# Steps per rotation anomaly

Lightburn 1.2.01
Ruida 6445S
RDC V26.01.16

Chuck style rotary axis
Steps per rev,
14-120 tooth pulleys. 120/14= 8.5714285
Gecko stepper drive 10 micro steps, 200*10=2000 steps per motor rev.
8.5714285 * 2000=17142.86 steps per rotary chuck rev.

Steps that must be entered in rotary setup to actually get correct 360 degree test is 17270

Why the anomaly? What am I overlooking? Is this bug? Or am I just stupid?

This anomaly works out to 127.14 micro steps or 12.72 full motor steps.

That equals just under 23 degrees discrepency at the stepper motor shaft.

Something is wrong. (might be me)

I am only an old retired carpenter but I believe my math is correct.

Not sure what’s happening but I suggest you break down the issue.

First test just the stepper motor in isolation. Does 2000 steps get you one full revolution?

Separately test the gearing reduction. Does each revolution of the smaller gear result in 42 degree rotation in the larger gear?

Doing that should pretty quickly tell you where the discrepancy is.

Every stepper that I have ever used is 200 steps or 1,8 degree. It is kind of baked into their very bones.
It certainly is not going to be 230 steps per rev. It just isn’t.
Gecko have to my knowledge been doing ten micro step drives since forever. Maybe their most modern drives may have gone the Chinese route. But these are definately 10 micro steps. These G201s I have been using for years. Rock solid.
As for gear or belt/pulley ratios it is basic physics. 14 tooth to 120 tooth. Hard to get it wrong.
If say, I made a miscount, and there were 15 teeth on the smaller pulley then the anomaly woud be considerably larger.
However I have made math mistakes before. If that is the case I would be happy to have the error highlighted.

I’m not questioning the theory. I’m looking to isolate where the discrepancy is introduced. Then next order of business is identifying cause.

If everything was working according the theory then there’d be no discussion in this case.

How are you testing these values? Is this through the “Test” button in rotary setup?

I do appreciate your taking the time to look at this.
I am using the test feature in the “Rotary Setup” and also confirming this with a 360 degree move.
Setting circumference at 360mm and jogging a distance of 360mm.

I can only see three possibilities.

1. I made a mistake in the math. This was my first question.

2. The stepper driver is not delivering ten micro steps. Highly unlikely as the same drive runs the Y axis when not in rotary mode and Y axis moves correct distance as per toothed belt calculations.

3. The controller is not putting out the stated number of steps.

I could attempt to measure the step number with an oscilloscope. Not sure I can do that but might give it a try.

I’d consider these other possibilities as well:

1. something isn’t configured the way you think it’s configured
2. something isn’t working the way it’s configured
3. tooth pitch is subtly different between gears
4. belt pitch doesn’t match tooth pitch
5. there’s some sort of mechanical variation not accounted for
6. LightBurn isn’t configured as you think it’s configured
7. LightBurn isn’t commanding what’s been configured
8. this is within tolerance of machine components or stepper motor function

If you can isolate the variance the answer should present itself fairly quickly.

Yes I think if I can manage to actually count the steps output that would narrow it down.
Just not certain with the equipment and skill available that I can achieve that.
I will make an attempt today.

If you can run the two tests I suggested that would be easier. No fancy equipment needed.

First test the gears:

1. Place something on the gears that points in a single direction while allowing you to rotate the gears.
2. Turn small gear one full rotation
3. Measure angle of larger gear. You could print an angle chart if you don’t have anything else that could be used. The longer the “pointer” the more exaggerated any measuring error.

Next test stepper motor:

1. With the pointer still attached, command what you think is 1 full rotation.
2. Measure and confirm rotation

If the error is only seen in one of the two tests then that eliminates the other side from the problem. If they’re in both side then you’ll need to account for that separately but should allow you to further narrow in.

Or, perhaps:

1. It’s firing off steps so fast the motor cannot follow them.

In that case, the motor will miss the occasional step and require more steps than you calculate to get all the way around.

Reduce the speed and acceleration for that axis by a factor of ten and see if that improves the results.

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Thanks for chipping in.
Slowed it right down. No difference.

AS accurately as I can measure one motor rotation turns the bigger pulley exactly 42 degrees As expected.
These toothed belts do not slip or exhibit any measurable backlash at these very light loads.

Even swapped out the stepper driver to see if that made a difference. No joy.

I failed in my attempt to count the pulses with an oscilloscope. Probably just my lack of expertise.

At the setting of 17270 it achieves exactly 360 degrees. At the calculated 17142.86 it falls 3 degrees short.

When all is said and done, it really does not matter in any practical sense. The rotary axis does exactly what it needs to perform the task… It was just my curiosity as to why the anomaly existed.

So I am going to leave it here. Thankyou very much for your input.

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Assume you have these on the Y axes…

What are your motor drivers set? 2000 steps/rotation?

@berainlb best idea is to view the actual motor shaft… I’m also in agreement, don’t see anything wrong with the old retired carpenter math that I can see…lol

Are these NEMA types … I’m not familiar with these…

This is how I set them up on my Ruida, maybe that will help… although I think much of a variance would make a ‘large’ difference… So your procedure was probably very similar.

Thanks for chipping in Jack.

All the info is found in the posts above.

I started building CNC machines about 18 years ago and Gecko were not new then.
They were the go to supplier for non industrial builders before all these Chinese drives started being available.
A long standing USA manufacturer.

As explained above I am abandoning this curiosity pursuit.
Laser is performing great and doing all I ask.
The fact that there is something unknown going on, bugs me but I will get over it.

Thanks for the link and information…

Can’t argue with success…

Good luck

Geckodrive controllers seem more complex than my stepper driver.
Which model is yours ?
could you try the binary divider instead of decimal, maybe *16 or *32
I have one motor on *32 and one on *16 which makes my motors much smoother.
A notchy motor can miss pulses.

The drive in question is the predecessor of to this one.

These drives do not have selectable micro step counts. Hard wired ten micro steps.
They believe ten is optimum and more is just diminishing returns.

I have never built or operated a stepper driven machine that has lost steps.
It is one of those often believed myths that only belong to poorly designed and setup machines.

I have, from servos to steppers, some on very expensive equipment. When you push it too hard, things fail. Such is life

I push my machine to it’s limits any way I can… if I lose steps, I got it running too close to the edge… for that job.

That is why a well designed and setup stepper machine has a lot of headroom built in.

So it does not get “close to the edge”

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