3. Engineering Metrology
■ Postprocess Inspection
■ In-process, on-line, real-time inspection
■ Dimensional Tolerances
– The smaller tolerance🡪high cost, but more
accurate
4. 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. Fernel🡪
distance between Paris and Amiens
5. 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 Paris
🡪39.37 in
■ In 1960, 1,650,763.73 wave length in
vacuum of the orange light given off by
electrically excited krypton 86.
6. 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
7. Line-graduated Instruments
■ Graduated🡪marked 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, [25 μm or 0.001 in],With
digital readout
■ Micrometers; sensitivity [2.5 μm or
0.0001 in],
8.
9. Line-graduated Instruments
■ Diffraction gratings; with two flat optical
glasses, interference fringes with 40
lines/mm (1000 lines/in) gratings. [2.5
μm or 0.0001 in]
■ Indirect-reading; dividers, calipers,
telescoping gage for holes and cavities.
10. Comparative Length-Measuring Instruments
■ Also called deviation-type instruments
■ Dial Indicator; using rack-and-pinion and
gear-train mechanism, accuracy [1 μm or 40
μin]
■ 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, [0.125
μm or 5 μin]
11. 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
12. 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
13.
14. Gages
■ Heat treated, stress-relieved alloy steels or
from carbide
■ C.E. Johansson 1900s Gage blocks [0.05 μm
or 2 μin]
– 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
15. 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
17. 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 years🡪ultraprecision ion-beam
machining 0.001μm
18.
19. 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
20.
21. 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
22. 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)
23. 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
24. 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
25.
26.
27.
28. 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.
