Surface plate

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 0.0115 mm per 2960 mm for a grade 0 plate. Surface plates are a common tool in the manufacturing industry and are often permanently attached to robotic-type inspection devices such as a coordinate-measuring machine. Plates are typically square or rectangular. 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 0.0115 mm per 2960 mm for a grade 0 plate. Surface plates are a common tool in the manufacturing industry and are often permanently attached to robotic-type inspection devices such as a coordinate-measuring machine. Plates are typically square or rectangular. 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. 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. Tools and workpieces may also cause damage when dropped on the surface plate. Also, damage can be caused when swarf and other debris has not been removed. This will result in erroneous measurements. Damage to the plate can only be corrected by resurfacing. Unlike most mechanical precision instruments, surfaces 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 around 1800. 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, born in 1803, had been an apprentice with Maudslay from 1825 but had left by the time he started his own business in 1833. He described this process to the British Association in 1840 in his paper 'On producing True Planes or Surfaces on Metals'—as he related during his chairman's address in 1856 at the inaugural meeting of the British Institute of Mechanical Engineers in Glasgow. His 1840 paper, and his past work for Maudslay, has led to some writers claiming Whitworth as the originator of the surface plate scraping technique, not Maudslay. 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 dampening, 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.