Quote Originally Posted by JAZZCNC View Post
Depends on how you want to use 4th axis but general a 4th axis is slow spinning and used for position more than speed in which case 20Khz is more than enough for a 4th axis.
Here's how to work out the max RPM 20Khz will give you.
20,000 pulses per second(20khz) / micro steps * 60 = RPM for example. 20,000 / 2000=10 * 60 = 600rpm.

So for instance lets say your 4th axis is using a 6/1 ratio then you'll get 100Rpm which if you want to use 4th like a lathe then isn't very fast. You could increase RPM by lowering the Micro stepping but then you lose some resolution and smoothness.

Often you'll only use 4th axis for postional moves with much higher ratio and spinning only a few rpm but with high acceleration and High micro steps to help little with smoothness and to lesser degree resolution.

It's trade off between RPM and Motor smoothness/resolution. However for slow spinning motor then 20Khz will be fine.
Hi Jazz
Thanks for the info. Would the kernel frequency of uccnc have to be adjusted to it's lowest setting (the settings available are 25khz 50khz or 100 khz) or is it not important and a higher kernel frequency could be set say 50 or 100 khz. I've not got a 4th axis yet but could see a rotary drive being useful in the future.
Cheers
Andrew