Quote Originally Posted by Toolmaker51 View Post
Right off, I think suggestions will center more on a variable DC motor.
Edit into original post the existing motor horsepower, RPM and frame size if known. A big, clear shot of the motor tag would be ideal.
Second questions will be of the lathe itself; metal or wood turning, range of speeds offered by the present belt change setup, possibly what variety electrical service you have.
Ooops, on re-reading you have a mill, not a lathe. Ideas related above still apply, mostly. Achieving low gear speed range will be bigger issue, whether a DC can deliver good torque.
3 ph moters can be purchased in C face you can make an adapter plate to fit your mill you will have to make sure the motor shaft will fit your pulley.