I suppose if I’d had my coffee I would have just attached the files .the first place…
CNC-Feed-Rate.pdf (619 KB)
Any updates? Better, worse, taking a break?
Hey Ryan,
Sorry about the delay, lots of crazy happening lately and have not even had a chance to go to the shop. I hope to be able to get back in there in the next few days and then I can get some answers and test things out.
Thanks!
Chris
I know I’m chasing old ghosts here but since this is not obvious and people ponder about this, I must spread the information as wide as I can 
Yes, the rotor can move out of place for more than one step and still return. The reason being that the steppers we use can not lose just one step. They lose them in increments of four. Stable rotor positions that align with the field are 0.64mm (16T pulleys, GT2 belt) apart.
This is easy to observe experimentally: Take a pulley off your stepper and close an adjustable wrench around the shaft. Note the position. Start turning the wrench while counting the clicks. 1.8deg stepper has 200 full steps per rotation. However you can only count 50 clicks until you’re back where you started from. There is a link to some references in this post.
True, but when it gets more than two, the closest point is two steps back.
Anttix is right, the point being that a “whole step” is a 90 degree phase shift of the magnetic field. Skipping steps will move to the next position where the magnetic field aligns which is 360 degree phase shift of magnetic field. There are 200 “whole steps” per revolution but only 50 N/S pairs around the circumference of the motor.
So with a 16 tooth gt2 pulley, each whole step is 0.16 mm but the magnetic field will cog by 0.64 mm. You can deflect by up to 0.32 mm and still return. More than 0.32 and you snap to the next multiple of 0.64 mm.
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Jamie is spot on here. Unstable point where the rotor can snap in either direction is right in the middle.
The interesting quality here is that the holding torque curve looks like a sine wave.
We are all saying the same thing. But if you move 181 degrees, it’s not going back to 0, it’s going to go to 360.
I wasn’t too clear why this theory matters. Sorry about that. There is an assumption earlier in this thread that stepper “give” should be so small that it’s not observable. This is not true.
It is theoretically possible for the rotor to move up to two steps and still rotate back to where it started from. It’s also fairly easy in practice to get the shaft to rotate by one step or even slightly more without skipping especially if the loading is not constant e.g. a human is trying to do their best to rock the gantry without overpowering the stepper.
One step or 1.8 degrees is already an observable amount. I just tested this on my MPCNC: If I move the gantry to a position where a pulley set screw is visible, then grab the carriage that rides on the rail and try to rock it, I can observe the pulley rotating slightly (the set-screw provides a visible “mark” on the pulley to track).
I’m continually impressed by the community support, especially the persistence/willingness to help solve issues.
I read this thread with great interest as I’m soon to embark upon a LowRiderV2 build myself.
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