Something happened today when I changed the microstepping value of my 5056D. Upon the suggestion of John Keling to go with 1/8 microstepping (from a setting of 1/64). (Btw I had written John asking about current setting, not microstepping)
Prior to today, this stepper had a motor tuning parameter of 101815 steps per unit (inch). I could get rapids in the 40ipm range. After the change in the microstepping value I immediately noticed the feedrates and DRO's were moving much faster than usual. I thought I had acheived some miraculous speed increase (something I have always hoped for with this lathe but I was only seeking to stop the squealing of the X stepper). I finally realized the distance indicated was approximately 4 times the real movement. So much for the speed increase. I quickly multiplied my steps per unit by 4 to get semi accurate (it is close, could be spot on). Unfortunately this comes at a cost to speed as I am now in the 406000 steps per unit. Which if my math is correct 1/406000=.00000246 theoretically (a ridiculous resolution if it was possible). Now the feedrates are barely in the 20's
If the above did occur (which I believe it did), since 64/8=8, why did my steps per unit need to be multiplied by 4?
Also if I changed my microstepping to say full or 1/2 step, I could acheive more speed (I am going to try it anyway) though I have been told this isnt so. With all of the reductions in this particular system needing 20000 steps per unit at full step, wouldnt the stepper speeds already be high to minimze resonance? Maybe they designed the system for the electronics of the era? The manufacturer claimed rapids in the 63ipm range. In the present configuration I can get nowhere near it. Besides resonance I havent ever read about any damage from microstepping settings. Anything detrimental?
I was under the impression after having this same debate many times here in different threads, that microstepping was purely for resonance and that in reality it wasnt about speed. I believe that is an incorrect assumption.
I have a lathe with a 2.5mm leadscrew and a 10:1 gear reduction and 1.8 degree steppers. If my calculations are correct 10 (10turns per inch on the screw) x 10 (gear reduction) x 200 steps per rotation = 20000 steps per unit at full steps. Correct so far?
I believe this is a theoretical resolution of 1/20000=.00005, correct?
So.. my guess is that the microstepping modes are mislabeled or your mode select switches aren't working correctly. It either wasn't really in 1/64th or it isn't now actually in 1/8th. Otherwise it makes no damn sense at all that it should change by a factor of 4 instead of 8.
As to the top speed changing, if the PC where maxing out on how many steps it could produce per second, then the change from 64 to 8 should allow the PC to "catch up". Your PC apparently doesn't have any problem with high speed step pulse generation.
Resonance can affect top speed. In fact, that your speed dropped in half, really makes it sound like mid band resonance. However...
If you were changing to half step or full step, then I would guess that it might be mid band resonance, because microstepping can help avoid that problem by providing a smoother motion from one step to the next. But a change from 94 to 8 doesn't seem like it would be enough to make much difference in mid band resonance.
Your math is correct as far as I can tell... but I'm not feeling well and so not at my best. Hopefully someone else will also check it.
Not sure what else to say... If 64 works better, go back to that. You might try reducing the drive voltage on the X axis or the amperage if that is easier in order to quiet it down.
James hosts the single best wiki page about steppers for CNC hobbyists on the net:
http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)