Best Practice for Surface Plate Mastering
Use this minimalist gaging technique to get improved, accurate measurement results.
Surface plates are simple and extremely versatile. They provide a broad, smooth, flat reference surface that can be used to inspect incoming, in-process or finished parts. When used in combination with various gages and accessories, such as height gages, gage blocks, angle plates and squares, they can check a wide range of parameters, including length, flatness, squareness, straightness, angle, feature location and runout.
Surface plates are available in a wide range of sizes, from about 12 × 12 inches to 6 × 12 feet, and can weigh as much as 10 tons. They are available in three grades, the flatness tolerances for each grade varying with the size of the plate: AA (laboratory grade), A (inspection grade) and B (toolroom grade). Many surface plates can be ordered with ledges and threaded inserts, both of which make it easier to clamp workpieces or accessories to the surface. Granite is the most common material used in their construction; it is harder and denser than steel, has very little internal stress and is less subject to dimensional change due to temperature variations.
Any of the checks mentioned above can be done using a height gage and indicator as the measuring instrument and a gage block stack as the zero reference. However, there is a gaging technique that anyone can adapt to improve the overall performance of the measurement. It’s one that will improve those measurements when gaging performance to 50 microinches or 1 micron is required.
The typical process for making a measurement on a surface plate is to place the part in one spot on the surface, the gage block stack in another and the height gage in a third. This relationship is shown in Figure A. The height gage is first moved to the gage block along path (a) then on to the part along path (b). Once on the part, the deviation between the gage block and the part is read on the indicator.