I have a VFD on my mill and DC motors with speed controllers on my
lathes but I don't think they give quite enough adjustment range to be
used with only one belt speed.
chuck

I find for general prototyping and small production run work, the "constant
torque" provided by the VFD is fine at the lower spindle speeds (versus the
"constant horsepower" that a mechanical vari-speed tries to provide which
gives you even higher torque at low spindle speeds). If I was pushing my
lathe and mill to the max cutting capabilities at the lower spindle speeds
perhaps the VFD would be limiting.

But with a 2HP motor on both my lathe and mill, I've never had any need for
more power at the lower speeds using the VFD's. I set up both VFD's to run
from 20% underspeed to 200% overspeed, giving me a 10 to 1 speed range. Add
in the back gears and that a very wide speed range on both machines.

But if you're happy with your mechanical vari-speed drives, stick with them.
In my case on the Clausing, I didn't like the extra noise and maintanence
hassles. On the Bridgeport I was tired of changing belt pulley settings, but
I didn't realize how much I would like the automatic spindle braking and
quick reversing until I had it.

Paul T.