this sample chapter is for review purposes only

Without some form of accurate measurement,modern industry could not exist. The science thatdeals with systems of measurement is called metrology. Today, industry can make measurementsaccurate to one microinch (one-millionth of an inch).

If a microinch were as thick as a dime, one inchwould be as high as four Empire State Buildings(about 5000′ total). An engineer once estimated,with tongue in cheek, that a steel railroad rail sup-ported at both ends would sag one-millionth of aninch when a “fat horsefly” landed on it in the middle.

In addition to using US Conventional units ofmeasure (inch, foot, etc.), industry is gradually converting to metric units of measure (millimeter,

meter, etc.), called the International System ofUnits (abbreviated SI). A micrometer is one-millionth of a meter (0.000001 m).

All of the familiar measuring tools are availablewith scales graduated in metric units, Figure 4-1. AnSI Metric (millimeter) rule is compared with con-ventional fractional and decimal rules in Figure 4-2.Metric-based measuring tools should offer no prob-lems for the user. As a matter of fact, they are ofteneasier to read than inch-based measuring tools.

Although you will measure in very tiny unitswhen you go to work in industry, you must firstlearn to read a rule to 1/64″ and 0.5 mm. Then, you can progress through 1/1000″ (0.001″) and 1/100 mm (0.01 mm) by learning to use micrometerand Vernier-type measuring tools. Finally, you can progress to 1/10,000″ (0.0001″) and 1/500 mm (0.002 mm) by using the Vernier scale on somemicrometers.

55

Chapter 4

Measurement

LEARNING OBJECTIVESAfter studying this chapter, you will be able to:� Measure to 1/64″ (0.5 mm) with a steel rule.� Measure to 0.0001″ (0.002 mm) using a

Vernier micrometer caliper.� Measure to 0.001″ (0.02 mm) using Vernier

measuring tools.� Measure angles to 0°5′ using a universal

Vernier bevel.� Identify and use various types of gages

found in a machine shop.� Use a dial indicator.� Employ the various helper measuring tools

found in a machine shop.

IMPORTANT TERMSdial indicatorsgage blocksgaginggraduationshelper measuring toolsInternational System of

Units

metrologymicrometer calipersteel ruleVernier caliper

Figure 4-1. This rule can be used to make measurements inboth US Conventional and SI Metric units.

4.1 THE RULEThe steel rule, often incorrectly called a scale, is

the simplest of the measuring tools found in theshop. Figure 4-2 shows the three basic types of rulegraduations. A few of the many rule styles areshown in Figure 4-3.

Machining Fundamentals56

4.1.1 Reading the Rule (US Conventional)A careful study of the enlarged rule section will

show the different fractional divisions of the inchfrom 1/8 to 1/64, Figure 4-4. The lines representingthe divisions are called graduations. On many rules,every fourth graduation is numbered on the 1/32edge, and every eighth graduation on the 1/64edge.

To become familiar with the rule, begin by mea-suring objects on the 1/8 and 1/16 scales. Once youbecome comfortable with these scales, begin usingthe 1/32 and 1/64 scales. Practice until you canquickly and accurately read measurements. Somerules are graduated in 10ths, 20ths, 50ths, and100ths. Additional practice will be necessary to readthese rules.

Fractional measurements are always reduced tothe lowest terms. A measurement of 14/16″ isreduced to 7/8″, 2/8″ becomes 1/4″, and so on.

4.1.2 Reading the Rule (Metric)Most metric rules are divided into millimeter

or one-half millimeter graduations. They are

Millimeters (mm) Centimeters (cm)

Half millimeters(0.5 mm)

Metric

Fractional inch

Decimal inch

Figure 4-2. Compare the metric (millimeter-graduated) rule withthe more familiar rules graduated in fractional and decimal inchunits.

Figure 4-3. Many different types of rules are used to make measuring quicker and more accurate. (L. S. Starrett Co.)

6” steel rule

Rule with adjustable hook

Narrow rule Small rule with holder

This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.

Chapter 4 Measurement 57

numbered every 10 mm. See Figure 4-5. The mea-surement is determined by counting the number ofmillimeters.

• Wipe steel rules with an oily cloth before stor-ing. This will prevent rust. If the rule is to bestored for a prolonged period, coat it withwax or rust preventative.

• Clean the rule with steel wool to keep thegraduations legible.

• Make measurements and tool settings fromthe 1″ line (10 mm line on a metric rule) orother major graduations, rather than from theend of the rule.

• Store rules separately. Do not throw them in adrawer with other tools.

• Use the rule with care to protect the ends fromnicks and wear.

• Use the correct rule for the job being done.

4.2 THE MICROMETER CALIPERA Frenchman, Jean Palmer, devised and

patented a measuring tool that made use of a screwthread, making it possible to read measurementsquickly and accurately without calculations. Itincorporated a series of engraved lines on the sleeveand around the thimble. The device, called SystemePalmer, is the basis for the modern micrometercaliper, Figure 4-6.

1 2

1 2

116

18

132

164

716

1116

1132

332

″ ″

″ ″

Figure 4-4. These are the fractional graduations found on arule. Measurements are taken by counting the number of graduations.

1.0 mm

0.5 mm

44 mm 69 mm

245.5 mm

Figure 4-5. Most metric rules are graduated in millimeters andhalf-millimeters. They are available in a variety of sizes.

4.1.3 Care of the RuleThe steel rule is precision-made and, like all

tools, its accuracy depends upon the care it receives.Here are a few suggestions:

• Use the rule for measurements only. Do notadjust screws or open paint cans with it. Becareful not to bend your rule.

• Keep the rule clear of moving machinery.Never use it to clean metal chips as they formon the cutting tool. This is extremely danger-ous and will ruin the rule.

• Avoid laying other tools on the rule.

Figure 4-6. The micrometer caliper, past and present.A—A drawing of the Systeme Palmer measuring device.B—These modern micrometer calipers operate on the sameprinciple as the original 1848 invention.

B

A


The micrometer caliper, also known as a “mike,”is a precision tool capable of measuring to 0.001″ or0.01 mm. When fitted with a Vernier scale, it willread to 0.0001″ or 0.002 mm.

4.2.1 Types of MicrometersMicrometers are produced in a wide variety of

models. Digital display is included in manymicrometers, making measuring easier. Some of themost popular models are the following:

• An outside micrometer measures externaldiameters and thickness, Figure 4-7.

• A chamfer micrometer will accurately measure countersunk holes and other chamfer-type measurements. With fastener

Figure 4-7. This digital outside micrometer can be used to measure in both US Conventional and SI Metric units.(Mitutoyo/MTI Corp.)

• An inside micrometer has many uses, includ-ing measuring internal diameters of cylin-ders, rings, and slots. The range of aconventional inside micrometer can beextended by fitting longer rods to themicrometer head. The range of a jaw-typeinside micrometer is limited to 1″ or 25 mm.The jaw-type inside micrometer has a scalegraduated from right to left. See Figure 4-8.

• A micrometer depth gage measures the depthsof holes, slots, and projections. See Figure 4-9.The measuring range can be increased bychanging to longer spindles. Measurementsare read from right to left.

• A screw-thread micrometer has a pointedspindle and a double-V anvil shaped to con-tact the screw thread, Figure 4-10. It measuresthe pitch diameter of the thread, which equalsthe outside (major) diameter of the threadminus the depth of one thread. Since eachthread micrometer is designed to measureonly a limited number of threads per inch, aset of thread micrometers is necessary to mea-sure a full range of thread pitches.

Figure 4-8. Inside micrometers. A—A conventional insidemicrometer. B—The caliper jaws on this inside micrometer allowquick and accurate measurements. The divisions on the sleeveare numbered in the reverse order of a conventional outsidemicrometer. (L. S. Starrett Co.)

Figure 4-9. A standard micrometer depth gage.


tolerances so critical on some aerospace andother applications, it is important that coun-tersunk holes and tapers on fasteners meetspecifications. A chamfer micrometer makes itpossible to check these critical areas.

Special micrometers are available for otherapplications. These micrometers are devised to handle nonstandard measurement tasks.

4.2.2 Reading an Inch-Based MicrometerA micrometer uses a very precisely made screw

thread that rotates in a fixed nut. The screw threadis ground on the spindle and is attached to the thimble. The spindle advances or recedes from theanvil as the thimble is rotated. See Figure 4-11. Thethreaded section has 40 threads per inch; therefore,each revolution of the thimble moves the spindle1/40″ (0.025″).

The line engraved lengthwise on the sleeve isdivided into 40 equal parts per inch (correspondingto the number of threads per inch on the spindle).Each vertical line equals 1/40″, or 0.025″. Everyfourth division is numbered, representing 0.100″,0.200″, etc.

The beveled edge of the thimble is divided into25 equal parts around its circumference. Each divi-sion equals 1/1000″ (0.001″). On some micrometers,every division is numbered, while every fifth divi-sion is numbered on others.

The micrometer is read by recording the highestnumber on the sleeve (1 = 0.100, 2 = 0.200, etc.). Tothis number, add the number of vertical lines visiblebetween the number and thimble edge (1 = 0.025, 2 = 0.050, etc.). To this total, add the number of thou-sandths indicated by the line that coincides with thehorizontal sleeve line.

Figure 4-10. This screw thread micrometer can measurethreads as wide as 7/8″. (L. S. Starrett Co.)

Figure 4-11. Basic parts of a micrometer caliper.

0.458″

Example 1

Add the readings from the sleeve and the thimble:4 large graduations: 4 � 0.100 = 0.4002 small graduations: 2 � 0.025 = 0.0508 thimble graduations: 8 � 0.001 = 0.008Total mike reading = 0.458″

0.289″

Example 2


AnvilSpindle

Lock nutSleeve Thimble

Ratchetstop

Frame


tional inch-based micrometer, reading the metric-based tool will offer no difficulties.

0.353″

Example 3


4.2.3 Reading a Vernier MicrometerOn occasion, it is necessary to measure more

precisely than 0.001″. A Vernier micrometer caliperis used in these situations. This micrometer has athird scale around the sleeve that will furnish the1/10,000″ (0.0001″) reading. See Figure 4-12.

Figure 4-12. A Vernier micrometer caliper includes a Vernierscale on the sleeve.

The Vernier scale has 11 parallel lines thatoccupy the same space as 10 lines on the thimble.The lines around the sleeve are numbered 1 to 10.The difference between the spaces on the sleeve andthose on the thimble is one-tenth of a thousandth ofan inch.

To read the Vernier scale, first obtain the thou-sandths reading, then observe which of the lines onthe Vernier scale coincides (lines up) with a line onthe thimble. Only one of them can line up. If the lineis 1, add 0.0001 to the reading; if line 2, add 0.0002 tothe reading, etc. See Figure 4-13.

4.2.4 Reading a Metric-Based MicrometerThe metric-based micrometer is read as shown

in Figure 4-14. If you are able to read the conven-

0.0002

0.0120

0.20000.0750

0.20000.07500.01200.0002

Reading is 0.2872″

Figure 4-13. How to read a Vernier micrometer caliper. Add thetotal reading in thousandths, then observe which of the lines onthe Vernier scale coincides with a line on the thimble. In thiscase, it is the second line, so 0.0002 is added to the reading.

5.00 mm

0.50 mm

0.28 mm

5.000.500.285.78 mmReading is

Figure 4-14. To read a metric micrometer, add the total readingin millimeters visible on the sleeve to the reading of hundredthsof a millimeter, indicated by the graduation on the thimble. Notethat the thimble reading coincides with the longitudinal line onthe micrometer sleeve.

Vernier scale lines


4.2.5 Reading a Metric Vernier MicrometerMetric Vernier micrometers are read in the same

way as standard metric micrometers. However,using the Vernier scale on the sleeve, an additionalreading of two-thousandths of a millimeter can beobtained, Figure 4-15.

results as the ratchet stop but permits one-handed use of the micrometer.

7.000 mm

0.500 mm 7.0000.5000.3100.0047.814 mm

0.310 mm

0.004 mm

Reading is

Figure 4-15. Reading a metric-based Vernier micrometercaliper. To the regular reading in hundredths of a millimeter(0.01), add the reading from the Vernier scale that coincideswith a line on the thimble. Each line on the Vernier scale is equalto two thousandths of a millimeter (0.002 mm).

4.2.6 Using the MicrometerThe proper way to hold a micrometer when

making a measurement is shown in Figure 4-16. Thework is placed into position, and the thimblerotated until the part is clamped lightly between theanvil and spindle. Guard against excessive pressure,which will cause an erroneous reading. Somemicrometers have features to help regulate pressure:

• A ratchet stop is used to rotate the spindle.When the pressure reaches a predeterminedamount, the ratchet stop slips and preventsfurther spindle turning. Uniform contactpressure with the work is ensured, even if dif-ferent people use the same micrometer. Referagain to Figure 4-11.

• A friction thimble may be built into the uppersection of the thimble. This produces the same

Figure 4-16. Proper technique of handling a micrometer.A—Use very light pressure when turning the thimble. B—Whenthe piece being measured must also be held, position themicrometer as shown, with a finger in the micrometer frame.

B


• A lock nut is used when several identicalparts are to be gaged. Refer again to Figure 4-11. The nut locks the spindle into place.Gaging parts with a micrometer locked at theproper setting is an easy way to determinewhether the pieces are sized correctly.

4.2.7 Reading an Inside MicrometerTo get a correct reading with an inside microm-

eter, it is important that the tool be held squareacross the diameter of the work. It must be posi-tioned so that it will measure across the diameter onthe exact center, Figure 4-17.

reverse order of the graduations on an outsidemicrometer. See Figure 4-18. The graduations underthe thimble must be read, rather than those that areexposed.

Figure 4-17. Using an inside micrometer. Extension rods can beadded to increase the tool’s measuring range.

Measurement is made by holding one end of thetool in place and then “feeling” for the maximumpossible setting by moving the other end from left toright, and then in and out of the opening. The mea-surement is made when no left or right movement isfelt, and a slight drag is noticeable on the in-and-outswing. It may be necessary to take several readingsand average them.

4.2.8 Reading a Micrometer Depth GageBe sure to read a micrometer depth gage cor-

rectly. The graduations on this measuring tool are in

Figure 4-18. A micrometer depth gage. When making mea-surements with a depth gage, remember that the graduationsare in reverse order. This gage indicates a depth of 0.250.

4.2.9 Care of a MicrometerMicrometers are precision instruments and

must be handled with care. The following tech-niques are recommended:

• Place the micrometer on the work carefully sothe faces of the anvil and spindle will not bedamaged. The same applies when removingthe tool after a measurement has been made.

• Keep the micrometer clean. Wipe it with aslightly oiled cloth to prevent rust and tar-nish. A drop of light oil on the screw threadwill keep the tool operating smoothly.

• Avoid “springing” a micrometer by applyingtoo much pressure when you are making ameasurement.

• Clean the anvil and spindle faces before use.This can be done with a soft cloth or by lightlyclosing the jaws on a clean piece of paper anddrawing the paper out.

• Check for accuracy by closing the spindlegently on the anvil and note whether the zeroline on the thimble coincides with the zero onthe sleeve. If they are not aligned, follow themanufacturer’s recommended adjustments.

• Avoid placing a micrometer where it may fallon the floor or have other tools placed on it.

• If the micrometer must be opened or closed aconsiderable distance, do not “twirl” theframe; gently roll the thimble with your palm.See Figure 4-19.

• Never attempt to make a micrometer readinguntil a machine has come to a complete stop.

• Clean and oil the tool if it is to be stored forsome time. If possible, place the micrometerin a small box for protection.


4.3 VERNIER MEASURING TOOLSThe Vernier principle of measuring was named

for its inventor, Pierre Vernier, a French mathe-matician. The Vernier caliper can make accurate measurements to 1/1000″ (0.001″) and 1/50 mm(0.02 mm). See Figure 4-20.

The following measuring instruments mayinclude a Vernier scale:

• Height and depth gages are used for layoutwork and to inspect the locations of features.See Figure 4-22.

• Gear tooth calipers are used to measure gearteeth and threading tools, Figure 4-23.

• Universal Vernier bevel protractors are usedfor the layout and inspection of angles, Figure 4-24.

Vernier measuring tools, with the exception ofthe Vernier bevel protractor, consist of a graduatedbeam with fixed jaw or base and a Vernier slideassembly. The Vernier slide assembly is composedof a movable jaw or scribe, Vernier plate, and clamp-ing screws. The slide moves as a unit along thebeam.

Unlike other Vernier measuring tools, the beamof the Vernier caliper is graduated on both sides.One side is for making outside measurements, theother for inside measurements. Many of the newerVernier measuring tools are graduated to make bothinch and millimeter measurements.

Figure 4-19. Micrometers must be treated carefully. Roll themicrometer thimble on the palm of your hand if the instrumentmust be opened or closed a considerable distance.

Figure 4-20. Vernier calipers can be used to make very accu-rate measurements. A—Standard Vernier caliper. B—Moderndigital calipers are easier to read than mechanical instruments.

The design of the tool permits measurements tobe made over a large range of sizes. It is manufac-tured as a standard item in 6″, 12″, 24″, 36″, and 48″lengths. SI Metric Vernier calipers are available in150 mm, 300 mm, and 600 mm lengths. The 6″, 12″,150 mm, and 300 mm sizes are most commonlyused. Unlike the micrometer caliper, the Verniercaliper can be used for both inside and outside mea-surements, Figure 4-21.

Figure 4-21. Vernier calipers can be used to make both internaland external measurements. (L. S. Starrett Co.)

A

B


4.3.1 Reading an Inch-Based Vernier ScaleThese measuring tools are available with either

25-division or 50-division Vernier plates. Both platescan be read to 0.001″.

On measuring tools using the 25-division Vernierplate, every inch section on the beam is graduated

Figure 4-22. Many instruments are equipped with a Vernierscale. A—Height gage. B—Depth gage. C—The digital readouton this type of height gage serves the same function as a stan-dard Vernier scale. (L. S. Starrett Co.)

Figure 4-23. Gear tooth Vernier calipers are used to measuregear teeth, form tools, and threaded tools. (L. S. Starrett Co.)

A

B

C


into 40 equal parts. Each graduation is 1/40″(0.025″). Every fourth division, representing 0.100″,is numbered.

There are 25 divisions on the Vernier plate.Every fifth line is numbered: 5, 10, 15, 20, and 25.The 25 divisions occupy the same space as 24 divi-sions on the beam. This slight difference, equal to0.001 (1/1000″) per division, is the basis of theVernier principle of measuring.

To read a 25-division Vernier plate measuringtool, note how many inches (1, 2, 3, etc.), tenths(0.100, 0.200, etc.), and fortieths (0.025, 0.050, or0.075) there are between the “0” on the Vernier scaleand the “0” line on the beam, then add them. Thencount the number of graduations (each graduationequals 0.001″) that lie between the “0” line on theVernier plate and the line that coincides (corre-sponds exactly) with a line on the beam. Only oneline will coincide. Add this to the above total for thereading.

The “0” line on the Vernier plate is:Past the 2: 2 � 1 = 2.000Past the 3: 3 � 0.100 = 0.300Plus 2 graduations: 2 � 0.025 = 0.050Plus 18 Vernier scale

graduations: 18 � 0.001 = 0.018Total reading = 2.368″

On the 50-division Vernier plate, every secondgraduation between the inch lines is numbered, andequals 0.100″. The unnumbered graduations equal0.050″.

The Vernier plate is graduated into 50 parts,each representing 0.001″. Every fifth line is num-bered: 5, 10, 15 . . . 40, 45, and 50.

To read a 50-division Vernier measuring tool,first count how many inches, tenths (0.100), andtwentieths (0.050) there are between the “0” line onthe beam, and the “0” line on the Vernier plate. Thenadd them. Then count the number of 0.001 gradua-tions on the Vernier plate from its “0” line to the linethat coincides with a line on the beam. Add this tothe above total.

Figure 4-24. A universal Vernier bevel protractor is used toaccurately measure angles. (L. S. Starrett Co.)

0.3000.050 0.018

2.000

2.0000.3000.0500.018

Reading is 2.368″

The “0” line on the Vernier plate is:Past the 2: 2 � 1.000 = 2.000Past the 2: 2 � 0.100 = 0.200Plus one graduation: 1 � 0.050 = 0.050Plus 15 Vernier scale

graduations: 15 � 0.001 = 0.015Total reading = 2.265″

4.3.2 Reading a Metric-Based Vernier ScaleThe principles used in reading metric Vernier

measuring tools are the same as those used for US Conventional measure. However, the readingson the Vernier scale are obtained in 0.02 mm preci-sion. A 25-division Vernier scale is illustrated inFigure 4-25, while a 50-division scale is described inFigure 4-26.

2.000

0.2000.050

0.015

2.0000.2000.0500.015

Reading is 2.265″


4.3.3 Using the Vernier CaliperAs with any precision tool, a Vernier caliper

must not be forced on the work. Slide the Vernierassembly until the jaws nearly contact the sectionbeing measured. Lock the clamping screw. Make thetool adjustment with the fine adjusting nut. Thejaws must contact the work firmly, but not tightly.

Lock the slide on the beam. Carefully removethe tool from the work and make your reading. Forprecise layout work, divider and trammel point set-tings are located on the outside measuring scale andon the slide assembly.

Dial calipers. These direct-reading instrumentsresemble Vernier calipers. They can be used to makeoutside, inside, and depth measurements (with theaddition of a depth attachment). A lock permits thetool to be employed for repetitive measurements.See Figure 4-27.

The beam is graduated into 0.10″ increments.The caliper dial is graduated into 100 divisions. Thereading is made by combining the division on thebeam and the dial reading.

The dial hand makes one full revolution for each0.10″ movement. Each dial graduation, therefore,represents 1/100 of 0.10″, or 0.001″. On the metricversion, each dial graduation represents 0.02 mm.

4.3.4 Universal Vernier Bevel ProtractorA quick review of the circles, angles, and units of

measurement associated with them will help inunderstanding how to read a universal Vernierbevel protractor.

• Degree (°)—Regardless of its size, a circlecontains 360°. Angles are also measured bydegrees.

• Minute (′)—A minute represents a fractionalpart of a degree. If a degree is divided into 60 equal parts, each part is one minute. Afoot mark (′) is used to signify minutes (e.g. 30°15’).

• Second (″)—Minutes are divided into smallerunits known as seconds. There are 60 secondsin one minute. An angular measurement writ-ten in degrees, minutes, and seconds appearsas 36°18’22″. This would read “36 degrees, 18 minutes, and 22 seconds.”

30.009.00

0.28

30.00 9.00 0.2839.28 mmReading is

Figure 4-25. How to read a 25-division metric-based Vernierscale. Readings on the scale are obtained in units of two hun-dredths of a millimeter (0.02 mm).

30.00

9.000.28

30.009.000.28

Reading is 39.28 mm

Figure 4-26. How to read a 50-division metric-based Vernierscale. Each division equals two hundredths of a millimeter (0.02 mm).

Figure 4-27. Dial calipers provide direct readings of measure-ments. (L.S. Starrett Co.)


A universal bevel protractor has several parts: adial, a base or stock, and a sliding blade. The dial isgraduated into degrees, and the blade can beextended in either direction and set at any angle tothe stock. The blade can be locked against the dialby tightening the blade clamp nut. The blade anddial can be rotated as a unit to any desired position,and locked by tightening the dial clamp nut.

The protractor dial is graduated into 360° andreads from 0° to 90° and then back down to 0°.Every ten degree division is numbered, and everyfive degrees is indicated by a fine line longer thanthose on either side. The Vernier scale is dividedinto twelve equal parts on each side of the “0.”Every third graduation is numbered (0, 15, 30, 45,60), representing minutes. Each division equals fiveminutes. Since each degree is divided into 60 min-utes, one division is equal to 5/60 of a degree.

To read the protractor, note the number ofdegrees that can be read up to the “0” on the Vernierplate. To this, add the number of minutes indicatedby the line beyond the “0” on the Vernier plate thataligns exactly with a line on the dial.

• Never force the tool when you are makingmeasurements.

• Use a magnifying glass or a jeweler’s loupe tomake Vernier readings. Hold the tool so thelight is reflected on the scale.

• Handle the tool as little as possible. Sweat andbody acids cause rusting and staining.

• Periodically check for accuracy. Use a mea-suring standard, Jo-block, or ground parallel.Return the tool to the manufacturer foradjustments and repairs.

• Lay Vernier height gages on their side whennot in use. Then there will be no danger thatthey will be knocked over and damaged.

4.4 GAGESIt is impractical to check every dimension on

every manufactured part with conventional mea-suring tools. Specialized tools, such as plug gages,ring gages, and optical gages are used instead.These gaging devices can quickly determinewhether the dimensions of a manufactured part arewithin specified limits or tolerances.

Measuring requires the skillful use of precisionmeasuring tools to determine the exact geometricsize of the piece. Gaging involves checking partswith various gages. Gaging simply shows whetherthe piece is made within the specified tolerances.

When great numbers of an item with severalcritical dimensions are manufactured, it might notbe possible to check each piece. It then becomes nec-essary to decide how many randomly selectedpieces must be checked to ensure satisfactory qual-ity and adherence to specifications. This techniqueis called statistical quality control.

Always handle gages carefully. If dropped ormishandled, the accuracy of the device could beaffected. Gages provide a method of checking yourwork and are very important tools.

4.4.1 Plug GagePlug gages are used to check whether hole

diameters are within specified tolerances. The double-end cylindrical plug gage has two gagingmembers known as go and no-go plugs, Figure 4-28.The go plug should enter the hole with little or nointerference. The no-go plug should not fit.

The go plug is longer than the no-go plug. A progressive plug gage, or step plug gage, has thego and no-go plugs on the same end. This gage isable to check the dimensions in one motion. SeeFigure 4-29.

0°20′

50°00′50°00′

0°20′Reading is 50°20′

In this example the “0” is past the 50° mark,and the Vernier scale aligns at the 20′ mark.Therefore, the measurement is 50°20′.

4.3.5 Care of Vernier ToolsReasonable care in handling these expensive

tools will maintain their accuracy.• Wipe the instrument with a soft, lint-free cloth

before using. This will prevent dirt and gritfrom being ground in, which could eventuallyaffect the accuracy of the tool.

• Wipe the tool with a lightly oiled, soft clothafter use and before storage.

• Store the tool in its case.


4.4.2 Ring GageExternal diameters are checked with ring gages.

The go and no-go ring gages are separate units, andcan be distinguished from each other by a groovecut on the knurled outer surface of the no-go gage.Refer to Figure 4-30.

On ring gages, the gage tolerance is the reverseof plug gages. The opening of the go gage is largerthan the opening for the no-go gage.

Figure 4-29. A step plug gage can check for oversize andundersize in a single test.

Figure 4-28. A double end cylindrical plug gage.

Figure 4-30. Ring gages. The larger sizes are cut away toreduce weight. (Standard Tool Co.)

4.4.3 Snap GageA snap gage serves the same purpose as a ring

gage. Snap gages are designed to check internaldiameters, external diameters, or both. There arethree general types:

• An adjustable snap gage can be adjustedthrough a range of sizes. See Figure 4-31.

• A nonadjustable snap gage is made for onespecific size. See Figure 4-32.

Figure 4-31. An adjustable snap gage. (Taft-Pierce Co.)

Gage size

Forchecking

O.D.

Forchecking

I.D.

Figure 4-32. Diagram of a nonadjustable snap gage. A—A com-bination internal-external nonadjustable snap gage. B—Anexternal nonadjustable snap gage.


• A dial indicator snap gage measures theamount of variation in the part measurement.The dial face has a double row of graduationsreading in opposite directions from zero.Minus graduations are red and plus gradua-tions are black. Both adjustable and nonad-justable indicating snap gages are available.See Figure 4-33.

On snap gages, the anvils should be narrowerthan the work being measured. This will avoiduneven wear on the measuring surfaces.

When working with gage blocks, keep the fol-lowing tips in mind:

• Improper handling can cause temperaturechanges in the block, resulting in measure-ment errors. For the most accurate results,blocks should be used in a temperature-controlled room. Handle the blocks as little aspossible. When you must handle the blocks,use the tips of your fingers, as shown inFigure 4-37A.

• When wringing gage blocks together to buildup to desired size, wipe the blocks and thencarefully slide them together. They shouldadhere to each other strongly. Separate theblocks when you are finished. Leaving gageblocks together for extended periods maycause the contacting surfaces to corrode. SeeFigure 4-37B.

Figure 4-33. A dial indicator snap gage. (L.S. Starrett Co.)

4.4.4 Thread GagesSeveral types of gages are used to check screw

thread fits and tolerances. These gages are similar tothe gages already discussed:

• Thread plug gage.• Thread ring gage.• Thread roll snap gage.

These gages are illustrated in Figure 4-34.

4.4.5 Gage BlocksGage blocks, commonly known as Jo-blocks or

Johansson blocks, are precise steel measuring stan-dards. Gage blocks can be purchased in various setsranging from a few commonly used block sizes tomore complete sets. See Figure 4-35.

Gage blocks are used to verify the accuracy ofmaster gages. They are also used as working gagesand for setting up machining work requiring greataccuracy. The Federal Accuracy Grades for gageblocks are shown in Figure 4-36.

Figure 4-34. Thread gages. A—Thread plug gage. B—Threadring gage. C—Go/no-go thread snap gage. (Standard Tool Co.and Taft-Pierce Co.)

C

B

A


• Wipe gage blocks with a soft cloth or chamoistreated with oil. Be sure the oil is one recom-mended by the gage manufacturer. See Figure4-37C.

4.5 DIAL INDICATORSIndustry is constantly searching for ways to

reduce costs without sacrificing quality. Inspectionhas always been a costly part of manufacturing. Tospeed up this phase of production without sacrific-ing accuracy, dial indicators and electronic gages arereceiving increased attention.

Dial indicators are designed with shockproofmovements and have jeweled bearings (similar tofine watches). There are two types of indicators: bal-anced and continuous. Balanced indicators can takemeasurements on either side of a zero line.Continuous indicators read from “0” in a clockwisedirection. See Figure 4-38.

Dial faces are available in a wide range of graduations. They usually read in the followingincrements:

• 1/1000″ (0.001″)• 1/100 mm (0.01 mm)• 1/10,000″ (0.0001″)• 2/1000 mm (0.002 mm)Much use is made of dial indicators for center-

ing and aligning work on machine tools, checkingfor eccentricity, and visual inspection of work. Dialindicators must be mounted to rigid holdingdevices, Figure 4-39.

A digital electronic indicator, Figure 4-40, fea-tures direct digital readouts and a traditional grad-uated dial for fast, accurate reading. Theseindicators are available as both self-contained andremote readout units.

Figure 4-35. A typical set of gage blocks.(Federal Products Co.)

Federal Accuracy GradesTolerance

Accuracygrade

0.5 AAA �.000001″ �.00003 mm�.000002″ �.00005 mm +.000004″ +.0001 mm –.000002″ –.00005 mm +.000006″ +.00015 mm –.000002″ –.00005 mm

AA1

A +2

A&B3

Formerdesignation

US Conventionalsystem (inch)

Metric system(millimeter)

Reference temperature: 68°F (20°C)One inch = 25.4 millimeters exactly

Figure 4-36. Federal Accuracy Grades for gage blocks.

Figure 4-37. Proper care of gage blocks. A—Handling gage blocks. B—Wipe blocks and slide them together. Do not leave blockstogether for extended periods. C—Wipe blocks with a soft cloth before storing. (Webber Gage Div., L.S. Starrett Co.)

The effect oftemperature

Handle blocks like this

Instead of this

Correct methodof wringing

gages together

Oil-treatedcloth

A B C


4.5.1 How to Use a Dial IndicatorThe hand on the dial is actuated by a sliding

plunger. Place the plunger lightly against the workuntil the hand moves. The dial face is turned until

the “0” line coincides with the hand. As the worktouching the plunger is slowly moved, the indicatorhand will measure movement.

The dial indicator can show the differencebetween the high and low points, or the total run-out of the piece in a lathe. When machining, adjust-ments are made until there is little or no indicatormovement.

4.6 OTHER GAGING TOOLSIndustry makes wide use of other types of gag-

ing tools. Most of these tools are used for specialpurposes and are not usually found in a schoolshop. However, since you might need to use them inindustry, it is important to learn about such tools.

4.6.1 Air GageAn air gage uses air pressure to measure hole

sizes and hard-to-reach shaft diameters, Figure 4-41.This type of gage is especially helpful when mea-suring deep internal bores. The basic operation of anair gage is illustrated in Figure 4-42.

Figure 4-38. The two basic varieties of dial indicators.A—Balanced indicators. B—Continuous indicators.(L. S. Starrett Co.)

Figure 4-39. Mounting this dial indicator on a magnetic basepermits it to be attached to any ferrous metal surface. A push-button releases the magnet.

Figure 4-40. This digital electronic indicator has numeric read-outs and a conventional graduated dial.(Federal Products Company)

A

B


There is no actual contact between the measur-ing gage and wall of the bore being measured. Thebore measurement depends on the air leakagebetween the plug and the hole wall. (The larger thebore diameter, the greater the leakage.) Pressurebuilds up and the measurement of the back pressuregives an accurate measurement of the hole size.

Change in pressure (air leakage) is measured bya dial indicator, a cork floating on the air stream, orby a manometer (U-shaped tube in which the heightof fluid in the tube indicates pressure).

4.6.2 Electronic GageAn electronic gage, Figure 4-43, is another type

of gaging tool used to make extremely precise measurements. Electronic gages are comparisongages: they compare the size of the work to a reference size. Some are calibrated by means of master gage blocks and others use replaceable gaging probes. These instruments measure in bothUS Conventional and SI Metric units.

4.6.3 Laser GagingA laser is a device that produces a very narrow

beam of extremely intense light. Lasers are used incommunication, medical, and industrial applica-tions. Laser is an acronym for light amplification bystimulated emission of radiation.

The laser is another area of technology that hasmoved from the laboratory into the shop. Whenemployed for inspection purposes, it can check theaccuracy of critical areas in machined parts quicklyand accurately. Refer to Figure 4-44.

4.6.4 Optical ComparatorThe optical comparator uses magnification as a

means for inspecting parts, Figure 4-45. An

Figure 4-41. Digital air gages are available with either US Conventional or SI Metric readouts. They can check eitherinside or outside diameters. A—An air gage set up to inspect aninternal dimension. The master ring shown with the gage isused to set zero on the readout. B—This gage has an air fork,which is used to check hard-to-reach diameters, such as crank-shaft journals. (Federal Products Company)

Nonadjustableequalizing jet

Zero setting valve

Cushionedand jewelled

indicatingmovement

Master

Gaging plugNonadjustable

master jet

Pressureregulator

Filter

Airsupply

Masterpressure

gage

Differentialpressure

meter

Figure 4-42. This diagram illustrates the operation of an airgage.

A

B


enlarged image of the part is projected upon ascreen for inspection. The part image is superim-posed upon an enlarged, accurate drawing of thecorrect shape and size. The comparison is madevisually. Variations as small as 0.0005″ (0.012 mm)can be noted by a skilled operator.

4.6.5 Optical FlatsOptical flats are precise measuring instruments

that use light waves as a measuring standard,Figure 4-46. The flats are made of quartz and haveone face ground and polished to optical flatness.When this face is placed on a machined surface anda special light passed through it, light bands appearon the surface, Figure 4-47. The shape of thesebands indicate to the inspector the accuracy of thepart. See Figure 4-48.

Figure 4-43. This electronic bore gaging system can deliverelectronic resolution as fine as 0.00001″ (0.0002 mm). Usingreplaceable gaging probes, the self-contained unit measuresdiameters ranging from 0.370″ to 2.900″. It also measures inmillimeters. It can be linked to a computer for statistical processcontrol (SPC) data collection. (Sunnen Products Company)

Figure 4-44. This laser is being used to inspect a part from acar’s automatic transmission. Manually, one person couldinspect no more than four units an hour. The laser can inspectover 120 parts an hour. (Ford Motor Co.)

Figure 4-45. This 50-power optical comparator permits a fastcheck of the tooth formation on a tap.

Figure 4-46. Optical flats are used for precision flatness, parallelism, size, and surface variations. (L. S. Starrett Co.)


4.6.6 Thickness (Feeler) GageThickness gages are pieces or leaves of metal

manufactured to precise thickness, Figure 4-49.Thickness gages are made of tempered steel and areusually 1/2″ (12.7 mm) wide.

Figure 4-48. Interference band patterns indicate surface flatness and variations.

Optical flat

Work

Optical flat

WorkWork

Work

Optical flatOptical flat

Concave, cylindrical(air wedge)

Concave, spherical(contact)

Flat surface(air wedge)

Convex, spherical(air wedge)

Lightsource

Optical flat

Jo-block

Toolmaker's flat

Ball bearingbeing inspected

Interferencebands

Interference bandsindicate difference

in size between ball bearing and Jo-block

Figure 4-47. Optical flat set-up. Optical flat is placed on top ofthe work and light is positioned above the flat.

Figure 4-49. Thickness or feeler gages.

Thickness gages are ideal for measuring narrowslots, setting small gaps and clearances, determin-ing fit between mating surfaces, and for checkingflatness of parts in straightening operations. SeeFigure 4-50.

4.6.7 Screw Pitch GageScrew pitch gages are used to determine the

pitch or number of threads per inch on a screw,Figure 4-51. Each blade is stamped with the pitch or


number of threads per inch. Screw pitch gages are available in US Conventional and SI Metricthread sizes.

4.6.8 Fillet and Radius GageThe thin steel blades of a fillet and radius gage,

Figure 4-52, are used to check concave and convexradii on corners or against shoulders. The gage isused for layout work and inspection, and as a tem-plate when grinding form cutting tools. See Figure4-53. The gages increase in radius in 1/64″ (0.5 mm)increments.

4.6.9 Drill RodDrill rods are steel rods manufactured to close

tolerances to twist drill diameters. They are used toinspect hole alignment, location, and diameter. Drillrods are available in both US Conventional and SIMetric sizes.

4.7 HELPER MEASURING TOOLSSome measuring tools are not direct reading and

require the help of a rule, micrometer, or Verniercaliper to determine the size of the measurementtaken. These are called helper measuring tools.

4.7.1 CalipersExternal or internal measurements of 1/64″

(0.4 mm) can be made with calipers, Figure 4-54. Acaliper does not have a dial or scale that shows ameasurement; the distance between points must bemeasured with a steel rule.

Round stock is measured by setting the calipersquare with the work and moving the caliper legsdown on the stock. Adjust the tool until the caliperpoint bears lightly on the center line of the stock.Caliper weight should cause the caliper to slip overthe diameter. Hold the caliper next to the rule tomake the reading, Figure 4-55.

An inside caliper is used to make internal mea-surements where 1/64″ (0.4 mm) accuracy is accept-able. Hole diameter can be measured by setting thecaliper to approximate size, and inserting the legsinto the opening. Hold one leg firmly against thehole wall, and adjust the thumbscrew until the otherleg lightly touches the wall exactly opposite the first

Figure 4-50. A thickness gage is used to check part clearance.

Figure 4-51. Screw pitch gages are made for both inch-basedand metric threads. (L. S. Starrett Co.)

Figure 4-52. A set of radius and fillet gages. (L. S. Starrett Co.)


leg. The legs should drag slightly when moved inand out, or from side to side.

Considerable skill is required to make accuratemeasurements with a caliper. See Figure 4-56. Muchdepends upon the machinist’s sense of touch. Withpractice, measurements with accuracy of 0.003″(0.07 mm) can be made. However, a micrometer orVernier caliper is preferred and must be utilizedwhen greater accuracy is required.

4.7.2 Telescoping GageA telescoping gage is intended for use with a

micrometer to determine internal dimensions,Figure 4-57. Sets of telescoping gages with varyingranges are available, Figure 4-58.

To use a telescoping gage, compress the contactlegs. The legs telescope within one another underspring tension. Insert the gage into the hole andallow the legs to expand, Figure 4-59. After theproper fitting is obtained, lock the contacts intoposition. Remove the gage from the hole and makeyour reading with a micrometer, Figure 4-60.

4.7.3 Small Hole GageA small hole gage is used to measure openings

that are too small for a telescoping gage, Figure 4-61. The contacts are designed to allow accurate

Figure 4-53. Using a radius gage. A—Various ways a radiusgage can be used. B—Using a radius gage holder.(L. S. Starrett Co.)

Figure 4-54. Inside and outside calipers. (L. S. Starrett Co.)

Figure 4-55. The outside caliper is read with a steel rule.

B


Figure 4-56. Using outside and inside calipers.(L. S. Starrett Co.)

Figure 4-57. A telescoping gage is used with a micrometer.

Figure 4-58. A typical set of telescoping gages.

Figure 4-59. Positioning a telescoping gage to measure aninside diameter.

Figure 4-60. After removing the locked telescoping gage, measure it with a micrometer.

B

A


measurement of shallow grooves, and small diame-ter holes. They are adjusted to size by the knurledknob at the end of the handle. Measurement is madeover the contacts with a micrometer, Figure 4-62.

TEST YOUR KNOWLEDGEPlease do not write in this text. Write your

answers on a separate sheet of paper.

1. Make readings from the rules.

Figure 4-61. Small hole gages are used to measure the diameter of holes that are too small for telescoping gages.

Figure 4-62. The correct way to measure a small hole gage witha micrometer.

2. Make readings from the Vernier scales shownbelow.

B

A

B C


Answer the following questions as they per-tain to measurement.

3. The micrometer is nicknamed _____.

4. One-millionth part of a standard inch isknown as a _____.

5. One-millionth part of a meter is known as a_____.

6. A micrometer is capable of measuring accu-rately to the _____ and _____ part of standardinch and (in metric versions) to _____ and_____ millimeters.

7. The Vernier caliper has several advantagesover the micrometer. List two of them.

8. A Vernier caliper can measure to the _____ partof the inch and (in the metric version) to _____millimeters.

9. List six precautions that must be observedwhen using a micrometer or Vernier caliper.

J

H

I

G

E

F

C

D


10. The Vernier-type tool for measuring angles iscalled a _____.

11. How does a double-end cylindrical plug gagediffer from a step plug gage?

12. A ring gage is used to check whether _____ arewithin the specified _____ range.

13. Gage blocks are often referred to as _____blocks.

14. An air gage employs air pressure to measuredeep internal openings and hard-to-reachshaft diameters. It operates on the principle of:a. Air pressure leakage between the plug

and hole walls.b. The amount of air pressure needed to

insert the tool properly in the hole.c. Amount of air pressure needed to eject

the gage from the hole.d. All of the above.e. None of the above.

15. The dial indicator is available in two basictypes. List them.

16. What are some uses for the dial indicator?

17. Name the measuring device that employs lightwaves as a measuring standard.

18. The _____ is used for production inspection.An enlarged image of the part is projected on ascreen where it is superimposed upon an accu-rate drawing.

19. The pitch of a thread can be determined with a_____.

20. Of what use are fillet and radius gages?

21. What are helper measuring tools?

22. How is a telescoping gage used?

23. Make readings from the micrometer illustra-tions.

A B C

D E F

G H I

J K L

this sample chapter is for review purposes only

Documents