Granite Before the
Second World War, metal was the standard material used for surface plates, however, the war efforts of various countries put a strain on the availability of metal. A monument and metal shop owner (Wallace Herman) in Dayton, Ohio, along with his inventive employee Donald V. Porter, started using
granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite, more accurately referred to as black
diabase, with the more wear-resistant surface plates being made of quartz-bearing granite. The quartz content of these granite surface plates increases the wear resistance of the plate as quartz is a harder stone. Black granite is dominantly used in machine bases, granite accessories, and custom applications for its superior stiffness, excellent vibration damping, and improved machinability. Quartz-bearing granite (usually pink, white, or grey) is often made thicker than black granite to provide equal load-bearing capabilities of the types of material used for surface plates, as it is not as stiff as black granite. Damage to a granite surface plate will usually result in a chip but does not affect the accuracy of the overall plane. Even though it is chipped, another flat surface can still make contact with the undamaged portion of a chipped surface plate, whereas damage to a cast-iron plate often raises the surrounding material above the working plane causing inspected objects to no longer sit parallel to the surface plate. Granite is also inherently stable, is non-magnetic, has excellent
vibration damping characteristics, and will not rust. On 3 August 1961, Federal Specification GGG-P-463B was issued to provide requirements in
United States customary units for igneous rock (granite) surface plates for use in precision locating layout, and inspection work. It encompassed new certification, recertification in the field, and recertification after resurfacing. GGG-P-463B was later revised and reissued on 12 September 1973 as GGG-P-463C, which provided common language and terms of classification for surface plate manufacturing and commerce. On 15 June 1977 an amendment was issued to the federal specification in order to include requirements in
metric units. Although GGG-P-463C was used extensively in American industry since its publication, the government did not issue any new revisions to keep up with advancements within industry. The
American Society of Mechanical Engineers (ASME) decided to form a committee to revise the federal specification in accordance with modern technologies. Most notably, a more complete glossary was added with currently accepted definitions, and a new format was used that should be more familiar to current users of the Standard. ASME also recognised the need for updates to incorporate modern concepts such as traceability and measurement uncertainty that have undergone considerable development since 1973. In June 2013, ASME replaced Fed Spec GGG-P-463C with the American National Standard (ANS) ASME B89.3.7 – 2013 Granite Surface Plates. Iso standard defines ISO8512-2 for granite surface plates, but it seems the current in use is still dating back 1990.
Cast iron Prior to World War II, almost all surface plates were made from ribbed
cast iron with the ribbing used to increase stiffness without incurring the weight of solid construction. The cast iron was
aged to reduce
stress in the metal in an effort to decrease the likelihood of the plate twisting or warping over time. Cast-iron surface plates are now frequently used on production floors as a tool for
lapping granite surface plates to achieve certain grades of accuracy. The metal allows itself to be impregnated with the lapping media over a large flat surface. Despite a fall in popularity among machine shops, cast iron remains the most popular material for master surfaces (different use from a surface plates) among laboratory
metrologists, machine builders, gauge makers, and other high-accuracy industries that have a requirement for gauging flatness. Cast iron that has been properly cast is more dimensionally and geometrically stable over time than granite or ceramics, is more easily worked to a higher grade of flatness, and provides a better bearing surface to assist the creation of other master standards. These specialized surface plates are produced in sets of three, by the company that 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==