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Can tighter flatness tolerances than Laboratory Grade AA be achieved?

April 26, 2017AA achieved flatness grade laboratory tighter tolerances

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Can tighter flatness tolerances than Laboratory Grade AA be achieved?

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Yes, but they can only be guaranteed for a specific vertical temperature gradient. The effects of thermal expansion on the plate could easily cause a change in accuracy greater than the tolerance if there is a change in the gradient. In some cases, if the tolerance is tight enough, the heat absorbed from overhead lighting can cause enough of a gradient change over several hours. Granite has a coefficient of thermal expansion of approximately .0000035 inches per inch per 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, the 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.