Length Measurement

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• Length Measurement

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Length measurement I

• Issues related to measuring length
• Fundamental dimension in Engineering Metrology.
• Generally confined to simple form.
• Separation of two points in two planes or
  surfaces.
• Length affected by flatness and parallelism of
  planes.
• Surface finish and form are the two fundamental
  limits for precise definition of length.

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• Measurement of length
• Reference artifacts e.g., Gage Blocks, Ring
  gages.
• Small measuring instruments e.g., Vernier
  Calipers, Micrometer, Height gage etc.
• Comparators
• Coordinate Measuring Machines

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Length measurement II

• Relationship to definition of meter
• Not practical to check instruments against
  standards.
• Gage blocks are a practical solution.
• Gage Blocks History
• In 1895, Swedish Engineer Carl E. Johansson
  conceived the idea of using built-up gages
  instead of several special gages for measurement.
• The original Johansson gage block set consisted
  of 102 blocks arranged in three series. These
  were made by a specially designed lapping
  process.
• By proper selection of individual blocks a wide
  range of different lengths could be built in very
  fine increments.

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Gage Blocks

• Purpose
• The essential purpose of gage blocks is to make
  available end standards of specific lengths by
  temporarily combining several individual elements
  each representing a standard dimension - into a
  single large bar.
• Major Requirements for Gage Blocks
• The individual elements must be available in
  dimensions
• needed to achieve any combination of sizes
  within the
• designed range and graduation of the set.
• The accuracy of the individual blocks, in every
  significant
• respect, must be within accepted tolerance
  limits.
• In the built-up combination, the individual
  blocks must be attached so closely to each other
  that the resulting bar will have a length which,
  for most practical purposes, is equal to the
  added sizes of the individual blocks of the
  assembly.
• The attachment of the elements to each other must
  be firm enough to permit a reasonable amount of
  handling as a unit, yet when taken apart, the
  individual pieces should be reusable without any
  harm to their original size or other essential
  properties.

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Gage Block Tolerance grade ASME B89.1.9 - 197

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Gage Blocks
The surface of the gage block viewed through an
optical flat. The straight pattern of the
interference fringes proves the excellent
flatness of the gaging surface
CROSS-SECTIONAL DIMENSIONS AND TOLERANCES OF GAGE
BLOCKS
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FLATNESS, PARALLELISM AND SURFACE ROUGHNESS
TOLERANCES OF GAGE BLOCKS
LENGTH TOLERANCES OF GAGE BLOCKS
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Gage block Materials

• High carbon, chromium alloyed steel
• Good hardenability.
• Minimum quenching distortions.
• Reasonable wear resistance.
• Relatively low prices.
• Easy to manufacture.
• Other properties like hardness, homogeneity and
  stability have been improved.
• Tungsten Carbide and Chromium Carbide
• High wear resistance.
• More expensive than steel.
• Fused Quartz
• Wear resistant.
• Can be polished to a high degree of smoothness.
• Do not produce protruding burrs when scratched in
  use.

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AVERAGE COEFFICIENT OF THERMAL EXPANSION OF
MATERIALS COMMONLY USED FOR THE MANUFACTURE OF
GAGE BLOCKS AND FOR COMPONENT PARTS IN
ENGINEERING PRODUCTION
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Composition of typical Gage Block sets
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Gage Blocks Method of use

• Wringing
• The steps in wringing together, rectangular gage
  blocks are as below
• Hold the blocks in a crosswise position.
• Exert some pressure and swivel them slightly
  until they adhere to each other.
• Slip the adhering blocks together into a parallel
  position with slight pressure.
• The wrung blocks form a single unit for the
  purpose of instrument setting and measurement.

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SUGGESTED PROCEDURE FOR SELECTING THE INDIVIDUAL
BLOCKS TO BE COMBINED INTO THE REQUIRED TOTAL
LENGTH
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Example for a combination of gage blocks into a
specific total length. Available set of 81
blocks. Desired dimension 1.7865 inch.
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Principle of dimensioning for limit gages
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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