Originally Posted by
digits
Without trying to sound like a smart arse, I think you are going to have to do some Googling and calculations of your own here. Stepper torques are usually provided by the people who sell them, but you will need to do some maths to work out what sized steppers you will need before you start spending.
From what I have read, a stepper and a timing belt won't make a great 4-th axis - all commercial systems seem to use worm gears, harmoinic drives or multi-kW direct drive servos. The problem with using a stepper without a worm is that the cutting force can easily exceed the stepper's holding torque and drive the rotary axis backwards. Worm gears tend to be self-locking and are pretty much impossible to back-drive.
I would also suggest you work out the torque you are going to be putting on your 5th axis due to the weight of your 4-th axis and that 3-jaw chuck.
As for cutting forces - I wish I had a handy link for you - I can't seem to find the ones I used. IMHO though 0.625" is pretty large actually, and given that you said you want to cut aluminium with small cutters, shouldn't you be looking for something with way,way more rpm than that Grizzly (and coolant!)?