Well, just to complete this thread I have finished the stepper mount plate and machined a pulley for the stepper.
With a belt connected the whole thing fits neat and I can spin the chuck under the stepper power from within Mach 3 ...treating it as the A axis.



If I move the A axis 3.75mm the chuck does a single revolution. If I move the x axis along as well then I can cut threads easily.

It’s slow and needs some lube on the cutter, but the result is pretty satisfying.

There are lots of things I can do with this tool, and given the cost and time I’m really happy with it.