Chapter 14: Optical Metrology

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Chapter 14 Optical Metrology

• Overview
• Principles of the microscope
• Applications
• Comparison of optical comparators and microscopes
• Optical considerations
• Applications of the optical comparator
• Accuracy of optical comparators

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Optical Metrology

• Two major divisions of microscopy
• Direct measurement linear measurement, surface
  topology and metallurgical measurement,
  measurement of features inaccessible by other
  means
• Positioning position the part then use another
  instrument to make the measurements. For
  placement, use the displacement method
• Two main instruments microscopes and optical
  comparators

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Principles of The Microscope

• If use electronic instrument ?setup
• None of these instruments measures directly ?
  calibrate against a standard
• Keep both eyes open when using a magnifier
• Eyepiece magnification 10X
• The recommended magnification
• Size of detail Magnification 0.1-0.01in. 6.6
  X-30X
• 0.01-0.001in. 20X 60X
• 0.001-0.0005in. 40X-90X 0.0005-0.00001in. 80
  X-150X

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Principles of The Microscope
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The Measuring Microscope

• Reticles standard ?limitation
• Magnification? ?field of view? ?microscope pick
  ups the reference feature mechanical/electronic
  instrument perform the rest of the measurement
• E.g. toolmakers microscope

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Microscope Reticles

• Applications surface topology, examination of
  grain size in metallurgy
• To set part features and read scales cross
  lines or cross hairs.
• Low accuracy.
• The line thickness 5?m-7.62 ?m ?setting to 1.27
  ?m use low magnification ( 2.5X-5X for
  objective 10X for the eyepiece)
• Narrow feature ? broken lines
• For picking up scale lines ? bifilar reticle
• For precise work ? angle lines (30O) ?symmetry

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Microscope Reticles
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Applications

• Surface topology applications
• Methods use angular cut or plane of light
• Requirement require great magnification (to
  2000X)
• Other applications
• Use optical instruments microscope
  ?inaccessible part features
• Use as an accessory to other instruments
• Locating part features, read scale without
  contacting
• Use with CMM or CNC machines locate the part
  features (center finder, edge finder)

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Applications
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Comparison of Optical Comparators and Microscopes

• An optical comparator projects a greatly
  magnified image of a part feature onto a screen
  for examination
• The advantage of optical comparator over
  microscope
• Larger field of view
• Many people can be involved in measurement and
  analysis
• Direct measure on the screen
• Simple photographic adapter (photograph screen)
• Less eyestrain ? reduce fatigue in measurement
• Disadvantages
• Bulk
• Expensive

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Optical Considerations

• In the basic optics system for a comparator
  increase in magnification ?increase the length d
  l l (xf) (xf) f(2m 1/m)
• Special problems of profile projection if we
  illuminate the object with converging beams of
  light ? distortion ?use collimated beam

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Special Problems of Profile Projection
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Optical Considerations

• The projection screen
• Translucent screen displays magnified images
  projected on the back side of the screen ?use
  for most of comparators, parallax phenomenon
• Opaque screen projection from the front (similar
  to a moving picture screen movie) ?highest
  accuracy, use for largest optical comparators
• Folded optics
• To solve the problem of magnification? ? the
  length of optical system?
• Solution1 design a system with shorter focal
  length f
• Solution 2 use a folding mirror long optical
  path, high accuracy, any work piece size and
  magnification

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The Folded Optical System
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Applications of the Optical Comparator

• Reasons for the importance of optical comparator
• Examine a large number of part features in two
  dimensions simultaneously
• Direct measurement ? reliable
• Can use imaginary construction points or ref.
  Points
• Convenience
• Four situations that CMMs provide better results
  than optical comparators
• When need to exam the 3rd axis in the same setup
  as the others
• When closer tolerances are required
• Need direct calculating computing capacity
• When available CMM has enough capacity for the
  work piece

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Direct Measurement

• When?
• Whenever possible
• The work piece suitable for profile projection
• Can display the longest feature on the screen
• Tolerances are within the range of the instrument
• Measure the length of objects on the screen using
  a drafting scale or a chart (tool room chart
  gage) ? compare arcs and measure angles , not for
  contour lines
• Limitation size of screen ? translation
• The optical comparator allows to check geometry
  at the same time of measurement

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A Tool Room Chart-Gage
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Tracer Technique

• Use for contour inspection
• Hold the workpiece on the worktable ? mount the
  tracer directly to the comparator ? image of the
  workpiece on the screen
• Limitation contour length ? follower probe
  attached to the tracer probe
• Variation reticle gage follower ? no limitation
  on the workpiece size or amplification
• The follower tracer ? tracer moves relative to a
  fixed gage draw on the screen
• The recticle-gage tracer, the gage draw on the
  instrument moves relative to a fixed ref. On the
  screen

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Tracer Technique
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Tracer Technique
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Tracer Technique
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Accuracy of Optical Comparators

• Two classes of errors instrument and operator
• Operator error ? use wrong setup or wrong
  calculations
• If error ? check arithmetic first ? other
  causes
• Accuracy, direct measurement typical errors are
• Accuracy of projection at the screen (0.003in.)
• Accuracy of reading at the screen (0.002 in.)
• ?reduce by decreasing magnification
• Accuracy, chart-gage errors
• Chart-gage errors ? ? magnification factor?
• Magnification
• Method of gage layout
• Dimensional stability of the gage material (
  glass, plastics, etc.)