grading criterion is tightness of tolerance on their sizes; thus higher grades are made to tighter tolerances and have higher accuracy and precision. 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) 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 The ANSI/ASME standard follows a similar philosophy as set forth in ISO 3650. See the NIST reference below for more detailed information on tolerances for each grade and block size. Also consult page 3 of: Commercial Gauge Block Tolerances (Length refers to the calibrated thickness) Manufacture Gauge blocks are usually made either from hardened alloy tool steels, ceramics or cemented carbides (such as tungsten carbide or tantalum carbide). Often the carbide has a hardness of 1500 Vickers hardness. Long series blocks are made from high-quality steel having cross section (35 × 9 mm) with holes for clamping two slips together. These are also available in carbon steel material. Steel blocks are hardened and tempered. The hardness is important because it slows down the gauge's rate of wear during use (this is why other kinds of gauges, such as pins, thread plugs, and rings, are also hardened.) The cutting of the blocks to size is accomplished with grinding followed by lapping.