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A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup. The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with accuracy up to 0.00001 in or 250 nm for a grade AA or AAA plate. Surface plates are a very common tool in the manufacturing industry and are often permanently attached to robotic type inspection devices such as a coordinate-measuring machine.
Accuracy and grade
There are varying grades used to describe the accuracy of some metrology equipment such as: AA, A, B and Workshop grade. While workshop grade is the least accurate, the plates are often held to a high degree of flatness.
Unlike most instruments of mechanical precision, surface plates do not derive their precision from more-precise standards. Instead they originate precision by application of the principle of "automatic generation of gages". In this process, three approximately flat surfaces are progressively refined to precise flatness by manual rubbing against each other in pairs with a fine abrasive. Any errors of flatness tend to wear off with this abrasion, since the only stable, mutually conjugate surface shape is a plane. Joseph Whitworth described this process to the British Association in 1840 in his paper The Mode of Producing a True Plane as he related during his chairman's address in 1856 at the inaugural meeting of the British Institute of Mechanical Engineers in Glasgow.
Surface plates must be calibrated on a regular basis to ensure that chipping, warping or wear has not occurred. A common problem with surface plates are specific areas or a section that is frequently used by another tool (such as a height gauge) that will cause wear to a specific point resulting in an uneven surface and reduced overall accuracy to the plate. Tools and workpieces may also cause damage when dropped on the surface plate or when material chips have not been removed. This will result in erroneous measurements and can only be fixed by resurfacing the plate.
Before the Second World War, metal was the standard material used for surface plates, however, after 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 started using granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite with less common surface plates being made of pink granite or diabase. The quartz content of surface plate determines the wear ability of the granite surface plate with higher quartz content being more resistant to wear. Pink granite contains much higher levels of quartz and is often thinner than black granite, which reduces the load-bearing capabilities of the surface plate.
Damage to a granite surface plate will usually result in a chip but does not affect the accuracy of the overall plane. Even though 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, non-magnetic, 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 recognized 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.
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 surface masters (different usage from a surface plate) 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 without the need to send the plates out for external rework. Despite the very stable structure, cast iron remains unsuitable even in high tolerance production applications for use as a normal surface plate due to thermal expansion encountered with regular use as an inspection tool. The nature and use of a surface master, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.
Cast iron unlike granite has also very uniform optical properties, and unlike glass or ceramic material very small light penetration depth which makes it a favorable material for certain optical applications.
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 cast iron.
- Whitworth, Joseph (April 1856). "The Institute of mechanical Engineers in Glasgow: Chairman's address". Practical Mechanics Journal. 2 1: 173–175. Retrieved 26 October 2011.
- B89.3.7 Granite Surface Plates.
- Society of Manufacturing Engineers, (1991). Fundamentals of Tool Design. Dearborn, Michigan: Society of Manufacturing Engineers. ISBN 0-87263-412-4
- Wayne R. Moore (1970), Foundations of Mechanical Accuracy, The Moore Tool Company
- Two surface plates made by Whitworth
- "Granite Surface Plates". L.S. Starrett Company. Retrieved 2012-01-27.
- "Frequently Asked Questions". L.S. Starrett Company. Retrieved 2007-02-15.