engineering metrology and instrumentation(8)
TRANSCRIPT
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
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.