I'll agree with everything
@matthewsx posted.
That said, I've converted two mills to 3 phase motors - a Grizzly round column (RF-30 clone) with dual belts, and my current PM knee mill, with a single belt and back gear. I find the variable speed feature to be very useful, for two main reasons:
1. Power tapping. The lowest speed available with belts is a lot faster than I feel safe to use. In addition, a VFD allows nearly instant reversing of the spindle direction. The knee mill does have a spindle brake, but it's inconvenient to get to when paying attention to the tap. The Grizzly did not have a spindle brake, so if I were to even try power tapping, I'd have to be very good at anticipating!
2. Convenience. With two belts, resetting speed on the Grizzly was kind of a headache. Oftentimes, the belt on the bottom would have to be repositioned above the other one. On that mill, I found the "sweet spot" belt combination and left it there nearly all the time - used the VFD to change speeds for different workpieces and cutters - everything from steel to plastic, 3" shell mills to ¼" end mills. I'm still learning the ins and outs of the knee mill, but it looks like I've again found the "sweet spot" belt setting.
As others have said, converting to a 3 phase motor and VFD will cost a few hundred bux, some time, and some rewiring. The existing forward-off-reverse switch will have to be rewired as a VFD input. It should NOT be left between the VFD and motor. And of course, you'll want to add a speed control pot somewhere handy.
Try your mill as is for a while. And consider your priorities. A DRO might well be a handier addition than a variable speed motor. It does look like you have a nice mill there!
