Surface plate

A surface plate is a solid, flat 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 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. Plates are typically square or rectangular. One current British Standard includes specifications for plates from 160 mm × 100 mm to 2500 mm × 1600 mm.

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 with surface plates 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 reduced overall accuracy to 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.

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.

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 colouring matter in between then hand scraping off the high points. Any errors of flatness are removed by this scraping, since the only stable, mutually conjugate surface shape is a plane. Joseph Whitworth, who had been an apprentice with Maudslay, 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. Whitworth, born in 1803, worked as an apprentice for Maudslay from 1825 but had left by the time he started his own business in 1833. His 1840 paper, and this past work for Maudslay, has led to some writers claiming Whitworth as the originator of the surface plate scraping technique, not Maudslay.

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