35.6 GAUGE BLOCKS
· The purpose of gauge blocks are to provide linear dimensions known to within a given tolerance.
· The requirements of gauge blocks are,
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- the actual size must be known
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- the faces must be parallel
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- the surface must have a smooth finish
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- the surfaces must be flat
· most gauge blocks are made by normal techniques, but the high accuracy is obtained by a process called lapping (discussed later)
· The materials gauge blocks are made from are selected for,
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- hardness
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- temperature stability
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- corrosion resistance
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- high quality finish
· type of gauge blocks
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- rectangular
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- hoke (square)
· there are four grades of blocks,
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- reference (AAA) - high tolerance (± 0.00005mm or 0.000002")
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- calibration (AA) (tolerance +0.00010mm to -0.00005mm)
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- inspection (A) (tolerance +0.00015mm to -0.0005mm)
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- workshop (B) - low tolerance (tolerance +0.00025mm to -0.00015mm)
· Original gauge block sets had lower tolerances and had a total of 91 pieces with values,
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0.010" to 0.100" in 0.001" steps
· An 81 piece set of gauge block was developed by Johansson(s??) and is capable of covering wider ranges of dimensions.
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0.1001" to 0.1009" in 0.0001" steps
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0.1010" to 0.1490" in 0.0010" steps
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0.0500" to 0.9500" in 0.0500" steps
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1.0000", 2.0000", 3.0000", 4.0000" blocks
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(2 wear blocks at 0.0500")
· An 83 piece set has also been developed and it has the values (in inches),
· The metric set has 88 gauge blocks (in mm),
· Most gauge block sets include thin wear blocks that should be included at the ends of a gauge block stack to protect the other gauge blocks.
· How to select gauge blocks for an application
· To assemble a gauge block stack,
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1. remove the gauge blocks required from the protective case
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2. clean of the oil that they have been coated in using a special cleaner. It is acceptable to handle the blocks, in fact the oil from your hands will help them stick together.
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3. one at a time, hold the blocks so that the faces just overlap, push the blocks together, and slide them until the faces overlap together. This will create a vacuum between the blocks that makes them stick together (this process is known as wringing).
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4. Make required measurements with the gauge blocks, being careful not to damage the faces
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5. take the blocks apart, and apply the protective coating oil, and return them to their box.
· When using gauge blocks, minimze the number used. Each block will have tolerance errors, and as the stack of blocks becomes larger, so does the error.
· Do not leave gauge blocks wrung together for long periods of time.
