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Surface Plate Calibrations
 |  | | Volume 2, No. 2 | Summer, 2008 |
| | | |  For many shops, inspection rooms and laboratories, surface plates are relied upon as the basis of accurate measurement. In the “Last Word” article in the Spring 2008 edition, basic cleaning, maintenance and accuracy checking were reviewed. Beyond that, comprehensive calibration by the manufacturer or an independent company is necessary from time to time. The interval varies by amount of usage, conditions within the surface plate's environment, and required accuracy,
Variations Between Calibrations
In some cases, there are variations between surface plate calibrations. These are often a cause for concern for the user, especially given the reliance of the surface plate as the first element in accurate measurement. Sometimes, factors such as surface change resulting from wear, or improper use of inspection equipment or use of non-calibrated equipment can account for these variations. The two most common factors, however, are temperature and support.
Temperature
One of the most important variables is temperature. For instance, the surface might have been washed with a hot or cold solution prior to calibration, and not allowed sufficient time to normalize. Other causes of temperature change include drafts of cold or hot air, direct sunlight, overhead lighting, or other sources of radiant heat on the surface of the plate.
There can also be variations in the vertical temperature gradient between winter and summer. In some cases, the plate is not allowed sufficient time to normalize after shipment. If at all possible, it is a good idea to record the vertical gradient temperature at the time the calibration is performed.
Surface Plate Support
Another common cause for calibration variation is a plate that is improperly supported. A surface plate should be supported at three points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered.
 Only three points can rest solidly on anything but a precision surface. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same three points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. All Tru-Stone steel stands have support beams designed to line up with the proper support points.
If the plate is properly supported, precise leveling is only necessary if your application calls for it. Leveling is not necessary to maintain the accuracy of a properly supported plate. | | | | | |
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