You need to be very careful when using a ( rheostat) speed controller for most hand held tools. The motors in most ,if not all, of these tools are universal motors (actually dc series motors).When you lower the speed you lower the torque,when this happens the current goes up and with it the temperture. They can overheat rapidly. This is not the same kind that maybe used with an ac motor to either vary the frequency or to chop off part of the voltage wave.
regards
jerry
regards
jerry