You don't say what the holding torque is for that motor, but for 4-5" material, my opinion is that even the 570 ozin NEMA 23's are too weak for a diameter that large.
You will need some sort of mechanical reduction. I recommend 2:1 for each inch of desired diameter. Example 3" diameter = 6:1 reduction.
With direct drive and 200 steps per revolution your step resolution would be near .078" at 5" diameter totally unacceptable. At eighth steps it would still be over .009" which is still poor. With 6:1 reduction and eighth steps resolution is around .0015. Assuming a commonly used .006 to .008 stepover with nicely detailed 3D files you want to have a minimum of 3-4 steps to get to your required stepover to not induce rounding error distortion.
Chucks seem to perform the best, but they must have a setscrew to prevent winding off, if threaded on. If used gently for on center 3D machining a spur may also work.
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