Geoff,

You might find this useful: http://www.mycncuk.com/forums/faqs-p...do-i-need.html

As Robin has already alluded, stepper motor holding torque is what it takes to move the rotor from a stationary position. Once it's moving the torque available is considerably lower and essentially flat until you get to what is called the corner speed after which it becomes inversely proportional to speed. Ideally you run your motors at or just below this corner speed. The corner speed itself is dependent on the rise time of the current in the coils, and that depends essentially on the ratio of the coil inductance to its resistance. As Robin said, one way to improve the corner speed is to drive the coils harder by applying a much higher voltage (most stepper coils are rated for 3 - 5v but will often be driven from 12 - 80v) and controlling the current to avoid burning them out - constant current drive (expensive, heat producing) or chopper regulation (cheaper, but noisy and prone to other side effects).

Sadly, very few stepper manufacturers provide graphs of torque vs speed at the rated current so its always a little bit of 'what has everyone else done'.