Gage Block Interferometry

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Gage Block Interferometry
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Gage Block Calibration

• Gage Block comparator
• Differential measurement
• Gage Block Interferometer

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Gage Block Comparator
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Gage Block Interferometer
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Gage Block Interferometry
O
Surface
S Source O Optical flat l - wavelength
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Continued
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Gage Block Interferometry
Pattern of fringes
Example
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Gage Block Interferometry
Gage Length Interferometer
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Gage Block Interferometry
Choose the mth and nth fringe, then
Length of gage l X Y (m f)l/2 nl/2 (m
n f)l/2 (m n) unknown, but being integers
their difference must be an integer, say M. l
(m n f)l/2 (M f)l/2 Suppose l is
approximated using nominal length L, L Ml/2 or
M 2L/l
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Continued
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• Problem
• Suppose that two blocks stand side by side on a
  finely finished surface and that an optical flat
  is placed across them, as shown in the previous
  slide. A is a gage block whose end faces are
  finished flat and parallel to the very high order
  which would be required of a gage block and B
  is a test block. Suppose that in the flat above
  A four fringes, straight and parallel, as
  shown, are seen, while above B seven fringes
  are seen as shown, then the height of the test
  block at each of the four corners, or at any
  other point could be determined.
• Since there are four fringes seen above A we
  know that the slope of the flat is at the rate of
  4 l/2 per ½ in. and in a direction
  perpendicular to the fringes.

Block A
Block B
Assumption l wavelength of light 0.000028 in
2 in
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• Height of point a
• Distance between blocks rate of change of slope
  of the flat for 2 in.
• Height of point c
• The point c is not higher than a as a is
  the point of contact. From the figure in the
  previous slide we can see that there are 5
  fringes between points a and c. Therefore,
• Height of point c Height of point a 5l/2
• Height of point b
• Even if b and d lie on the same horizontal
  plane as a, there would be a gap due to the
  inclination of the flat. Therefore,
• Height of b Height of a rate of change of
  slope for 0.5 in.(width of block
• B actual depth of gap
  ( 3l/2)

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• Height of point d
• Similarly, working diagonally from a to d we
  get