I'm sure there have been a few other guys with this question..
I couldn't find much though, so here we go. I apologize if I'm missing something obvious, and if you do find other threads where this has been beaten to death, please link me to them!
I'm converting an RF-40 type machine to CNC. Got it all going with the stock leadscrews, then took it apart to install ballscrews. At the same time, I found an incredible deal on a brand new, but incomplete, RF-45. It's missing a motor and table, maybe a few other minor bits, but I have a complete column and head assembly that I can bolt onto my RF-40.
So the plan is to use my usable machine to convert the new head to belt drive, do whatever else I need to move the head up/down as my Z axis, rather than the quill on my current machine, then swap the column and head into my base/table.
Long story short, here's what I thinking for the belt drive. I have a brand new 2hp 3ph 3500rpm Toshiba motor and a VFD. The VFD manual tells me it'll run continuously with full torque at 6Hz. Seems a bit of a stretch, but if that is the case, theoretically I can run the motor from 7000rpm@120Hz, down to 360rpm@6Hz. Obviously those are pretty extreme limits, but I'm considering just going with a single 0.75 ratio on the belt drive, which would give me spindle speeds of 270rpm to 5250rpm.
For my purposes, I anticipate I'll run between 1000 and 3000rpm most of the time.
Does all of that sound reasonable? I know, ideally I'd be running within 20% of the motors rated speed the whole time, but.. Running a single speed belt drive rather than 2 different ratios would make the system much simpler.