Surface plate

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A surface plate is a solid, flat table that can be confidently used as a datum for marking out, measuring workpieces, or setting up tooling. Considering that it is the baseline for all future measurements and that tolerances as low as ±1 micrometre or .000050" may be in use, its precision is critical. A well maintained surface plate is a must in any manufacturing application. Before you begin using a surface plate be sure to wipe it down with some Starrett cleaning solution or something of equal quality to remove any built up residue.

There are varying grades that are applied to the accuracy of some metrology equipment, A, B and Workshop grade are some of these. While workshop grade is the least accurate it is still required to be more accurate than the name workshop may suggest.

The table may be made from ribbed cast iron, the ribbing increases strength while reducing the weight. It will be aged and stress relieved to decrease the likelihood of it moving (twisting or warping) over time.

Glass is an alternative material and was used during the second world war when material and production dead lines were either in short supply or over stretched. Glass can be suitably ground and has the benefit that it chips rather than raises a burr like cast iron.

Granite is the best material of all as not only does it chip instead of burr, it is inherently stable. It is aged to the point where any further movement in the material will not affect any measurements based from it. Another benefit of the Granite is that it will allow you to work with material that is magnetic without interference. It is two times stronger than steel and you don't have to worry about it rusting.

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, other that having an item dropped on the surface, is that a specific area or section will be used with another tool such as a height gage and that wear will occur in that area. This will result in erroneous measurements and can only be fixed by resurfacing the plate.

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.