This paper will review tolerancing schemes for spherical radii, including the historical origins. It finishes with recommendations for specifying tolerances for spherical radii.

1. POWER TOLERANCE VERSUS RADIUS TOLERANCE

There are two main paths for tolerancing spherical radii: power tolerance and linear radius tolerance. Both measure change relative to a nominal value, but the metrology used is the key difference.

Power Tolerance: Controls radius through increments of sag change relative to a test plate of known radius. The sag difference between the known radius and the surface under test is measured, from which the radius of the surface can be calculated... 

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