What power stepper motors do I need?

[Monty_jon]

Hi

I have a Clarke CMD1225 milling machine which I would like to convert to CNC, what size stepper motors would be suitable to drive the X ,Y lead screws and the quill feed?

I was looking at some 3.1Nm but I’m not sure if they have the power to move the table?

[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.

[Robin Hewitt]

3.1Nm holding torque tells you very little, it's like judging a car's performance by the quality of the hand brake.

Our American chums started this by buying motors purely on holding torque. "I've got Zillion ounce inch motors, the reeel McCoy! Incidentally, why can't I feed faster than 30 inches per minute?" :heehee:

What you want is a graph showing torque against steps per second for the voltage you plan to drive it with. It will be there somewhere, if it isn't, try somewhere else :tup:

[Monty_jon]

Thank you,

Just out of interest, what sort of machining accuracy should I expect with the above CNC’d ?

[Robin Hewitt]

Just out of interest, what sort of machining accuracy should I expect with the above CNC’d ?

Clamp a DTI to the bed.
Run a tool up against it.
Lean on the spindle and measure the deflection.
That tells you what is possible.
Achieving 'possible' is the tricky bit :naughty: