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Granite surfaces plates are key instruments in inspection labs across the country. The calibrated, extremely flat surface of a surface plate enables inspectors to use them as a baseline for part inspections and instrument calibration. Without the stability afforded by surface plates, many of the tightly toleranced parts in various technological and medical fields would be much more difficult, if not impossible, to manufacture correctly. Of course, to use a granite surface block to calibrate and inspect other materials and tools, the accuracy of the granite itself must be assessed. Users can calibrate a granite surface plate to ensure its accuracy.
Clean the granite surface plate prior to calibration. Pour a small amount of surface plate cleaner on a clean, soft cloth and wipe the surface of the granite. Immediately dry the cleaner off the surface plate with a dry cloth. Do not allow the cleaning liquid to air-dry.
Place a repeat measuring gauge on the center of the granite surface plate.
Zero the repeat measuring gauge to the surface of the granite plate.
Move the gauge slowly across the surface of the granite. Watch the gauge's indicator and record the peaks of any height variations as you move the instrument across the plate.
Compare the flatness variation across the surface of the plate with the tolerances for your surface plate, which varies based on the size of the plate and the flatness grade of the granite. Consult federal specification GGG-P-463c (see Resources) to determine if your plate meets flatness requirements for its size and grade. The variation between the highest point on the plate and the lowest point on the plate is its flatness measurement.
Check that the largest depth variations on the surface of the plate fall within the repeatability specifications for a plate of that size and grade. Consult federal specification GGG-P-463c (see Resources) to determine if your plate meets repeatability requirements for its size. Reject the surface plate if even a single point fails repeatability requirements.
Stop using a granite surface plate that fails to meet federal requirements. Return the plate to the manufacturer or to a granite surfacing company to have the block re-polished to meet specifications.
Perform formal calibrations at least once per year, although granite surface plates seeing heavy use should be calibrated more frequently.
Formal, recordable calibration in manufacturing or inspection environments is often conducted by quality assurance or an outside calibration services vendor, although anyone can use a repeat measuring gauge to informally check a surface plate prior to use.