A thick slab of black or pink granite, or cast iron, lapped flat to within millionths of an inch and graded by flatness: AA for lab, A for inspection, B for toolroom use. Height gauges, indicators, parallels and parts sit on it so measurements are taken from a known plane. It is kept clean, covered and free of hammering, and periodically recalibrated.
Put the part on the surface plate and run the height gauge over it, I want to see how flat that face really is.
Nothing gets hammered, filed or set down hard on the granite; a chip in the plate is a chip in every measurement after it.
Also heard as
- granite plate
- granite
- the plate
Related terms
Test indicator
A lever-type dial indicator with a swiveling stylus, used to align setups and compare surfaces.
Jo blocks
Shop name for gauge blocks: lapped steel or ceramic blocks, accurate to millionths, stacked to make any length.
Dykem
Shop name, from a trade name, for blue layout fluid painted on metal so scribed lines show up bright.
Parallels
Matched pairs of ground steel bars used to raise a part in the vise to a known, level height.