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 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. The importance of the high-precision surface plate was first recognised by Henry Maudslay around 1800. Any errors of flatness are removed by this scraping, since the only stable, mutually conjugate surface shape is a plane. 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. Instead they originate precision by application of the principle of "automatic generation of gages". Unlike most mechanical precision instruments, surface plates do not derive their precision from more-precise standards. Damage to the plate can be corrected only by resurfacing, which requires specialised techniques and equipment depending on the grade of the plate. This will result in erroneous measurements. Also, damage can be caused when swarf and other debris have not been removed. Tools and workpieces may also cause damage when dropped on the surface plate. 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. Surface plates must be calibrated regularly to ensure that chipping, warping, or wear has not occurred. While workshop grade is the least accurate, all grades of surface plates are held to a high degree of flatness. There are varying grades used to describe the accuracy of some metrology equipment such as: AA, A, B, and Workshop grade. Plates are typically square or rectangular, although they may be cut to any shape. 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. 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. A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup.
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