35' Engineering Metrology and Instrumentation

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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

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Engineering Metrology

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

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Engineering Metrology

• Postprocess Inspection
• In-process, on-line, real-time inspection
• Dimensional Tolerances
• The smaller tolerance?high cost, but more accurate

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Measurement Standard

• Inch, foot based on human body
• 4000 B.C. Egypt Kings Elbow0.4633 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

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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.

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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

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Line-graduated Instruments

• Graduated?marked to indicate a certain quantity
• Rules Steel rule (machinists rule), bar or
  tape, 1 mm or 1/64 in
• Vernier calipers after P. Vernier 1600s, caliper
  gages, 25 mm or 0.001 in,With digital readout
• Micrometers sensitivity 2.5 mm or 0.0001 in,

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Line-graduated Instruments

• Diffraction gratings with two flat optical
  glasses, interference fringes with 40 lines/mm
  (1000 lines/in) gratings. 2.5 mm or 0.0001 in
• Indirect-reading dividers, calipers, telescoping
  gage for holes and cavities.

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Comparative Length-Measuring Instruments

• Also called deviation-type instruments
• Dial Indicator using rack-and-pinion and
  gear-train mechanism, accuracy 1 mm or 40 min
• 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 mm or 5 min

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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

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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

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Gages

• Heat treated, stress-relieved alloy steels or
  from carbide
• C.E. Johansson 1900s Gage blocks 0.05 mm or 2
  min
• 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

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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 makers rule)
• At least 10 times accurate than the tolerance
• Sensitivity
• Stability (drift) capability to maintain
  calibrated status

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Precision vs. Accuracy

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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
  0.001mm

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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

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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

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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)

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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

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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

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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.

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End of Metrology ME 300 Ch 35