This technical bulletin discusses the accuracy and calibration of granite surface plates, based on the American national standard (ANSI) ASME B89.3.7-2013, Granite Surface Plates. Surface plates are made of various types of granite and are massive, stiff, and hard. The top work surface is lapped flat and smooth to provide a critical reference surface for many types of dimensional measurements. The quality of surface plates had been standardized for decades through the Federal Specification GGG-P-463c, which was made inactive in 2013 when the ASME B89.3.7 standard was published.
The focus of this technical bulletin is the surface plate flatness, which is the most important metrological characteristic of surface plates and is subject to routine calibration over time. ASME B89.3.7 defines standardized tolerance grades for surface plates, with the highest accuracy plates being Grade AA, Grade B plates being the lowest accuracy plates, and middle grade plates, Grade A, often used in inspection areas. Customers can order the size and quality of the plate needed.In practice, both the overall flatness and local variation in flatness need to be calibrated on a surface plate. Calibration is done by checking conformance of the surface plate to the two different tolerance values. The decision rule for statements of conformity to these specifications is Simple Acceptance with a test uncertainty ratio, TUR ? 4 for both Grade A and Grade B,