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A surface plate is a flat, level plane used as a reference to determine if other objects are precisely flat and level. Surface plates are usually used in the manufacturing industries, though there are meteorological and optical applications for the measurement of tools as well. Most surface plates are made of cast iron or granite, though some plates are created from glass or other materials.
Before the mid-twentieth century, most surface plates were cast iron. Cast iron surface plates had a ribbed surface to decrease their weight while retaining accuracy. Within the tooling industries, cast iron surface plates have been largely abandoned due to changes in plate shape from thermal stress and less abundance of iron metal. When temperature is not an issue, as in meteorological and optical applications, cast iron is still the preferred material. The fact that cast iron is nearly impenetrable by light also makes it particularly well-suited for precision optical work.
Currently, granite surface plates are the norm in the manufacturing industries. Wallace Herman, owner of a monument and metal shop in the United States, was the first to use granite as a material for surface plates. His change to granite was spurred by the unavailability of metal after World War II. Since he already had a surplus of granite from his tombstone business, his change to granite as a material quickly caught on. Granite surface plates have the advantages of being non-magnetic and rust-resistant. Today, a granite surface plate usually has a high quartz content for increased durability.
Glass surface plates are relatively rare. Glass as a material for surface plates was used during times of metal shortages, such as during World War II, when metal was in high demand for military weaponry. Glass surface plates must be laminated and toughened in order to prevent them from shattering. These surface plates are surprisingly durable and, although they may chip, are accurate.
Regardless of the material, all surface plates should be calibrated at least annually to ensure accuracy. Surface plate calibration can detect inaccuracies from wear and tear on the plate. Wear to surface plates can occur from temperature variations, failure to rotate the plates regularly or breakage. Chipping also becomes an issue and can cause measurement errors, as can warping from overuse or continual temperature fluctuations. Any inaccuracies found during calibration can be corrected through reconditioning of the surface plate.
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