Surface plates are flat planes used as a reference for determining flatness and levelness in manufacturing, meteorological, and optical industries. Cast iron was commonly used before mid-20th century, but now granite is the norm due to its non-magnetic and rust-resistant properties. Glass surface plates are rare but were used during metal shortages. All surface plates should be calibrated annually to ensure accuracy.
A surface plate is a flat plane used as a reference for determining whether other objects are exactly flat and level. Surface plates are usually used in manufacturing industries, although there are also meteorological and optical applications for tool measurement. Most surface slabs are made from cast iron or granite, although some slabs are made from glass or other materials.
Prior to the mid-20th century, most surface plates were cast iron. The cast iron surface plates had a ribbed surface to reduce their weight while maintaining accuracy. Within the tool industries, cast iron surface plates have largely been abandoned due to changes in plate shape due to thermal stress and lower iron metal abundance. When temperature is not an issue, such as in meteorological and optical applications, cast iron is still the material of choice. The fact that cast iron is almost impenetrable to light also makes it particularly suitable for precision optical work.
Currently, granite slabs are the norm in manufacturing industries. Wallace Herman, a monument and metal shop owner in the United States, was the first to use granite as a surface slab material. His move to granite was prompted by the metal’s unavailability after World War II. Since he already had a surplus of granite from his stone business, his move to granite as a material quickly caught on. Granite surface plates have the advantages of being non-magnetic and resistant to rust. Today, a granite surface slab usually has a high quartz content for greater durability.
Sheets with a glass surface are relatively rare. Glass as a surface plate material was used during times of metal shortages, such as during World War II, when metal was in high demand for military weapons. Glass surface plates must be laminated and tempered to prevent cracking. These surface plates are surprisingly tough, and while they can chip, they’re accurate.
Regardless of material, all surface plates should be calibrated at least once a year to ensure accuracy. Platen surface calibration can detect inaccuracies due to platen wear. Surface plate wear can occur due to temperature changes, failure to rotate the plates regularly, or breakage. Chipping also becomes a problem and can cause measurement errors, as can deformation from excessive use or constant temperature fluctuations. Any inaccuracies found during calibration can be corrected by reconditioning the plate.
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