mechanical instruments for measurement

7/21/2019 MECHANICAL INSTRUMENTS FOR MEASUREMENT

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1.( )irect Length Meaure!ent *ith Mechanical Appliance

1.(.1 Uni+eral ,ernier Caliper -age

,ernier calipergage is an instrument used in making measurements of length dimensions

on physical ob'ects. The main elements in its construction are shown in (ig.&.&:

(ig.&.& )niversal vernier caliper gage with knife !'aws" edges for internal measurements and tongue !stem" for depth

measurements.

In (ig.&.*, the vernier scale !main scale" is divided into &+ e%ual divisions and thus the leat

count of the instrument is .1mm. /oth the !ain scale and the +ernier scale readings are taken

into account while making a measurement. The !ainscale reading is the #irtreading on the main

scale i!!ediatel" to the le#t of the 0ero of the vernier scale ! mm", while the +ernier scale

reading is the mark on the vernier scale which exactly coincidewith a !ar on the main scale !+.-

mm". The reading is therefore .- mm.

(ig.&.* The reading here is .- mm.

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The sliding 'aw indicationcan be on a +ernier !(ig.&.a", on a round dial !(ig.&.b", or

with digital readouts !(ig.&.c". ith digital readouts is eaierto read and le sub'ect to human

error than reading verniers.

a" b" c"

(ig.&. #liding 'aw indication: a" on a vernier/ b" on a round dial/ c" with digital readouts.

0ccording to the highetlimit of the length dimension that can be measured, #T0# &+-1

23 establishes as follows limit dimensions: &4+, *++, ++, 4++, 2++, &+++, &4++, *+++ mm.

The clai#ication of the vernier caliper gages is made according to the next criteria:

a2 destination, where can be for external and internal measurements, for depth measurements,

for gear tooth and for grooves/

$2 vernier division value, which are with a resolution of +,& mm, +,+4 mm and +,+* mm.

1.(.( Caliper height gage

The$aicdesign principles of vernier caliper gages are applied in caliper height gageas

well. The primary use of caliper height gages is in the field of surfaceplatework as a layout tool,

for marking +ertical distances and for measuring height di##erence between tep at various

levels.

In distinction to the caliper gages, the vernier height gages !(ig.&.3,", have a ingle'aw only,

because the surface plateon which the instrument base rests is functioning as the re#erenceplane.

The vernier height gages are made with *idebases and with bars of trongcross section which

carry the same kind of graduations found on the beams of vernier calipers.

These gages usually have offset 'aws whose contact surfaces can be brought to coincide with

the reference plane when the slide position indicates $ero height
http://www.tresnainstrument.com/product/ec05.htmlhttp://www.tresnainstrument.com/measuring_instrument_images/vernier_calipers/d/vernier_calipers_vc01.jpg


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a" b" c"

(ig.&.3 5aliper height gages: a" 6ernier hight gage/ b" 7ial hight gage/ c" 7igital height gage.

(or !aring purposes, cri$er can be attached to the contact 'aw. These scribers are

designed to have the edge substantially at e%ual level with the contact surface of 'aw, in order to

make the height of the scribed line coincident with the dimension indicated by the gage.

1.(.3 -ear Tooth ,ernier Caliper

0 further application of the vernier caliper principle is found in the gear tooth caliper

!#T0# &-841-". These measuring tools are used to check the pitchlinethicneof gearteeth

by measuring the tooth chordat specific ditance!chordal addendum" from the top of the gear

tooth. The gage consists of t*o independently actuated vernier calipers, each having its own

movable slide, but the beams and the stationary 'aws are made of a common single piece. One of

the slides has the form of a plate, called the tongueof the instrument, which contacts the topof the

gear tooth. 9y moving this slide, the gage can be ad'usted to operate at the desired addendum

distance. The econd slide, integral with the movable 'aw, carries out the actual chordal thickness

measurement at the pitch line.

a" b"

(ig.&.4 a" 6ernier gear1tooth caliper with two vernier scales/ b" Measuring the thickness of gear teeth.

1.(.3 ,ernier )epth -auge

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perating on the principles of vernier caliper gages, +ernier depth gage!(ig.&.;" differ in

the design application of the basic concept, resulting in the reversal of the usual process:

a"b"

(ig.&.; a" 6ernier depth gauge/ b" 7ial depth gauge.

&. The lide is connected with the cross beam of the instrument which, by contacting a

reference plane on the ob'ect surface, establishes the datu! of the measurement/

*. The end #aceof the beam takes over the function of the movable 'aw, in contacting the

ob'ect element whose distance from the datum is to be measured.

The use of the vernier depth gage is not limited to actual depth measurements, although these

constitute the ma'or application for the instrument. )itancewhich are referenced from, and are

nor!al to a flat surface of the ob'ect, can also be measured conveniently with the aid of vernier

depth gages.

1.(.4 Micro!eter

The eential element of measuring instruments operating on the micrometer principle is a

cre* with preciel" controlled lead, having a pitch of usually & 4 mm. The screw of themicrometer is integral with the measuring pindle&whose face establishes the measuring contact

with the ob'ect. The distance of that contact face from a fixed datum constitutes the measuring

length, which is then displayed by the scale graduations of the micrometer.

The !icro!eter !(ig.&.


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(ig.&.- 0 micrometer screw gauge.

The#irtsignificant #igure is taken from the lat graduation showing on the lee+edirectly to

the le#t of the re+ol+ing thimble. =ote that an additional half cale division !+.4 mm" must be

included if the mark $elo* the main scale is +ii$le between the thi!$le and the !ain scale

division on the lee+e.The remaining t*o significant figures !hundredths of a millimeter" are taken

directl"from the thi!$le opposite the main scale.

a" b"

(ig.&.2 a" The reading is -.2 mm/ b" The reading is -.-* mm.

In (ig.&.2a the lat graduation visible to the le#t of the thi!$le is - mm and the thi!$le

lines upwith the !ain scale at 2 hundredths of a millimeter !+.2 mm"/ therefore the reading is

-.2 mm. In (ig.&.2b the lat graduation visible to the le#t of the thi!$le is -.4 mm/ therefore the

reading is -.4 mm plus the thimble reading of +.** mm, giving -.-* mm.

1.(.6 Micro!eter #or E5ternal Meaure!ent

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The !ot common application for micrometers is the measurement of length dimensions

between t*oparallel end surfaces on theouterside of an o$7ector feature. This is currently known

as e5ternal or outidemeasurement!(ig.&.


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Disc micrometermeasuresforming tools,cutting edges,

narrowslots.

The disc micrometer is also called aflange micrometer.

This micrometer finds its application inmeasuring flanges and hard to reach areas.

The 60 degreecomparatormicrometer isusually called ascrewthreadcomparator

micrometer orpitchmicrometer.

Measuring the pitch diameter of a screwthread.

The pitch diameter, the most importantmeasurement of a screw thread.

1.(.8 Micro!eter #or Inide Meaure!ent

The micrometer type instruments have many useful applications for inide measurements as

well. These comprise the !eaure!ent of an ob'ect feature $ounded by *all having parallel

elements in the gaging plane, such as the dia!eterof a cylindrical$oreor the*idthof a parallel

> sided groo+e.

Internalmeasurements, particularly of $ore diameters, are affected by more +aria$le than

the measurement of length between outside surfaces. S!allbore si$es pre+ent the introduction of

inide micrometers. The depth of the $ore&when its diameter must be measured at a greater

distance from theopenend, cancauedifficulties for micrometer applications.

(ig.&.&+ Inside micrometer set with interchangeable

measuring rods and handle for inserting the instrumentinto deeper holes.

The inide !icro!eter consists of a

headpart comprising the micrometer cre*&and

of independent measuringrod !(ig.&.&+". The

rod are !ade in different length, in tepof

(4mm, can be ae!$led with the headpart by

means of a threaded connection and are

accurately positioned on a shoulder.

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The !allet bore which can be measured with this type of micrometer is 4 mm in

dia!eter&and the !a5i!u! diameter depends on the available rods, a practical upper limit being

about 9 mm. 0 handle can be attached to the head to permit measurement at greaterdepths

inside a bore.

(ig.&.&& #mall hole gage for transfer of actual bore

dimension which is then mesurable with a regular

outside micrometer.

/ore dia!eter !eaure!ent $" tran#er.

S!all bores !?+,4 > ?&* mm" cannot be

measured directly because of pace limitations.

It is possible, however, to tran#er the inside

dimension to be measured by uing an

appropriate !ean which will represent the

re+ere replica of the inside length. The

resulting physical outide length can then bemeasured with a standard outidemicrometer.

The !all hole gage!(ig.&.&&", consisting of a plit $allwhich can be e5pandedto the i0e

of the dia!eterof the$oreto be measured. Rotating the knurled knob of the handle ad+ancethe

inide screw whose conical end causes the ball hal+e to eparate to the re'uired extend. The

actual pread of the balls is!all, in the order of3!!&and therefore these gages are upplied in

et encompassing a *ider range of dimensions.

The telecoping gage!(ig.&.&*" has a tu$ularmember to which a handle is attached at

right angle position.

(ig.&.&* The use of a telescoping gage in taking

the si$e of a bore for ultimate measuring by an

outside micrometer.

The tu$ular member has either one or t*o

plunger, which are under pring pressure and

telecope into the #i5ed tube. The #ree ends of the

plungers or, in the case of a single plunger, the closed

end of the fixed member too, have pherical forms andare hardened to serve as contact elements. hen

introduced into the hole to be mesured, the previously

retracted plungers are releaed to extend to a length

e'ual to the dia!eterof the ob'ect.

#ubse%uently, the thus #i5ed length of the

telescoping rode can be !eaured with a regular

outide micrometer.


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(ig.&.&& Three1point contact, internal micrometer with

self1centering properties. !@eft" general view. !Aight"

Bartial section showing the mechanism for the

controlled expansion of the contact plungers.

Three % point contact internal !icro!eter.

#everal of the di##icultie connected with bore

diameter mesurements by a micrometer can be

reduced when using the three:point internal

micrometer, shown in (ig.&.&&. The el# %

aligning property of this instrument is

particularly useful when measuring deepbores,

for which purpose an e5tenion part can be

attachedto the $aictool.

Ad+ancing the cone along its axis pread the engaged measuring points radiall",resulting in a larger envelope circle&while the cone movement in the inverse sense causes the

spring loaded contact members to retract. The cone is attachedto a pindlewhose a5ialposition is

ho*n on the !icro!eter sleeve and thi!$le.

The three1point internal micrometers are a+aila$le in i0e from 6 to 3 !! bore

diameters. It is advisable to recali$rate these instruments periodicall", using a standard ringgage.

)sually these micrometers are procured in et, to cover a more extended range of measurable bore

diameters.

1.(.9 Special Micro!eter Intru!ent

The following examples of measuring instruments, based on the application of the

micrometer screw as the measuring member, should illustrate the wide uses of the micrometer

principle.

(ig.&.&* Indicator micrometer with ad'ustable

tolerance marks.

Indicator !icro!eter !(ig.&.&*". The narro*

range indicator, whose mechanism is coupled with

the !o+a$le anvil of the !icro!eter frame, has

graduation in &( or &1 !! increments. The

!alletgraduation of the thi!$leis in thouandth

and the lee+e can have or notvernier graduation.

hen is making measurements with the instrument without vernier the !icro!eter screw is

advanced to the thouandth mark nearet to the #inal si$e as ignaledby the !o+e!ent of the

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indicator pointer from its rest position. 0t this point the !eured si$e is readby combining the

i0e shown on the thi!$leand the poitionof theindicatorpointer.

The built1in indicator improves the repeat accuracy of the micrometer measurements and

adds to the versatility of the instrument applications since:

a" The contant measuring force, as checedby the pointer movement, is particularly valuable

for co!parati+e and repetiti+e mesurements/

b" Meauringerrors caused by !itae in e+aluating the +ernierpositing are eliminated/

c" The micrometer can be ued as an ad'ustable napgagewith added indicating potential,

when the range of si$e variations does not exceed the measuring spread of the indicator. The

ad7uta$le indexes on the indicator sector can be etto the li!it si$es of the o$7ect/

)epth !icro!eter are used to measure the ditance of an ob'ect #eaturefrom a #lat referencesurface !(ig.&.&".

(ig.&.& 7epth micrometer for measuring the

distance of protruding features from a flat reference

surface.

Cxamples of application are the mesurement of

tep&the depth of flanges or the$otto!surface in a

$ore, and the heightof an ob'ect feature in relation to

a re#erence surface, when accesibility permits it to be

i!ultaneoul" contacted by the $ae member and

the pindle face.

/ench !icro!eter !(ig.&.&3" can substantially improve the precision of micrometer

mesurements, particularly when the ob'ects to be measured are !all.

(ig.&.&3 9ench micrometer for the sensitive

measuremenof small parts.

The ta$leposition of the instrument during the

!eure!ent, permits a more precie locating of the

work and the hea+"base adds to the rigidit" e%uipped

with largediameterpindleand thi!$le, permitting a

finer pitch thread for higher eniti+it", and direct

reading in ten:thouandth, or smaller.

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Micro!eter head without a #ra!e as an integral part, have wide applications in the

instrument $uildingfield. ne or the more#re'uentuses is the controlleddisplacement of lide,

either ingle or cross lide!(ig.&.&4".

(ig.&.&4 Micrometer heads as independent units and

mounted on an instrument slide.

5ommonly, the micrometer will !o+e

the lide against a !oderate spring preure,

which is applied to aure a positive contact

point on the lide face. Spring exerting a

uni#or! force along the entire slide traverse

are preferred.

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mechanical instruments for measurement

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