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14.12 on UM2. Possible bug: Extruder motor hickups during brim/raft print.

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I am a newcommer here, I just bought an Ultimaker 2. Pre-installed version was 14.09. I then updated to the recent 14.12, not 14.12.1.

I noticed some hickups of motor extruder during brim and raft printing. Filament stops for several cm of path. These hickups were not present in 14.09.

By sensing the filament at the extruder input with my fingers, the motor clearly goes back few mm, very fast, but doesn't resume position. That's why several cm of filament are missing on the brim/raft.

I started several times the same print file, always with the same hickups.

These hickups were not present two days ago, when I printed from the same file, still with 14.12 firmware.

If I play with speed during print, reducing it to 85%, hickups disapears.

There is not problems durint print of the part itself, which is done at a different speed.

This remember me when I design real time systems, with dual acces to variables, either from processes or from interrupts, this without protection scheme :mrgreen:



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I would check your bed levelling to make sure it's proper. If the bed is too close to the nozzle the pressure will be higher which can cause the motor to skip back.

The skip back is mechanical, it's not controlled by the electronics (more so than controlling the amount of current the motor receives), that's why you're not seeing the printer compensate for it after it happens.


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I tried with another part, which is smaller.

Still same behaviour of the motor. Now, it does it even during print of the part body.

I tried to keep pushing by hand the filament. It is clearly pulled back by the motor.

I also sensed the axle protruding from the extruder. If it is the motor axle, it is also going back.

And again, lowering the speed from 80mm/s to 65% (about 50mm/s) solved the problem.

I will do more tests.



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