Measuring Instruments

Metrology science of weights and measures

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

10 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 error the apparent shift of an object when viewed from different angles

Calibration 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 care

Use 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.

Variables (Errors)

1. Observational errors misreading the gage parallax

2. Manipulative errors holding the gage incorrectly not locating datums properly mounting the part on the wrong datums not aligning the gage properly measuring pressure /ratchet stops

3. Bias errors rounding off

4. Gage errors precision loss (sluggish or sticking indicator) accuracy loss (incorrect gage block stack, bent micrometer frame, out of calibration error

5. Part errors Within the piece variation (taper, roundness, etc.) dirty parts poor surface finish flaws/ burrs

Direct reading instruments

1. Steel Rules

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

3. Micrometers (Can be Vernier, Dial, or Digital Reading) Outside frame, anvil, spindle, sleeve, thimble Inside Depth

Comparison 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 types 1. spring calipers 2. telescoping gages 3. small hole gages

4. adjustable parallels 5. planer gages

6. thickness gages 7. radius gages

Attribute Gages Go - No Go gages that only tell if the part is good or bad. They don’t provide a measurement of how good or bad.

8. Squares Machinists Combination Square Set

solid beam square Cylindrical Square

Indicators – 2 Types 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”

Indicator Bases mounting must be rigid magnetic bases flexible link holding arms height gage applications

Surface Plates

Primary measurement is height measurements are from the plate surface, up

Ideal plane for dimensional measurement should be perfectly flat nothing is perfect

Surface plate materials 1. Cast iron magnetic if damaged, raised material is produced on surface easily rust 2. Granite far superior produce no raised material lower in price retain flatness longer than cast iron do not rust

Care of surface plates no unnecessary objects on the surface clean before and after use keep it covered placing and removing objects from the plate

Accessories Gage blocks (rect. and square) Angle plate (Knee) V-blocks Fixed parallels Planer gage 1,2,3 Blocks

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)

1. Plate protractor discrimination of 1 degree

2. Bevel protractor part of the machinists combination set - discrimination of 1 degree

3. Universal bevel protractor - vernier measure to accurately to within 5 minutes of a degree (remember there are 60 minutes in one degree

4. Sine bar 5” and 10” center spacing of cylinders

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 blocks permit comparison between working measurement & instruments of mfg.

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 blocks

Range in thickness from .050 to 4.0”

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

Grade on tolerance = +/- .000002”

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

Use as few combinations as possible when stacking

Care 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 blocks

Main cause of gage block wear is wringing poorly cleaned blocks

Checking with optical flats

Wear blocks used in applications where direct contact is made usually .050” or .100”

Accessories scribes bases screw sets

Other machines Optical comparator Coordinate measuring machine (CMM)