3, it is found that the factor for4 degrees 10 minutes, the value in the tablenearest to the true value, is 3 minutes 56 sec-onds. Accuracy sufficient for inspection of anordinary thread plug is obtained by droppingthe seconds and assuming that the correctionfactor is an even 4 minutes. Assume that theangle of one flank as actually measured in theprojector or microscope is 30 degrees 13 min-utes and that the angle of the other flank reads29 degrees 57 minutes. Then the true valueswill be 30 degrees 13 minutes+4 minutes =30degrees 17 minutes, and 29 degrees 57 minutes+4 minutes= 30 degrees 1 minute. The correctionfactor is always added to the observed angle tofind the angle of the thread in the axial plane.Since the accuracy of a projector or a toolmaker’s microscope generally is not closer than2 minutes, it is not necessary to use correction

Figure 193. Correction factor for thread angle.


Figure 194. Common visual errors found in thread gages.

factors for flank angle of 60-degree threads un-less the helix angle is 3 degrees or more. Whenthe helix angle is less than 3 degrees, the cor-rection factor is less than 2 minutes.

8.11.3.5 In addition to checking for the valueof the flank angle, the thread should also bechecked for general appearance while in the pro-jector or microscope. Common faults in threadform are shown on figure 194.

8.113.6 There is no exact standard for thestraightness of a thread flank. However, theflank of a good thread gage will not show anyappreciable deviation in straightness whenviewed in a projector or microscope. In noinstance should the difference between the highpoint and the low point of the flank be as greatas the permissible lead error (taken withoutconsideration of plus and minus sign).

8.11.4 Checking angle with a tool maker’smicroscope. Thread angles may also be in-spected with a tool maker’s microscope. Thereare no special precautions in the use of this in-strument other than that the work and themicroscope lenses should be clean, and thatknife-edge profile members must be broughtagainst the thread flank gently.

8.11.4.1 Microscopes which do not haveknife-edge members for contacting the threadflank must be tilted through the helix angle ofthe thread, and flank angle correction factorsused to determine the true angle of the threadin the axial plane.

8.11.4.2 Microscopes with knife-edge con-tacting members have an advantage over theother type because direct measurement in theaxial plane gives results which are more accu-rate, particularly when the helix angle of thethread is large. This instrument is also used tocheck the general appearance of the thread form.

8.11.5 Root. Thread root can be checkedconveniently at the same time that the angle ischecked. This can be done in two ways. Thefirst way is to measure the distance between thelowest points on the straight surfaces of theflanks by means of a projector or microscope.(This is the distance X as shown on sketch Aof figure 195). The second way is to place a

special test tool with a point truncated to havea flat equal the maximum permissible width ofthread root into the threads when the gage hasbeen mounted on a light box, or to place linesmarked on a special screen for projector or

Figure 195. Root inspection of thread gages.

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impractical to provide clearance at the root ofthe “Not Go” gage, the root should be as sharpas possible (generally about 0.002 inch inwidth).

8.11.6 Pitch diameter. S i n c e m a t i n gthreads, when assembled, generally bear alongtheir flanks, the positioning of the flanks is themost important element determining the fit ofthe threads. The most satisfactory method formeasuring the relative positioning of the threadflanks across the diameter is to measure overthree precision wires placed between the threadflanks as shown on figure 196.

8.11.6.1 The best size of wire to use for this

Figure 196. Three-wlre method of measuring pitch

diameter of thread plug gages.

Figure 197. Pitch Iine.

microscope between the images of the threadteeth. This is shown on figure 195, where thedotted lines represent the test tool or the specialforms drawn on the instrument screen. It isimportant to remember that the root of a “Go”plug gage should be cleared below a width offlat equal to p/4 or less, in order to make surethat the gage does contact the minor diameterof the component. The purpose of a “Not Go”plug gage is to measure the pitch diameter ofthe threads, and it does not make any particulardifference whether the relief is slightly wideror narrower than nominal. The relief shouldbe deep enough to clear a sharp “V” as shown onview D, in order to be sure that the componentnever wedges in the gage because of binding ata point other than the pitch diameter. On veryfine pitch threads (32 and finer) where it is

measurement is that which will contact thethread flanks halfway along their length (asreferred to the fundamental thread triangle).The effect of angle errors is eliminated by theuse of such wires. Best size wires contact theflanks along a line known as the pitch line.This line is defined as the line which equallydivides the lands and the spaces of basic Ameri-can National Thread Form (see fig. 197). Thedistance between the pitch lines on oppositesides of the thread is known as the pitchdiameter.

8.11.6.2 To derive the formula for determin-ing the best size wire for any given pitch ofthread, consider the sketch shown on figure 198.Here, g is the radius of the wire, a is the halfangle of the thread, and p is the pitch of thethread. By definition of the pitch line, the dis-

tance between the two points of contact is

and the distance indicated in the sketch by g is

times the secant of a. The diameter of the

Figure 198. Best size wire.

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measuring wire, G must be twice this amount

In the case of 60-degree threads,

computing this equation: G =0.57735p.8.11.6.3 The method used to compute C, the

constant to be subtracted from the reading overthe wires to obtain the value of the pitch diam-eter is best understood by dividing the analysisinto two steps.

a. The first step is to find Z, the distance fromthe top of any wire to the vertex formed by thethread flanks at their theoretical sharp V inter-section (see figure 199). If g is the radius ofthe measuring wire used and a is the anglewhich the thread flanks make with the vertical,this distance is computed as follows:

Z=g+g cosec a

b. The second step is to find W, the distancefrom the pitch line to the vertex formed bythe theoretical intersection of the thread flanks(see fig. 200). By definition, the distance be-tween the thread flanks at the pitch line will

be Therefore, the top side of the triangle

shown on figure 200 must equal in length.

The required distance equals:

The distance from the top of the wire to thepitch line will obviously be equal to the differ-ence between the two distances previously com-puted:

Figure 199. Height of wire above vertex.

Figure 200. Height of pitch line above vertex.

In taking a measurement by means of thethree-wire method, the difference between thetop of the wire and the pitch line on both sidesof the threaded piece must be taken into con-sideration.

Therefore:

C= 2 (Z-W)2g= G

P= (for single thread screws)

C=G (1 plus cosec a)–

E, the pitch diameter, equals M, the measure-ment over the wires, minus C

Therefore:

E=M plus –G (1 plus cosec a)

The above equations do not take into accountthe effect of the helix angle on the measurementof the pitch diameter with wires. However,this effect is so slight for all threads of theAmerican National Coarse and Fine PitchSeries that the National Bureau of Standardshas established the practice of neglecting thecorrection for helix when measuring the pitchdiameter of these threads.

8.11.6.4 However, when Multiple Startthreads or National Special threads with un-usual combinations of pitch and diameter aremeasured, it often is necessary to take thisfactor into account. In such cases, the con-stant printed on the wire container does notapply, and the constant must be computed fromspecial equations. Since the pitch line must liein a plane through the axis of the threads bydefinition, as shown on figure 200, the distancefrom the pitch line to the vertex at the root ofthe threads will be the same regardless of the

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helix, and the equation specified in 8.11.6.3 willbe accurate regardless of how steep the helixangle is. However, a correct computation forthe distance from the top of the wire to thevertex cannot be made unless figure 199 is as-sumed to represent a view in a plane perpendic-ular to the axis of the thread measuring wire.This plane will also be normal to the helix ofthe threads because the thread wires alinethemselves approximately with the helix.Therefore, the angle assumed for solving thetriangle shown in this view should not be a, thebasic value of the half angle of the thread inthe plane through the axis of the threads. In-stead, it should be a’, the value of the half angleof the threads in the plane normal to the helix.(For an explanation of this relationship, seefigure 193 and the accompanying text. )Therefore, the formula given in 8.11.6.3 wouldbe more accurate if a´ were substituted for a inthe first term, as follows:

C=G (1+cosec a´ )–

With this correction, the equation for the pitchdiameter becomes:

E=M + –G (l+cosec a´)

When substituting values in formulae specifiedin 8.11.6.4, use the actual measured values of thethread angle instead of the basic values. Fora’ use the average of the two flank angle valuesobtained by observing the flanks in the linethrough the helix. For a use the averagevalue of the flank angles in the axial planeas computed by the formulae given in figure193. If a wire which will contact the flankswithin a few thousandths inch of a pitchline is not available, it is preferable to measurethe thread angle directly in the axial plane bymeans of the tool makers’ microscope, by meansof test tools, or by computing the value aftertaking measurements over various sizes of wires.Assuming the basic pitch diameter, compute thevalue of the helix angle at the point of contactof the wire which is to be used in measuringthe pitch diameter. (Use the actual diameter atthe point of contact, instead of the term E andequation specified in 8.11 .3.1.) Using this valueof the helix, compute the value of the half angle


in the plane normal to this helix with the for-mulas shown on figure 193. According to thepractice of the National Bureau of Standards,the formulas specified in 8.11.6.3 may be used tocompute the pitch diameter except when the

value of cos a cot a is greater than 0.00015

inch. When measuring American Nationalthreads with best size wires, this condition doesnot occur unless the helix angle is greater than 6degrees. In other words, the effect of the helixangle may be neglected unless this value isgreater than 6 degrees.

8.11.6.5 The following are the principalsteps in measuring the pitch diameter of athread plug gage:

a. Remove the gage handle.b. If the thread gage has an average pitch

diameter tolerance (between 0.0002 and 0.0005inch ), the measuring instrument used should beone with which 0.0001 inch can be read directlyand 0.00001 inch can be estimated. The measur-ing instrument must also have means for con-trolling the contact load at 1 or 2½ pounds.

c. Set the pressure regulator for 2½ poundsif the threads to be checked are 20 pitch orcoarser. Use a l-pound load if the pitch is finerthan 20.

d. If the plug is small (about 1 inch or lessin diameter), stand it on the elevating table ina vertical position. Then bring the anvils to-gether until they almost touch the major di-ameter and slide the three measuring wiresinto position between the threads in the rela-tionship shown on figure 196. Adjust the an-vils to bear on all wires as determined by “feel”in sliding the wires. If the plug is heavy, itcan be set up with the major diameter in con-tact with the elevating table. This will permitthe plug to roll back and forth on its majordiameter to center itself between the anvils.The three wires are stood vertically betweenthe plug and the anvils when the measurementis taken by this method. Plugs which are toolarge to be checked when resting on the majordiameter can be set in place with the face hori-zontal and with a small shaft between the faceof the plug and the elevating table in a mannersimilar to that used for measuring large plainplugs (see fig. 162). Be sure that each wire is

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in firm contact with an anvil by moving theplug a bit and by pushing each wire a littlewith the fingers before the anvils are broughtup to full pressure. The wires must be free toadjust themselves in proper alinement with thethreads. No rubber bands, threads, wires orother supports should bind the wires while theactual measurement is made. If the wires arestood in a vertical position they should nottouch the elevating table during the actualmeasurement. Measure several points aboutthe circumference of the gage, record the maxi-mum diameter to five decimal places, and sub-tract from it the constant given on the containerfor the wires. The result will be the pitchdiameter accurate to four decimal places. Thisprocess should be followed at the front, center,and back sections of the gage. The pitch di-ameter at all three points must be within thelimits specified on the gage drawing.

8.11.7 Major diameter. The major diam-eter of a thread plug should be measured at thefront, center, and back by means of a precisionmeasuring instrument in the same manner thata plain plug is checked. Measurement forroundness should be made by turning the gageto different positions and recording the largestdiameter as before.

8.11.8 Concentricity. The major and thepitch diameters of all thread plug gages must beconcentric within the tolerance limits of themajor diameter. The concentricity of thesetwo elements can be checked with the aid ofthree measuring wires and a “V” block. Standthe wires in the “V” block, two on one side andone on the other, parallel to the ends of the plug.Place the plug on the wires and rotate it with anindicator gage in contact with the major diam-e t e r .

8.11.9 Finish. Thread gages have poor fin-ishes more often than other types of gages. Thesurfaces of the threads should be inspected witha magnifying glass or jeweler’s glass of aboutfour power. If any chatter marks (smallridges), cracks, unfinished spots, or other ir-regularities are revealed, the gage should be re-jected. If the applicable gage drawingrequires that the thread flanks be lapped, the

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gage should be rejected unless this operation hasbeen performed.

8.12 INSPECTION OF THREAD RINGGAGES

8.121 Thread form. The form of internalthreads can best be inspected with casts of cop-per amalgam, dental wax, or plaster of paris.The casts are inspected by the same methodsused for ordinary external threads.

8.12.1.1 Before the cast is made the ringgage should be thoroughly cleaned with a goodsolvent and dried. If a corrosion inhibitor isnot available, or if the casts tend to stick, thegage can be painted with a very thin coat ofgrease, such as a thin mixture of gasoline andvaseline. Great care should be taken to makethe coat of grease as thin and as uniform as

8.12.1.2 Copper amalgam is heated, pressedwith mortar and pestle, and then worked into aplastic state. The excess mercury can besqueezed out with a small piece of chamois.The amalgam is worked into the threads insmall amounts with the aid of a dental tool.After enough thickness has been built up toform a solid piece considerably higher thanthe thread crests, the cast is allowed to set fora time and then is removed. A sharp rap willassist in loosening the cast from the ring.

8.12.1.3 Dental wax used for casts prefer-ably should not be melted and poured in, butshould be softened in warm water and pressedinto the threads with the fingers. The gageshould be warmed slightly before pressing inthe wax in order to prevent sudden chilling ofthe wax.

8.12.1.4 When plaster of paris is used a cor-rosion inhibitor must be added to the waterused to mix the plaster in order to prevent cor-rosion of the gage. A solution of potassiumbichromate (K2Cr2C7 crystals) added in a pro-portion of 1½ ounces to a quart of water willserve for this purpose.

8.12.1.5 A plaster of paris cast is made byclamping the lower portion of the gage betweentwo flat plates, as shown on figure 201. Theplaster is mixed with water until the resultingmixture flows easily (50 grams of plaster to 40cc. of liquid will give a good mix for most

Figure 201. Method of making cast of thread ring gage.

threads). It is then poured into the gage untilthe maximum depth equals about one-quarterof the ring diameter. A bolt or other headedobject can be placed headfirst in the liquid inorder to facilitate removal and handling ofthe cast after it is hardened.

8.12.1.6 The cast should be pulled shortlyafter it solidifies. It must be inspected imme-


diately after removal. Casts made of plasterof paris generally will retain their form onlyabout ½ hour after hardening.

8.12.1.7 Another way to make casts of in-ternal threads is to place a piece of soft lead onthe threads, place a mandrel over the lead, andsqueeze the mandrel and gage together in a vise,as shown on figure 202. This method is notquite so accurate as the other method, but it hasthe advantage of being very rapid and easilyperformed.

8.12.1.8 When placed in the projector or mi-croscope the cast must be adjusted until theimage of the threads is horizontal. The castcan be twisted to adjust for the helix angle byrotating until the shadows of the threads are asnarrow and sharp as possible.

8.12.2 Lead. The lead of thread ring gagescan be inspected by making a copper amalgamcast of the threads as specified in 8.12.1.2. Thelead of the threads on the cast is measured bymeans of a precision lead checking machine. If

Figure 202. Making lead cast of thread ring.

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Figure 203. American gage design standard thread ring locking device. 1, Locking screw; 2, Sleeve; 3, Adjusting screw; 4, Body;

5, Adjusting slots; 6, Adjusting slot terminal hole; 7, Locking slot.

it is not practical to set up the cast in a lead 8.12.3 Pitch diameter. The pitch diametermeasuring machine, the cast may be measuredwith a contour projector or a tool maker’smic roscope .

8.12.2.1 The angle is measured with the con-tour projector by alining the image of onethread flank with a 30-degree line on the screen,setting the lateral micrometer at the zero point,and noting the distance the micrometer must beadvanced to aline the flanks of the other threadswith the W-degree line. The two requirementsin positioning the cast for this operation arethat the cast must be horizontal and with thethread axis parallel to the line of motion ofthe lateral micrometer.

8.12.2.2 The placement of the cast in the pro-jector or the microscope will be greatly simpli-fied if a slotted plug of the same size as theminor diameter of the ring is available. Thematerial used for the cast is forced between theplug and the ring at a point where it will fill theslot and the adjacent threads. After the casthas hardened the plug is taken out and the castremoved. The cast should not be removed byunscrewing it with the plug. In the actual in-spection, the cast is reinserted in the slot of theplug and the plug mounted between the centersof the microscope or projector.

of adjustable thread ring gages is adjusted byfitting the ring to a setting plug of known di-mensions. The various parts of the threadring adjusting assembly are shown on figure203. The principal steps in the adjusting oper-ation are the following:

8.12.3.1 Clamp the handle of the setting plugbetween the jaws of a vice or to some conven-ient object.

8.12.3.2 Turn the locking screw counter-clockwise enough to loosen the locking assem-bly and then turn the adjusting screw clock-wise until the ring gage is large enough to bethreaded on the setting plug.

8.12.3.3 Turn the adjusting screw counter-clockwise until the ring can just be rotated onthe plug with firm pressure of the hands. Alittle oil on the check plug will avoid excessivewear without materially affecting the accuracyof the adjustment

8.12.3.4 Turn the locking screw in a clock-wise direction until it is fully tightened, androtate the ring on the plug to see whether it isstill a good lit.

8.12.3.5 Generally a ring gage must be fittedby alternately adjusting both the adjustingscrew and the locking screw until the proper

138


fit has been obtained. The locking screw mustbe tightened securely before each trial of thefit of the ring.

8.12.3.6 Turn the ring on the plug a dis-tance of about two threads on each side of thering and note whether there is any shake.Shake indicates either a bellmouth condition orthe presence of a lead error in the ring. Ineither case this is sufficient reason for rejectingthe gage.

8.12.4 Concentricity of pitch and minor di-ameters. It is necessary to check the concen-tricity of the pitch and the minor diameters ofall thread ring gages because lack of concentric-ity will have the same effect as an undersizedminor diameter. The manufacturer can be de-prived of a large proportion of the tolerance onthe minor diameter of the component, or thegage may even fail to function if proper con-centricity is not maintained.

8.12.4.1 The following procedure generallyis the most satisfactory in making this check:

a. Thread a full form setting plug almostcompletely into the ring, allowing a little morethan one full thread turn of the plug to standup above the face of the ring.

b. Chuck the ring on a rotating face plateand center the ring by running a test indicatorover the major diameter of the protrudingthread on the setting plug.

c. Remove the thread setting plug and rotatethe ring, using the test indicator to determinethe concentricity of the minor diameter. Theconcentricity should be such that the measuredminor diameter, minus the total variation inindicator reading, is not less than the minimumlimit of the minor diameter as specified on thegage drawing.

8.12.4.2 If a large number of thread ringgages of the same size are frequently inspectedin the laboratory, it is worth while grinding acylindrical pilot on a thread setting plug forthis size. This cylindrical section will greatlyfacilitate centering the thread ring when it ismounted on a face plate.

8.12.4.3 If the thread ring gage has flangeswith precision ground outside diameters, thetest for concentricity can be made more quicklyand accurately. In this case, thread the set

plug completely into the ring and mount theplug between centers. Rotate the plug and in-dicate over the outside diameter of the flangefor concentricity. Remove the thread plugand insert the plain plug used for checkingminor diameter and again mount the plug be-tween centers and indicate over the outsidediameter of the flange. A properly made gagewill be concentric in both instances.

8.12.5 Minor diameter. This dimension isinspected with a check plug, if one is providedwith the gage. If not, it generally can bemeasured with precision internal micrometers(John Bath Co., or equivalent), gage blockswith accessories, or adjustable limit gages.

8.13 INSPECTION OF TAPEREDTHREAD PLUG GAGES

8.13.1 Angle and relief. The thread formof tapered thread plugs can be inspected bymethods similar to those used on ordinarythread plugs.

8.13.2 Lead. If a precision lead checkingmachine with attachment for measuring thelead of taper threads is not available, it maybe necessary to check the lead on a contourprojector. This is done by placing the plugbetween centers and rotating the stage (or thethread measuring fixture) through the helixangle. Aline the image of a thread flank onthe right-hand end of the gage with one of the30-degree lines on the projection screen. Ad-vance the micrometer until the image of theflank of the next thread tooth is in line withthe 30-degree line on the projection screen.Note the distance by which the micrometer wasadvanced in making this change.

8.13.2.1 Divide the included taper per inchby two, multiply by the lead and then multiplyby the tangent of the half angle of the thread.Add this quantity to the difference in microme-ter readings determined in 8.13.2 in order toobtain the lead from the first thread to the sec-ond thread on the thread plug. In measuringthe lead from the first thread to the thirdthread, two times this correction factor must beadded to the differences in micrometer read-ings, and in measuring from the first thread tothe fourth thread, three times this correctionfactor must be added to the difference in themicrometer readings, etc.

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Figure 204. Measurement of pitch diameter of taper thread

gages by the two-wire method.

8.13.2.2 If a Bausch and Lomb projector orsimilar projector is being used, the micrometerincorporated in the thread measuring fixtureshould be used rather than the longitudinalmicrometer on the cross slide table for thismeasurement.

8.13.3 Pitch diameter and taper. T h etaper of a tapered thread plug makes it im-practical to measure over three wires when thegage is in proper alinement for measuring thepitch diameter. For this reason, the pitchdiameter is measured over two wires. Thediameter at any given point along the axis isdetermined by placing one wire at this pointand by taking the average of two measurementsmade with the other wire in two different posi-tions, each position being a distance of p/2above or below the point where the pitchdiameter is being determined (see fig. 204).

8.13.3.1 The first step in measuring the pitchdiameter of tapered thread plugs is to mount theplug between centers and to indicate over thefront face of the rotating plug to check for per-pendicularity in the same way plain tapered

plugs were checked. If the face is not perpen-dicular, the gage must be set up between cen-ters, clamped into position, or mounted accu-rately vertical by some other method. Theback face and the surface of the step should alsobe checked for perpendicularity with the axis.The gage should be rejected if these surfacesare not perpendicular within limits such thatthe error will not be noticed when observationis made to see whether these surfaces are flush

or below the component when the gage is inactual use. The length of the gage and thedepth of the step can be measured directly withm i c r o m e t e r s .

8.13.3.2 In order to establish the diameterof the plug with reference to one end or a givenpoint along the axis of the plug, the wire used tomeasure the pitch diameter must be placed at aknown distance from the front face of the plug.This is accomplished by placing a conical gageblock accessory, having an included angle equalto that of the thread, on a stack of gage blocks,and rotating the thread plug until the point ofthe accessory fits exactly between two threads(see fig. 205). The proper location is not de-termined by feel, but by sighting through thethreads until the conical point shuts out light onboth thread flanks when it is pushed forwardinto the threads. A mark is made on the threadflank directly above the point where the con-tact was made, in order to locate the point. Theheight of the gage blocks used in this operationshould be a little greater than one and one halftimes the pitch to permit measurement near thefront face of the plug and still leave room fora wire on the opposite side of the plug a halfpitch lower than the established height.

8.13.3.3 The next step is to set the gage upin a hold-down fixture and to place it betweenthe anvils of a measuring device with the plugturned so that the mark made in Prussian bluewill be directly opposite one of the anvils. Oncethis position has been established, it is impor-tant not to rotate the gage until all measure-ments have been completed. One measuringwire then is placed under the locating mark inthe position formerly occupied by the conicalpoint of the gage block accessory. Two read-ings over the wire are taken, one with a wire on

the opposite side placed ½p lower than the fixedwire, and the other with the wire on the oppositeside placed ½p higher than the fixed measuringwire. The fixed wire then is removed and rein-serted between the thread flanks at a point asnear as possible to the back face of the gage.Two readings are again taken as before.

8.13.3.4 The computations necessary to fig-ure the pitch diameter and the taper frommeasurements made in this manner are ex-

140

Figure 205. Locating height of fixed measuring wire.

plained in figure 206. The tolerance on thepitch diameter is specified at only one point onthe gage drawing. However, this is generallyconstrued to mean that the pitch diameters at allpoints along the length of the gage should notdiffer from the basic pitch diameter at the pointin question by an amount greater than the toler-ance on the pitch diameter given in the drawing.The basic pitch diameter at the point in questionis computed by triangulating from the givenbasic pitch diameter using the basic taper.

8.13.3.5 The permissible variations in taperare sometimes construed to be limited only bythe restrictions on the pitch diameter. How-ever, a tapered thread gage should preferablyhave a taper which conforms to nominal within0.0001 inch over the length of the gage. This isbecause close conformity to the specified taperwill permit more even wear on the gage andwill provide a better bearing surface for per-forming the gaging operation.

8.13.3.6 In order to illustrate the methodsused to compute the pitch diameter and thetaper, the work required to do this for the ta-


Figure 206. Computing pitch diameter and taper

of taper thread plug.

141


pered thread plug gage specified in figure 207will be given. Assume that the following actualmeasurements were made on this plug:

M1= 1.03843 K= 0.4613M2= 1.03394 H= 0.1200 (selected)M3= 1.04744 p= 0.071429 (specified)M4=1.05191 L= 3 threads (selected)

The nominal value of the pitch (p) can be usedinstead of attempting to measure this distancedirectly because any lead error in the gage willnot have any appreciable effect on the computa-tions. The value of p can be found by referringto any of several standard tables of thread ele-ments. If these are not available, it can befound by dividing one by the number of threadsper inch, in this case, 1/14.

Taper (T) =1.03843 1.051911.03394 1.04744

2) 2.07237 2) 2.09935

1.03618 1.049671.049671.03618

0.01349

0.01349

3×0.071429=0.06295 incl. T. P. I.E l, the pitch diameter at the height 0.3343

inch, =1.04967

– 0.06192

0.98775E2, the pitch diameter at the large end of thegage, =

0.4613–0.2143

0.2470–0.1200

0.1270

0.1270X 0.06295

0.00799+ 0.98775

0.99574Since this differs from the nominal pitch di-ameter of 0.99560 by only +0.00014, the pitch

diameter of the gage is within drawing specifi-ca t ions .

The error in taper equals:

0.06295– 0.0625

0.00045 per inchThe error in taper over the length of the

gage equals:0.00045

X 0.4613

about 0.0001Therefore, the requirement that the taper errorbe within 0.0001 for the length of gage is ful-filled. We also know that the actual pitch di-ameter at the front face of the gage differs fromthe basic by:

+ 0.00014– 0.0001+ 0.00004

8.13.3.7 The time required to measure thepitch diameter and the taper of tapered threadscan sometimes be shortened by the use of ataper thread checking fixture. The construc-tion of this fixture and the manner in which it isused are shown on figure 208. The roll which isrested against the front face of the plug shouldbe of proper size to contact the face of the plugnear its center. This is to eliminate the effectof any possible error in the perpendicularity ofthe plug face. Other than the roll diameter,the dimension of the fixture can be any conven-ient length, provided the proper calibration ismade. A sketch illustrating the geometry in-volved in making this calibration is shown on

figure 209. The calibrated value of DC as com-puted by this method should be marked on eachfixture for all sizes of rolls commonly used.The manner in which the fixture is actuallyused will be illustrated by describing the meas-urement of the pitch diameter of a 1¼–11½N. P. T. plug gage. The steps in this processare as follows:

a. Select the roll whose diameter is closest tothe diameter of the plug at the small end, andfasten it in place on the fixture by means of aholding screw.

b. Place two wires under the gage far enoughapart to maintain a base for supporting the gage

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Figure 207. Specification—¾–14 N. P. T. thread plug gage.

Figure 208. Set-up for measuring pitch diameter with

taper fixture.

Figure 209. Calibrating taper thread fixture.

and push the face of the gage tightly againstthe roll. It is important to make sure that theface of the gage remains fully in contact withthe roll during the entire measurement.

c. Place a measuring wire between the threadsat the top of the gage and determine the dis-

143


tances from the top of the wire to the bottom ofthe fixture by indicating over the wires with anindicator or measuring device which has a flatanvil. Take measurements near the front, cen-ter, and back of the gage.

d. Rotate the gage and take additional mea-surements in order to check out-of-round.

e. Assuming a given basic pitch diameter of1.5571 at the small end of the plug and a valueof DC for the roll and fixture in question of0.9164, compute the nominal distance betweenthe top of the wire and the base of the fixture asfollows :

Find the basic measurement over the wires byadding the wire constant for 11½ pitch to thegiven pitch diameter.

0.075311.55710

1.63241

Add this to the calibrated value of DC and mul-tiply by the cosine of 1 degree, 47 minutes, 24seconds.

0.916401.63241

2.54881 X 0.99951= 2.54756

Therefore, if the plug being checked was madeexactly to the basic dimensions given on thegage drawing, the height of the measuring wireabove the base of the fixture would be 2.5476inches, measured to the nearest 0.0001 inch.

f. The amount of error in the gage is foundby taking the difference between the heights

Figure 210. Taper thread checking fixture.

Figure 211. Hold-down wire for taper thread checking fixture.

actually measured over the top of the wire and2.5476.

8.13.3.8 Another type of fixture for measur-ing the pitch diameter of tapered threads isshown on figure 210. Although this fixturewill be somewhat more expensive to manufac-ture accurately than the one described above,it has an advantage in that only one roll is re-quired. This roll can be moved up and down,making it possible to contact the front face ofall sizes of plugs exactly in the center. Afurther advantage is that only one calibrationnumber (DC ) is needed for all sizes of plugs.

8.13.3.9 The holding wires shown on figure211 are screwed to the sides of the fixture in aposition such that the wires will rest betweenthe threads at the top of the plug. If the wiresare made snug enough to give a bit of tension,this will hold the gage in position and permitthe checker to use two hands in taking themeasurements. Usually two holding wires,one on each side of the fixture, are sufficient.

8.13.4 Major diameter. The major diam-eter of a tapered thread plug can be measuredover rolls by placing gage blocks over the threadcrests and then measuring across them by thesame method used to measure plain taperedplugs (see fig 212). The computations are thesame except that a factor to allow for the thick-ness of the gage blocks, 2t sec A, must also besubtracted from the over-all reading.

This gives the following formula for findingthe diameter of the plug at the base or at any

144


Figure 212. Measuring major diameter over rolls.

point level with the stacks of gage blocks usedto support the rolls: (see fig. 213).

D = major diameter small end of gaget = thickness of gage blocksr = radius of measuring rollsD1= over-all measurement over the rollsA = half angle of taper of the plug

8.13.4.1 The actual value of A can be com-puted from the over-all measurements by use ofthe following formula:

In the case of American National pipe taperthreads, the included taper should be 0.0625per inch of length, and the half angle of tapershould be 1 degree, 47 minutes, 24 seconds.

8.13.4.2 The major diameter of taperedthread plugs can also be checked by means of

a sine bar and roll in the same way that a plaintapered plug would be checked, or by placingthe plug in the fixture previously describedfor measuring the pitch diameter of taperedthreads. If the latter method is used, themajor diameter of the plug is set directly on thesurface of the fixture, and indicator readingsare taken over the thread crests at the top of theplug. The height of a gage with exactly basicmajor diameter above the base of the fixture canbe found by adding the basic value of the majordiameter at the front of the plug to the cali-brated value of DC and then multiply the sumof these two figures by the cosine of 1 degree 47

8.13.5 Convolution. It is sometimes re-quired that the threads on a tapered thread pluggage be cut back or convoluted one full turnnear the ends of the gage in order to avoid feath-

8.14 INSPECTION OF TAPEREDTHREAD RING GAGES. The thread formand the lead of tapered thread ring gages canbe inspected by the same methods used on plainthread ring gages. However, relief or clear-ance at the root of gages for pipe threads isalways required instead of being optional as inthe case of ordinary “Go” thread gages. Thepitch diameter is checked by screwing the ringon a check plug. The ring should be turneddown as tightly as moderate hand pressure will

allow. The check plug then is stood upright

Figure 213. Measuring major diameter pipe threads by roll

and gage block method.

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with the ring still assembled, and an indicatoris passed over the back or the steps of thecheck plug and the steps of the ring to see howclosely they aline themselves. It is generallypermissible for tapered plugs to go 0.005 incheither way beyond the limits specified for thesteps on the gage drawings. In checking thepitch diameter of the tapered thread ring, it isvery important to make sure that all the threadsare clean. The minor diameter of taperedthread rings is checked by means of taperedcheck rings.

8.15 INSPECTION OF THREAD SNAPGAGES. The lead and the thread form ofthread snap gages can be inspected by removingthe gaging members from the frame and check-ing them with the same instruments used tocheck thread plug gages (see fig. 214).

8.15.1 The pitch diameter is adjusted orchecked by passing a thread check plug betweenthe gaging members in the assembled gage. Itis common practice to set the gaging membersclose enough together so that the snap gage willjust fall away from the check plug of its ownweight when the check plug is held horizontallyand the snap gage is passed over it in an in-verted position. The most critical adjustmentcan be obtained by tightening the anvils untilthe gage will hang from the check plug when itis exactly vertical, but will fall off if the plug isrotated through a slight angle. (See fig. 215).

8.15.2 The gaging members should bechecked for proper pitch relationship with eachother by inserting the thread check plug be-tween the gaging members of the assembledgage and sighting between the check and themembers to see whether contact is being madeon both flanks of each of the annular teeth onthe thread rolls. If so, the gaging members are

Figure 214. Thread snap gages.

Figure 215. Setting thread snap gage.

properly alined. If the gaging members arebearing only on the thread flanks on one side,this will indicate that the gaging members arewedged against the frame of the gage and areprevented from assuming proper pitch aline-ment. This condition is cause for rejecting thegage because it can create considerable errorin the pitch diameter when it is applied tocomponents.

8.15.3 The check plug should be tried withonly a thread or two engaged at each end of thesnap gage in order to check the snap gage forpa ra l l e l i sm.

8.15.4 Care should be taken to see that alllock nuts and screws are fully tightened. Thefinal check of the pitch diameter adjustmentshould be made after all locking devices havebeen fully secured.

8.15.5 The minor diameter can be checkedafter the pitch diameter has been adjusted bypassing gage blocks between the gaging mem-bers. The blocks should be tried near each endof the rolls in order to detect any possible taper.Another type of adjustable thread gage, usingthree tapered thread rolls, is shown on figure110. This gage is preferred by some branchesof the Armed Services for the inspection oftapered threads because it permits greater visi-bility. It is adjusted in the same manner asthe thread roll gage described in the precedingparagraphs.

8.16 INSPECTION OF QUALIFYINGTHREAD GAGES. Qualified threads are

146

threads in which the thread form itself mustbe a specified axial distance from a shoulder orother part of the product. The purpose of thisis to assure that the product will be in a certainangular position when it is assembled. Anexample of qualified threads are the threads ofa rifle barrel, which must be positioned withrespect to the shoulder so that the barrel willalways be in an upright position when it isscrewed into the gun as far as it will go.

8.16.1 Checking qualifying thread gageswith a contour projector. This operation isperformed by clamping the gage in the pro-jector in a horizontal position as shown onfigure 216. Any angular reference surface,such as a slot in the shoulder of the gage, mustbe set up in the angular position shown in thedrawing. The axis of the gage must be locatedparallel with the line motion of the supporttable. This can be done by placing the imageof the major diameter along the vertical line onthe projection screen and adjusting the positionof the gage until the thread crests just touchthe line.


8.16.1.1 A ball point is then set on blocksso that the center of the ball point is exactly onthe axis of the gage. The ball point is thenplaced between the thread flanks at the pointfrom which the thread location is dimensioned,and brought into focus. The distance from thecenter of the ball point to the shoulder of thegage is measured by moving the projector sup-port stage. (This is the dimension shown asL in figure 217. ) The purpose of the ball pointis merely to focus the projector at the centerlineof the gage.

8.16.1.2 When the threads are not qualifiedwith respect to some precision surface on thegage which is positioned accurately at the timethe gage is clamped in the holding fixture, thelength L can be used to establish the referencepoint. For example, when gages which checkthe position of the thread on the product bymeans of scribed lines are being inspected, thegage is rotated in the holder until the length Lis the same as that given on the drawing. Thegage is clamped in place, and a scriber is set

Figure 216. Set-up for checking qualified threads on a projector.

922173 0 - 51 - 11 147


Figure 217. Measuring Iength to reference surface.

up on blocks the same height as the centerline ofthe gage. This scriber is used to check the meanscribed line on the gage or to mark the gage ifit has not yet been marked.

8.16.1.3 If maximum and minimum lines areto be checked or marked, this can be done bysetting up the scriber at heights such that thedifference between its height and the heightof the centerline equals the sine of the specifiedangle times the radius of the shoulder.

8.16.1.4 In general, the gaging surface orthe scribed lines should be set up in the basicposition and the distance to the qualifiedthreads L will be measured if a tolerance isplaced on the length of the threads on the gagedrawing. The procedure specified in 8.16.1.1is an example of this method. On the otherhand, if a tolerance is placed on the gagingsurf ace or scribed lines in the gage drawing,then the distance to the thread should be set upin the basic position and the location of thegaging surfaces or scribed lines should be

measured. An example of this method is givenin 8.16.1.2.

8.16.2 Checking qualifying threads with athread template. The first step in checkingqualifying thread gages by means of a threadtemplate (see fig. 218) is to clamp the gage ina position which is accurately horizontal. Thiscan be done by resting the major diameter ofthe gage on a flat precision surface such as aparallel or a gage block, tightening the gagein position, and then indicating over the majordiameter at the top of the gage to check theparallelism. The parallelism of the pitch di-ameter when the gage is in this position should

Figure 218. Thread checking template.

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Figure 219. Special angle iron for supporting thread

template.

be checked by indicating over a thread measur-ing wire at each end of the gage.

8.16.2.1 The thread template must be sup-ported by a flat precision surface, parallel withthe surface plate, and exactly as high as thecenterline of the gage. The height of the cen-terline is established by subtracting one-halfthe major diameter from the height of thethread crests. The support for the templatemay be a small parallel supported by two stacksof gage blocks in the case of small gages. Anangle iron with an adjustable bar attached tothe face is a convenient arrangement to use forlarge gages (see fig. 219).

8.16.2.2 The template itself bears a singlethread tooth with the distances from the inter-section at the lower end of the flanks to eachend of the template accurately calibrated andmarked. (These are the dimensions marked Lon fig. 218. ) By intersection is meant the theo-retical point at which the bottoms of the flanksmeet the straight front surface of the template.

8.16.2.3 The template is placed on the sup-porting surface and the tooth is located betweenthe thread flanks near the point from which thelocation of the qualified thread is dimensioned(see fig. 220). The front surface of the tem-plate should be made snug against the majordiameter of the gage being inspected. Thetooth of the thread template is slid against athread flank which is on the pressure side ofthe gage threads. This pressure side is the sidewhich will “fetch up” or tighten against thecomponent threads when the gage is screwedin the product as tightly as possible. Thisusually is the side nearest the shoulder of thegage. Gage blocks are placed along the sup-porting surface between the template and thesurface of the gage from which the qualified

threads are dimensioned. If this surface is noton the side facing the threads, it may be neces-sary to place an angle iron or a parallel againstthe surface in question to contact the gageblocks. A precision square should be used tomake certain that this piece is perpendicular tothe crests of the threads. If the combinationof gage blocks selected is too small, a slightamount of light will be seen between the edgeof the thread template and the flank of thethread on the gage. If the stack of gage blocksbeing used is too long, they will not fit betweenthe template and the reference surface. Whena combination which appears to be a good fithas been found, stacks of blocks 0.0001 inchlonger and a 0.0001 inch smaller should be triedto make sure that the fit cannot be improved.It will be easier to make this check if a piece ofwhite paper is placed under the threads.

8.16.2.4 In the case of large gages and gageswith steep helix angles, where the lead and theangle cannot easily be checked by the usualmethods, the thread template is used.

8.16.2.5 Angle is checked by sighting be-tween the flanks of the template and the gagewith a jeweler’s glass. If any light passes be-tween the two, the degree of error can be deter-mined by trying different thread templatesrepresenting the maximum and minimum limitsof the angular tolerance.

8.16.2.6 Lead is checked by subtracting oradding multiples of the theoretical lead to thedimension used to qualify the threads, placingthe thread tooth of the template between theappropriate threads, and rechecking the dis-

Figure 220. Set-up for checking qualifying thread gage

(top view).

149


tance to the reference surface. In the case ofplug gages, the thread tooth with the shortestlead is used as a basis for computing the ob-served value of the thread qualifying dimen-sion. This is because the thread tooth with thegreatest minus lead error will determine theposition of the gage in the product.

8.16.2.7 In checking qualifying threadgages, it is important to remember that taperin the pitch diameter affects the location of thegage in the component. The part of the gagewith the largest pitch diameter will wear rap-idly, and will cause the gage to function differ-ently as it wears. For this reason, the per-missible taper in the pitch diameter of a newgage should be considerably less than the pitchdiameter tolerance.

8.17 INSPECTION OF SEGMENTALTHREAD GAGES. Segmental thread gagesare gages which have the ordinary type ofthread form but which have gaging surfaceswhich constitute an arc rather than the com-plete circumference of a circle. In short, seg-mental thread gages constitute a portion of thefull cross section of the ordinary thread plug orring gage. The purposes of this type of con-struction are to permit the gage to open andclose as in the instance of certain types of snapand indicator gages, or to make the gage lighterand easier to handle as in the instance of verylarge thread gages for artillery.

8.17.1 The particular difficulty in inspect-ing segmental thread gages is that the form ofthe gage makes it impossible to take measure-ments directly across the diameters. Gener-ally, the major diameter is computed by meanssimilar to those used for measuring externalradii on profile gages, and the pitch diameter iscomputed by finding its position with respect tothe major diameter.

8.17.2 One of the most common ways of de-termining the major diameter, for example, iS

to place the thread crests against a flat precisionsurface and to take a measurement over tworolls as shown on figure 221. On this figure, M

equals the major diameter, m equals the distancebetween the outer surfaces of the rolls, and Gequals the diameter of the precision rolls. Inthe case of small segments, the gage can be

Figure 221. Measuring major diameter of segmental

thread gage.

placed face downward on a tool maker’s flat andthe measurement over the rolls can be made witha bench micrometer. In the case of large gages,particularly where it is necessary to clamp thegage upright for checking qualified threads, thegage is clamped in an upright position with theaxis horizontal, and the face of an angle ironthen is pressed against the thread crests of thegage. The position of the upper roll is deter-mined by means of a height gage, while theposition of the lower roll is determined bybuilding a stack of gage blocks beneath it untilit is wedged snugly between the crests of thegage and the face of the angle plate.

8.17.3 The pitch diameter can be found byplacing the gage flat with the threads upper-most. A thread measuring wire suitable for thepitch in question is placed between the threadflanks, and the difference between the top of thewire and the crests of the thread is measuredwith a height gage. The difference multipliedby two and added to the major diameter as pre-viously determined, gives the effective measure-ment over the wires. The pitch diameter thenis found by subtracting the same constant whichwould be used in determining the pitch diam-eter by the ordinary three-wire method. Thischeck should be made at three or more pointsalong the length of the gage.

8.17.4 The form of segmental threads canbe checked by placing the gage in a contourprojector or a tool maker’s microscope. Theposition of the gage should be adjusted so that

150

the images of the thread crests fall along thehorizontal line of the projection screen. Thegage is then twisted until the thread flanks formthe sharpest and narrowest outline obtainable,and the thread form is inspected in the sameway used to check the form of an ordinary plugwhich has been turned through the helix angle.

8.17.5 If the lead is measured by opticalmeans it should be remembered that the read-ings taken must be multiplied by the secant ofthe helix angle unless the supporting stage andthe lateral micrometer have been turnedthrough the helix angle.

8.17.6 In the case of large segmental threadgages, thread form can be checked most easilyand accurately by means of an appropriatethread template.

8.18 INSPECTION OF ACME THREADGAGES. Specifications for Acme threads aregiven in Handbook H28, Screw Thread Stand-ards for Federal Services.

8.18.1 Ordinarily, Acme thread gages canbe inspected by the same methods used to inspectthread gages for American National threads.The one point of difference is that the minordiameter of Acme threads must be measured.This is done by measuring the distance betweenthe thread crests and thread roots, multiplyingby two, and subtracting this amount from themeasured value of the Major diameter. Thedistance between the crests and roots generallycan be found by means of the ordinary depthmicrometer. In questionable cases and in caseswhere there are close tolerances on the minordiameter this distance can be found by placingthe gage horizontal and taking measurementswith an indicator height gage and gage blocks.

8.18.2 The distance between the major andthe minor diameters should be measured atmore than one point about the circumference ofthe gage in order to make certain that thesediameters are concentric.

8.18.3 When Acme thread measuring wiresare not available for determining the pitch di-ameter, ordinary thread measuring wires orprecision rolls which will rest somewhere nearthe center of the Acme thread flanks can be usedinstead. The distance from the tops of thewires or rolls to the pitch line can be determinedfrom the equations specified in 8.11.6.3. In this


case G will represent the diameters of the wiresor rolls, and a, the half angle of the threads, willbe 14 degrees, 30 minutes.

8.18.4 When suitable thread measuringwires are not available. or when it is desired tomeasure the pitch diameter of a multiple start“Not Go” thread plug gage where one or morethread starts have been removed, making itimpossible to measure pitch diameter withwires, the pitch diameter can be measured witha tool maker’s microscope or with a contourprojector. This is done as follows:

8.18.4.1 Measure the major diameter. Placethe plug in the projector and rotate the stagethrough the helix angle.

8.18.4.2 Aline the thread crests with thehorizontal line on the screen and then raisethe image of the threads until the horizontalline coincides approximately with the pitchline, noting the exact amount which the plugwas raised in doing this.

8.18.4.3 Measure the thickness of the threadtooth between the points on the flanks inter-cepted by the horizontal line. Multiply thisvalue by the secant of the helix angle.

8.18.4.4 Compute the difference between thiswidth and the value of p/2 for the pitch ofthread in question. ( Remember that in thecase of multiple start threads p equals the leaddivided by the number of thread starts. )

8.18.4.5 Divide this difference by two andmultiply by the cotangent of the half angle ofthe thread.

8.18.4.6 If the thickness of the thread toothat the point where it was measured was greaterthan p/2, subtract the figure obtained in 8.18.4.5from the amount which the plug was raised asspecified in 8.18.4.2. If the width of the toothwas less than p/2, add this figure to the amountthe plug was raised.

8.18.4.7 Multiply the figure obtained in8.18.4.6 by two and subtract it from the valueof the major diameter. This will give thepitch diameter.

8.18.5 This measurement should be taken atleast two different points about the circum-ference of the plug to make sure that the majordiameter and the thread flanks are concentric.

8.19 INSPECTION OF MODIFIEDSQUARE THREAD GAGES. The major

151


diameter of these threads is checked withmicrometers or with a bench micrometer,depending on the tolerance. If the lead is sogreat that the anvils of the measuring instru-ment will not contact the major diameter atboth sides of the plug at once, gage blocks canbe placed on the thread crests and the measure-ment can be taken over them.

8.19.1 The minor diameter can remeasuredwith a micrometer depth gage or by setting theplug up in an accurately horizontal position andmeasuring the distance between the thread rootson the top side and on the bottom side with anindicator height gage. It should be notedwhether the minor diameter is concentric withthe major diameter within the diametral toler-ances specified.

8.19.2 The great degree of helical interfer-ence encountered in modified square threadsusually makes it impractical to check them bymeans of a projector. For this reason, thethread form and the width of the thread filletsgenerally are checked by means of a threadtemplate. The set up for doing it is specifiedin 8.16.2.5. If the modified square threadsbeing inspected are not qualified, the face ofan angle iron placed in a position perpendicularto the surface supporting the thread templatecan be used as a base for positioning the gageb l o c k s .

8.19.3 To measure the width of the threads,the thread template is placed against the flankof a thread tooth on the gage and a stack ofgage blocks is fitted between the template andthe angle iron. The tapered template thenis positioned against the thread flank on theother side of the same thread tooth on the gageand the distance between the template and theangle iron again is measured with gage blocks.The difference in the lengths of the gage blockstacks, minus the width of the thread tooth onthe template, will give the width of the threadtooth of the gage.

8.19.4 The thread angle of the gage ischecked by using thread templates made to themaximum and minimum permissible angles.As in the case of Acme threads, this inspectioncan be made more accurately by placing a pieceof white paper under the set-up and using ajeweler’s glass.

8.20 INSPECTION OF FLUSH PINGAGES

8.20.1 General. The general operationalcheck is especially important in the case of flushpin gages. The following steps should betaken:

a. Make sure that all pins move freely, butwithout shake or side play. Pins should befree to move at least inch further than thelimits shown on the drawing, when tolerancesare close. When tolerances are large, leeway of inch or greater is preferred.

b. The corner of the pin at the step end andthe corners of the feeler edges should be sharp.Any burr should be removed by stoning exactlyparallel to the surface with a hard Arkansass tone .

c. If the component drawing is available,check dimensions such as diameter of pin,amount of chamfer, and size of radius for inter-ference with the component. Also notewhether there are any obstructions to preventthe gage body from resting on the proper por-tion of the component.

d. When the length of the pin is checked besure to indicate over the top of the pin and thesteps for parallelism.

e. Where the pin is free to rotate, an indicatoranvil should be placed near the top edge of thepin and the pin rotated to check the squarenessof the top surfaces with the pin and the body.

f. If the flush pin is removed from the bodyof the gage for any reason, they should bemarked so that they can be reassembled in thesame relative position. (This does not apply toa lock pin which engages the flush pin at the siderather than in a slot through the center of theflush pin.)

g. Before disassembling the gage, notewhether the lock pin holding the flush pin inposition is tapered. If so, drive it out in theproper direction.

h. A chamfer is frequently specified at thecontact end of the flush pins. These chamferscan be inspected on a contour projector.

8.20.2 Depth measuring flush pin gage.This gage is checked with two equal stacks ofgage blocks as shown on figure 222. Startingwith block combinations which would bring the

152


Figure 222. Set-up for checking depth measuring flush pin gage.

pin flush with one of the steps if no errorexisted; other combinations of gage blocks aretried until a flush condition is reached. Theflush condition is determined by running an in-dicator over the top of the pin and the step.The height of the gage blocks required to makethe pin flush is the required dimension, Theother gaging dimension is found by repeatingthe process at the other step.

8.20.3 Length measuring flush pin gages.This type may be measured accurately with apin of the same diameter as the flush pin (orslightly smaller), which has square ends andan accurately known length. The pin shouldbe long enough to project from the gage whenit is placed in the bore from the lower side andpushed snugly against the flush pin. Afterinsertion of this pin, the gage is checked withtwo equal stacks of gage blocks using the samemethods as for depth measuring flush pin gages(see figs. 223 and 224).

8.20.3.1 If pins of the type described in thepreceding paragraphs are not available, the in-

spection can be made with a depth micrometer.The micrometer is clamped upside down nearthe top of an angle iron and its position ad-justed until the anvil is horizontal (see fig.225 ). Then the flush pin gage is placed on theanvil of the micrometer and the micrometerspindle advanced until the pin is flush with thesteps as shown by indicating. If the gagemaker’s tolerances are large, the reading of themicrometer will give the dimension to sufficient

Figure 223. Set-up for checking Iength measuring flush pin

gage—pin method.

153


Figure 224. Checking Iength measuring flush pin gage—

pin method.

Figure 225. Set-up for checking Iength measuring flush pin

gage—micrometer method.

accuracy. If the gage maker’s tolerances areclose, lock the depth micrometer in position, re-move the flush pin gage and measure the differ-ence in the heights of the micrometer spindleand the anvil by means of an indicator heightgage and gage blocks (see fig. 226).

8.20.3.2 This type of gage may be measuredaccurately by removing pin and measuring itslength as well as the distance between the baseand the steps of the body in a bench microme-ter. The length from the base to the minimumstep, minus the length of the pin equals theminimum gaging dimension; and the distance

from the base to the maximum step, minus thelength of the pin equals the maximum gagingdimension. When this method is used the endsof the flush pin must be perpendicular to thea x i s .

8.20.4 Post type flush pin gages. Thistype of gage is inspected by trying gage blockcombinations between the anvils and noting thelengths of blocks required to bring the top ofthe flush pin level with the maximum and theminimum steps (see fig. 227). As before, thelengths of the gage blocks indicate the gagingdimensions.

Figure 226. Checking length measuring flush pin gage—

micrometer method.

Figure 227. Post type flush pin gage.

154

8.20.4.1 There are several additional checkswhich must be made on gages of this type.The following are the most important:

a. Check the anvils for parallelism by plac-ing the gage blocks near each edge in turn andnoting changes in position of the pin.

b. Check the surfaces of the anvils forsquareness to the axis of the flush pin. Thiscan be done by swinging the movable arm toand fro over the gage blocks.

c. Be sure that the stationary anvil fits thebase of the gage securely. In particular, thereshould be no space between the shoulder of thestationary anvil and the base.

d. Be sure the top edges on the stationaryanvil are rounded unless there is a functionalreason for not doing so.

8.20.4.2 A very common design error on thistype of gage is to make the length of engage-ment of the flush pin too short. The length ofthe flush pin in the bushing should preferablybe as great as the distance from the flush pinto the anvil on the arm. (In fig. 227, dimen-sion A should be as great as dimension B.) Ifthe length of engagement is not adequate callthe designer’s attention to this fact.

8.20.5 Flush pin gages for location ofholes. When gages of this type have a mov-ing pin with more than one diameter, the pinshould be rotated in position with an indicatorin contact with each of the diameters except theone which fits the body of the gage to check thediameters of the pin for concentricity.

8.20.5.1 Moving pins having a tapered sec-tion must be removed and placed in a “V” blockto check concentricity. If a plate is clamped onthe “V” block so as to close one end of the “V”and a ball with about the same diameter as the

Figure 228. Conditions commonly found on tapered pins.


pin is placed in the “V” at this end, the pin canbe rotated without incurring axial movement ifit is pressed firmly against the ball. An indi-cator in contact with the tapered section thenwill register only the variations due to eccen-tricity of the tapered section.

8.20.5.2 It is especially important that therebe no shake in flush pins of this type.

8.20.5.3 If the tapered section of a pin isused for locating purposes a contour projectormust be used to check the condition at the inter-section of the taper and the straight pilot sur-face. The following are particularly importantrequirements:

a. The intersection must be free of all sharpedges and abrupt changes in contour whichmight prevent the pin from locating properly.

b. The intersection must be concentric withthe rest of the pin.

c. The finish must be good.8.20.5.4 A radius is permitted, provided it

is symmetrical and even. Common acceptableand unacceptable conditions of this portion ofthe gage are shown on figure 228.

8.20.5.5 From the above requirements, it willbe noted that the pin must be rotated through atleast half a turn in the projector.

8.20.5.6 The next step for all types of loca-tion flush pin gages is the measurement of thediameter of the flush pin at the section whichenters the component. In particular, it shouldbe noted whether there are any flats about thecircumference of the pin.

8.20.5.7 The gaging dimensions are meas-ured by placing a wedge or an adjustable sup-port under the gaging pin to bring the top ofthe flush pin exactly level with one of the stepsas shown by indicating. An indicator heightgage and gage blocks then are used to measurethe distance from the centerline of the pinto thesurface used to support the component (see fig.229 ). This distance is one of the gaging dimen-sions of the gage. If the support for the gagingpin is not extremely secure, it is wise to indicateover the top of the flush pin and the step afterfinding the gaging dimensions to be sure theyare still flush. This process is repeated withthe flush pin level with the other step to findthe other gaging dimensions.

155


Figure 229. Flush pin for measuring location of bolos.

8.20.5.8 Both ends of the gaging pin shouldbe checked for parallelism with the surfacewhich supports the work.

8.20.5.9 Where it is possible to place gageblocks directly on a gaging surface which sup-ports the component, the procedure . can beshortened considerably by so placing the blocksand bringing the gaging pin down on the blocksuntil a combination is found for which the flushpin is level with one of the steps. The height ofthe blocks, plus one-half of the measured diam-eter of the pilot of the gaging pin is the requireddimension.

8.20.6 Tapered flush pin gages. For in-spection purposes these gages generally aredivided into two classes: those in which theflush pin has a slight taper and close tolerances,and those in which the angle is large and toler-ances are not very close.

8.20.6.1 The first type is inspected by remov-ing the pin, standing it upright in a hold-downfixture, and measuring the diameter and thetaper of the tapered section over rolls byexactly the same methods used for measuringplain tapered plug gages. The length from theapex or from a datum diameter to the flat endof the pin should be determined.

8.20.6.2 The length from the base of thebody to the maximum and minimum steps thenis measured with a bench micrometer. Thesedistances, subtracted from the distance fromthe datum or apex to the flat end of the pin,will give the maximum and the minimumdepths to the datum or apex as specified on thecomponent and gage drawings.

8.20.6.3 The purpose in taking measure-ments over the rolls when the taper of theflush pin is slight is that slight variations inthe taper or in the diameter of the tapered sec-

tion will have a large effect on the positioningof the flush pin in the component. In the caseof steep tapers, these factors will not have somuch effect on the positioning of the flush pin.

8.20.6.4 When the angle at the bottom of theflush pin is large and when the tolerances arenot close, the angle can be checked by meansof a contour projector or a sine bar.

8.20.6.5 The dimension for this type of gagecan be measured by means of a channel-barfixture (see fig. 230). Two precision rolls areplaced on the channel-bar base and the channel-bars brought together and locked in positionwith the rolls between them so as to hold therolls rigidly in position. When the diameterof the tapered surface of the flush pin is large, itmay be necessary to space the rolls a fixed dis-tance apart by placing a gage block betweenthem. The tapered surface should contact therolls at about the center of its length, or in thesection where the pin will bear on the compon-ent if it does not make contact over the fulllength of the tapered section.

8.20.6.6 Two equal stacks of gage blocks areplaced on the channel-bars and the gage placedon top of them. The gage must be centeredso that both sides of the flush pin touch therolls. Various combinations of gage blocks aretried until the top of the flush pin is flush withone of the steps. This is repeated for the otherstep.

8.20.6.7 The computations necessary to de-termine the distance from the bottom of thegage to the apex of the pin are explained infigure 231. In this figure, the following symbolsare used:

D = the distance from the apex to the bodyW= the height of the channel barsZ = the height of the gage blocksV = the width of the block separating rollsA = the half angle of the taper of the flush

pinr = the radius of the rolls

8.20.7 Goose neck flush pin gages. The di-mensions of this gage are checked by themethod generally used for other types of flushpin gages; namely, gage blocks are tried be-tween the anvils until the combinations whichwill make the flush pin flush with each of thegaging steps are found.

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Figure 230. Set-up for checking small conical flush pin gage.

Figure 231. Computing Iength to apex on tapered flush

pin gage.

8.20.7.1 The set-up should simulate asnearly as possible the conditions under whichthe gage will be used. For example, surfaceswhich bear on a component in use should be lo-cated on precision reference surfaces for themeasurements.

8.20.7.2 Sometimes one surf ace of a gooseneck flush pin gage rests on the componentduring inspection. On such a gage this surfaceshould have a finely ground finish that is prop-erly positioned with respect to both the flushpin and the anvils.

8.20.7.3 When a fairly close check of thecenterlines of the anvils is desired or when it isnecessary to establish the distance from thecenterline of an anvil to some other portion ofthe gage, the measurement generally can best bemade by means of a contour projector.

8 . 2 1 I N S P E C T I O N O F P R O F I L EGAGES

8.21.1 Template gages for profile ofradius.

157


8.21.1.1 Checking radii where great accu-racy is not required. A template or profilegage having a radius with a fairly large toler-ance (0.005 inch or greater) can be easilychecked by means of a contour projector. Pro-jection screens are available having a series ofradii engraved on the glass. The image of theradius on the gage is matched with a radius onthe screen and the value of the screw radiusnoted. If a projection screen of this type is notavailable, or if the radius being inspected islarger than those given on the screen, it is pos-sible to make a drawing of the radius in questionat the appropriate magnification (see 8.5.5). Atool maker’s microscope can also be used fordirect measurement of a radius. The accuracyto which it can be used varies with the length ofarc. In general, this method can be used whenthe tolerance on the radius is 0.001 inch orgreater. The geometry of a measurement ofradius by direct measurement with a projectoris given in figure 232. The horizontal line onthe projector screen (the line AB in the dia-gram) is placed so as to intercept the arc at anydesired height. When the measurement is atonly one point, this line should be placed sothat the points A and B are near the ends of thearc. Measure the distance D between the pointsA and B. Move the support table until thehorizontal line on the screen is tangent to thebottom of the curve, noting the displacement.This is the distance H. The radius then can be

Figure 232. Measuring radius with projector.

158

Figure 233. Set-up for checking radius with indicator.

found by substituting the values for D and H inthe formula shown on figure 232.

8.21.1.2 Checking radii where accuracy isrequired. When the tolerance on the radiusis close, a convenient and accurate method is toclamp the profile gage to the rotating face plateof precision bench centers, the face plate of aprecision indexing head, or an indexing plate.The contact point of an indicator is brought incontact with the radial surface of the gage andthe face plate rotated over the length of thearc on the gage. The position of the gage isadjusted until the indicator shows a minimumof variation when the face plate is rotated.When the minimum variation of the indicatorhas been obtained the gage is located so thatthe origin of the radius coincides with the cen-ter of the face plate. The distance from theradius of the gage to the origin (center of faceplate) then can be measured by using gageblocks and rolls or gage blocks and an indi-cator height gage. The location of the originwith respect to straight precision surfaces onthe gage can be checked by rotating the faceplate until the surface in question is horizontalwhen indicated, and then checking the distancefrom the surface to the center of the face plateby means of an indicator height gage and gageblocks. The uniformity of the radius can bechecked by noting the variations in the indi-cator readings. These variations must not ex-ceed the tolerance on the radius as specified inthe gage drawing. If there is no tolerancespecified on the radius, the uniformity of theradius must be held as closely as the tolerance

on the position of the origin. Another set-upfor this measurement is shown on figure 233.This method takes longer than the one specifiedin 8.21.1.2, and it is generally used only whenan indexing plate is not available.

In this arrangement, a tool maker’s button isattached to the work plate and the profile gageis clamped in a position such that the distancefrom the center of the button to the arc on thetemplate gage is equal to the basic radius speci-fied on the gage drawing. Adjust the positionof the profile gage on the work plate until thereis no variation in the indicator readings whenthe work plate is rotated. The actual radius ofthe gage then can be determined by measuringthe distance from the radius on the gage to thecenter of the button by means of an indicatorheight gage. If uniform indicator readingscannot be obtained when the gage is rotated,the radius of the gage is not uniform. In suchcases, the position of the gage on the plate isadjusted to give a minimum variation in indi-cator reading. The distances of any close tol-erance surfaces from the origin of the radiuscan be found by measuring the distances fromthese surfaces to the center of the tool maker’sbutton with an indicator height gage. In caseswhere the equipment described above is notavailable, or where the radius is so large thatthe methods are not practical, a tool maker’sbutton can sometimes be located at the originon a large horizontal plate, and a stack of gageblocks and a roll, whose total length plus the

Figure 234. Checking radius with fixed

gage blocks.

button and


radius of the fixed button equals the nominalradius of the gage, is placed between the gageand the fixed roll as shown on figure 234. Theblocks and the movable roll are turned throughthe entire angle subtended by the radius. Ifthe blocks and roll are a snug fit over the entiresurface of the radius, the radius is satisfactory.If they are not an exact fit, the error in theradius can be determined by trying different,combinations of gage blocks. Errors in theorigin can be determined by moving the fixedbutton to new positions.

8.21.1.3 Checking radii with gage blocks,tool maker’s buttons, and parallels. Thismethod is generally used when great accuracyand dependability of measurement are required,or when the radius being measured is so largethat it cannot be measured by the methods de-scribed in previous paragraphs. Template gages should be examined for any indication ofwarping before they are clamped in positionfor measurement. They should be clamped soas to be free of stress. It is generally sufficientto check radii at three different points along thearc. In order to be sure that the entire gagingsurface of the gage is uniform, it should bechecked with an indicator placed between twobuttons mounted on a plate as shown on figure235. The variations in indicator readingsshould be considerably less than the toleranceon the radius. This is known as the check foruniformity of radius. The following symbolsare used in the formulae given in the ensuingpages:

R= Radius being measuredr = radius of precision roll or buttonG=diameter of precision roll or buttonM= measurement taken over the rolls

Other symbols are distances as shown on figures.The formulae used in checking profile gagescan be either of two types: those involving trig-onometric functions and those which involveonly algebraic terms. Generally, the trigono-metric formulae require less time to solve thanthe algebraic. However, both types of equationare given for most of the gages discussed in theensuing pages for the benefit of those who arenot thoroughly familiar with trigonometry.The use of the equations involving trigono-

159


Figure 235. Checking uniformity of radius.

metric terms usually requires finding the sine ofthe angle subtended by the rolls and usingstandards tables to find the cosine of this anglefor use in computing the dimension desired.To obtain the accuracy generally required forprofile gages, seven place tables, giving thevalues of the functions of the angles for each10 seconds of arc, or eight place tables, givingthe values of natural sines and cosines for each1 second of arc, should be used. If such tablesare not available, the relationships Cos A=

and tan A= –1 can be used.A practical book to use is “Natural Sines andCosines to Eight Decimal Places,” Special Pub-lication No. 231, prepared by the Coast and Geo-detic Survey, available from the Superintend-ent of Documents, Washington, D. C. Theterm S given in many of the equations is not adimension which can be shown on the drawing,

160

but merely an algebraic quantity. S representsseveral terms in the equation which always re-main the same, once any given gage has beenclamped in position. The advantage in usingthis term is that after it has once been computedit can be used in all solutions of equations in-volving various positions of the button. Thiswill save the time required to compute the termscomprising S each time a new position for thebutton is selected. The methods used to derivethe algebraic formulae are shown on figures 239,240 and 241. The methods used to derive thetrigonometric formulae are shown on figures257,258,259, and 260.

8.21.1.3.1 Checking an inside radius withno precision reference surface. This is doneby clamping a parallel to a surface plate andplacing two buttons on one side of the parallel,spaced apart by a stack of gage blocks longenough to place the buttons near the ends ofthe radius being checked. The length of thegage blocks equals dimension 21 shown on fig-ure 236. The sides of the gage blocks shouldnot touch the sides of the parallel. The profilegage is placed against the buttons and the posi-t-ion of the buttons and the blocks adjusted untilthey are a snug fit. Then the gage is clamped

into position. One of the buttons then is placedat the center of the radius and the dimension his determined by finding the combination ofgage blocks which fits snugly between the rolland the parallel. The roll should be pushed to

Figure 236. Checking inside radius (with no precision

reference surface).

and fro during thissettles at the point

operation to be sure that itfarthest from the parallel.

Knowing the dimension, h, the radius can bedetermined by solving the formula shown onfigure 236.

8.21.1.3.2 Checking an inside radius wherethe radius is tangent to a straight surface.The steps in this operation are as follows:

a. Clamp a parallel to a surface plate anduse a stack of gage blocks of any convenientlength H, to locate the gage in a position suchthat the straight surface is parallel to the par-allel bar.

b. The length n shown on figure 237 is thedistance from the edge of the button to thepoint where the radius and the straight sectionof the gage are tangent. If this point of tan-gency is dimensioned accurately with respect tosome other precision surface on the gage, usethis precision surface to position a second par-allel perpendicular to the first. Knowing theexact distances from the parallel to the referencesurface, and from the reference surface to thepoint of tangency, it is easy to compute the valueof n when various combinations of gage blocksare placed between the parallel and the button.

c. If the point of tangency is not specificallylocated with reference to some other precisionsurface on the gage, the second parallel (theone which is vertical, shown on fig. 237) is

Figure 237. Checking inside radius (where radius is tangent

to straight surface).


clamped in position perpendicular to the firstwithout regard to its exact distance from thegage. The distance y from the parallel to theorigin then can be established by placing theroll in position between the gage and a stack ofblocks of convenient length h and trying vari-ous combinations of gage blocks between theroll and the second parallel until the distancebetween the roll and the second parallel hasbeen determined. (Small values of h are bestin performing this operation. ) The value ofn which corresponds to the selected value of hthen is computed from the following equations:

The nominal value of n can also be computedby substituting the selected value of h in thealgebraic equation given in figure 237 andsolving for n. This value of n, subtractedfrom the distance between the roll and the par-allel will give y, the distance from the parallelto the origin.

d. The second parallel should be placed ex-actly 90 degrees to the first parallel. This canbe done by placing a precision square and agage block in the corner formed by the par-allels. A second gage block 0.0001-inch largerthan the first is used to check the uniformity ofthe space between the square and parallel inthe same manner an angle plate is checked forsquareness.

e. Select at least three different values of nat equal intervals. Compute the correspondingvalues of h from the equations shown on figure237. Insert gage blocks equal in length to theselected values of n plus y (the distance fromthe origin to the second parallel), between theroll and the second parallel. Try various com-binations of gage blocks between the roll andthe first parallel and note how they comparewith the nominal computed value of h.

f. In the equations, S is merely a mathemati-cal quantity used to simplify the mathematicalwork. Its value will remain the same for allpositions of the roll. The value of the angle Achanges for each new position of the roll, andis determined by computing the sine and find-ing the corresponding angle in trigonometric

161


tables when the amount of n is known, or bycomputing the value of the cosine when thevalue of h is known.

g. If the measured values of h with blocks donot conform closely to the computed values ofh, an indication of the nature and the amount ofthe error in the gage can be obtained by chang-ing the position of the gage to correct for devia-tions in h. For example, if the deviations ofthe measured values from the computed valuesbecome progressively greater as h is checkedfrom right to left in a set up such as that shownon figure 237, the gage can be loosened andtapped lightly on the left end until h is thesame as nominal on the left as well as the right.This will place the straight surface out of par-allel. If all values of h now check closely alongthe full length of the curve, the fault in the gagelies in the straight surface, which is tapered.The amount of the taper can be measured bytrying gage blocks between the parallel andeach end of the straight section.

h. If the errors in h do not increase progres-sively in any one direction, an analysis cansometimes be made by setting the tool maker’sbutton against blocks which are equal in lengthto the nominal values of h, and measuring thevariations in N.

i. The permissible tolerance in the positionof the roll will depend on the dimensioning ofthe gage drawing. Generally, the tolerance of hwill be limited by the following three factors:

(1) The tolerance on the position of thestraight surface tangent to the radius.

(2) The tolerance on the vertical position ofthe origin.

(3) The tolerance on the radius. The varia-tions in h should not exceed the tolerance ofany of these dimensions, where they are givenon the gage drawing. The variations in nshould not exceed the tolerance on the horizon-tal position of the origin.

j. When there is a tolerance on the radius, itmay be necessary to solve the equations shownon figure 237 for both the maximum and theminimum limits of R, and to see whether thelengths of the gage blocks are within the corre-sponding extremes of h and n. In these cases,it also may be desirable to measure the radius

directly, according to the method shown onfigure 236.

k. The most important consideration in de-ciding whether a template gage is within tol-erance is the accuracy of the curvature,including its tangency with other surfaces. Inthe gage under consideration, the uniformityof the radius, and the alinement of the straightsurface are much more important than thelength or location of the straight surface or theradius with respect to other portions of thegage.

1. If a gage appears incorrect with one com-bination of blocks and roll, a set-up based on aslightly different assumption may make it pos-sible to find the true condition. It is difficultto give general instructions for doing this be-cause there are several different factors whichaffect the accuracy of a complicated profilegage. The checker should draw on his ownexperience and try experimental set-ups whichappear to be best suited for the type and thecondition of the particular gages on hand.Generally, the gage can be brought into toler-ance most easily by shifting the assumed posi-tion of the origin.

8.21.1.3.3 Checking an inside radius wherethe radius is not tangent to a straight sur-

Figure 238. Checking inside radius (where radius is not

tangent to straight surface).

162

face. Equations for use in checking this typeof gage are shown on figure 238. The deriva-tion of algebraic formulae given in this figureis shown on figures 239,240, and 241.

This gage can be checked by methods similarto those used for the template gage previouslydescribed. However, it generally is easier toanalyze the errors if the gage is thrust againsttwo located rolls instead of establishing theposition by placing the straight surface paral-lel. The principal steps in this operation areas follows:

a. Clamp two parallels on a surface plate sothey are accurately perpendicular to each other.

b. Place one roll on the plate in a positionwhere it can contact the gage at the intersectionof the radius and the straight surface. Fix theroll an exact distance from each parallel by

Figure 239. Computing distance from origin to straight surface.

Figure 240. Computing vertical distance from origin to

center of roll.


Figure 241. Computing distance from parallel to roll.

means of two stacks of gage blocks of con-venient size.

c. Knowing h, the length of the lower stack ofgage blocks, compute the value of n from thefollowing equations:

Subtract from n the height of the second stackof gage blocks. (These are the blocks whichwould be horizontal shown on fig. 238. ) Thisgives y, the distance from the parallel to theorigin, which may be used to compute thelengths of gage blocks for other positions ofthe roll.

d. With these figures, compute the exact posi-tion of a roll which will contact the radius ofthe gage near the other end of the arc, and setanother roll in this position by means of gageblocks. In this arrangement one stack of gageblocks will cross another. This can be done byplacing a roll at the point of juncture, as shownon figure 242.

e. Place the gage on the surface plate so thatit is snug against the two rolls and clamp it inp o s i t i o n .

f. Check dimension H at both ends with gageblocks to be sure the straight surface of the gageis not tapered.

g. Assume at least three values of n, spacedat equal intervals, and compute the correspond-

922173 O - 51 - 12 163


Figure 242. Setting template gage in position.

ing values of h from the equations shown onfigure 238.

h. Establish the values of n by means of gageblocks and check the corresponding values of hwith block combinations (see fig. 243).

i. As before, the variations in the observedpositions of n and h should not exceed the toler-ances given on the gage drawing for the posi-tion of the origin. If a tolerance is given on thegage drawing for the radius, it may again benecessary to solve the equation for both themaximum and the minimum values of theradius.

8.21.1.3.4 Checking an outside radiuswhere the radius is tangent to a straightsurface.

a. Clamp two parallels on a surface plateexactly perpendicular to each other.

b. Using gage blocks of any convenient

164

length H locate the gage so that the straight sur-face is parallel to one of the parallels at a dis-tance H.

c. Check the gage by methods similar to thosedescribed in the sections on profile gages withinside radii. Use equations shown on figure244. The equations for finding n when h isknown are as follows:

d. The limitations for the variation in thevalue of h are as previously described.

8.21.1.3.5 Checking an outside radius where the radius is not tangent to a straightsurface. This type of gage is checked bymethods almost exactly the same as those usedby the preceding gage, using formulae shown


Figure 243. Checking radius on template gage.

on figure 245. The formulae for finding n whenh is known are:

8.21.1.4 Checking template gages with spe-cial indicator fixture shown on figures 246through 249 will effect a very great saving intime when used to check template gages whichincorporate precision radii. The indicator isheld in proper relationship to the center of thefixture by means of an adjustable support andbar. This bar can be detached to permit theuse of bars of other lengths or to permit the useof an arm for supporting the gage in set-upswhere it is desirable to move the gage and tohold the indicator stationary. In making sucha fixture, it is extremely important to obtain avery close fit between the pivot at the center ofthe fixture and its bushing. A dial indicatorgraduated in units of 0.00005 inch should pref-erably be used with this device.

Small template gages often can be checkedmore easily by bolting them to an arm of afixture and rotating them past a stationary in-dicator. A typical procedure is shown on fig-ures 248 and 249.

a. Clamp the pivot block assembly at theedge of the surface plate with the axis of thepivot pin in a horizontal position. It may benecessary to place parallels under the fixtureto obtain a convenient height.

b. Clamp the gage to be checked to the armand locate any straight precision surfaces withrespect to the origin (center of the pivot) bymeans of gage blocks and an indicator heightgage as shown on figure 248.

c. Adjust an indicator height gage to a zeroreading over gage blocks equal to the height ofthe origin plus the radius specified on the gaged r a w i n g .

d. Place the arm of the fixture in a verticalposititon. Place the contact point of the indi-cator on the radius of the gage and note theindicator reading. Rotate the arm and note any

165


Figure 244. Checking outside radius (where radius is tangent

to straight surface).

variation in the indicator readings over theentire length of the arc.

e. If the indicator reading does not remainconstant, readjust the position of the gage onthe arm until constant readings are obtained.The distance which the gage is moved indicatesthe amount of error in the radius of the gage.

8.21.2 Template gages for profile of angle.The angular surfaces on profile gages generallyare checked by clamping the gage to an angleplate, mounting the angle plate on a sine plate,and checking the angles with an indicatorheight gage as specified in 8.5.3. For example,consider the gage shown in figure 250. Thesteps in checking the angles and the location ofthe intersection of this gage are as follows:

8.21.2.1 Clamp the gage to an angle plateand place the angle plate on a sine plate. Theangular surface on the gage should be higherthan the angle plate in order to permit indi-

cating over the full width of the gaging surface.8.21.2.2 Indicate over the surface c and ad-

just the position of the gage until this surfaceis accurately horizontal. Then clamp the gagesecurely in position on the angle plate. Thesurface g should not touch the sine plate unlessit is a precision surface which is supposed to beaccurately parallel to c.

8.21.2.3 While the gage is still mounted withthe surface c horizontal, place a precision rollin the angle formed by f and e. Use an indi-cator height gage to find the distance Q from thetop of the roll to the surface c (see fig. 251).

8.21.2.4 If f and d are precision surfaces,turn the angle plate 90 degrees and indicateover f and d to be sure that they are perpen-dicular to c.

8.21.2.5 Turn the angle plate back and setthe sine plate at the angle A. Indicate over

Figure 245. Checking outside radius (where radius is not

tangent to straight surface).

166


Figure 246. Set-up for checking a large radius with a special indicator fixture.

Figure 247. Checking a Iarge radius with a special indicator fixture.

167


Figure 248. Set-up for checking a small radius with a special

indicator fixture.

Figure 249. Checking a small radius with a special

indicator fixture.

Figure 250. Checking template gage for location of

intersection.

Figure 251. Measuring distance to apex on template gage.

1 6 8

MIL-STD-1209 September 1963

Figure 252. Checking template gage for accuracy

of angles,

the surface e (see fig. 252). If it is not hori-zontal try various combinations of blocksuntil it is, and note corresponding error in—angle.

8.21.2.6 Compute the value of x, the loca-tion of the intersection of surface e withsurface f, by means of the formula shown onfigure 250. In this computation, use themeasured value of angle A.

8.21.3 Profile plug gages. The gages gen-erally are used as check gages for special re-ceiver gages and receiver profile gages bymeans of blueing. Being reference gages,profile plugs usually are made to very closetolerances and must be inspected carefully bymeasurement over rolls.

8.21.3.1 Measuring plug with plain radius.The formula shown on figure 253 for thismeasurement can also be used to check convexradii on template gages where there are nostraight precision reference surfaces.

When A is positive use

When A is negative use

The set-up used may be either a measure-ment over rolls or a check by means of achannel bar fixture. For measurements overrolls, a plug gage of this type generally restson a gage block wrung on a tool maker’s flat.

Two stacks of gage blocks of equal height areselected to support the rolls at the desiredheight and measurements are then takenover the rolls. The gage is tilted at several. angles and rotated to test the uniformity ofradius for each selected combination of sup-porting gage blocks. When tolerances areliberal, micrometers can be used for thismeasurement. When tolerances are close, auniversal measuring machine is a suitableinstrument. Plugs with plain radii sometimesare permitted to have a flat at the bottom ofthe curved section. In such cases, the gagemust always be tilted enough during meas-urement to prevent the flat from bearing onthe gage blocks. These gages should not beclamped in position, or forced down withexcessive pressure, because of possible injuryto the gage blocks. The object of resting thebottom of the plug on gage blocks is to permitat least one measurement to be taken nearthe bottom of the radius with the outsideblocks lower than the central blocks. Itshould be noted that the value of A shownon figure 253 is negative when the outer setsof blocks are shorter than the central blocks.When A becomes negative the accuracy ofthe R value will decrease as the contact ele-ments of the rolls approach the tangent ofthe center block. Use the second formulashown on figure 253 for negative values of A.

8.21.3.2 Measuring profile ogive checkplugs with single radius. These gages are

Figure 253. Checking outside radius (where there is

no precision reference surface).

169SUPERSEDES PAGE 169 OF 12 DECEMBER 1950


Figure 254. Measuring profile ogive check gage.

checked by holding them upright over a toolmaker’s flat or hold-down fixture and measur-ing over rolls as shown on figure 254. Beforeany measurements are taken, the axis of thegage must be accurately vertical. If the gagehas a precision ground flat at the top of thegaging member, it is possible to check the per-pendicularity of this flat with the axis of thegage before mounting the gage for measurementover the rolls. The gage is first mounted be-tween centers and rotated with an indicator incontact with the flat in a set-up similar to thatused for checking the squareness of the frontface of a taper plug (see fig. 163). The point ofthe gage is put in a female center and the otherend is held by a male center. If the flat on thegage is perpendicular to the axis, the gage canbe established in a vertical position by adjust-ing the fixture shown in figure 254 until the flatis parallel to the base of the hold-down fixture.The height of this horizontal surface above thetool maker’s flat is then determined with an

indicator height gage in order to compute thevalue of the dimension L when selected lengthsof gage blocks support the rolls (see fig. 255).

Gage blocks suitable for measurements at five ormore points at convenient intervals along thesurface of the gage then are selected. The cor-responding values of L are substituted in theequations shown on figure 255 to determine theproper value for the measurements over therolls. Time will be saved if the first measure-ment is taken with the rolls in the position near-est the point at the bottom of the gage. If themeasurement over the rolls does not fall withinthe specified tolerance, vary the height of thegage blocks supporting the rolls until the meas-urement is correct. Take all the other measure-ments over the rolls with the gage blocksaltered by exactly the same amount, and notewhether the measurements over the rolls fallwithin tolerance. The effect of changing theheight of the stacks under the rolls is to shiftthe origin of the radius. If the change in thesize of the gage blocks is less than the toleranceon the horizontal dimension which determinesthe position of the origin on the gage drawing,

Figure 255. Measuring profile check plug (single radius).

170

and all subsequent measurements taken over therolls are within tolerance, the gage is acceptable.If the required change in the gage blocks tobring the measurements within limits is greaterthan the horizontal tolerances on the origin, thegage is not acceptable and the change in the sizeof gage blocks from basic will indicate the errorin the location of the origin. The purpose oftaking the first measurement at the lower posi-tion of the rolls is that errors in origin, the typemost commonly found have their greatest effectat this point. If the position of the gage is

changed to make the measurement over the rollscorrect at this point, it is highly probable thatmeasurements over rolls at all other points willbe within tolerance and that further adjust-ments in the position of the gage will be unnec-essary. When the receiver corresponding to

the check in question is used in conjunction witha snap gage for checking ogival length, theerror in the horizontal placement of the origin

(the dimension L shown on fig. 255) must not begreater than the tolerance on the swell diameter,D, unless another tolerance on the origin is ex-plicitly stated on the gage drawing. The per-pendicularity of the flat near the swell diametermust be accurate within the tolerance on the lo-cation of the origin. When the receiver corre-sponding to the check in question is used onlyfor a profile check and no tolerance is given forthe origin on the gage drawing, a variation ofa few thousandths of an inch in a directionwhich makes the check longer is permissible.It is generally assumed that the measurementsover the rolls at any point along the axis of thegage should not vary from basic by an amountgreater than the tolerance on D, the diameteracross the top of the gage. However, wherethere is a tolerance given for R on the gagedrawing, the gage may sometimes be foundwithin the limits by solving the formulae shownon figure 255 for both the maximum and theminimum values of R.

8.21.3.3 Measuring profile ogive checkplugs with double radius. The larger radiusof these plugs is checked with the same methodsand formulae used to inspect the profile ogivegage with a single radius. The smaller radiusR 1, which determines the form of the gage


near the point, is measured by the methodspreviously described but with the formulaeshown on figure 256. The first readings shouldbe made at three points: the swell (largest)diameter, the point of tangency of the tworadii, and at a location near the point of thegage. These measurements can be used as abasis for relocating the gage in case there isan error in origin. After this, the intermediatepoints are checked. Figures 257, 258, 259, and260 explain the methods used to derive thetrigonometric formulae shown on figure 256.The first formula for cosine B shown on figure257 applies to an oblique triangle. The deri-vation can be found in textbooks on trigo-nometry. The formulae for R1 should not beused when the rolls are in a position over a sur-face determined by R, or vice versa. When L isgreater than (R+ r) sin C, the rolls are in aposition to measure R1. On figure 256, thevalues of angles A, B, and C will remain thesame for any given gage. The value of the

Figure 256. Measuring second radius of profile check plug.

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Figure 257. Computing point of tangency of double radius.

Figure 258. Computing variable angle in double radius

measurement.

angle E must be recomputed for each differentposition of the rolls.

8.213.4 Measuring plugs with convex andconcave radii. These measurements are madeby standing the gage in a vertical position andmeasuring over precision rolls supported bygage blocks in a manner similar to that usedfor profile ogive gages. The limiting values ofmeasurements over the rolls, corresponding tothe maximum and the minimum tolerance lim-its given on the gage drawing, can be computedfrom the formulae shown on figures 261 and262. The inspector usually can determinewhether the rolls are resting on the convex or

Figure 259. Computing position of roll in double radius

measurement.

the concave surface by visual inspection. How-ever, in doubtful cases or when a very smallgage is being inspected, use can be made of thefact that the roll rests on the convex surfacewhen L1 is less than:

The rolls rest on the concave surface when L2

is less than:

On gages of this type a reading should be madeat the point of tangency of the two radii becauseerrors occur at this place more frequently thanat other points on the gage. In formulae shownon figures 261 and 262, angles A and C, changefor each new position of the rolls but angles Band D remain the same for any given gage.

8022 INSPECTION OF FIXTUREGAGES -

8.22.1 Flush pin type fixture gages. Thefollowing procedure should be followed ininspecting this type of gage:

822.1.1 Check the gage for general func-tional characteristics. This includes movingall flush pins to see that they operate freely,making sure they can move below the minimumsteps and above the maximum steps, checkingfor shake in the flush pins, making sure that thecorners of the pins at the gaging end are sharpas well as the f aces of the steps, making sure that

Figure 260. Computing measurement over rolls in double

radius measurement.

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Figure 261. Measuring outside radius of profile plug with

outside and inside radi i .

Figure 262. Measuring inside radius of profile plug with

outside and inside radii.

all screws are tight and checking all dowel pinsfor proper fit. This last point is particularlyimportant because the dependability and accu-racy of any gage is limited by the fit of the dowelpins used in assembly. The dowel pins can bechecked by loosening the screws and tappingthe block and the pins lightly. If the positionof the block has shifted in the least when the

screws are retightened after this operation, thedowel pins do not fit properly.

8.22.1.2 The hardness check must be madeat more different points than on most othertypes of gages. In addition to the gaging sur-faces, all surfaces which are made to a slide fit,all surfaces which support the work, and allother portions of the gage which will be subjectto wear should be checked for hardness. Theshoulder of small pins, generally those less than inch in diameter, should be drawn to a hard-ness of about C58 to C60. All other surfacessubject to wear should be hardened to C63 toC66, unless other specifications are given on thed r a w i n g .

8.221.3 Clamp the gage on an angle ironand use an indicator height gage to locate thepoints which determine the position of the com-ponent with reference to the gage. The sidesor base of the gage should never be used as ref-erence places. If a long straight precision sur-face on the gage is used for locating the compo-nent, the gage can be set up by indicating overthis surface and adjusting the position of thegage on the angle iron until this surface is hori-zontal. If no such surface exists, the height oftwo precision surfaces should be measured bymeans of an indicator height gage and gageblocks and the position of the gage adjusteduntil the centerline from which these points aredimensioned is horizontal. Select for this pur-pose the two surfaces which have the closestlocation tolerance. In general, the gage shouldbe clamped as close to the surface as possible inorder to minimize errors in measuring heightsand to provide a more stable set up. Toolmaker’s clamps are better than C clamps, par-ticularly on small gages, because they have lesstendency to spring the base out of position.

8.22.1.4 Once the locating surface or thecenterline of the gage has been established, thegage should be tightened securely in position onthe angle iron and should not be disturbed untilthe inspection has been completed. Clampsshould be positioned so that the checker will nothave to move the clamps more than is absolutelynecessary when the angle iron is set indifferent positions to permit inspection of thevarious parts of the gage. If it is necessary to

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move the clamps, at least three clamps should beused. Only one clamp should be loosened andrelocated, leaving two clamps securely fastenedat all times.

8.22.1.5 Check the distances between the lo-cating points. This usually is done with an in-dicator height gage and gage blocks. When thedistance between two points is at an angle theunit should be set up on a sine plate so that thedimension in question is vertical. When thismethod is impractical gage blocks are tried be-tween the pins until the length which providesa snug fit between the pins is found. The blockshould not be forced between the pins becausesmall pins are easily bent enough to throw themoutside of close tolerance limits.

8.22.1.6 Check the angular position of thepins by setting the sine plate so that the pins arehorizontal. Indicate over both ends of eachpin. Check for shake at this time, using theindicator to measure the shake.

8.22.1.7 The gaging dimensions of flush pinsare checked by using a sine plate to bring theflush pins vertical, with the steps at the top.This can be done by turning the angle iron 90degrees after each pin is checked for angularity.Determine the height of the fixed pin or surfaceto which the end of the flush pin is dimensioned,add the dimension given on the drawing to theheight of this reference surface and combinegage blocks equal to the sum. Slide the upperblocks sideways so that they overhang as shownon figure 263 and bring the flush pin down onthe top of the blocks to see whether the pin isflush with the step within the tolerance on thegage drawing. This check must be made onboth steps, using the maximum and the mini-mum dimensions given on the gage drawing forthe initial heights of the gage blocks.

8.22.1.8 The top of the flush pin and thesteps should be checked for parallelism at thesame time. Parallelism must be such that nopoint on the top of the flush pin exceeds thegage maker’s limits for the dimension of theflush pin.

8.22.1.9 When several flush pin fixture gagesof the same size and design are checked, con-siderable savings in time will be effected byprocuring master gages in the form of a com-

Figure 263. Checking flush pins and steps on fixture gage.

ponent, one for the maximum component limitsand another for the minimum limits. Thesemasters can be inserted in the gage and the posi-tion of the pin with reference to a step meas-u r e d .

8.22.2 Indicator type fixture gages. Thesegages are checked by the same general methodsdescribed in the preceding paragraphs. How-ever, where the flush pins were checked withrespect to the steps on the preceding gage, set-ting gages or gage blocks are placed betweenthe work locating surfaces of the indicator typegages, and the gaging pins and indicatorschecked for total range which should exceed thecomponent tolerance. The dial indicatorsthemselves are checked in accordance with8 .23 .1 .

8.22.2.l The spindle of the indicator shouldbe parallel to the axis of the gaging pin againstwhich it rests.

8.22.2.2 On this type of gage, all surfaceswhich contact the component should be in properalinement, even though specific tolerances forthem are not given on the gage drawing.

8.22.2.3 The location and parallelism of cen-terlines generally are established by means ofan indicator height gage and gage blocks.

8.22.2.4 The tension of any springs used tomove the gaging pins should be sufficient toprovide proper positioning of the pin yet not

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so great that it is difficult to insert and removethe component.

8.22.2.5 The inspection of indicator fixturegages which measure concentricity is largely amatter of checking alinement. All “V” sup-ports, centers, and bearings should be alinedboth vertically and horizontally. All bearingsand other moving parts should rotate freely.

8.22.2.6 The final step in inspecting concen-tricity gages is to place a master or a com-ponent in a gage, and be sure it rotates freelyand that all supporting members make contactproperly. It is easy to note whether bearingsare making proper contact by attempting to ro-tate them when the master is standing still.

8.23 INSPECTION OF MISCELLANE-OUS GAGES

8.23.1 Dial indicator gages. General in-formation concerning standards for dial indi-cators is to be found in Commercial StandardsBooklet CS (E) 119–45 published by the De-partment of Commerce and available from theSuperintendent of Documents, Washington 25,D. C.

8.23.1.1 The proper inspection and repairof dial indicators requires considerable special-ized knowledge. For this reason it is advisableto assign the responsibility for checking andrepairing all dial indicators to one person in alaboratory.

8.23.1.2 The following steps must be takenin inspecting dial indicators:

a. Check for sticking, This is done by mov-ing the spindle slowly from the rest position tothe maximum limit and return by means ofhand pressure.

b. Check for spindle looseness by pushing thespindle back and forth in a direction per-pendicular to its axis. Check for rack pin sidemovement by attempting to rotate the spindle.The specifications given for these checks meanthe total amount by which the indicator needlevaries from its rest position when the checksare made. Note whether the needle comes torest at the same point each time pressure isre leased .

c. Note whether the spring pressure is ex-cessive. Experience will help the operator tojudge the “feel” imparted by proper indicatingspring tension.


d. Set the indicator up in a precision benchmicrometer in a horizontal position such thatthe spindle is in line with the movable anvil andbears on this anvil (a special fixture can be con-structed for this purpose if many indicators areto be checked). The calibration of the indi-cator is checked by advancing the micrometer tovarious points and noting whether the changesin the indicator are the same as the changes inthe micrometer reading. On an indicatorwhich has been graduated 0-50-0 a check shouldbe made at the points 0, 20 and 40 to the limitof range of the indicator. In moving back inthe other direction checks should be made bythe points 50, 30, 10, and 0. Indicators withother graduations should be checked at the cor-responding points. After the limits of the in-dicator scale have been reached at this check andmeasurements are taken in the other direction itshould be noted whether there is any backlashand if there is, this should be entered on the tagas a separate defect.

8.23.1.3 The following specifications are fol-lowed in classifying the indicators:

Class I—Excellent.a. Does not stick.b. Repeats to zero with indicator at rest.

On indicators graduated to 0.0001 inch or lessthe tolerance for this requirement is one grad-uation. On indicators where the graduationsare 0.0005 inch or more the tolerance is 0.0002i n c h .

c. No excessive spring pressure.d. Spindle play and rack pin side play held

within standard specifications. If the gradua-tions are 0.0001 inch or less the pointer variationmust be held within 0.000025 inch for this check.If the graduations are 0.0005 inch or greater thepointer variation should not exceed 0.0002i n c h .

e. Accuracy of calibration within one unitgraduation plus or minus at any point coveringthe entire range of indicator.

Class II—Good.a. Does not stick.b. Most points within the standard specifica-

tion listed under class 1 and all other pointsnearly within specifications. (No explanationis needed on the tag. )

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c. Accuracy of calibration substantiallywithin specifications for class I.

Class III—Fair.a. Does not stick.b. Spindle looseness, rack pin side play and

repeat error held substantially within tolerance.c. Accuracy of calibration within one grad-

uation plus or minus at any point covering atleast one-half the number of revolutions of theentire range of the indicator. The defects ofcalibration should be noted on the tag to beattached to the indicator for informing theuser over what range the indicator is accurate.

Class IV—Poor.Indicator requires repair for any one of the

following reasons:a. Sticks.b. Backlash present when applied pressure on

stem is released slowly.c. Stem play outside tolerance limits as noted

under class I.d. Rack pin side movement outside tolerance

limits as noted under class I.e. Does not repeat at zero as specified under

Class I.f. Calibration outside tolerance limits as

noted in class III.8.23.2 Special indicator snap gages. B y

this class of gage is meant special gages inwhich the component is placed in a receiver andslid under a movable gaging member whichactuates an indicator gage, and all other specialindicator asemblies which would not ordinarilybe considered indicator fixture gages. The fol-lowing are the principle steps to be taken inchecking gages of this type:

8.23.2.1 Check the dial indicator in accord-ance with 8.23.1.2 if it has not already beenchecked. If the dial indicator has been in-spected, check the information on the tag to seewhether the class of indicator is satisfactory foruse on the part in question. If the indicator isclass III, make sure that it is positioned in thegaging assembly so as to permit the use of therange over which the calibrations are accurate.

8.23.2.2 Check the alinement of the anvils.This can be done visually by noting how thecheck gage fits between the anvils. When thealinement of the anvils must be held closely, a

contour projector or an indicator height gageis used.

8.23.2.3 Make sure that the range of the gageis sufficient to permit measurements of com-ponents which are undersize and componentswhich are oversized. The range of an indicatorgage should be at least twice the componenttolerance.

8.23.2.4 Inspect checks for setting the gage,if provided, and try them in the indicator gageassembly. If checks are not provided or if thechecks are not of the same form as the part, trya component part in the gage to see whether thegage has been made and designed to permitproper functioning.

8.23.2.5 If the drawing specifies that thecomponent limits are to be marked on the in-dicator dial this can best be done with an ordi-nary red pencil. Carbon tetrachloride can beused to remove these pencil marks if the com-ponent limits are changed. When markinglimits on the indicator dial use a setting gageequal to the minimum component limit and asetting gage equal to the maximum componentlimit for establishing the position of the limitlines on the dial. Do not mark off spaces onthe dial on the basis of the dial graduations.Using the dial graduations will introduce errorsof the indicator mechanism into the readingswhereas establishing the limits directly bymeans of checks will tend to eliminate theseerrors.

823.2.6 Check all working parts to see thatthey are a free fit. Make sure there is no shakeor side movement where this would affect thefunctioning of the gage.

8.23.2.7 If any of the gaging members arekept in position by means of springs, check thespring pressure to make sure that the pressureis not too little to assure proper contact nor toogreat to permit easy functioning of the gage.

8.23.2.8 Sharp edges on the anvils or on thegaging member near the point where the com-ponent enters may make it very difficult to insert

the component. Stone all such edges sufficientlyto permit easy entry of the component.

8.23.3 Spanner gages. Spanner gages arealso commonly known as multiple pin gages forlocation of holes. The steps employed in theirinspection follow:

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8.23.3.1 Test each pin with the fingers tomake sure it is tight.

8.23.3.2 Measure the diameters of’ the pinsand check them for out-of-round. When thetolerances on these diameters are close and it isimpracticable to position them between the an-vils of a bench micrometer or other precisionmeasuring instrument, the diameter can best bechecked by making small snap gages from stacksof rectangular gage blocks. This is done bybuilding a stack of blocks equal to the maximumdimension of the diameter and then wringinga block to each side of the stack in a positionsuch that the two added blocks will overhangthe stack and effectively serve as the anvils of asnap gage (see 8.5.2.7 and fig. 139). Theprocess is repeated with other combinations ofblocks until the size of the pin is determined tothe nearest 0.0001 inch. In checking for diam-eter and out-or-round the inspector should alsonote whether there are any flats near the ends ofthe pins.

8.23.3.3 Check for parallelism of all pins.This is done whenever possible by mountingthe gage on an angle plate and by indicatingalong all pins. When this is not practical,stacks of gage blocks calculated to fit snugly be-tween the pins are inserted, and the inspectorsights between the sides of the pins and theends of blocks to see whether they are parallel.

8.23.3.4 Check for canted pins. This is doneby turning the angle plate over 90 degrees andindicating again for parallelism. When it isimpractical to indicate, gage blocks should beplaced between each pin and two other pins ifpossible.

8.23.3.5 Check the center distances betweenthe pins. This is done, whenever possible, bymeans of an indicator height gage. When thismethod is impractical, stacks of gage blocks areplaced between the pins. It is important not toforce blocks between pins because small pinscan easily be bent enough to throw them outsideof close tolerance limits.

8.23.3.6 If there are more than two pins onthe gage, it is best to select the pins with thesmallest tolerances and greatest distance be-tween centers as reference pins.

8.23.3.7 If all pins or many pins in a mul-tiple pin gage appear to be outside of the speci-

fied limits, try using two different pins forreference. Sometimes an error in the locationof one reference pin will make all other pinsappear to be improperly placed, even thoughthey may be satisfactory.

8.23.4 Dovetail gages. The first step in theinspection of dovetail gages is to check theangle. This may be done by measuring overrolls of different sizes. Because the angles mayhave compensating defects, each angle should bemeasured individually. A way to check theangle of either a male or female dovetail gageis to set the gage upright on the angle plate andto measure the height of three or more differentsizes of rolls as shown on figure 264.

8.23.4.1 Another method is to use a set-upsimilar to that shown on figure 264, but insteadof using different sizes of rolls, use one size rolland check different positions by placing variouscombinations of gage blocks between the rolland the angle iron. The increase in the heightof the roll, divided by the increase in theblocks, will equal the tangent of angle A. Itis necessary to check the angles of the sides ofthe dovetail independently, as specified in the

Figure 264. Checking angle of dovetail gage.

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Figure 265. Checking female dovetail gage.

previous two paragraphs, instead Of measuringthe angles simultaneously by placing blocks be-tween two rolls in the case of female gages, orby measuring over two rolls in the case of malegages. If the two angles are checked at once,they may appear to be within tolerance, whenin fact, both angles are out of tolerance indirections such that the error in one angle tendsto nullify the effect of the error in the othera n g l e .

8.23.4.2 The distance between the apexes onfemale dovetail gages can be measured byplacing two precision rolls between the anglesand by placing a stack of gage blocks betweenthe rolls. Rolls should be a size which willpermit them to make contact with the sides ofthe angles about halfway along their length.The length of the stack of gage blocks can becomputed from the formulae shown on figure265. If the stack of blocks is not a snug fit be-tween the rolls, the change in the length of theblocks required to bring about a good fit willindicate the amount by which the distance be-tween the apexes differs from the basic dimen-sion N. This check should be made at both endsof the gage. It is important not to use muchforce in pushing the gage blocks between therolls.

8.23.4.3 The distanm between the apexesof the male dovetail gage is obtained by takinga measurement over rolls, as shown on figure266. This measurement should never be madeby measuring directly across the apexes.

8.23.4.4 When a male dovetail gage is to beinspected by fitting it to a female check, this

is done by placing a coat of blueing on thefemale gage and by trying the male gage ateach of its ends. At least ¾ of the blueingshould be removed. The male gage should alsobe inserted completely into the female to seewhether there is any shake or side play.

8.23.4.5 Large inaccurate radii on dovetailgages can be checked with a radius gage.Where the radii are small, or when they aredimensioned with fairly close tolerances, a con-tour projector usually provides the best meansof checking.

8.23.4.6 Tapered dovetail gages can bechecked rapidly and accurately through theuse of a precision indexing plate. The stepsto be taken in using this method are as follows:

a. Check the precision sides (angles) of thegage by rubbing a little blueing on a flat surfaceand drawing the sides of the gage across thissurface. If the gage has a good finish, butdoes not seem to make a good bearing when theblueing test is made, it may be necessary tomeasure the gage over a pair of precision ballsas specified in 8.23.4.8. If a faulty bearingappears to be caused chiefly by a rough finish,the gage should be rejected.

b. Clamp the gage onto the plate with thelarge end horizontal and at the same height asthe center of the plate (see view A, fig. 267).When the indexing plate method is used, theend surface of the dovetail must be precisiong r o u n d .

c. When a large number of gages of the samesize are to be inspected, a great amount of timewill be saved if a block with a straight precisionground edge, or a small parallel is clamped orbolted to the indexing plate just above the posi-

Figure 266. Checking male dovetail gage.

178


Figure 267. Measuring width of tapered dovetail on anindexing plate.

tion taken by the gage. The straight precisionsurface of this block should be established in aposition through a centerline of the indexingplate. Then when each gage is placed on theplate, alinement can be established easily andrapidly by placing the top end of the dovetailagainst the block.

d. Rotate the indexing plate clockwisethrough an angle equal to 90 degrees minus C,the half angle of the taper as specified in thegage drawing. Hold a precision roll under theside of the gage and use an indicator heightgage to determine whether this roll is hori-zontal. If it is not, determine the value of thehalf angle of taper by changing the angularposition of the indexing plate until the roll ishorizontal.

e. Use three sizes of rolls or three combina-tions of gage blocks and a roll to determine thevalue of the angle B, the angle made by thedovetail side in a plane perpendicular to thesides of the dovetail. In order to checka tapered dovetail gage properly, it is importantto understand the difference between the angleof the side and a plane perpendicular to thesides of the gage, and the same angle in theplane perpendicular to the axis of the gage.This relationship is shown on figure 268. Theangle of the dovetail side in the plane perpen-dicular to the sides of the gage is angle B, asshown in section BB. The same angle in theplane perpendicular to the axis of the gage isthe angle B´ as shown in section AA. Theangle actually found by measurement with rolls

will be the angle B. However, since the angleof the sides of the dovetail usually is specifiedon the gage drawing in the plane perpendicularto the axis, it is necessary to compute the angleof value of B’ after having measured the valueof B. This can be done by means of the for-mulae shown on figure 268.

f. Use an indicator height gage and gageblocks to measure the dimension L, the distancefrom the bottom of the roll to the top of thecenter of the indexing plate.

g. Rotate the indexing plate through anangle equal to 90 degrees plus the half angle ofthe taper as referred to the original position ofthe plate.

h. Check the half angle of taper and the angleof the dovetail sides, and the distance from thetop of the roll to the top of the center of theindexing plate by the methods previouslydescribed.

i. Use the formula shown on figure 267 todetermine the distance between the apexes ofthe gage at its large end. In this figure, t equalsthe thickness of the gage (see fig. 268), G equalsthe diameter of the roll used when the dimen-sions L and K were established, angle C equalsthe half angle of taper as actually measured,

922173 O - 51 - 13 179

Figure 268. Computing relationship of angles of tapereddovetail gage.


Figure 269. Checking taper dovetail on sine plate.

and angle B is equal to the angle made by thedovetail sides in the plane perpendicular to thesides (see fig. 268).

8.23.4.7 If an indexing plate is not availableor if the gage to be inspected is too large formounting on the indexing plate, a sine platecan be used instead. The steps required tomake the inspection of the gage with this deviceare as follows: As before, check the sides of thegage for flatness.

a. Place an angle iron on its side as shownin view A of figure 269 and clamp the dovetailin position with the large end horizontal.Clamp a sine plate to the angle iron in the posi-tion shown.

b. Use an indicator height gage and gageblocks to measure the distance Q, the distancefrom the end of the dovetail to the center of theupper roll on the sine plate. The dimension Pshould equal the nominal size of the sine plate.

c. Turn the angle iron at right angles so thatit rests on the sine plate as shown in view Bof figure 269. Measure the half angle of taperof each side of the dovetail by establishing thesine plate at this angle, and by indicating overrolls placed against each side of the dovetail tosee whether they are horizontal. (Do notchange the position of the gage on the angleplate ). Place blocks under the left side of theplate at the same time blocks are placed underthe right, if necessary, to get clearance with thesurface plate in checking the lower half angleof the dovetail.

d. While the sides of the dovetail are in ahorizontal position, determine the amount ofthe, dovetail angles by measuring over threedifferent sizes of rolls or by measuring over a

roll after gage blocks have been inserted in backof it in a manner similar to that used to measurethe angles of a straight dovetail.

e. If the angle of the dovetail is specified onthe gage drawing in the plane normal to theaxis of the gage, instead of in a plane normal tothe sides of the gage, use the formula shown onfigure 268 to convert the angle measured asspecified in 8.23.4.6 to the corresponding anglein the plane normal to the axis.

f. While the sine plate is set up in the posi-tion to make the rolls horizontal measure thedistance K, the perpendicular distance from thetop of the upper measuring roll to the bottom ofthe right hand roll of the sine plate. Alsomeasure the distance L, the perpendicular dis-stance from the bottom of the lower measuringroll to the bottom of the left hand roll of thesine plate.

g. Substitute the measurements taken as de-scribed above in the formula shown on figure269 in order to find N, the distance between theapexes of the dovetail at the large end. Theangle B is the angle of the dovetail side asmeasured in the plane perpendicular to thesides (see sec. BB of fig. 268). The term tequals the thickness of the dovetail gage asshown on the same figure.. All other dimen-sions are shown on figure 269.

8.23.4.8 Tapered dovetail gages can also bechecked by measuring over pairs of precisionballs. This method takes longer than measure-ment over rolls, but it is a more thorough checkbecause readings can be taken at several pointsalong the precision surfaces of the gage, thusdetermining the amount of errors in thestraightness of the angle and the taper. Thegage to be inspected by this method must havean extremely smooth finish, otherwise, the ballswill settle into small depressions and give falsereadings. The taper is measured by setting the

gage in the manner shown on figure 270 and tak-ing measurements over a pair of precision ballswhich are supported by gage block stacks. Thetaper in the gage is found by comparing thedifferences in readings with the differences inheights in the same way that the taper of a plaintapered plug is computed. If the function ofthe gage is such that the centerline of the half

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Figure 270. Measuring distance between apexes of tapered dovetail-ball method.

angles of taper must be accurately square withone end of the gage, it may be necessary to checkeach half angle of taper independently by themethods specified in 8.23.4.7. This is becausethe included angle of the taper may be correctwithout having the proper relationship to theends of the gage. The angle of the dovetailsides is also found by measuring over two balls.Combinations of gage blocks are placed betweenone of the balls and the angle plate, leaving theother ball in ‘the same position for all measure-ments of one angle (see fig. 271). To measurethe angle on the other side of the gage, the gageblocks are moved to this side, keeping stationarythe ball which originally was moved. Thismakes it possible to check each angle indepen-dently of the other.

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These measurements should be taken in aline parallel with the faces of the dovetail gage,not in a line through the centers of the balls.This can be accomplished by placing the angleplate which supports the gage parallel with theanvils of the measuring instrument and clamp-ing it to the elevating table. The table binderat the base of the elevating table should beloosened in order to permit the assembly to bepositioned properly between the anvils. Thispositioning can be accomplished by pushingthe assembly against the movable anvil andby tightening the micrometer thimble until theindicator pointer shows that the proper pres-sure has been obtained against the oppositeanvil. Make sure that base of the elevatingtable is clean enough to slide freely. The size


Figure 271. Measuring angles of sides of tapered dovetail-ball method.

of the gage blocks, divided by the increase inthe measurement over the balls after the inser-tion of the blocks, will give the tangent of theangle of the dovetail side. This will be in aplane perpendicular to the axis of the dovetail(see angle B1 of fig. 268). Having measuredthe angle of the taper and the angle of the sides,it is possible to compute the distance betweenthe apexes by using the formulae shown on fig-ure 272. The answer obtained by solving thegiven equation for N will be the distance be-tween the apexes in a plane perpendicular tothe ends of the gage and at a height equal tothe height of the centerline of the balls wherethe particular measurement in question wastaken.

8.23.5 Spline gages. These gages can bechecked by placing them between the centersof a precision indexing head (see fig. 158) orbench centers with sine bar attachment. Thefollowing principal operations are performed:

8.23.5.1 Measure the outside diameter of thegage. When there is an even number of splinesthis can be done by direct measurement withhand micrometers or with a precision measur-ing instrument. When there is an odd number

of splines the gage is inserted between the cen-ters and rotated until a spline is uppermost.The height of the top of this spline is estab-lished by means of an indicator height gage andgage blocks. Then the gage is rotated throughan arc equal to the width of one spline and the

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height of the spline on the bottom of the gageis established by means of the indicator heightgage and gage blocks. The difference in theseheights will give the measurement of the out-side diameter.

8.23.5.2 The gage should be rotated at leasta half revolution with an indicator set so as totouch the top of the splines in order to makesure that the splines are concentric with thecenters of the gage.

8.23.5.3 Check the minor diameter (the di -ametral distance between the bases of thesplines). If there is an even number of splines,this can be done by placing rolls between splineson opposite sides of the gage and taking a directmeasurement. The rolls used for this purposeshould be small enough so that they will bear onthe minor diameter instead of the sides of thespline but large enough to protrude beyond themajor diameter. If there is an odd number ofsplines, or if tolerances are close, this dimensioncan be measured by means of an indicator

Figure 272. Computing distance between apexes—ballmethod of measurement.

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height gage and gage blocks in a manner similarto that used to find the outside diameter.

8.23.5.4 Check the minor diameter for con-centricity with the rest of the gage by placinga roll successively in each of the spaces betweenthe splines and noting whether readings remainthe same when the gage is rotated under theindicator with the roll held in position againstthe minor diameter and one of the spline sides.

8.23.5.5 Establish the height of the center-line. This can be done by finding the heightof a spline at the top of the gage by means of aheight gage, and subtracting one half of theoutside diameter as previously measured. Ro-tate the gage until the splines have the relation-ship to the centerline which is given on the gagedrawing. Usually the drawing will show thecenterline passing through the center of aspline. In this case the position of the gage isestablished by adding gage blocks equal to halfthe width of a spline to a stack of blocks at theheight of the centerline, and by indicating overthe blocks and the top side of one spline untilthey are the same height.

8.23.5.6 Clamp the gage in position with onespline horizontal as determined by the methoddescribed in the preceding paragraph. Thenobserve the angle of the indexing head.

8.23.5.7 Move the indicator back and forthand from side to side on the upper side of thespline to make sure that it is horizontal. Re-peat this process on the lower side of the splineand measure the width of the spline by compar-ing the readings obtained on the upper andlower sides. The width can also be measureddirectly with micrometers if there is sufficientspace and if tolerance permits.

8.23.5.8 If there is an angular tolerance onthe position of the splines, rotate the indexinghead until the next spline reaches a horizontalposition. Note the angular reading. The dif-ference between this reading and the reading ob-tained in the step specified in 8.23.5.6 will repre-sent the relative angular position of the splines.Repeat this process on each spline.

8.23.5.9 If there is not an angular toleranceon the position of the splines it must be as-sumed that each spline must be central with itsbasic centerline within the tolerance applied


on the width of the spline. To make the checkin this case, rotate the indexing head throughthe angle specified on the drawing and then in-dicate over the sides of the spline to see whetherthe distances from the basic centerline to thesides of the spline are within tolerance. (Thedistance from the centerline to each side shouldbe one half of the width of the spline and thetolerance on each individual side will be onehalf of the tolerance on the width of thespline.)

8.23.6 Spline ring gages. Spline ringgages generally are checked by clamping themon the face plate of an indexing head and usingmethods similar to those employed on the splineplug gage. However, when it is frequently nec-essary to check gages to the same drawing in agiven laboratory, it is sometimes worth while tomake up special fixtures similar to that shownon figure 273. In this figure a sample of thetype of gage being checked is shown on the right,while the fixture, with a gage placed in positionfor inspection, is shown on the left. The prin-cipal features of fixtures of this type are a pinof the same size as the bore of the gage for sup-porting the gage, a horizontal step higher thanthe centerline by the same amount as the sidesof the splines for locating the gage, and a mainsupporting body with precision ground sidesequal in number to the number of the splines forindexing the assembly to the angle specified forthe splines. The chief steps employed in in-specting a gage with a fixture of this type are asfollows :

8.23.6.1 Points of particular importance inmaking the general functional check are to makesure that the spline blades cannot be wiggledand to make sure that the dowel pins are a goodfit by tapping them lightly.

8.23.6.2 Measure the width of the splinesand the spline blades by means of a hand mi-crometer or a bench micrometer, depending onthe tolerance.

8.23.6.3 Mount the spline gage on the fixtureand rotate the gage until the height of onespline side is the same as the step on the fixture.Check by running an indicator over both sur-faces.

8.23.6.4 Check the angle between the splinesby turning the fixture over on another side andby indicating over the side of the spline whichis now horizontal. The side of the spline shouldbe parallel as well as at the proper height. Careshould be taken not to disturb the gage from itsposition on the fixture during this operation.

8.23.6.5 Rotate the fixture so that the thirdspline is horizontal and check as before.

8.23.6.6 Check the centrality of the splineswith respect to the blades by setting a planergage or a stack of gage blocks at the height ofthe spline sides. Then place a stack of gageblocks on the planer gage or on the first stackof gage blocks and indicate over the top of theblocks and sides of the blade. The height of thesecond stack of blocks must equal the differencebetween the distances of the blade side and thespline side from the centerline. Each of theblade sides is checked in succession by rotatingthe fixture to each of its three sides. Duringthis check it is important to check the bladesides for parallelism of the entire surface, aswell as proper position. It might not be con-sidered necessary to have the splines central andthe sides of the blades parallel as long as thesplines themselves are correct and the blades area good fit in the body of the gage. However, itis desirable to have the blades conform exactlyto specifications in order to make replacementpossible when the gage has become worn.

8.23.6.7 Check the lower sides of the splinesand the blades for parallelism by methodssimilar to those used for the top sides.

8.23.6.8 Determine the height of the center-line by subtracting half the width of the splinesfrom the height of the horizontal step on thefixture. Subtract from this the radius speci-fied on the drawing for the distance from thecenterline to the tops of the splines, and setthe height gage to zero at this point. Rotatethe gage until one spline is under the pin in avertical position. Indicate over the spline andnote any variation in the position of the top ofthe spline from basic. When a gage of thistype is being checked on the face plate of anindexing head, the radius to the tops of thesplines can be checked just as easily by placingeach spline in a vertical position over the cen-

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Figure 273. Checking spline ring gage.

terline. However, the spline must be placedunder the centerline when the fixture is usedbecause the spline will always bear on the pinwhen it is in the upper position.

8.23.6.9 Check the distance from the center-line to the base of each spline by methods sim-ilar to that used to determine the distance fromthe radius to the top of the spline. If the toler-ance on the position of the surface at the baseof the spline is not very close, this distance canbe determined by measuring from the top ofthe spline to the base with a scale or with amicrometer depth gage. Check the central holein the gage with check plugs or with a precisioninternal measuring device.

8.23.7 Tapered spline gages. Taperedspline gages are checked by clamping them tothe face plate of an indexing or dividing platewhich is small enough to be set up on a sineplate. The assembly is set up on an angleequal to the half angle on the taper and anindicator height gage is passed over the taperedsurfaces to see whether they are horizontal.

In other respects these gages are checked likeordinary spline gages.

8.23.8 Large gages. The particular pointto watch for in inspecting large gages is tomake sure that the gage is set up for inspectionin such a way that it will not be sprung out ofshape due to the pressure of the clamps or dueto the effect of its own weight. Trouble of thissort often can be avoided by placing blocksor small parallels between the gage and thesupporting surface at points where it isclamped, and by supporting long gages at bothends and at the center by parallels.

8.23.8.1 Another difficulty commonly foundin inspecting large gages is that these gagescannot be mounted on an angle iron. Thismakes it difficult to turn the gage through rightangles.

8.23.8.2 A typical case of this sort is the loca-tion ring gage shown on figure 274. This gageis checked by clamping it in a vertical positionand inserting check plugs in the ring bushings.The position of the gage is adjusted until the

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Figure 274. Checking Iarge locating ring gage.

corresponding check plugs on opposite sides ofthe gage are the same height. The differencesin heights of check plugs in the same verticalalinement then are measured by means of anindicator height gage, and these measurementsare compared with the chordal dimensions be-tween the centers of the rings as given in thegage drawing. The gage then is rotatedthrough 90 degrees and the chordal dimensionsrunning at right angles to the first dimensionsare checked in the same way.

8.23.8.3 The problem of rotating the gagethrough exactly 90 degrees is solved by clamp-ing a precision square on the gage as shown onfigure 274. An indicator is run along one edgeof the square and the position of the square isadjusted until this surface is horizontal. Afterthis, the square is tightened securely in posi-tion. The location ring gage then is loosenedand turned until the other surface of the square

is horizontal. The gage is then tightened intoplace in a new position which is exactly 90degrees from its former position.

8.23.8.4 In order to inspect a point gage inexcess of 80.0 inches a specially constructed Ibeam 200.0 inches long is used. The top surfaceis scraped, the beam specially heat treated, andthe edge of the top surface is also scraped atright angles to the top to permit alinement ofthe “V” blocks in which the point gage rests.Two precision angle plates are also used in con-junction with the beam as stops, permitting amaximum length of 192.0 inches to be utilized.The first step in checking a point gage of 100.0inches would be to locate one angle plate byclamping it securely to face of beam and placingtwo master length gages, one 60.0 inches longand one 40.0 inches long against the angle platenow in place. The second angle plate can belocated by inserting a 1.0 inch precision blockbetween the master length gages and the angleplate. Secure second plate. Placing the 100.0inches point gage in place and using precisionblocks, its exact length is determined by thevariation of the original 1.0 inch dimension.

8.23.9 Cams. When cams on a gage areused only for functional purposes such as hold-ing the work in position, it generally is sufficientto check their extreme angular positions andnoting the rise with a rule.

8.23.9.1 A cam with a slow rise can be meas-ured accurately by placing it on a horizontalindexing plate, as shown on figure 275. Thisface plate of indexing head must be adjustedaccurately parallel to the surface plate on whichthe height gage is resting.

8.23.9.2 The cam on the indexing head isthen centered by indicating on some precisioncylindrical surface, and adjusting the positionof the cam until the indicator reading remainsconstant. The surface of the internal diameterusually is used for this purpose.

8.23.92 The cam surface is inspected byplacing the gaging tip of an indicator over theexact center of the cam by indicating over the

top of a precision ball until the maximum heightis found. When the cam is mounted on a shaft,the ball is rested in the center at the end of theshaft. When the cam is hollow, it may be nec-essary to insert a mandrel and place the ball in

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Figure 275. Checking a cam.

the center at the end of the mandrel. the use of gages which are checked for wear.8.23.9.4 Having located the indicator at the

center of the cam, a parallel is clamped in posi-tion against a side of the height gage base.Whenever the indicator is moved, the base of theheight gage is kept snug against this parallel.By this arrangement all measurements are takenon a line through the center of the cam, eventhough the height gage is moved to and fro inmaking the measurements.

8.23.9.5 The cam profile is measured by de-termining the height of any convenient pointon the cam surface with an indicator height gageand gage blocks. The indexing plate is rotatedthrough any convenient but accurately estab-lished angular distance and the height of thecam surface is measured again. The differencein heights equals the rise for the angle in ques-tion. The indicator should be moved across theentire width of the cam surface to determinethe perpendicularity with the axis of the cam.

8.24 CHECKING GAGES FOR WEAR.The checker should be thoroughly familiar with

All general rules and practices for reinspectinggages are subject to exceptions because of con-siderations of component design and function.Gages should never be permitted to wear to apoint where the interchangeability or the effec-tiveness of the component is endangered. Ingeneral, gages which have been designed with awear allowance are permitted to wear to the sizeof the component limit. Gages which do nothave a wear allowance and are not adjustableare usually allowed to wear 5 percent of thetotal component tolerance, or 0.0001 inch,whichever is greater, beyond the componentlimit. This practice must not be followed ifthe component dimension is critical. If thiscondition exists, the gage must not be used afterit has worn beyond the component limit evenif the gage has to be discarded after only briefuse.

8.24.1 Plain plug gages. Plain “Go” pluggages are measured for wear in two differentways, depending on their use. Those which

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are used for through holes only are measured ata point of the length of the gaging memberfrom the entering end of the plug. Plugs forgeneral use and specifically those for blind holesare to be measured at a point inch from theentering end of the gage. If the plug is to beused on a specific blind hole where the diametermust be strictly within limits at a distance lessthan inch from the bottom of the hole, themeasurement must be taken, of course, at a dis-tance which will assure proper inspection at theprescribed depth.

824.1.1 The diameter of the plug, as meas-ured at the points specified in 8.24.1, should notbe less than the minimum limit of the compo-nent diameter.

824.1.2 “Not Go” plain plug gages are meas-ured for wear at a point inch from the en-tering end of the gage. The gage is permittedto wear below the gage maker’s limits by anamount equal to the gage maker’s toleranceunless this permits an unusual number of com-ponent rejections. When component tolerancesare close, front taper in a “Not Go” plug shouldnot be permitted because of the frequency withwhich the plug will become jammed in the com-ponent.

8.24.2 Plain ring gages. Plain ring gagesare also measured for wear at different points,depending upon their use. When the compo-nent can be dropped completely through thering gage, the check for wear is made at points of the width of the gage from each end.Gages which come up against a shoulder orwhich are issued for general use are measuredfor wear at points inch in from each of thefaces of the ring. If the portion of the compo-nent on which the diameter must be closely con-trolled is closer to the shoulder than inch, themeasurement for wear must be taken at a pointthat distance away from the end of the gage.

8.24.2.1 Plain “Go” ring gages should not beallowed to wear beyond the maximum compo-nent limit.

8.24.2.2 Plain “Not Go” ring gages are meas-ured for wear at points -inch from each ofthe ends of the gage. The diameter may wearbeyond the maximum gage maker’s limit by anamount equal to the gage maker’s tolerance.

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8.24.3 Thread plug gages. Generally,thread gages need be checked for wear only onthe pitch diameter because the major diameterdoes not wear as rapidly as the pitch diameter.The pitch diameter generally is beyond thelimits if the thread angle or lead has wornsufficiently to be beyond their limits.

8.24.3.1 To measure a thread plug gage forwear, place a thread measuring wire of theproper size between the thread flanks at apoint two full thread turns from the enteringend. The other two wires are placed on theother side of the plug, one a half turn aboveand the other a half turn below the single wirein the usual fashion.

8.24.3.2 The pitch diameter of the “Go” gageis permitted to wear undersize by an amountequal to 5 percent of the pitch diameter toler-ance prescribed for the component threads. Ifthe component tolerance is less than 0.002 inch,the pitch diameter of the thread plug gage ispermitted to wear below the minimum gagemaker’s limit by 0.0001 inch.

8.24.3.3 “Not Go” thread plugs may wearbelow the minimum gage maker’s limit by anamount equal to the gage maker’s tolerance.

8.24.4 Thread ring gages. Since the flanksof a thread ring gage wear more rapidly nearthe pitch line than near the major diameter, aplug with truncated thread crests should beused as a means of inspecting for wear. Themajor diameter should be about halfway be-tween the pitch diameter and the major di-ameter of the ordinary setting plug. A fullform setting plug will bear on the thread flanksof a worn ring near the major diameter indi-cating a good fit even though the thread ringis badly worn near the pitch line. The truncatedplug, on the other hand, will fit loosely in thering if the ring has been badly worn near thepitch line.

8.24.4.1 The truncated plug is used by enter-ing it in the thread ring two full thread turnson each side of the ring. If there is any ap-preciable shake on either side of the ring, thering is no longer satisfactory. If the ring gageis adjustable, it should be readjusted to a goodfit on a full form set plug before the truncatedplug is used.

8.24.4.2 If a truncated plug is not available,the wear of the thread ring can be estimatedby making a cast of the threads (detailed in-structions on methods of making casts will befound in 8.12.1). The cast is placed in a con-tour projector and the image of the most badlyworn portion of the thread flanks alined withthe “V” formed by the 30-degree lines on theprojection screen. The distance between theworn and unworn portions of the flank shouldnot exceed three-quarters of the gage maker’stolerance on the pitch diameter if the threadring is to be considered satisfactory for furtheruse.

8.24.5 Thread concentricity gages. Theplain diameters of these gages are measured ata point inch from the beginning of the plainsection and the pitch diameters of the threadsare measured two full threads from the begin-ning of the threaded section.

8.24.6 Snap gages. The flatness and paral-lelism of the anvils of snap gages should besuch that the perpendicular distance betweenthe anvils at any point on their surfaces isneither greater than nor less than the limitsof the gage maker’s tolerance.

8.24.6.1 The flatness of the anvils can bechecked by means of an optical flat (for meth-ods see 8.5.4). If moderate errors in flatnessare permissible this check can be made with aknife-edge straightedge or a test indicator.Parallelism can be checked by placing gageblocks or precision rolls in various positionsbetween the anvils.

8.24.7 Indicator gages. Check the dial in-dicator itself in accordance with the instruc-tions given in 8.23.1. If conditions are suchthat a thorough check is not considered neces-sary, test the indicator for sticking and cali-brate a few points by placing different combina-tions of gage blocks in the gaging position andnoting whether or not the indicator readingscorrespond to the size of the blocks. Loosenthe screw which holds the indicator and shift itsposition slightly to prevent excessive wear inone section of the indicator gear rack. In


rechecking an indicator gage, be sure that wearhas not occurred in such a way that a discrep-ancy exists between the dimension for whichthe gage is set and the dimension which thegage actually measures. This generally occurswhen the indicator is set with gage blocksinstead of setting gages of the same form as thecomponent. For example, a small componentmay wear a hollow in the large anvil of an indi-cator gage. When gage blocks are used forsetting, they may bridge the hollow, whereasthe component rests in the hollow, thus permit-ting the acceptance of oversize components andthe rejection of satisfactory components whichappear to be near the minimum limit. Thissituation arises when the gaging members arerounded and gage blocks are used for setting.If a flat is worn on the radius of the gagingmembers the indicator will be set to this flat,whereas the component may rest on the radius,thereby causing a false reading.

8.24.8 Flush pin gages. Flush pins shouldmove freely at all times. If they do not, takethe gage apart and clean it. In general, thegage is satisfactory if the total shake of the flushpin is not more than 0.0005 inch. Under ordi-nary circumstances, the end of the flush pinwhich contacts the component is permitted towear until the feeler surface of the pin is0.0002 inch lower than the point originally spec-ified by the gage maker’s limits. In checkingthe position of the pin by means of gage blocksit should be noted whether the setting obtainedin this way coincides with the true meas-urement or whether worn spots cause a falsereading similar to that previously described forindicator gages.

8.24.9 Fixture gages. All moving partsshould move freely but without shake or playsufficient to interfere with the proper function-ing of the gage, and all supporting membersare to be checked for misalinement due to wear.All gaging members are to be checked to seewhether their form is still sufficiently accurateto permit proper gaging. Flush pins and indi-caters must conform to the requirements pre-viously described for these gages.

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8.25 CALIBRATING THREAD MEAS-URING WIRES. It is essential that allthread measuring wires be calibrated by the lab-oratory or by the Bureau of Standards beforetheir initial use. They should also be recali-brated frequently because thread wires makevirtually point contact with threads duringmeasurement and their positioning is subjectto a wedging action. Therefore, slight irregu-larities on the surfaces of the wires can easilycause errors which exceed the desired accuracyof measurement.

8.25.1 Thread measuring wires should pref-erably be calibrated on a comparator or meas-uring machine which reads directly to 0.00001inch, and which can be adjusted to measure witheither 1- or 2½-pounds pressure. The pressureselected for any given wire should be the sameas the pressure under which the wire is to beused.

8.25.2 The diameter of a thread measuringwire is measure by placing the wire on a

precision lapped, hardened steel plug 0.750 inchin diameter between the anvils of the measuringmachine. Measurements are taken with oneside of the wire bearing on the anvil and theother bearing on the plug. A set of three wiresshould have the same diameter within 0.00002inch, and this common diameter should bewithin 0.0001 inch of that corresponding to thebest size wire for the thread on which the wireis to be used. Each wire should be measured atseveral positions around the circumference ofthe wire over a ½ inch length near the center ofthe wire to be sure that the diameter is withinthe limits over that entire portion.

8.25.3 The roundness of the wires must alsobe checked by resting each wire in a “V” grooveof a round object of hardened and lapped steel.When the wire is rotated and measured at vari-ous points about its circumference, the varia-tions in the measurements over the wires shouldnot exceed 0.00002 inch. Special plugs with aflat on one side and thread grooves of variousdepths on the other are available for this pur-pose. However, if such a plug is not on hand, ahardened and lapped thread plug gage willserve, provided the thread flanks are known tobe accurately straight.

8.25.4 The wires should also be checked forstraightness by rolling them along a toolmaker’s flat and noting whether any light passesbetween the wire and the flat. A slight amountof bend in wires for fine pitches, say 32 pitchand finer, is not objectionable because the meas-uring pressure will straighten them if the con-cave side is placed against the anvil of a benchmicrometer. However, there should be no bendwhatsoever in wires for the coarse pitches.Kinks or sharp bends should not be present infine pitch wires even though these imperfectionsmay appear to be very slight when inspectedvisually.

8.26 CALIBRATING GAGE BLOCKS.It is difficult to determine the length of timegage blocks should be used before recalibrationbecause some sets of gage blocks may be usedmuch more than others. Each laboratoryshould occasionally check its own gage blocksroughly in order to decide whether they shouldbe returned for calibration. Precision gageblocks should be inspected at least every 6months, even when they are not used.

8.26.1 The following suggestions will behelpful in reinspecting used gage blocks, or inchecking new blocks:

a. Clean the blocks by washing in Stoddardsolvent ( Varsol ). Avoid so far as possible theuse of benzol or carbon tetrachloride becauseof their toxicity.

b. Stone the gaging surfaces of the blockswith a hard Arkansas stone until all burrs havebeen removed, and then wipe clean with a cha-mois or a cloth which leaves no lint. Speciallarge flat stones are available commercially forthe removal of burrs. Blocks can be checkedfor presence of burrs with an optical flat or bygently attempting to wring them to anotherblock.

c. Check for flatness by placing an opticalflat on the gaging surfaces and noting the curva-ture of the interference bands. The displace-ment of a band from the point of greatest cur-vature to a point inch in from the edge ofthe blocks, in terms of band widths, multipliedby 11.6, is the error in flatness in millionths ofan inch. (See 8.5.4 for full details. ) This testshould be made both along and across eachblock.

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d. Check for parallelism by wringing theblock and a master block side by side on anoptical flat. Place a second optical flat on topof the two blocks and adjust its position so thatthe bands on the master block have no error inparallelism across its width. Compensationfor any error in parallelism of the master blockis made by appropriate orientation of itsfringes. If for example the master block shownon figure 276 had an error in parallelism of0.000005 inch its fringe pattern would have tobe rotated 0.4 of a band. The direction of rota-tion depends on the location of the contact be-tween the top flat and the blocks. If it is at thebottom as shown on figure 276 the bands have tobe rotated clockwise. (See fig. 276 for an actualphotograph of this set-up. ) The bands on themaster block are brought into the desired posi-tion by pressing on the upper optical flat withthe fingers and by shifting its position horizon-tally until it is balanced at a point which pro-duces the desired interference pattern. Theangle of the interference bands on the blockbeing inspected then indicates the error in theparallelism of this block. On figure 276 theblock being inspected is the one on the right.Since the interference bands on this block arehorizontal within approximately one third ofthe band width, it can be assumed that thefaces are parallel within about 4 millionths ofan inch. Assuming that the upper flat is bear-

Figure 276. Set-up for checking parallelism of gage blocks.

Figure 277. Horizontal check for gage block parallelism.

ing on the front edges of the blocks the patternindicates that the left side of the block beinginspected is lower than the right side. On figure277, the length of the block being inspected isabout 6 millionths of an inch less at the outsideedge than at the edge near the master block(assuming that the optical flat is bearing on thefront edges of the blocks). This test should berepeated with the optical flat placed so as tomake the interference bands accurately verticalon the master block. On figure 278, the lengthof the block being inspected is about 12 mil-lionths of an inch less near the lower edge thanit is near the top (assuming that the flat is bear-ing on the right-hand edges of the blocks) andno error in parallelism exists on the masterblock.

e. Check the length of the blocks in a com-parator to an accuracy of 0.000005 inch orbetter.

Figure 278. Vertical check for gage block parallelism.

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f. Since considerable experience is requiredto use optical flats properly for precision meas-urement of length, and since a certain amountof error is necessarily introduced when anoptical flat is used without an interferometer,it is best to check the length of gage blockswith a high magnification comparator. How-ever, if an instrument of this type is not avail-able, or if the inspector desires to experimentwith optical flats, the following method shouldbe used:

Wring the block being inspected and amaster block of the same nominal length endto end on an optical flat, as shown on figure 279.Place another flat on top of the blocks andadjust its position until a minimum number ofvertical interference bands is produced. Thisupper flat should be placed symmetrically withrespect to the blocks, so that it has no tendencyto tilt. Heavy pressure should not be used inthe adjustment.

g. Place set-up out near the edge of the mon-ochomatic light source and view the inter-ference bands from a position as nearly ver-tical as possible. Count the number of darkbands on the master block. Fractions of aband (the distance between the centers of thedark bands) at the end should be included inthe count. On figure 279 the band count isapproximately 2–½. If the flat is bearing onthe left edge of the blocks, the block being in-spected is between 23 and 35 millionths of aninch lower than the master block and if it isbearing at the right end the block is the shineamount higher. Having determined the ap-

Figure 279. Rough check of difference in Iength of twogage blocks.

Figure 280. Check of difference in length of two gage blocks.

proximate difference between the lengths ofthe blocks by means of a precision measuringinstrument or as described above, a more exactcomparison can be made with the set-up shownon figure 280. The two blocks are wrung onan optical flat side by side with the markinguppermost, and a second flat is again placed ontop of them. The upper flat is tilted enoughto give a few horizontal interference bands asif flatness were being checked. Since we pre-viously determined that there is between twoand three bands difference between the twoblocks, we know that any given band on theblock being inspected is displaced between twoand three band widths from the band on themaster block of the same order of interference.If we select a band through the center of theunknown block the corresponding band on themaster block is between two and three bandsaway in a direction determined by the loca-tion of the point where the top flat bearson the blocks. On figure 280 the point ofcontact is at the bottom and the band onthe master block corresponding to one on theunknown block is between two and three bandsabove it. The exact difference in length is de-termined by counting the whole and fractionalbands between the corresponding bands andmultiplying by 11.6 millionths of an inch. Inthe figure the difference is 2.3 bands or 27 mil-lionths of an inch. Since it is the practice ofthe Ordnance Corps to measure the length ofblocks near the center, the fringe count shouldbe made between a band at this point on theunknown block and the corresponding fringeon the master block.

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8.26.2 “A” precision gage blocks.8.26.2.1 Reference point. The measured

length is in each case the perpendicular distancefrom one gaging surface to a point on the othergaging surface near the middle of and about inch from the edge to the right of or near-est to the size marking.

8.26.2.2 Length. The tolerance on length is±0.000004 inch for blocks up to 1 inch in lengthand ±0.000004 inch per inch of length forlonger blocks. On blocks within ±0.000004inch of the nominal size, or ±0.000004 inch perinch, observational errors of ±0.000004 inchon blocks up to 1 inch in length, and of±0.000004 inch per inch of lengths on longerblocks are possible in the measurements oflength. On blocks differing from the nominalsize by more than the above amounts, additionalmeasurements are made and the observationalerror is ±0.000003 inch. For blocks havingflatness errors exceeding 0.000010 inch, the ob-servational error is indefinite. These possibleobservational errors are taken into account indetermining acceptability of blocks. Gageblocks should be accepted as of “A” quality ifdeviation from nominal length is within the fol-lowing limits:

1 inch or less ----------------- ±0.000007 over 1 to 2 inches, inch ------- ±0.000014 over 2 to 3 inches, inch ---------±0.000020 over 3 to 4 inches, inch --------±0.000025

8.26.2.3 Parallelism. The error in paral-lelism indicates the correction to be applied tothe measured length to obtain the length at apoint about inch in from the opposite edge.The error in parallelism is not reported if itdoes not exceed ±0.000002 inch. Gageblocks should be accepted as of “A” quality ifthe error in parallelism does not exceed±0.000008 inch, or if the algebraic sum of thelength and parallelism errors does not exceedthe above limits for deviation from nominallength.

8.26.2.4 Flatness. The errors in flatnessalong the length and across the width are meas-ured along lines midway between the edges, butdo not include the inch of surface nearestthe edges. Flatness errors indicated by the


letter “a” are measured along a line inchfrom an edge with respect to the centerline.The flatness of gages less than 0.100 inch inlength is measured when the gages are wrungdown on an optical flat. The errors in flatnessare not reported if they do not exceed0.000002 inch across the width and 0.000003inch along the length. Gage blocks should beaccepted as of “A” quality if the flatness error

does not exceed 0.000006 inch across the width or0.000008 inch along the length.

8.26.3 “B” precision gage blocks, or usedgage blocks.

8.26.3.1 Reference point. The measuredlength is in each case the perpendicular distancefrom one gaging surface to a point on the othergaging surface near the middle of and about inch from the edge to the right of or nearestto the size marking.

8.26.3.2 Length. The tolerance on lengthis ±0.000008 inch for blocks up to 1 inch inlength, and ±0.000008 inch per inch of lengthfor longer blocks. On blocks within ±0.000008inch of the nominal size, or ±0.000008 inchper inch, observational errors of ±0.000005inch on blocks up to 1 inch in length, and of±0.000005 inch per inch of length on longerblocks are possible in the measurements oflength. On blocks differing from the nominalsize by more than the above amounts, addi-tional measurements are made and the observa-tional error is ±0.000003 inch. For blockshaving flatness errors exceeding 0.000010 inch,the observational error is indefinite. Thesepossible observational errors are taken into ac-count in determining acceptability of blocks.Gage blocks should be accepted as of “B”quality if deviation from nominal length iswithin the following limits:

1 inch or less ---------------- ± 0.000011Over 1 to 2 inches, inch ------- ± 0.000020Over 2 to 3 inches, inch ------- ± 0.000028Over 3 to 4 inches, inch ------- ± 0.000035

8.26.3.3 Parallelism. The error in paral-lelism indicates the correction to be applied tothe measured length to obtain the length at apoint about inch in from the opposite edge.The error in parallelism is not reported if itdoes not exceed ±0.000005 inch. Gage blocks

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should be accepted as of “B” quality if theerror in parallelism does not exceed ±0.000010inch, or if the algebraic sum of the length andparallelism errors does not exceed the abovelimits for deviation from nominal length.

8.26.3.4 Flatness. The errors in flatnessalong the length and across the width aremeasured along lines midway between theedges, but do not include the inch of surfacenearest the edges. Flatness errors indicated bythe letter “a” are measured along a lineinch from an edge with respect to the centerline.The flatness of gages less than 0.100 inch inlength is measured when the gages are wrungdown on an optical flat. The errors in flatnessare not reported if they do not exceed 0.000005inch. Gage blocks should be accepted as of “B”quality if the flatness error does not exceed0.000008 inch across the width or 0.000010 inchalong the length.

8.27 HELIX ANGLES8.27.1 American National coarse and fine

thread series are shown in tables II and III,respectively.

TABLE II. AMERICAN NATIONAL COARSEThread Series

8.27.2 Chart for determining helix angles.A chart showing graphically the relationshipsof diameters, leads and helix angles of screwthreads is shown on figure 281. If the values

TABLE III. AMERICAN NATIONAL FINE THREADSERIES

of any two of these dimensions are known, thevalue of the third can be found by means of thechart. The horizontal lines represent the pitchdiameter in inches. The values of these linesmay be read in the columns either at the extremeleft or the extreme right of the chart. Thevertical lines represent the lead. The valuesmay be read at the top of the chart if the leadis known in inches, or at the bottom of the chartif the number of thread turns per inch isknown. The diagonal lines represent the helixangles. The values of these lines can be deter-mined from the columns of figures at the left,at the center, or at the right of the chart.

Example: To determine the helix angle of al/2-20NF–2 screw thread: The basic pitch di-ameter is 0.4675 inch. Find the point repre-senting the value 0.4675 on the diameter scaleat the left of the chart and follow a horizontalline to the right of this point until the verticalline representing 20 thread turns per inch isfound. From this intersection follow a lineparallel to the diagonals until the scale repre-senting the values of the helix angle is inter-sected. The helix angle line directly below theplotted line represents a value of 2 degrees Ominutes. Therefore, the actual value of thehelix angle in question is found by interpola-tion to be 1 degree 57 minutes.

8.27.3 Thread flank angle correction factorsare shown in tables IV and V.

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TABLE IV. 60 DEGREE INCLUDED THREAD ANGLE


TABLE V. 29 DEGREE INCLUDED THREAD ANGLE

922173 O - 51 - 14195


Figure 281. Chart for determining the helix angles of screw threads.

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TABLE VI. THREAD LEAD CHECKING

TABLE VII. EQUIVALENT HARDNESS NUMBERS 8.27.4 Thread lead checking is shown intable VI.

8.27.5 Equivalent hardness numbers areshown in table VII.

8.27.6 Key to table of equivalent hardnessnumbers

1. “Rockwell” hardness tester, C scale,“Brale” penetrator—150 kg. load.

2. “Rockwell” superficial, 15–N scale, “NBrale” penetrator—15 kg. load.

3. “Rockwell” superficial, 30-N scale, “NBrale” penetrator—30 kg. load.

4. “Rockwell” superficial, 45–N scale, “NBrale” penetrator—45 kg. load.

Copies of this Military Standard may beobtained by directing requests as follows:

For Department of the Army agencies, toChief of Ordnance Corps or as directed by thatactivity.

For Department of the Army contractors, tothe Contracting Officer.

For Department of the Navy activities, to the

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- - -


Commanding officer, Naval Supply Center,Norfolk 11, Va.

For Department of the Navy contractors, tothe Bureau of Supplies and Accounts, Wash-ington 25, D. C.

For the Marine Corps, to the QuartermasterGeneral, Headquarters, U. S. Marine Corps,Washington 25, D. C.

For the Department of the Air Force activi-ties and Air Force contractors, to the Com-manding General, Air Materiel Command,Wright-Patterson Air Force Base, Dayton,O h i o .

Copies of this Military Standard maybe ob-

tained for other than official use by individuals,firms, and contractors from the Superintendentof Documents, U. S. Government Printing Of-fice, Washington 25, D. C.

Both the title and the identifying symbolnumber should be stipulated when requestingcopies of Military Standards.

Custodians:Army—Ordnance Corps Navy—Bureau of Ordnance Air Force

Other interest: All other Army technical services and Navy

bureaus.

198

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