Kossel Mini Issue - Bigger Prints, Z

Hi everyone!

I recently assembled my first ever delta printer at home: a Kossel Mini (Anycubic). Assembly took really long but I finally managed it. After a few test runs and glitches, I was able to get a small prints (those with a small radius) done on it fine.

The problem is that when a print is too wide (bigger radius or closer to the edges of the build plate), the printer does something really weird: it goes too low from a certain edge and lifts up almost 2-3 inches off the built plate subsequently (I've posted a video of this here - [youtu.be] - since I might not be able to explain it best in words). It tries to go out of the bounds of the printer as well it seems. Then the stepper motors make a harsh clucking sound (because the motor tries to push it away from the center but there is nowhere for the extruder to go).

I'm not sure if this is a leveling issue since another print I did which was smaller and just around the center of the built plate came our perfectly fine, and if it is I'm not sure how to fix the leveling since all the axes that I checked the leveling on, seem leveled. I leveled it to 0.27 using a gauge.

Any suggestions would be really helpful to either solve or diagnose the problem.

Thanks,
A

It is just a really bad calibration / leveling. The rough sounds are missing steps. Once you miss steps then all the head movement from that time on is wrong because there is a difference between the real head position and the position assumed by firmware.

If the printer is baldly calibrated then the print head position error is growing when more far away from the center. So even a really badly leveled printer can print OK in the center but it completely fails at the print bed edges.

Calibrate the printer using a numerical method (you need a Z-probe). Use this if you can handle a bit of math with maxima. Otherwise search for dc42's calibration calculator web page.

I had this problem too - it was because I had the linear rails too high. If you watch it when it starts skipping, you will probably see one of the carriages hitting the bottom stop.

I simply loosened all of the screws on the bottom stop and the rails, moved it down a bit, and tightened them up again. I left the top stop with the switch were it is, as the adjustment screw will prevent the carriage from going off the top anyways.

Hope this helps,
John

John707:
A01 has written "it goes too low from a certain edge" which I interpret as "grinds into build plate".
But you are probably right. If the red dots on the rails are bottom stops then it looks like the carriages are hitting them.

hercek: Yes, I have big gashes in the buildtak from when it was grinding into the build plate. The behavior in the video is exactly what my printer was doing.