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Weird Layer Shifting

Posted by Hauser 
Weird Layer Shifting
November 26, 2015 10:49AM
Hi all,

For the last 3 or 4 weeks I have been working on get my Prusa i3 on shape.
I have nearly disassembled it, cleaned it up and assembled it again, I've recalculated e-steps for the extruder and also extruder multiplier for the current filament I'm using (3mm PLA), I've optimized temperature and also surface adhesion.
Everything seems to be working again and I was very satisfied for the relatively small pieces I printed when everything was ready. However, a few days ago, I tried to print a larger piece. After more than one hour of printing, I found the piece as you can see in the attached picture. It doesn't seem like a layer shifting, which I experienced before, but like a total change of coordinates in the middle of the print.

Does somebody know what the cause could be and how to fix it? I followed a couple of visual guides but I haven't found anything similar. I don't really know how to address the problem. Any help would be appreciate it. Thanks in advance!
Attachments:
open | download - IMG_5365.JPG (594.4 KB)
Re: Weird Layer Shifting
November 26, 2015 11:26AM
First, this has nothing to do with calibration or the extruder. The number of steps per mm doesn't change and can't do this.

Your print appears to be shifted only in the Y axis. If there is something interfering with the free motion of the Y axis, such as berings binding, a loose belt, or pulley slipping on its shaft, you can get this sort of layer shift, and that is the first thing to check.

Your printer's bed moves in the Y axis. In most printers where the bed moves in the Y axis, the extruder moves in the X axis. The Y axis moving mass is typically much greater than the X axis moving mass, so the Y axis motor is subjected to a greater load than the X axis motor. The increased moving mass in the Y axis is harder to control. If you try to accelerate too fast the motor doesn't have the torque required to move the bed as fast as the controller is telling it to move and the result is skipped steps and shifting of the print layers. This sort of problem can be remedied by reducing acceleration/jerk in the Y axis. Those settings are in the machine's firmware. Why didn't this show up before? It is usually very dependent on the geometry of the print. Some things will print fine and others won't depending on how much the Y axis has to start and stop and reverse direction.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Weird Layer Shifting
November 29, 2015 04:36AM
Also being only one direction in the Y, it might be a binding of the motion in one direction.
Re: Weird Layer Shifting
November 30, 2015 02:33AM
Hi again,

Thank you both for your responses. These are really interesting points which I didn't think about before. I'll have a look at the acceleration/jerk settings on the prints and give it a try again. I thought it was something I could modify in Slic3r, but I've just checked it and I didn't see the option...
Re: Weird Layer Shifting
November 30, 2015 11:15AM
Check you current/voltage on your Y stepper driver. Make sure it's correct and also check cooling. Sometimes a hot driver board can cause this problem in my experience.
Re: Weird Layer Shifting
December 01, 2015 08:40AM
You can change your speeds in slic3r, but not just in the Y direction.
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