35. Engineering Metrology and Instrumentation

Measurement Standards Line-graduated Instruments Measuring Straightness, Flatness, Roundness

and Profile Coordinate Measuring and layout Machines Gages Optical Instruments Automated Measurement Geometric Dimensioning and Tolerancing

Engineering Metrology

Measurement of dimensions– Length– Thickness– Diameter– Taper– Angle– Flatness– profiles

Engineering Metrology

Postprocess Inspection In-process, on-line, real-time inspection Dimensional Tolerances

– The smaller tolerancehigh cost, but more accurate

Measurement Standard Inch, foot; based on human body 4000 B.C. Egypt; King’s Elbow=0.4633

m, 1.5 ft, 2 handspans, 6 hand-widths, 24 finger-thickness

AD 1101 King Henry I yard (0.9144 m) from his nose to the tip of his thumb

1528 French physician J. Ferneldistance between Paris and Amiens

Measurement Standard 1872, Meter (in Greek, metron to

measure)- 1/10 of a millionth of the distance between the North Pole and the equator

Platinum (90%)-iridium (10%) X-shaped bar kept in controlled condition in Paris39.37 in

In 1960, 1,650,763.73 wave length in vacuum of the orange light given off by electrically excited krypton 86.

Measurement Standard Sensitivity (Resolution); the smallest

difference in dimensions that the instrument can detect or distinguish

Precision; the degree to which the instrument gives repeated measurements of the same standard (sometimes called accuracy)

Standard measuring temperature 20 0C Instrument, gage

Line-graduated Instruments Graduatedmarked to indicate a

certain quantity Rules; Steel rule (machinist’s rule), bar

or tape, [1 mm or 1/64 in] Vernier calipers; after P. Vernier 1600s,

caliper gages, [25m or 0.001 in],With digital readout

Micrometers; sensitivity [2.5m or 0.0001 in],

Line-graduated Instruments

Diffraction gratings; with two flat optical glasses, interference fringes with 40 lines/mm (1000 lines/in) gratings. [2.5m or 0.0001 in]

Indirect-reading; dividers, calipers, telescoping gage for holes and cavities.

Comparative Length-Measuring Instruments

Also called deviation-type instruments Dial Indicator; using rack-and-pinion and gear-

train mechanism, accuracy [1m or 40 in] Electronic gage; using sensors (strain gage,

inductance or capacitance), – LVDT (Linear variable differential transformer) for

small displacement Laser Scan micrometer; non-contact

measurement; high temperature, too elastic or brittle material, on-line measuring, [0.125m or 5 in]

Other Measurements

Straightness; Knife edge, dial indicator, autocollimator (like a telescope with a light beam that bounces), Optical (transits and laser)

Flatness; dial indicator, precision steel square, interferometry (if not flat, light fringes are curved)

Roundness; – V-block with dial gage, read TIR(total indicator

reading) – Circular tracing; platform rotates

Other Measurements Profile

– Template, profile gage to check shape conformity

Measuring screws and gear teeth– Threaded plug gages, screw-pitch gages

(similar to radius gages), micrometer with cone shaped points, snap gages

Optical contour projector Coordinate measuring machines

Gages Heat treated, stress-relieved alloy steels or from

carbide C.E. Johansson 1900s Gage blocks [0.05m or

2 in]– Grade 0.5 (AAA)-reference gage, very high precision

work– Grade 1 (AA)-laboratory grade, for calibration of

instruments– Grade 2 (A+)-precision grade, tool room and

inspection– Grade 3 (A)-working grade, use in production

Plug gage, GO gage, NOT-GO (NO-GO) Pneumatic (air) gage

Selection of Measuring Instruments Accuracy

– The degree of agreement of the measured dimension with its true magnitude

Magnification (amplification) Precision Resolution

– the smallest dimension that can be read on an instruments

Rules of 10 (gage maker’s rule)– At least 10 times accurate than the tolerance

Sensitivity Stability (drift); capability to maintain calibrated

status

Precision vs. Accuracy

Tolerance Tolerare; put up with, endure Impossible to make perfect parts Too small tolerance, cost is high Boeing 747-400 has 6 million parts,

measurement of 28 features, 150 million measurements

NIST (U.S. National Institute of Standard and technology); tolerance shrink by a factor of 3 every 10 yearsultraprecision ion-beam machining 0.001m

Importance of tolerance

Parts from the same machine can be different– Speed of operation– Temperature– Lubrication– Variation of incoming material– Other factors

ISO system; definitions

Definitions Allowance; the specific difference in

dimensions between mating parts Basic size; dimension from which limits of

size are derived Bilateral tolerance; deviation from the basic

size (+ or -) Clearance; the space between mating parts Clearance fit; fit that allows for rotation or

sliding between mating parts Datum; theoretically exact axis, point, line or

plane

Definitions Feature; Physically identifiable portion of a part,

e.g. hole, slot, pin, chamfer Fit; the range of looseness or tightness Geometric tolerancing; tolerances that involve

shape features of the part Hole-basis system; tolerances based on a zero

line on the hole Interference; negative clearance Interference fit International Tolerance grade (IT); a group of

tolerances that the same relative level of accuracy within a grade (varies depending on basic size)

Definition Limit dimension; Maximum and minimum

dimension of a part, MMC(maximum material condition); condition

where a feature of size contains the maximum amount of material within the stated limits of size

Nominal size; Positional tolerancing; A system of specifying the

true position, size, and form of the feature of a part, including allowable variation

Shaft-based system; Standard size Transition fit; Fit with small clearance or

interference that allows for accurate location of mating parts

Tolerances

Unilateral tolerancing Zero line; reference line along the basic

size from which a range of tolerances and deviations are specified.

Limits and Fits See the separate lecture note

Micrometer

The micrometer is a basic precision measuring tool. To a machinist, this tool is indispensable.

The engineer should also realize the importance of being able to use this tool properly. This discussion should help in mastering the use of the common "Mike," whether it is an outside, inside or depth type.

End of Metrology ME 300 Ch 35