Gauge blocks or other standards serve as the known dimension and are used to judge the unknown accuracy of the micrometer. Gauge blocks or other standards used to calibrate a micrometer must have a certificate of traceability issued by an agency such as the National Institute of Standards and Technology of the U.S. Department of Commerce. The accuracy of gauge blocks is typically plus or minus 0.000002 of an inch. The accuracy of a micrometer is typically plus or minus 0.0001 of an inch.
Ted Doiron and John Beers in "The Gauge Block Handbook" note the widespread use of gauge blocks as a measuring standard and the most common method to calibrate precision devices such as micrometers. The basic design of gauge blocks has not changed in the past 70 years. However, the use of gauge blocks has increased due to an increased need for precision measurement.
Requirements for gauge blocks are specified by such organizations as American National Standards Institute, the International Standards Organization and the American Society of Mechanical Engineers.
The accuracy of gauge blocks is specified by a system of four tolerance grades. Grade 0.5 is within one-millionth of an inch of the specified nominal length. Grade One has an accuracy of one half of Grade 0.5. Grades 0.5 and One are generally used as masters to check other gauge blocks. Grade Two has an approximate accuracy of one half as great as Grade One. Grade Three has an accuracy approximately one half as Grade Two. Grades Two and Three are generally used to calibrate measuring devices such as micrometers.
Recommended procedures for the calibration of micrometers is detailed in the "Erie County Technical School Calibration Manual." The micrometer must be inspected for any sign of damage such as a bent anvil, chipped measuring faces and loose or missing parts prior to the calibration procedure. All parts of the micrometer must be kept clean during calibration. The micrometer must move freely without excessive play in travel. If excessive play is present, the play must be eliminated by tightening with a spanner wrench.
A dimension of the gauge block will be measured with the micrometer. The reading must be within 0.0005 of inch. If the reading falls outside of this range, the micrometer must be adjusted with the spanner wrench. The procedure will the repeated with the gauge block. Three readings within the tolerances must be obtained using the gauge block. If the readings fall out of tolerances in the three attempts, the micrometer is considered out of tolerance and should be removed from service.