Flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface. This flatness measurement commonly carries a tolerance and may include a grade designation.
The flatness tolerances for three standard grades are defined in the federal specification as determined 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.
For standard sized surface plates, we guarantee flatness tolerances that exceed the requirements of this specification. In addition to flatness, ASME B89.3.7-2013 & Federal Specification GGG-P-463c address topics including: repeat measurement accuracy, material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection, installation of threaded inserts, etc.
Tru-Stone granite surface plates and granite inspection plates meet or exceed all of the requirements set forth in this specification. At present, there is no defining specification for granite angle plates, parallels, or master squares. Our standard tolerances for these items are the tightest in the precision granite industry.