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tukl

UM2: large z-Jumps

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Hi!

in the last few days I experience a lot of crazy z-jumps on my UM2. They can happen everywhere when you print the same file multiple times. After a while, the z-axis increases a bit too much and the print is gone...

I made a video:

have a look at 3:10, the jump occurs at 3:16.

I tried reformatting the sd card, change of the sd card.. any ideas why this happens?

Thank you so much!

Sebastian

 

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Very strange. Are you sure it isn't a slicing error? Definitely email illuminarti the gcode file, I think he will see the jump in the gcode as well.

 

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I send you the file.

After the fifth try, it came to a complete print, so I think it is not the gcode itself. I had this in the past several times with other files, but not that often like now...

 

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Could be a SD communication problem. It's rare, and it's related to the flat-cables under the machine.

Can you show a photo of the underside of the machine? Maybe something is wrong there.

 

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Take a picture before removing the covers. That alone might be enough information. Then remove both covers - it is very easy. The larger cover has only 2 screws. The smaller cover I have not removed yet.

Maybe keep the two cables farther apart? Maybe there is cross talk? Daid understands the problem better than I do.

 

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Don't be afraid to take those 2 covers off. They come off very easy.

Inside that black netted cable are 2 gray ribbon cables. I would disconnect them from both ends, then reconnect them. I think Daid needs to see which side of the gray cables is the red stripe, right Daid?

 

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I don't think that the issue is with the SD card, reader or connection from the controller to the stepper board. If that were the case, you would see erratic movements on all axes. It's hard to see a condition in that chain that would only affect one axis.

I think something is causing erroneous "step" signals to the Z axis driver.

I suspect that current flowing through the wires crossing the card is inducing the step signal.

I would try moving the cables away from the stepper card as much as possible with focus on the wires to the Z stepper. I would also try a print without the fan on to see if the behavior changes. The fan is driven by a PWM signal and could be the source that is inducing the additional steps.

The only other guess is that you have a bad stepper board that has a driver on the Z axis that is too sensitive and is picking up noise that you can't eliminate.

Good luck.

 

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Yes, anon makes a good point about it only seeming to affect the one axis. Looking at your photo with the cover off, I don't see anything that looks particularly bad about the ribbon cable to the SD reader. The stepper cables look a bit odd, but I guess it's just the angle of the photo. I only see three actually plugged in, and they look to be going to the wrong places, but that can't be right, or the printer wouldn't work at all. So, I guess it's just the photo.

The X stepper motor cable passes behind the z-motor and goes up to the x-stepper on the back plate of the printer. The Y stepper and the extruder stepper cables should pass across the board, and go up the back left corner of the printer. The Z cable should be a very short one (since the motor is right there) and just go straight to the z motor. Check that, and check that the wires didn't get trapped/damaged under the edge of the electronics cover; there's a channel for the wires to run through, but they can escape and get caught if you aren't careful during assembly.

 

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It might actually be all 4 axes that are acting strangely. In the video the printer does some bizarre stuff. I think it's skipping a whole layer and then the extruder tries to over extrude and a fat string comes out. This is because the E values are not relative so that when you skip several layers, now the E position is much farther along and it overextrudes a fat string. The next time this happens it skips maybe 10 layers and another fat string of filament comes out at the same time. Both of these happen after 3:00 in the video - maybe the first one is at 3:10 possibly. The second skip is only a few seconds later.

So although anon's comment is clever and good "thinking out of the box", I think in this case, Daid's theory is better.

So what to do next? It's time to get your hands dirty TUKL. Simply disconnect both of those gray cables (mark them carefully so you can put them back. Then maybe try to re-arrange them slightly and connect them back in. Just the act of disconnection and reconnection might be enough. Hopefully Daid will come back and inspect your photo - I don't know what the "wrong" way is (red to red? red away from red?).

 

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Also you might want to try another SD card. Get one of the same manufacture, and size and type if you can. I don't know which types of SD cards the UM can read.

If you print the exact same file twice does it skip in different places of the print?

If you put the SD card back into your pc, can you use winmerge or another program to compare that the file on the PC is identical to the file on your computer? It seems like a few lines got deleted or corrupted.

http://winmerge.org/

 

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gr5 has a point. It's hard for me to tell from the video if all axes are affected. If it appears they are then the SD card "system" would be suspect as it is common to all axes.

Although, I would also expect to see random negative Z movements or x/y movements completely scattered about the platform unless Marlin somehow constrains them.

 

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Looks like you have an old ribbon cable. This cable is known to cause these problems, we've made an adjustment to it to fix this issue.

You should contact support for a free new cable. "Marrit Hoffmans" knows about this problem, so I recommend including her name in the ticket.

 

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