My newer prototype has axis motor mounts with multiple bolt patterns to be able to mount NEMA 23, 24, and 34 motors; all you have to do is switch the coupler on the motor shaft. I've been planning for a while to use both steppers and the Teknic servos, my main problem is that this machine has so many axes that using appropriately sized Teknic servos on every axis would cost more than the rest of the machine. So I'm leaving it open to use steppers, Teknic servos, and/or O-Drive servos once they're a bit more developed. There is a company called CUI that makes various small shaft encoders that would be perfect for this, they mount easily.
Sadly, nobody makes decent affordable spindle motors for this type of stuff, which is why I'm using RC motors. The spindles will have their own P4 angular contact bearings at 45 and 65mm ID's, so the motor shaft will only have to take the force of the pulley, I'm trying to put a reinforcement bearing in as well to take some of the pulley load off of the motor shafts.
Based on what you said, I will need to use lower KV motors and limit high current to a few peaks. The power supply is a stack of four server PSUs, so I can do (theoretically) 3000 watts at just over 60 amps. Regular household power outlets are 120v 15 amp, which limits them to about 1500 watts for practical use before they trip breakers. But the 240v appliance outlets work with the PSUs as well, which will be adequate to drive all four PSUs at max load.
Using the 9.8/KV formula with a 6374 90KV motor, a maximum of say 50 amps to the spindle motor VESC would do (9.8/90) * 50 = 5.44 Nm = 4 ft-lbs. I hope that's within the saturation current.
The 90KV motor would limit this spindle to around 4000 RPM, but I'll only be using half of that with a spinning lathe chuck. The milling motor would need a higher KV, perhaps 130.
The peak of 4 foot pounds is kind of disappointing; it means I would only be able to take skim cuts on a 3 inch radius part. I guess I can use bigger motors and have a 2 speed pulley to compensate.
I have two videos of the mill-turn first prototype, here's the second one;