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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)