What power stepper motors do I need?

[Jonathan]

Everyone says 'What power steppers do I need' when steppers are sold by torque and other ratings are more important... oh well.

I have exactly the same milling machine and naturally converted it to CNC a few weeks after buying it. The feedrates were good using the 3Nm steppers you suggest. If I put the heavy vice and or rotary table on it does have to slow down, but not significantly.

Are you going to replace the ACME screws with ballscrews? I'm going to at some point as it can eliminate the problems caused by so much backlash and will also go faster due to the better efficiency of ballscrews. Instead of mounting the stepper motor to the quill feed it is much better to add a ballscrew to the Z-axis by attaching the end of the screw to a plate machined to fit round the spindle. The ballscrew pushes on the plate to move the spindle up and down. You'll also want to be able to disengage the screw when changing taper tooling as otherwise the force from hammering on the drawbar to release the tool is transmitted to the ballnut/screw.

There are plenty of threads on this forum about converting similar machines that are worth reading.