Q5 - What is meant by "grades" of accuracy?

A5 - There are three grades available for granite surface plates as per Federal standard GGG-P-463c. Laboratory Grade "AA", Inspection Grade "A", and Toolroom Grade "B". If the flatness tolerance is known for laboratory grade "AA" then grades "A" and "B" are two and four times that of "AA" respectively. The following chart lists the tolerances for the more common sizes of granite surface plates.

SIZE Laboratory "AA" Inspection "A" Toolroom "B"

12"x12" 0.000050" 0.000100" 0.000200"
12"x18" 0.000050" 0.000100" 0.000200"
18"x24" 0.000075" 0.000150" 0.000300"
24"x36" 0.000100" 0.000200" 0.000400"
36"x48" 0.000200" 0.000400" 0.000800"
48"x72" 0.000350" 0.000700" 0.001400"
48"x96" 0.000500" 0.001000" 0.002000"

Q6 - What is meant by repeatability/repeat check measurement?

A6 - Repeat check measurement is performed to determine the variation which may exist on the plates surface. Since the surface of a plate can consist of peaks and valleys equaling the full flatness tolerance, it is possible that the repeat reading can exceed the full flatness of the plate. An example of this situation would be if your measuring instrument sits on the slope of a peak and your part sits in the valley, this would cause an angle which could register error values that can exceed the full flatness found on a plates surface. Repeat measurement is not to be confused with flatness. It is no NO way the same as flatness and a repeat measurement gauge CANNOT be used for the verification of flatness. Federal Standard GGG-P-463c defines flatness as "all points of the work surface shall be contained between two parallel planes, the base plane and the roof plane, separated by a distance no greater than that specified
for the respective grade"