Strange print shift issue

Hi everybody

I have a strange issue on my kossel mini.
everything was working good, I was printing all of my parts without any issue.

And now most of the prints result with a shift.
The shift is always on the Z tower, and always on the same direction.

The strange fact is: the shift is always on the same layer for one gcode !!!
I've tried several changes to find the source without finding any clues which could explain it.

Here is some examples, from left to right:
1: original gcode which was great on previous print
2, 3 and 4: another gcode
5 and 6: another gcode



My configuration is:
Kossel mini with Marlin Firmware
Octoprint print server on raspberry 2 (OctoPi iso)

I've tried printing from the SD card to avoid any octoprint bug.

All the gcode was generated with Cura 15.02.1

And now at the end of the print the belt is going outside of the smooth pulley



But this is not happening when the print is shifting.

I'll try to do another print and check the belt and pulleys at the moment of the shift.

But for now, does anyone has any idea about what happening ?

If the belt is shifting on the top pulley, that suggests to me that either it isn't tight enough, or there is an alignment issue between the carriage and the top pulley.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

During the first shifting prints the belt was still in the pulley.
I've tried tighting it more without any change in the shifts.
The alignment is good as the printer was working well during 2 months with very great accuracy ! And nothing moves between this time and now, I've already check the alignment 2 times since the issue is present.

The very strange fact for me is the shifting is always at the same layer for a gcode, but at different layer for differents gcode, this is not a usual issue with hardware problem.

well ...after following a print from start to the issue I think I finaly find the source.

If you look the top right side of the tail the plastic is doing a little bubble, and the hotend is still stuck a short time when going back to the left.



I'll do some tests to confirm this feeling.

Do you have any advice to avoid this kind of plastic bubble? lower the hotend temp ?

Set zhop to .5mm or so. It should help a bunch. Its under cura advanced settings. Also what speed are you printing? Have you checked the filament diameter? Set the average of 5m in 10cm increments in cura diameter.

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