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What are the grades that granite surface plates come in and how are they different?

The grading system for granite surface plates is based on the degree of flatness and tolerance that is accomplished. The American Society of Mechanical Engineers (ASME) offers required specifications that go beyond the quality of flatness in order to standardize the use of granite surface plates. These additional qualities include: support point location, surface finish, methods of inspection, repeat measurement accuracy, material properties of granites, and more. However, the three main grades for standard granite surface plates follow the federal specifications for flatness tolerance. Those grades are: Laboratory Grade AA which means the plate has a flatness tolerance equal to (40 + diagonal squared/25) x .000001" (unilateral), Inspection Grade A which has a flatness tolerance equal to Laboratory Grade AA times two, and Tool Room Grade B which has a flatness tolerance equal to Laboratory Grade AA times four.
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