Surface plate

Flat surface used as a reference plane

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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 a workpiece, therefore one primary surface is finished extremely flat with tolerances below 11.5 μm or 0. mm per mm for a grade 0 plate. Surface plates are a common tool in the manufacturing industry and are often fitted with mounting points so that it can be an integrated structural element of a machine such as a coordinate-measuring machine, precision optical assembly, or other high precision scientific & industrial machine. Plates are typically square or rectangular, although they may be cut to any shape.

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, all grades of surface plates are held to a high degree of flatness.

Surface plates must be calibrated regularly to ensure that chipping, warping, or wear has not occurred. A common problem is wear to particular areas, such as that caused by the frequent use of a tool in one place (such as a height gauge), that causes an uneven surface and reduces overall accuracy of the plate, this may be greatly accelerated if abrasive dust is present. Tools and workpieces may also cause damage when dropped on the surface plate. Also, damage can be caused when swarf and other debris have not been removed. This will result in erroneous measurements. Damage to the plate can be corrected only by resurfacing, which requires specialised techniques and equipment depending on the grade of the plate.

History

Unlike most mechanical precision instruments, 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 manually rubbing them against each other in pairs with colouring matter in between, and then hand-scraping the high points. Any errors of flatness are removed by this scraping, since the only stable, mutually conjugate surface shape is a plane.

The importance of the high-precision surface plate was first recognised by Henry Maudslay. He originated the systems of scraping a cast-iron plate to flatness, rubbing marking blue between pairs of plates to highlight imperfections, and of working plates in sets of three to guarantee flatness by avoiding matching concave and convex pairs.

Joseph Whitworth described this process to the British Association as he related during his chairman's address in at the inaugural meeting of the British Institute of Mechanical Engineers in Glasgow. His paper, and his past work for Maudslay, has led to claims that Whitworth is the originator of the surface plate scraping technique, not Maudslay.

Plate material

Granite

Before the Second World War, metal was the standard material used for surface plates; however, the war efforts put a strain on the availability of metal. Wallace Herman and Donald V. Porter started using granite instead. 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 increases the wear resistance of the plate. Black granite is used for machine bases, granite accessories, and custom applications due to its superior stiffness, excellent vibration damping, and improved machinability. Quartz-bearing granite is often thicker to provide equal load-bearing capabilities, 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 plane. Even with a chip, another flat surface can still make contact with the undamaged portion of the 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 stable, non-magnetic, and rust-resistant.

Cast iron

Prior to World War II, most surface plates were made from ribbed cast iron to increase stiffness without adding weight. Cast iron is now frequently used on production floors as a tool for lapping granite surface plates to achieve grades of accuracy. Despite the drop in popularity, cast iron remains the most popular material for master surfaces among laboratory metrologists and other high-accuracy industries. Cast iron, properly cast, is more dimensionally and geometrically stable over time than granite and is easier to work to a higher grade of flatness. These specialized surface plates are produced in sets of three for regular verification and refinement.

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Cast iron has uniform optical properties, making it favorable for certain optical applications.

Glass

Glass is an alternative material used during World War II when manufacturing capacity was limited. Glass can be ground suitably and has the benefit of chipping rather than raising a burr, which is a problem with gray cast iron.

Accessories

The surface plate is used with accessories such as squares, straight edges, gauge blocks, sine bars, sine plates, dial indicators, parallels, angle plates, height gauges, etc.

Calibration of surface plates

Granite surface plate calibration should be done routinely to maintain flatness and ensure measurement accuracy over time. The interval between calibrations depends on the environment. Lapping or resurfacing is provided when needed to bring measurements within grade guidelines, which involves polishing the surface with abrasive paste. Calibration also includes cleaning and light polishing.

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