Superior Electric Steppers

[Agathon]

My gut instinct for a machine like that with direct drive steppers, would be something in the 4Nm range to give plenty safety margin.

I think personally I'd consider altering things to use a belt drive with some Nema23 steppers, which should give you better performance. With big Nema34 motors, for best performance you've really got to use high voltage drivers, but you're then still limited at torque drops of quite quickly with big steppers.

Thanks. I had considered driving via a belt just in order to make the machine more compact. Would you gear up in order to increase the speed of the ball-screw relative to the stepper?

[m_c]

Opposite way!
You want to decrease speed of the ballscrew in relation to the stepper, so you get more torque at the screw. Somewhere around a 2:1 ratio would be a good starting point.

Try running some figures through the spreadsheet to see what kind of speeds you can get with different ratios, while still having usable torque to handle the cutting forces.

[Agathon]

Opposite way!
You want to decrease speed of the ballscrew in relation to the stepper, so you get more torque at the screw. Somewhere around a 2:1 ratio would be a good starting point.

Try running some figures through the spreadsheet to see what kind of speeds you can get with different ratios, while still having usable torque to handle the cutting forces.

OK, that's certainly how I would have dealt with torque issues with a conventional motor, but I thought the whole point was that torque went down as speed increases. I see that gearing down, by say 1:2 would give twice the torque, but getting the speed up to 1800 mm/min (900rpm for the motor) for rapids will mean that the motor is well outside it's optimum torque region. Or is it the case that in practice this wouldn't be an issue?

[m_c]

OK, that's certainly how I would have dealt with torque issues with a conventional motor, but I thought the whole point was that torque went down as speed increases. I see that gearing down, by say 1:2 would give twice the torque, but getting the speed up to 1800 mm/min (900rpm for the motor) for rapids will mean that the motor is well outside it's optimum torque region. Or is it the case that in practice this wouldn't be an issue?

I'm about to head out, so this is a very brief explanation.
Generally smaller motors the torque doesn't drop off as quick, especially on relatively lower voltage power supplies. Larger motors can maintain a similar torque drop, however you're into high voltage drivers, which carry an additional cost. So you can quite often get better performance from a smaller motor spun faster from a lower voltage, than you can with a large motor run from a less than ideal voltage.

And that is probably as clear as mud, but I've got to go!

[Agathon]

I'm about to head out, so this is a very brief explanation.
Generally smaller motors the torque doesn't drop off as quick, especially on relatively lower voltage power supplies. Larger motors can maintain a similar torque drop, however you're into high voltage drivers, which carry an additional cost. So you can quite often get better performance from a smaller motor spun faster from a lower voltage, than you can with a large motor run from a less than ideal voltage.

And that is probably as clear as mud, but I've got to go!

Thanks again - I think I get it. I thought I was good on electrics and motors until I started reading up on steppers!