Surface plate: materials, grades and use

A surface plate is a heavy, rigid plate with a flat upper face that gives a reference plane for marking out, measurement and inspection of flat surfaces and of heights (with a height gauge or dial indicator).

Materials

• Cast iron: fine-grained, stress-relieved and ribbed underneath for stiffness; it can be scraped and is self-lubricating. It is the usual choice and is widely used in machine tool workshops.
• Granite: hard, and a scratch or chip does not raise a burr; it is non-magnetic, rust-free, has little thermal expansion and good wear resistance, so it is preferred in inspection rooms.
• Glass or ceramic: for small plates in light use.

Grades

Plates are made in grades 00, 0, 1 and 2 (IS 2285): 00 and 0 for laboratory and inspection, 1 for the inspection room, and 2 for workshop use. The permissible flatness error increases with size and with grade number.

Manufacture

The plate is cast, aged, rough machined and finished by grinding and then by hand scraping. The flat surface is produced by the three plate method: three plates are scraped in turn against each other (A against B, B against C, C against A) until all three mate perfectly; only true planes can fit each other in every combination.

Uses

Marking out, checking flatness with a straight edge, checking squareness and parallelism with a dial indicator and angle plate, as a base for sine bars and slip gauges, and for measuring heights with a vernier height gauge. The plate is kept clean and covered, supported at three points and checked regularly.