Originally Posted by
digits
I suppose the first question I need to ask is how much spindle power do you actually need - and better still, if you know it, what sort of torque at what sort of speed? Those homeshopcnc servos are good for 600W continuous (~0.8HP) and can be driven cheaply with a G320.
I'm guessing though that you'd rather have a 3-phase induction motor for your spindle. If you want to simply index without cutting, unless you have very irregular parts , I can't see that you'd need much torque at all for the indexing. So, why not just drive your main spindle with a drive capable of indexing and add an encoder for feedback? If you don't need 3HP to index, couldn't you poweroff your VFD, disconnect it from the motor and then use a cheap AC-servodrive (e.g. Granite Devices) to index at only 100-200W of power?
Two motors can be made to work, but IMHO it is a bad idea to keep them both mechanically connected at the same time - couldn't you just physically disengage the indexing motor from its belt when you change modes?