Z height randomly dropped mid-print, causing nozzle to dig into the print

Machine: CR-10S PRO with TM3D marlin firmware

Problem: nozzle moves downward mid-print, damaging the print and machine


  1. Printing a model of height of around 250mm throught octoprint.
  2. At around Z-height of 6mm-12mm, nozzle move downward, destroying the print. Printer is then turned off to prevent damage.
  3. Printed the same model for 3 times on Octoprint, the z-height (6-12mm) and location of the occurance seems random. Printed the same gcode on sd card and it was fine.
  4. Printed another model with Octoprint and it happened again. Printing the same gcode on sd card and it is fine.

What have been tried to solve it:
I don't think the downward movement is homing action, as the printer homes by homing X, then Y, then Z; while what was observed is that the nozzle was moving sideway and downwards.
Slice the model again.
Print a different model.
Print the same gcode on sd card and it was fine.

Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, octoprint.log, serial.log or output on terminal tab, ...)
octoprint 20190403.log (290.2 KB)

Hi @ktnlee01,

there are quite a lot of communications errors in the log file.
The serial.log would be quite handy here. (You may have to activate the serial logging: Where can I find OctoPrint's and OctoPi's log files?)
Also check the USB cable for quality: OctoPrint randomly loses connection to the printer with a "SerialException"
And also try printing from OctoPrint without SD card in the printer.

Thank you!
Here is the serial.log file.
serial.log.zip (577.1 KB)

As I assumed, there are communication errors:

2019-04-03 10:39:34,006 - Send: N0 M110 N0*125
2019-04-03 10:39:34,009 - Recv: �qX�J&MA��m8&�XA��('JAa}�q��=��0�n i��AP|&2��Ha�8|Bd�~B=1A���\Q�e.�Ji�]t���Ma�}am^6�Kaq](��%��>aiP|P�*2+X6��Jaa��p|B�aAW�f�M@&%a�a4R�&$6f��8%�JA�����Kr��jr���:&2�}a��o6�Ef�M@&-�Kr��Kr��%h_H�x�t&2/�a3M@�~�t�J �A���%%�F�4�M@6���N_����mB��a�oS6�sa+'J A�AI�JB��aI8|�6az�/�>?�c��6e,=S6�]��6,|>@y�{&}\r��&>@yh�&{|rF�M@�JB�`��jB����5!yK��
2019-04-03 10:39:34,012 - Recv: aD�xQa
2019-04-03 10:39:34,015 - Recv: :�j ��9Qs�>�;e]2'J ��`q�'Jq}�NA��r��8b��%`���_�p|B�q}��~ra�
2019-04-03 10:39:34,016 - Recv:

Also, the SD card slows down the communication and prosessing:

2019-04-03 10:57:16,236 - Send: N401 G1 X132.595 Y112.334 E27.1796*100
2019-04-03 10:57:16,238 - Recv: Not SD printing
2019-04-03 10:57:16,573 - Recv: ok N397 P0 B0
2019-04-03 10:57:16,587 - Send: N402 G1 X135.640 Y112.298 E27.4076*96
2019-04-03 10:57:16,924 - Recv: ok N398 P0 B0
2019-04-03 10:57:16,938 - Send: N403 G1 X136.525 Y112.304 E27.4738*107
2019-04-03 10:57:17,228 - Recv:  T:205.00 /205.00 B:59.99 /60.00 @:78 B@:29
2019-04-03 10:57:17,238 - Recv: Not SD printing
2019-04-03 10:57:17,259 - Recv: ok N399 P0 B0