Gauge Blocks

Each gauge block is a precisely formed tool to judge distances between opposing faces. Often, gauge blocks come in sets that consist of blocks of various sizes, along with two wear blocks. The block are length are actually slightly shorter than what is stamped on them, because the stamped length includes the length of one wring film. This overall length is known as the interferometric length.

In use, the blocks are removed from the set, cleaned of their protective coating and wrung together to form a stack of the required dimension, with the minimum number of blocks. Gauge blocks are calibrated to be accurate at 68 °F (20 °C) and should be kept at this temperature when taking measurements. This mitigates the effects of thermal expansion. The wear blocks are included at each end of the stack, whenever possible, because they protect the gauge blocks from being damaged in use.

Gauge Block Grades
They are available in various grades depending on their intended use. Various grading standards include: JIS B 7506-1997 (Japan)/DIN 861-1980 (Germany), ASME (US), BS 4311: Part 1: 1993 (UK). Tolerances will vary within the same grade as the thickness of the material increases.

reference (AAA): small tolerance(±0.05 μm or ±0.000002 in) used to establish standards
calibration (AA): (tolerance +0.10 μm to −0.05 μm) used to calibrate inspection blocks and very high precision gauging
inspection (A): (tolerance +0.15 μm to −0.05 μm) used as toolroom standards for setting other gauging tools
workshop (B): large tolerance (tolerance +0.25 μm to −0.15 μm) 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 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