Mini Kossel Layer Shifting

Hello. This is a cross-post from the Google Groups delta and Duet forums.

• Original post: [forum.duet3d.com]
• Follow up post: [groups.google.com]

The short of it is that I've been experiencing layer shifting issues on my Mini Kossel and I'm struggling to figure out where they're coming from. On the calibration cube that I've been printing I get layer shifting where the layer patterns change. For example, if I print with 3 shells, then the outline of the 4th layer will not perfectly match the outlines of the 3rd layer. Here's a picture where you can see the shifts and other artifacts:




I actually managed to get rid of the shifting at the very top and bottoms (where the shells are), but the shifting in the middle, where the letter boundaries are, still remains.



I've torn the printer down and rebuilt it twice, but the layer shifting has persisted through it, and always in the same places. My guess is that there's some mechanical binding occurring somewhere. Here are some things I did during the rebuilds:

• I measured all of my beams and used the ones that were most similar in size.
• I tightened the top and bottom frames with belt clamps to make sure there were no gaps and there was no play during assembly.
• I cleaned and lubricated my rails.
• I measured my delta arms and made sure they were equal in length.
• I calibrated my FSRs and their mounts so that they are sensitive to a light but firm touch.
• I replaced all of my timing belts.
• I put rubber bands around the base of the delta arms in case there was any chatter.
• I tried swapping the X, Y, and Z axes, but the shifting remained in the same locations.
• I tried using a RAMPS 1.4 board instead of the Duet 2 WiFi.

During this process I had some findings:

• One of my delta arms had a ball joint that had come unscrewed. I reset the joint and matched the length as best as possible to the others.
• With the tools I have, the best accuracy I can get measuring my arms is to within 0.2 mm. Mine all measured in at 214.4 mm.
• One of my rails has a carriage that is missing a number of bearing balls. I've ordered some new ones, but the rail still glides well enough for now.
• The linear guide rails seem a bit beat up (they were secondhand), but they slide smoothly.

The layer shifting persisted after the rebuild, so I'm starting to run out of ideas to try. Here are my remaining ideas:

• Make some new arms. I'm currently waiting on parts for this. I expect this to take a while given the current state of lead times from China.
• Try magnetic arms.
• Try new stepper motors.

Finally, here is a picture of the printer. It's my first build, so please go ahead and suggest anything, even if you think it's obvious.



Any thoughts on what I can or should check next? Thank you!

Kamil