Kossel mini Y coordinate error after Nth layer

Hello, I have build a Kossel mini, calibrated and everything works perfect except I am experiencing weird problem some times (would say 1 from 10 prints). As you can see on the attached screenshot, whole coordinate system is "moved" along Y axis and next layer is printed in bad Y position (would say +2 mm or so). The part on the screenshot is cleaned since this error happened printing last layer, but this also happened on 5th or 6th layer once before and parts were ruined.

What got to my mind was loose belt or Z tower motor missing step, but the belt is tight and I am printing fine even larger parts.

Any clue what could be wrong? (I am using Repetier firmware and Slic3r for slicing, also using octoprint with raspberrypi to print wirelessly)

Thank you,

Michael

I think the option of skipped steps could still be the cause. you could try turning the acceleration down slightly or the power on the driver up slightly and see if the problem goes away. (alternatively the power on that stepper driver could be up very high and it could be thermal shutdown kicking in but this is less likely - see if the stepper drivers feel hot after running for a bit.

cheers

Tony

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DuetWifi.: advanced 3d printing electronics

Thank you for your hint. I checked my firmware configuration and noticed this:

#define MAX_ACCELERATION_UNITS_PER_SQ_SECOND_X 1000
#define MAX_ACCELERATION_UNITS_PER_SQ_SECOND_Y 1000
#define MAX_ACCELERATION_UNITS_PER_SQ_SECOND_Z 100

I think this is NOT good setting for a delta bot style printer since all motors should have same values right?

Yes, they should be the same. But different values should not result in missed steps, only in uneven movement over different cartesian X/Y positions. Lover values are safer but even values of about 10000 should be usable with a delta printer. So you already have it very conservative. I would be surprised if fixing acceleration helps you with the sudden layer shift.

Edit: Stepper driver overheating or a pulley slipping or maybe even too low current on one stepper looks more probable.
Strange that the error is very nice (about the same over the whole top area of the part). Stepper skips (or pulley slides) lead also to Z-height error and not only X/Y error. All the X/Y/Z errors because of that are about the same magnitude. It does not look like your part has any Z-error there.

Edited 1 time(s). Last edit at 06/25/2014 03:23PM by hercek.

Thank you for your input. I will check recheck steppers and drivers. Yes the error is even across the whole part. X and Z is OK, just the Y axis is shifted about 2 mms.

Edited 1 time(s). Last edit at 06/26/2014 03:32AM by vochomur.

That indicates your problem is something else. I do not know what it could be if the generated gcode itself does not contain the shift.

@Michael,

Did you ever get to the bottom of this? I have something extremely similar going on. I've replaced the stepper drivers and done 101 other things to no avail. When I see this behaviour I hear a 'clunk' as if steps have been missed (or a stepper gone into overheat mode) but they certainly haven't.

I have some level of repeatability but it's on a 0.1mm print and its a ways up, so the test cycle is pretty long. I'm generating the prints using Slic3r 1.2.9 and slowly going through all the options of what it could be...Slic3r is the next suspect but I'm looking for clues...

DAVE

Hey Dave,

hard to say. Since then, I have made lots of upgrades and tunes including switching to 32bit board, rewiring, flashing and reconfiguring newer version of firmware (repetier), stretching belts etc. I do not have these layer shift problems now, but unfortunately do not know the reason .

From what you write, I would say something may get overheated (longer 0.1mm prints?), do you cool your board/drivers? I haven't had cooling before, but now I actively cool the board/drivers with a fan and as said I have not experienced these problems since then.

Michael

If you have a consistent problem on one part you can try to turn it like 90 degree and then slice it for that. If the printing error is then also turned 90 degree it is the slicer. If it continues to be in the same direction it is your printer.

My guess is overheating of the stepper drivers. Slipping belts and missing steps on a kossel is really rare and I would expect it to be present much more consistently. Like - It does not wait 50 layers before presenting it self.

To speed up the trouble shooting process you can print 3 different things and space them far apart and close to the towers. That will stress your system a lot more and provoke the error faster.

Edited 1 time(s). Last edit at 07/22/2015 09:31AM by LarsK.

Michael/Lars,

Thanks for the rapid replies. I've actually replaced the stepper drivers and _still_ had the behaviour - indeed, I even replaced the RAMPS board :-( I also tried KISSlicer and saw it too...so I guess its printer mechanics or firmware. I guess a fan on the drivers can do no harm and it might show the problem....theres one under the base already but it's not well directed.

Its also rather bizzare that it always occurs at pretty much the same point the the print...rotating the part is also a great idea.

I guess I could/should update the firmware - I'm curently running a T3P3 variant and I'd prefer to not have to do that, but its running a fair bit behind head so there's a fairly good chance new things have been introduced.

I'll report back, thanks guys, new things to try.

EDIT: Tried a fan, no effect. Now updated to Marlin: Release 1.02....print in progress, watch this space.

DAVE

Edited 1 time(s). Last edit at 07/23/2015 06:30AM by mubes.

Maybe I am misreading your post, but remember that the fan should be cooling the stepper drivers ( those on the ramps board) . the stepper motors them self can be allowed to get quite warm without problems.

Lars,

Yep, that's where I aimed it. Absolutely no effect :-( I also tried printing from SD in case it was a comms problem....no change there either :-(

FOUND IT. Well, at least the system is fully tested now :-) It wasn't the firmware, and it wasn't voltages, the stepper motor, the direction of the wind or the 1001 other things I also tried.

For information, in case someone else has the problem; It was a silly thing that I should have spotted a lot earlier (as these things often are). The fill density was set pretty low and filament temp high, with the end result that there was some blobbing on the first passes over the top layers. Because the print was relatively big the blobs cooled and hardened, and because it was 0.1mm layer height the head got displaced sideways when it passed over to do the next layer.

Obvious, with hindsight.

Thanks for the help! Its always useful to have someone to give you another point of view on these things.

DAVE

Hey Dave,

glad you found the problem. Haven't been printing PLA lately, or just small objects. Now I got back to larger PLAs and got the Y shifting again. Indeed there is too much flow I think, no big blobs but strings and the lines are too thick, I will try to tune the temp and flow and will see if the shifting gets fixed.

Will post an update.

Michael

So playing with temperature/flow did not solve anything. Tried higher/lower temps but the occasional Y position shift still persists.