Measuring InstrumentsMeasuring Instruments

MetrologyMetrology

science of weights and measures

DiscriminationDiscrimination

degree to which a measuring instrument divides a basic unit of length

10 to one rule for discrimination10 to one rule for discrimination a measuring instrument should

discriminate 10 times finer than the smallest unit that it will be used to measure

instrument lands between two lines - do not use to measure

Parallax errorParallax error

the apparent shift of an object when viewed from different angles

CalibrationCalibration Process by which a measuring

instrument is compared to a known standard

The international prototype kilogram is a platinum iridium alloy, created in 1879, that is the standard by which all the world’s weight measurements are calibrated. It is lock in a Paris vault and is only brought out on very rare occasions

The length of a meter is defined by the distance light travels in a vacuum in 1/299,792,458 seconds

Handle precision measuring instruments with careHandle precision measuring instruments with care

Use of precision measuring instrumentsUse of precision measuring instruments keep calibrated when possible, take readings while

the instrument is in contact with the work piece

on cylindrical work pieces, check twice - 90 degrees apart

best temperature is 68 degrees F.

Direct reading instrumentsDirect reading instruments

1. Steel Rules1. Steel Rules

2. Calipers (Can be Vernier, Dial, or Digital Reading)2. Calipers (Can be Vernier, Dial, or Digital Reading) Outside Inside Depth

3. Micrometers (Can be Vernier, Dial, or Digital Reading)

3. Micrometers (Can be Vernier, Dial, or Digital Reading) Outside

frame, anvil, spindle, sleeve, thimble Inside Depth

Comparison measuring instrumentsComparison measuring instruments comparing to a known standard represents a known distance used in conjunction with an instrument

that has the capability of showing measurement

sometimes referred to as transfer measurement

direct reading instruments should be used whenever possible

Common typesCommon types

1. spring calipers 2. telescoping gages 3. small hole gages 4. adjustable parallels 5. radius gages 6. thickness gages 7. planer gages (similar to adjustable

parallels)

8. Squares8. Squares

a. machinists combination square set protractor centering head

b. solid beam square c. cylindrical square

9. Indicators9. Indicators

balanced vs. continuous readings mounting must be rigid

magnetic bases flexible link holding arms height gage applications (test)

2 types of indicators2 types of indicators a. dial

some can be used to make linear measurement discriminations range from .00005 to .001” ranges are from .003 to 2.0”

b. test used for comparisons only (no direct

measurements) discrimination approx .0005” range of about .030”

Angle Terminology Angle Terminology

acute angles = < 90 degrees right angles = 90 degrees obtuse angles = > degrees full circle = 360 degrees 1 degree = 60 minutes of an arc 1 minute = 60 seconds of an arc radian = metric unit of angular

measurement

Angle measuring instruments (not for measuring angularity)

Angle measuring instruments (not for measuring angularity)

1. Plate protractor1. Plate protractor

discrimination of 1 degree

2. Bevel protractor2. Bevel protractor

part of the machinists combination set - discrimination of 1 degree

3. Universal bevel protractor3. Universal bevel protractor includes a vernier scale measure to accurately to within 5

minutes of a degree (remember there are 60 minutes in one degree

4. Sine bar4. Sine bar

5” and 10” center spacing of cylinders

To find angles with the sine bar:To find angles with the sine bar: elevate the sine bar with adjustable

parallels until the indicator reads zero at each end of the workpiece

parallel is removed and measured with a micrometer

transpose the sine bar elevation formula and solve for the angle

sin of the angle = elevation / sin bar length (5 or 10”)

Gage blocksGage blocks

permit comparison between working measurement and instruments of mfg.

Uses:Uses:

calibration of measuring instruments establishment of precise angles (with

sine bars) positioning of machine tools and

cutting tools used to set snap gages

Common set has 81 to 88 blocksCommon set has 81 to 88 blocks

Range in thickness from .050 to 4.0”Range in thickness from .050 to 4.0”

3 Grades3 Grades

grade 1 = lab grade 2 = inspection grade 3 = shop

Grade on tolerance = +/- .000002”Grade on tolerance = +/- .000002”

Tolerance on a stack of 30 gage blocks assembled to represent 20”?

Tolerance on a stack of 30 gage blocks assembled to represent 20”?

Use as few combinations as possible when stackingUse as few combinations as possible when stacking

CareCare

avoid unnecessary contact with hands

clean with proper solvents handle and lay on lint free cloth don’t leave blocks rung for extended

periods of time spray with a preservative

Wringing gage blocksWringing gage blocks

Main cause of gage block wear is wringing poorly cleaned blocks

Main cause of gage block wear is wringing poorly cleaned blocks

Checking with optical flatsChecking with optical flats

Wear blocksWear blocks

used in applications where direct contact is made

usually .050” or .100”

AccessoriesAccessories

scribes bases screw sets

Other machinesOther machines

Optical comparator Coordinate measuring machine

(CMM)