Z Axis Acceleration Tuning

I got to thinking about the Z axis acceleration setting and the negative impacts to carve time having it set wrong. I’m trying to debate about it’s worth experimenting with the setting.

Per the GRBL wiki:

the simplest way to determine the values for this setting is to individually test each axis with slowly increasing values until the motor stalls. Then finalize your acceleration setting with a value 10-20% below this absolute max value. This should account for wear, friction, and mass inertia. We highly recommend that you dry test some G-code programs with your new settings before committing to them. Sometimes the loading on your machine is different when moving in all axes together.

Has anyone happened to have done this? I’m trying to think this through before experimenting. I’ve got a faster Z axis with a higher torque stepper, I should be able to go relatively high with the accel. I’ve got it set at 100 and I feel that’s low. I’ve tried playing it out with numbers and haven’t had an “Eureka!” moment yet.

I have the fast screw linear z but the stock stepper…would it be safe to set $122 at say 75?

Similar, ACME rod direct drive, Nema23/269oz running on 2,6A/24V
Acceleration Z = 500 (3000mm/min max speed)

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262oz 2:1 ratio pulleys, 100. I just started going up by 50 till I was missing steps (around 200). And I thought well 100 is very safe and twice as fast. I’m sure I could push to 150 with no problems.

@PhilJohnson I have the same linear drive as you and, as mentioned, I have mine set to 100 as well. But how did you come up with 100? Did you experiment?

@HaldorLonningdal Those numbers seem a little higher than I’d think would be possible but still better than stock numbers. Did you test with those?

At 95 steps/mm, 2X microstepping, that means 1 rotation is 400 steps.

At 60 ipm (1524 mm/min), that means the stepper is turning at 362 rpm. The stepper has a theoretical max RPM of 444 before torque dropoff (per the calculations at this site…makes me want a new stepper). At that “max” RPM, you would be doing 1869 mm/min or 73.6 ipm.

Here’s the torque curve that StepperOnline posted:

I’m comfortable with my 60 ipm plunging. I’m not comfortable with the acceleration though.

Only max feed rate which topped out at about 3600mm/min then dialed back to 3000mm/min
At that time my acceleration on Z = 125
I have since upped that to 500 with no faults whatsoever.

So I just purchased these steppers (went ahead and ordered 4):


These have a lower inductance than the ones from StepperOnline and therefore should perform better at 24VDC than the StepperOnline motor based on the research that I’ve done.

Looking at the torque curves, at the 362 rpm level, the StepperOnline has about 50 oz-in of torque. The Automation Technologies has about 120 oz-in of torque (with 8X microstepping, assuming a linear 2X relationship between 48VDC and 24VDC). I’ve emailed them asking if they have a curve with a 2X microstepping.

At some point, I plan to actually measure all these motors on a dynamometer and get real curves at different voltage levels. We’ll see if that ever happens.

In any case, I plan to do the acceleration testing this weekend and see what this stepper is capable of then, once I swap out steppers, I’ll do the same test again (and may even tweak/test the X and Y accel values as well, even though I hope those are belt limited and not motor limited).

Ok, did some testing this morning.

The Z starts losing steps on a jog with an acceleration value of 1100 mm/sec^2.

At 1000, the motion is pretty smooth still but kind of the edge of missing steps.

Using GRBL’s advice, a value of about 800 would be good just to give some margin. I’m going to go with 750 mm/sec^2 on my machine and see how that goes. This should benefit 3D carving quite a bit.

Now, each machine will be different. The Z axis loading may change the accel for each motor. I’m confident that my machine probably has a worse loading than most people’s machine due to the mounts that I am using so I don’t see a reason why at least 500, if not 750, cannot be used by others.

I’ll redo this test when I change my steppers.

I’d say you’re safe up to probably 250-300. Change $122 and do some jogging. You’ll know if it can’t do it.

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Video showing the differences of acceleration values.

Thanks, I’ll play with it some and see what works good.

I did more testing on this today since my new motors came in. The results I got were surprising, to say the least.

All acceleration testing was done using 2000 mm/min jog speed (limited by the GRBL max rate value)

Inventables stock stepper motor: 10,000 mm/sec^2 was the only value that I could ever produce stalling.
269 oz-in StepperOnline: 1100 mm/sec^2
270 oz-in Automation Technologies: 3000 mm/sec^2

I ended up upping my value to 1000, I think, and I’ll watch it. I did a day of carving at 750 this past week and had no issues. I’m now interested in what the max acceleration from a GRBL perspective is and if some values don’t play nice and cause calculation abnormalities (for instance at 3000 on the 270 is a stall but 4000 or 7000 worked just fine…)

Now, I also did some max feed rate testing on my linear slide. These were all with a 750 acceleration setting (accel changes max feed rate, at 2000 accel, the Inventables stepper achieved only 120).
Stock 140 oz-in stepper: 145 ipm
269 oz-in: 85 ipm
270 oz-in: 100 ipm

This really shows how well matched the Inventables steppers are to the machine and the lower DC voltage. These are also unloaded (nothing cutting) so I’d expect the Inventables to potentially drop quicker while under more load from plunging/cutting. This also really has me wanting to see torque curves of this motor as it’s gotta be pretty flat for that level of performance.

Based on this, and the minimal price difference, I think the Automation Technologies steppers are a better value. ~18% increase in max feed rate and much higher acceleration potential. They seem better matched. The one downside, the lead wires aren’t long enough to reach the terminals on the Xcarriage. I need to figure out how I want to remedy that.

I’m waiting for my new belts (staying at 6mm wide and 2mm pitch but going to 3rd gen belt) before I switch all my steppers out.

I’m willing to try more experiments because the results aren’t quite resonating yet.