You want between 4 and 5 volts only.
Easy way is to clip onto a NIMH pack if you have one with side-by-side configuration, then you just clip on the battery bars.
I use an old CPU Power supply on the 5 volt rail. It's about 4.8 volts at the motor.
The speed of the motor (via input voltage) also affects the way it will cut. Too slow or too fast and you won't get a nice finish.
BTW, most lathes with slave motors are wired backwards, make sure you check which way it is running or you will kill the bit in a second!