Geometric Dimensioning and Tolerancing Template - PowerPoint

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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
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 tolerancehigh 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.
  Ferneldistance 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
  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.
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
 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 mm or 0.001 in],With
  digital readout
 Micrometers; sensitivity [2.5 mm or
  0.0001 in],
Line-graduated Instruments
 Diffraction gratings; with two flat optical
  glasses, interference fringes with 40
  lines/mm (1000 lines/in) gratings. [2.5
  mm 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 [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]
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 [0.05 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
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 yearsultraprecision ion-beam
    machining 0.001mm
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

				
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