It is the same as we calculate the RPM for magnetic drilling machine.
The magnetic drilling machines are provided with speed gears and some machines are also equipped with variable speed control. As per the diameter of the annular cutter and the type of annular cutter the speed of the magnetic drilling machine has to be set.
The formula for calculating the speed is:
d = diameter of the annular cutter in mm.
v = cutting speed of the annular cutters. See below chart for cutting speeds.