Set your vfd up to run your motor at whatever speed the motor is rated at. Run your mill at Hi speed. Switch to lo when you need lower speed and/or higher torque. You will find it easier to turn the speed down using your vfd though you will lose some torque doing so. Assuming you have the original motor on your mill, if the motor runs for a long time at lower speeds using the vfd it may not get adequate cooling. I’ve never run my mill long enough to detect any significant heating of my milk’s motor but I rarely mill anything other than small angles or bars.

What I like most about vfds is they have dynamic braking so you can quickly stop your mill and don’t have to wait for it to coast down. Most modern vfds come with braking resistors so there’s no wiring or mechanicking needed. Just set a few parameters in the vfd and you’re good to go.

I’ve had Hitachi brand vfd on both my Bridgeport mill and 16” lathe for well over 10 years. They’ve been real workhorses.

Dan