It makes sense, what I understand is when a 200 steps stepper motor is used for the 4th axis, it stays dead locked until the rated torque is reached. So basically I could machine a stock on 200 faces with the ultimate precision (which is a 3 to 6 microns variations as stated by Neale - more than enough for me, as I'm not planing to machine air bearing for instance) dictated by the belt (which is the weakest link in the chain). The stepper will never act like a spring unless micro stepping comes into the equation. To me it sounds like a good option to drive the C axis.

Regarding your particular project and the servo motors, I understand your not happy with how it performs when the spindle takes the C axis role. Is that because the servos tend to act like a spring all the time, no mater the position, due to the lack of steps?

Also, there are these hybrid servo (servo stepper) motors. Do they also come with 200 steps + microsteps + all the other benefits of a servo motor? Or are they just as described by you, and 'hybrid' is just marketing? It's a bit confusing for me. If just marketing, I understand a 200 steps stepper would still be the best option for the C axis, provided there is no brake involved, and provided one must never ever go beyond the rated torque? Also, I'm sure there should be some electrics which could shut down everything in case a step is skipped (but this is not important right now).

I want to connect 2 motors to the headstock spindle. One for turning and one for the C axis. For turning I've got a 2.2kW AC. I'll invest in some 3kW VFD and make an indexer (not sure yet how). But at least I know the direction I should investigate. Now I'm looking for a good compromise for the C axis. The machinery will act either as a lathe, or as a 4 axis milling machine, but NOT at the same time. The AC motor will be permanently engaged (but powered off while in milling mode). I need to find a good way to disconnect the C axis motor while in lathe mode (even if I have to manually disengage the belt), so there would't be any backlash involved. The lathe and the mill would share the X Y axis motors. A BT30 mill would be fitted on the Z axis, to take care of the milling operations. I hope it makes sense.

Please don't hesitate to write down anything you think it may help (or not :) ).

Cheers!