Gage Block Calibration Tolerances
Gage (gauge) blocks are available in various grades, depending on their intended use. The grading criterion is tightness of tolerance on their sizes; thus higher grades are made to tighter tolerances and have higher accuracy and precision.
Tolerances will vary within the same grade as the thickness of the material increases.
• reference (AAA): small tolerance (±0.05 um) used to establish standards
• calibration (AA): (tolerance +0.10 um to −0.05 um) used to calibrate inspection blocks and very high precision gauging
• inspection (A): (tolerance +0.15 um to −0.05 um) used as toolroom standards for setting other gauging tools
• workshop (B): large tolerance (tolerance +0.25 um to −0.15 um) used as shop standards for precision measurement
More recent grade designations include (U.S. Federal Specification GGG-G-15C):
• 0.5 — generally equivalent to grade AAA
• 1 — generally equivalent to grade AA
• 2 — generally equivalent to grade A+
• 3 — compromise grade between A and B
and ANSI/ASME B89.1.9M, which defines both absolute deviations from nominal dimensions and parallelism limits as criteria for grade determination.
Generally, grades are equivalent to former U.S. Federal grades and they are as follows:
• 00 — generally equivalent to grade 1 (most exacting flatness and accuracy requirements)
• 0 — generally equivalent to grade 2
• AS-1 — generally equivalent to grade 3 (reportedly stands for American Standard - 1)
• AS-2 — generally less accurate than grade 3
• K — generally equivalent to grade 00 flatness (parallelism) with grade AS-1 accuracy