Granite surface plates are used as a reference plane for making dimensional measurements, such as flatness and height. They are widely used by industrial quality-control personnel. The surface plate must be calibrated to determine how flat it is.
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Created in 1955 by J.C. Moody, this method of calibration is widely accepted because it costs less, can be performed by semi-skilled technicians and provides sufficient accuracy for most applications.
An autocollimator with its mirrors and a straight edge is the most common equipment used to calibrate a surface plate. Alternatives include other angle-measuring equipment, such as a laser interferometer with electronic levels.
Prior to calibration, the surface plate must be thoroughly cleaned. A series of measurement guidelines are drawn on the plate. Lines are drawn corner to corner along the length and width. Lines are also drawn diagonally corner to corner, from mid-length to mid-length and from mid-width to mid-width. These lines are used as guides for placing the mirrors during measurements.
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