[philso]

techhelpbb is correct that some sort of feedback necessary to control the speed of a DC motor accurately.

An alternative is to use a 3-phase motor with a 3-phase motor controller. The motor RPM will follow the output frequency (not the PWM frequency) up to the maximum torque rating of the motor. Add a 10-turn potentiometer and you get infinitely variable speed without changing belts. It worked great on the drill press at the last place I worked at.