taig and sherline mills to the UK

The email bit isn't part of Free Agent.

Jim.

Problem was that the pins were asigned incorrectly in the software. Should have checked first. If I increase the speed too far they move and then stop, while making a grinding sound. Is that just the limit of the motor?

(This sounds a bit too close to the topic that caused Mr.Stevenson to go into _BIG INFANT_ mode, way,way back in the history of this NG!)

Any stepper motor will only start from stationary with fairly slow pulses, of the order of milliseconds. Once you've got them moving, you can accelerate them. However, although they may step quickly after they have been accelerated, you can't start them from cold at that high speed.

Are you driving your steppers directly from a Windows program, or via an intermediate controller using, say, PIC's?

If the former, then you're going to run into trouble if you try to drive them quickly (even after a proper acceleration). The reason is that Windows goes to sleep (as far as you're concerned) for 10's of milliseconds at a time whilst it gets on with its own housekeeping. If this happens when your motors have been accelerated, they'll stop when the big pulse caused by the Windows housekeeping comes along, but then they won't restart. The only solution is to keep you fastest stepping pulse to milliseconds. However, on the basis that our machines remain stationary for 99% of the time, a few more milliseconds taken up when cuttting is neither here nor there!

Another possible area for you to investigate is that the torque available from the motors decreases with decreasing step time, and is especially a problem if you're using resistance-loaded driving circuits because your L/R comes into play.