Surface plates used in precision locating, layout and inspection work are generally manufactured from two different types of materials Cast Iron or steel, or the most common material, granite. Granite is an igneous rock that could be of the type biotite granite, biotite hornblende granite, biotite muscovite granite, diabase, hypersthene gabbro or muscovite-biotite granite-gneiss. The various types of granite have different physical qualities and characteristics which generally are all suited for inspection work.
Milstandard GGG-P-463c specifies a variety of sizes for granite surface plates. 12" x 12" is the smallest size of surface plate considered by the standard. Surface plates have two characteristics that are normally measured in a field calibration: Flatness and Repeat reading.
Repeat Measurement specification states that a measurement taken anywhere on the surface of a plate will repeat or be equal to all other identical measurements within the stated tolerance. This is NOT the same as flatness. The flatness specification states that all points on the surface shall lie between two parallel planes separated by "X" distance, where "X" is the flatness tolerance. The flatness tolerance alone does not guarantee that accurate measurements can be taken on a plate, since the surface can consist of peaks and low spots equal to the full flatness tolerance. If the piece being checked bridges several peaks, and the gage rests in a low spot, then the measurement will have an error equal to the full flatness tolerance of the plate. If the gage rests on a slope, the error can be even greater than the full flatness tolerance.
The three standard grades of accuracy defined by the Federal Specification are:
(Tool Room) Grade B
(Inspection) Grade A
(Laboratory) Grade AA
The flatness tolerances for these grades are defined by the following formula:
Laboratory Grade AA = (40 + diagonal squared/25) x .000001" (unilateral)
Inspection Grade A = Laboratory Grade AA x 2
Tool Room Grade B = Laboratory Grade AA x 4.
In addition, Federal Specification GGG-P-463c deals with such issues as: repeat measurement accuracy material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection, installation of threaded inserts, which I will not all re-detail herein. It is of note that there is no defining specification for granite angle plates, parallels, or master squares, so manufacturers specifications should be utilized.
Manufacturers recommend that a plate be calibrated upon receipt, as soon as it has had a chance to normalize after shipment. Most surface plates when purchased new come with a Certificate of Accuracy, the only way to be absolutely certain that you have an accurate plate, which is within the stated tolerance is to have it calibrated on-site and in the environment that it will be used.
Many manufacturers recommend that a new plate receive a full recalibration within 1 year of purchase. If the plate is going to receive heavy use, it may be advisable to shorten this interval to six months.
A problem which arises when an untrained technician attempts to resurface a plate on-site, is the use of Repeat Measurements alone to certify a plate.
The instruments which are used to verify repeatability are NOT designed to check flatness. When set to zero on a perfectly curved surface, it will continue to read zero, whether that surface is perfectly flat, perfectly concave or convex 1/2"! They simply verify the uniformity of the surface, not the flatness. Only a plate which meets both the flatness specification AND the repeat measurement specification truly meets the requirements of Federal Specification GGG-P-463c.
Granite has a coefficient of thermal expansion of approximately .0000035"/1"/1°F. As an example: A 36" x 48" x 8" surface plate has an accuracy of .000075" (1/2 of Grade AA) at a gradient of 0°F (top and bottom are the same temperature). If the top of the plate warms up to the point where it is 1°F warmer than the bottom, the accuracy would change to .000275" convex!!! Therefore, ordering a plate with a tolerance tighter than Laboratory Grade AA should only be considered if there is adequate climate control.
Whenever scratches or nicks appear on a granite plate burrs can occur. They can be removed with a flat silicon carbide stone. Serious flaws, holes or nicks in a plate may be able to be repaired by qualified technicians.
When your plate is being recalibrated ensure that the technician is using one of the approved tools to include a repeatometer for repeat reading and one of the following for flatness: autocollimator, electronic levels, laser interferometer, master straight edge or planikator, or laser beam and detector.
Note that a surface plate does not need to be level in order to use or calibrate the plate, however certain calibration tools may require the plate to be level for the calibration tool to work. It is legitimate to downgrade the grade of a surface plate if the situation meets your specific metrological needs. Rotating your surface plate between calibrations, especially for plates that are often used is also a good idea to distribute the wear more evenly over the surface..