Proper Support of Granite Surface Plates
Improper support of granite surface plates is a common problem throughout the manufacturing industry. Proper support of a granite surface plate is easily accomplished, and will help to assure greater accuracy, repeatability, and reproducibility. Improper support will cause the surface plate to warp, and if the plate is moved from the location at which it was calibrated, the calibration of the surface plate is nullified. Quality Control Sales & Services, Inc. receives many calls requesting the method to properly support surface plates. Please refer to the following explanation for details on properly supporting surface plates.
Reasons for the 3 point support method
There is no written specification regarding the movement of plates and recalibration requirements due to the fact that it is understood that a 3 point support does not transfer anomalies between 2 surfaces. This holds true for all things (such as height masters, riser blocks, etc.), not just surface plates. The phenomenon that causes things such as surface plates to warp is uneven support, which at its worse will cause "rocking" of the plate on a two high point axis, and it will rest on a third point. This is not possible with a 3 point support.
While there is no written specification that addresses the moving of surface plates, please read the following excerpt¹:
Q8.1 How should a surface plate be supported?A. At only 3 points, preferably located 1/5 the length and width from the ends and sides. Only 3 support points can rest solidly on anything other than a precision surface. The surface should be supported only at these three points while being lapped, and it should be supported only at the same 3 points when being used. Any attempt to use more than 3 points will cause the plate to receive its support on various combinations of three points, which probably will not be the same 3 points at which it was supported while being lapped, thus causing errors.
As you can see from the excerpt, the important thing is that the plate is supported at 3 points. The relationship of the stand to the floor becomes irrelevant with the proper 3-point support. The most important thing is that the 3 point support present at the time of calibration is the same 3 point support at the time of use. For this reason, it is generally best to physically attach the support to the plate to ensure that the support present at the time of calibration is also the support present at the time of use. Without 3 known points of support, it would not be possible for a surface plate manufacturer to lap and calibrate a surface plate prior to shipment.
The following is an excerpt of Federal Specification GGG-P-463c: Plate, Surface, Granite, Inch and Metric. This specification lists the recommended support points for granite surface plates. Please note that the specification recommends 1/5 to 1/4 length or width, while QCSS recommends that the support be as close to 1/5 length and width as possible. This adds to the stability of the plate when working with heavy items.
Excerpt from Federal Specification GGG-P-463c, paragraph 3.2.5:
3.2.5 Supports. Unless otherwise specified, support of the surface plate shall be by three fixed feet, located according to figures 5 and 6 to support the work surface properly, and to minimize sag and warp. When the three fixed supports have special requirements due to their location, abnormal load and/or vibration conditions, the supports and their locations shall be specified on the purchase order (see 6.2(e) and appendix 40).²
- Rectangular plates. The support pads shall be located no less than one-fifth or more than one-fourth of the length and width in from the ends and sides, respectively, with the exception that the single pad at one end shall be located in the center (see figure 5).
¹ Excerpt taken from "Questions and Answers About Surface Plates" by the Rahn Granite Surface Plate Company.
² Figure 6 , paragraph 6.2e, and appendix 40 omitted for brevity.