Unit15 Comparison Measurement

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Krar • Gill • Smid

Technology of Machine Tools6th Edition

Comparison

MeasurementUnit 15



15-2

Objectives

• Explain the principle of comparison

measurement

•

Identify four types of comparators anddescribe their use

• Measure to within .0005-in. (0.01-mm)

accuracy with a dial indicator, mechanicaland optical comparator, or air and electronic

gages



15-3

Comparison Measurement

• Process consisting of comparingmeasurement of part to known standard or

master of exact dimension required• Comparators

– Any instrument used to compare size ofworkpiece to known standard

– Incorporate some means of amplification tocompare part size to set standard

• Standard usually gage blocks



15-4

Dial Indicators

• Used to compare sizes and measurements toknown standard and check alignment ofmachine tools, fixtures, and workpieces prior

to machining

• Many work on gear and rack principle

– Rack cut on plunger or spindle is in mesh with a

pinion which is connected to a gear train – Any movement of spindle magnified and

transmitted to pointer over graduated dial




15-5

Dial Indicators

• Generally two types

– Continuous-reading dial indicator

• Numbered clockwise for 360º• Regular-range

– Only 2 ½ revolutions

• Long-range indicator

–

Digital and used to indicate table travel – Dial test indicator

• Reads both right and left from 0

• Equipped with tolerance pointers





15-7

To Measure with a Dial Test

Indicator and Height Gage

1. Clean face of surface plate and vernierheight gage

2. Mount dial test indicator on movable jaw

3. Lower movable jaw until indicator point just touches top of gage block resting on

surface plate

4. Tighten upper locking screw on vernierand loosen lower locking screw



15-8

5. Carefully turn adjusting nut until indicatorneedle registers ¼ turn

6. Turn bezel to set indicator to zero

7. Note reading on vernier and record it

8. Raise indicator to height

of first hole to measure9. Adjust vernier until

indicator reads zero

10. Note vernier reading again

and record it11. Subtract first reading from

second and add height ofgage block

Metric Dial Indicator





15-10

To Measure with a Mechanical

Comparator• Clean anvil and master gage of required size

• Place master on anvil

• Carefully lower gaging head until stylustouches master and indicates movement ofthe needle

• Lock gaging head to the column• Adjust needle to zero using fine adjustment

knob and set limit pointer on the face



15-11

To Measure with a Mechanical

Comparator• Recheck setting by removing master and

replacing it

• Set tolerance pointers to upper and lowertolerances of part being checked

• Substitute work being gaged for master and

note reading – Reading to right of zero, work too large

– Reading to left of zero, work too small

– Needle stops between tolerance pointers, OK



15-12

Optical Comparators

• Projects enlarged shadow onto screen whereit is compared to lines or to master form thatindicates limits of dimensions or contour of part being checked

• Fast, accurate means of measuringworkpiece with master

•

Suited to checking extremely small or odd-shaped parts

• Available in bench and floor models



15-13

Principle of the

Optical Comparator


Available in the following

magnifications: 5X, 10X,31.25X, 50X, 62.5X 90X,

100X and 125X

Note: Use chart of same magnification as lens!



15-14

To Check Angle of 60º Thread

Using an Optical Comparator1. Mount correct lens in comparator

2. Mount tilting centers on micrometer cross-slide stage

3. Set tilting centers to the helix angle of thethread

4. Set workpiece between centers

5. Mount vernier protractor chart and align ithorizontally on screen



15-15

6. Turn on light switch

7. Focus lens so clear image appears onscreen

8. Move micrometer cross-slide stage until

thread image centralized on screen9. Revolve vernier protractor chart to show

reading of 30º

10. Adjust cross-slides until image coincideswith protractor line

11. Check other side of thread in same manner



15-16

Mechanical-

OpticalComparator

• Reed-type comparator

• Combines reed

mechanism with light

beam to cast shadowon magnified scale

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Permission required for reproduction or display.



15-17

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

FIXED BLOCK

• Fixed steel block and free-reed blockhave reeds (2 pieces of spring steel)attached

•

Upper ends of reed joined andconnected to a pointer

• Movement of spindle attached tofree-reed block will move it up anddown (moving pointer to right orleft)

• Light beam through aperture,movement of free block cause targetto interrupt light beam, castingshadow on scale

Principle of the

Reed Mechanism



15-18

Cutaway Section Showing Reed Mechanism


Upper part of reed deflects greater

distance than the lower part



15-19

To Measure with a Reed

Comparator1. Raise gaging head above required height,

and clean anvil and master thoroughly

2. Place master gage or gage block buildupon anvil

3. Carefully lower gaging head until end ofspindle just touches the master

– Shadow will begin to appear on left side ofscale

4. Clamp gaging head to column



15-20

To Measure with a Reed

Comparator• Turn adjusting sleeve until shadow coincides

with zero on the scale

• Remove gage blocks and carefully slide

workpiece between anvil and spindle

• Note reading

– If shadow to right of zero, part is oversize

– If shadow to left of zero, part undersize



15-21

Air Gages or Pneumatic

Comparators• Used to compare workpiece dimensions

with master gage by means of air pressure

or flow• Two types:

– Flow or column

• Indicates air velocity

– Pressure

• indicates air pressure in system



Pressure-type air gage



15-22



Column-Type Air Gage

• Air passed through filter andregulator (10 psi)

• Air flows through tapered tube withfloat

• Rate of flow proportionalto clearance

• Gage set to master

• Workpiece larger than

hole size, float rises – Smaller, float falls

15 23



15-23


Pressure-Type

Air Gage

15 24



15-24

Advantages of Air Gages

• Holes may be checked for taper, out-of-roundness, concentricity, and irregularity

• Gage does not touch workpiece• Gaging heads last longer than fixed gages

• Less skill required to use this type

• Gages may be used a machine or bench• More than one diameter may be checked at

same time

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Unit15 Comparison Measurement

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