page 177

• Next, knowing the gauge blocks are all very close in size, the stack of 8 blocks are wrung together into one pile, and compared to the master block using a comparator. The difference in heights, divided by eight, is the error in each block.

35.6.2 Compensating for Temperature Variations

•As gauge blocks change temperature, they also change size. The metals chosen for gauge blocks do resist this dimensional change, but will generally undergo some.

•The gauge block sets will carry dimensional readings, as well as rated temperatures. It is advised that all readings be taken at these temperatures, but if this is not possible, then some estimate of the dimensional change can be done.

•Basically this is done by using the difference between specified measurement temperature, and actual measurement temperature. This difference is multiplied by the coefficient of linear thermal expansion to give the change in size. This is obviously for small changes in temperature.

•Typical coefficients of linear thermal expansion is,

Steel 9.9 - 13.0 * 10-6 in./(in.°C) (typical is 11.5)

Bronze 16.7 * 10-6 in./(in.°C)

Aluminum 23.0 * 10-6 in./(in.°C)

Chrome carbide 8.4 *

Tungsten carbide 4 *

Cervit (?) -0.2 *

• Note the units are also ppm/°K

35.6.2.1 - References

Doiron, T., NIST, Personal Correspondence.

35.6.3 Testing For Known Dimensions With Standards

•When a dimension is well known, it can be measured by comparison to standards, using high precision, but limited range comparison instruments.

•Most gage blocks are steel which has a non-trivial coefficient of thermal expansion. But, considering that many parts are made of steel, these blocks will expand at approximately the same