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 the 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 x 100 mm to 2500 mm x 1600 mm.

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.

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.