will be using them, so the plates may be regularly verified and refined, including by the Whitworth three plate method, without the need to send them out to be reconditioned. Despite its high stability, cast iron remains unsuitable for use as a normal surface plate in high-tolerance production applications because of thermal expansion. The nature and use of a master surface, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable. Cast iron, unlike granite, has very uniform optical properties and, unlike glass or ceramic, very small light penetration depth which makes it favorable for certain optical applications. Glass Glass is an alternative material and was used during World War II when material and manufacturing capacity were in short supply. Glass can be suitably ground and has the benefit that it chips rather than raising a burr, which is a problem when using gray cast iron. Accessories The surface plate is used in conjunction with accessories such as a square, straight edge, gauge blocks, sine bar, sine plate, dial indicator, parallels, angle plate, height gauge, etc. Calibration of surface plates Granite surface plate calibration should be performed routinely to maintain proper flatness and ensure measurement accuracy over time. The intervals between calibrations depends on the environment where the surface plate is located. When needed, lapping or resurfacing is provided to bring measurements within grade guidelines. This procedure involves polishing the surface with an abrasive paste to remove all unwanted material. Granite surface plate calibration also includes cleaning and a light polishing. References Bibliography Further reading External links Two surface plates made by Whitworth Metalworking measuring instruments Quality control