Grinding Wheel Surface Speed and Max Safe RPM
The maximum RPM a grinding wheel diameter may turn at its rated surface speed, and whether a grinder's fixed speed stays under it: SFPM = pi x wheel diameter (in) x RPM / 12, so max RPM = rated SFPM x 12 / (pi x diameter). A 7 in wheel rated 6,500 SFPM tops out at 3,547 RPM, so a 3,450 RPM bench grinder runs it at 6,322 SFPM -- within rating; a 10 in wheel rated 9,000 SFPM on a 3,600 RPM machine hits 9,425 SFPM, OVER its rating. As the wheel wears its safe RPM rises but the machine speed is fixed. Per ANSI B7.1; the wheel blotter rating and the machine nameplate are the authority -- never mount a wheel on a faster machine.
Formula and source
SFPM = pi x wheel_diameter_in x RPM / 12; max_rpm = rated_max_sfpm x 12 / (pi x wheel_diameter_in); actual_sfpm = pi x wheel_diameter_in x grinder_rpm / 12; within rating when grinder_rpm <= max_rpm.
Surface-speed identity, first-principles; the wheel's rated maximum operating speed (SFPM) comes from the wheel blotter and the machine speed from the nameplate. Per ANSI B7.1.
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