Surface Plate - Accuracy and Grade

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.