Missing steps while cutting

rowblue
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Missing steps while cutting

Post by rowblue » Thu Jul 19, 2012 12:24 pm

First, I think a subforum for troubleshooting actual operations might be useful. Now on to the problem.

Last night I started playing with cutting acrylic/Plexiglas. It seems that my steppers are occassionally missing steps. It happens to the Z most often but my Y also misses steps at random. I think it could be happening due to thermal shutdown on the Z, while the Y may just be slipping inside the gear. Also sometimes one looks like it leads to the other. As in the Y misses a step, causes the Z to try to cut into previously uncut material and possibly miss a step that way. Anyone else struggle with this while getting the machine zeroed in?

potatotron
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Re: Missing steps while cutting

Post by potatotron » Thu Jul 19, 2012 3:47 pm

Yes! I'm not the only one!

I have this problem and so far haven't been able to isolate it, I'd say it happens once every 200-400 or so moves[*]. I have it on the X and Y axis but don't think I've seen Z misstep, but like you say it seems like if one axis missteps then the other one will too.

I've re-verified the voltage settings on the stepper drivers, added .1uf and 47uf filter capacitors to each stepper driver's motor power supply input and .1uf to each steppers logic power supply input, and added small delays ("G4 P.1") between movements. I've also tried decreasing the seek & feed rates and that seems to have helped some, but has not eliminated the problem. The only other things I can think to try are another version of GRBL (I'm currently using 0.7D, I will try 0.8E) and replacing the power supply.

It's slowly driving me (more) insane so any insight will be appreciated.

[*] My ShapeOko is a pick and place machine. Each pick/place cycle is 10 steps and I can get it to place between 20 & 40 pieces before this happens, but it seems like it will always happen at some point.

vios
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Re: Missing steps while cutting

Post by vios » Thu Jul 19, 2012 4:22 pm

Rowblue and potatotron,

Which stepper driver both of you are using.
I am using Synthetos Grbl shield and never have this problem.

Improbable Construct
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Re: Missing steps while cutting

Post by Improbable Construct » Thu Jul 19, 2012 4:23 pm

Rowblue,

Cutting plexiglass is always tricky. I am not a pro, but I have made a lot of things from acrylic.
Some things to keep in mind:
1. Acrylic is usually not uniform in thickness. +/- 10% is not uncommon.
2. Acrylic may be softer/harder at any point in the material.
3. Softer material tends to melt, harder tends to crack.

Finding the thickest point in the material and setting your Z to that height will keep the bit from going too deep on the first pass. Getting the feed speed correct takes some trial and error. You want to go fast enough keep chips coming off the bit without melting it. Plunging is even tricker! I have found a 2 flute bit to be the best. 1 flute takes to big of a bite and 4 flutes tends to melt the acrylic and gum up. I run acrylic at 400mm/min feed, 75mm/min plunge, and a 2 flute upcut spiral bit. As a reference I run MDF at 800-1600mm/min depending on the bit.

If you are not stalling the motors by running the feeds too fast, then I would guess you are having thermal shutdown issues. Have you tried active cooling of the motor drivers? A small fan may make a world of difference.

Potatotron,

It sounds like your problem is most likely the motor drivers overheating. I run a different kind of controller on my ShapeOko but have had issues with the motor drivers on my ORDbot overheating an causing it to miss steps. See if a fan on the motor drivers helps.
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potatotron
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Re: Missing steps while cutting

Post by potatotron » Thu Jul 19, 2012 7:44 pm

Improbable Construct wrote: It sounds like your problem is most likely the motor drivers overheating. I run a different kind of controller on my ShapeOko but have had issues with the motor drivers on my ORDbot overheating an causing it to miss steps. See if a fan on the motor drivers helps.
I have heatsinks and a fan on my boards; my probe tells me they never get over 105 degrees F. I'm currently using the Pololu A4988 boards but have some SparkFun Big Easy Drivers from another project and I may try swapping those.

I don't want to hijack this thread, however, so back to rowblue's problem -- what kind of cooling do you have on your boards? Maybe they're overheating like Improbably Construct suggests.

rowblue
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Re: Missing steps while cutting

Post by rowblue » Fri Jul 20, 2012 12:00 pm

I am running the synthetos grblshield with the 400 step/rev motors from Sparkfun. I have not tried any cooling yet, I suppose thermal shutdown could be a culprit. When I ran the motors initially to find the min and max current per the procedure on the synthetos website I didn't run them for nearly as long as I do during a job. I will take a shot with a fan this afternoon.

Would turning down the voltage from the power supply help? I think I am running it right around 24V now, I thought I read you could drive steppers up to 20x the rated voltage.

I was cutting around 30ipm = ~725mm/min with plunge at 10 ipm = 254 mm/min so maybe it is as simple as slowing down. The machine didn't feel/sound all that loaded at those feeds but I will try slowing it down. What indication might you get if the motors are stalling, would you hear them whining but not turning? I guess it may be hard to diagnose because they only miss a few steps at a time.

I will try cooling, then slowing down then both and report back.

edwardrford
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Re: Missing steps while cutting

Post by edwardrford » Fri Jul 20, 2012 1:38 pm

rowblue - a couple of thoughts:

1.) Yes, try some active cooling on your grlbShield. Those drivers don't tend to overheat anywhere near as often as the Pololu drivers, but it's still possible and cooling definitly will not hurt the situation.

2.) The few times that I've cut acrylic, I found the best setup to be taking light passes at a very high speedrate. What size/type of bit are you using?

If you are using a standard endmill (2 flute perhaps?) my suggestion would be to keep your z-axis at the same speed, possibly even dropping it down to 200mm/min. Try bumping up your X/Y rates to beetween 1000-1200mm/min. For your depth, change it to no more than 1.5mm per pass, less might be better if you're going to run your X/Y faster.

Good luck!

-Edward
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rowblue
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Re: Missing steps while cutting

Post by rowblue » Fri Jul 20, 2012 2:28 pm

I'm using a 1/8" 2-flute endmill at 0.4mm per pass (CamBam default possibly?). Thanks for all the quick suggestions so far. Sounds like I will be busy testing this evening.

EDIT: Isn't the thermal shutdown for these drivers pretty high? The data sheet for the drivers shows 150C for the thermal shutdown measured as die temperature . If I run the job and touch the drivers it should be apparent right?

aldenhart
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Re: Missing steps while cutting

Post by aldenhart » Fri Jul 20, 2012 7:02 pm

Running the motors at lower voltages will make the problem worse, not better. Stepper driver chips consume the most current when in transition, and the transitions are faster with higher voltages. Many PSs have an adjust. I usually tune them up to as close to 30v as I can get. Do not exceed 30v.

Burke LaShell
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Re: Missing steps while cutting

Post by Burke LaShell » Fri Jul 20, 2012 9:25 pm

If I run the job and touch the drivers it should be apparent right?
Not necessarily. I found that a small fan blowing across the heat sinks solved my problems. It's a pretty cheap and easy test that's worth trying first.

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