Stepper motor torque/speed

A friend and I are trying to determine how practical to use a threaded rod design in a RepStrap. I know that it can be done (McWire) but we're trying to figure out what sort've transversal speeds we can get.

Does anyone know the math to generate a torque/speed curve for a stepper motor, or have a link? I think I've read about this before, but it's been a while. For some reason I'm having poor luck with search engines as well.

Thanks,
Kyle

Hmm, I've run a simple calculation and am looking for a sanity check.

According to my back-of-the-envelope calculations, Darwin is only running its steppers at about 10rpm, or 4000 steps/min! Could anyone verify this, or let me know if I'm missing something? This is assuming a transversal rate of 14mm/s, which I thought I remembered from somewhere.

(14mm/s)/(25.4mm/in)/(.509*2pi in/rev)*(60s/1min) = 10.3 rpm.

Thanks,
Kyle

I think you are out by a factor of 2: 0.509" is more like the diameter of the pulley not the radius.

An easier calculation is that Darwin steps are about 0.1mm so that's just 140/400 * 60, about 21 RPM, not fast for a stepper.

But 14mm/s is not fast, I extrude at 16mm/s and move at 32 mm/s and hope to improve that when I move to Darwin. I think 64mm/s is needed to get the same sort of performance as a commercial machine.

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