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. For example, a grade 0 surface plate may only have a 3.5 μm (0.00014 in) deviation from flatness over a 250-by-250-millimetre (9.8 by 9.8 in) area. 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. Plates are typically square or rectangular, although they may be cut to any shape.