MK3 Extruder Skipping  

RE: MK3 Extruder Skipping

It seems I should probably also try this test for the extrusion speed as I have also been struggling with the extruder motor skipping just about no matter what I tried or tested and I have very seldom seen a case of the filament being grinded with the motor still able to turn.  That said, I do not have more that one machine to compare so I guess my results will be somewhat subjective.

By the way though, the only thing that recently seemed to help and which for a long time I have forgotten that I saw an article about, was to swap the nozzle with a non-Prusa version.  I remember there was such a discussion somewhere mentioning that the Prusa nozzles are profiled on the inside so as to improve MMU experience or something, but the fact that they do not have a constant diameter all the way through improves the changes of filament blockages.  This eventually did seem to help my case, but I am now wondering whether the root cause is not still something else, like the extruder motor having an issue and therefore not being able to overcome the smallest need for increased torque.

RE: MK3 Extruder Skipping

@danie

Its not the nozzles that are profiled different.  Its the heatbreak profiled differently to a standard e3d one for the MMU.  Changing that is a hotend teardown to a far greater extent than simply changing a nozzle.

RE: MK3 Extruder Skipping

Posted by: @danie

[...] By the way though, the only thing that recently seemed to help and which for a long time I have forgotten that I saw an article about, was to swap the nozzle with a non-Prusa version.  I remember there was such a discussion somewhere mentioning that the Prusa nozzles are profiled on the inside so as to improve MMU experience or something, but the fact that they do not have a constant diameter all the way through improves the changes of filament blockages.

That is a reference to the Prusa version of the E3D V6 heatbreak which has a 2.2-2.0mm step internally to improve MMU performance. Many users find that replacing the Prusa version with a standard E3D version with a straight 2.0mm bore helps. However, it's worth noting that all Prusa i3 Mk3 series printers shipped since 2018 have had the stepped heatbreak, so it's not a "problem" in all cases. There are likely a combination of issues causing friction in the filament feed and replacing the heatbreak is one of many possible mitigations that reduce friction just enough to get rid of the skipping. There are other things you can try before replacing the heatbreak, and there's no guarantee replacing the heatbreak will fix the problem completely if you have other issues.

 This eventually did seem to help my case, but I am now wondering whether the root cause is not still something else, like the extruder motor having an issue and therefore not being able to overcome the smallest need for increased torque.

I've put together some notes on the problem that cover all of the issues I've encountered discussing related problems here on the Prusa forums.

Trouble Tree?

Is there a trouble tree or more detailed trouble shooting guide for this issue?  I have been having skipping/no feed issues for a few months, and have replaced all the teflon, cleaned the hot end, with no improvement.  After I started observing more closely, it seems my issue occurs on prints with long layers, and not on prints with short layers that have lots of lifts.  This seems to suggest to me it is the stepper motor not being able to keep up with a constant feed rate with no breaks.  Sometimes it skips, and some times it seems to be feeding (feed rollers turn), but very slowly.

Clicks on long runs indicate volumetric rate issues

Posted by: @reddog

Is there a trouble tree or more detailed trouble shooting guide for this issue?

I'm not aware of anything formal, sorry. I just try to dump everything I encounter into my notes pages.

[...] After I started observing more closely, it seems my issue occurs on prints with long layers, and not on prints with short layers that have lots of lifts.  This seems to suggest to me it is the stepper motor not being able to keep up with a constant feed rate with no breaks.  Sometimes it skips, and some times it seems to be feeding (feed rollers turn), but very slowly.

That points to a possible issue with your maximum volumetric speed (MVS). If shorter runs print well, it may be because the nozzle has not fully come up to speed on the shorter extrusions. As it hits the longer runs, you might be hitting a flow rate that exceeds what your combination of hotend, nozzle, and filament can melt and push through. A quick test is to simply slow down. Reduce speeds 50% using the front knob when you notice the clicking. If it goes away, you know this is a good place to start. Bumping temps can help, but may also introduce stringing or bridging quality issues. Excessive cooling can also contribute to problems if the fan is not properly aligned.

In general, the Prusa presets are a bit optimistic with the MVS speeds. For a 0.4mm nozzle, I'd drop it from Prusa's 15mm^3/s to 11.5mm^3/s. I've got more notes on this topic here that might be of interest.

Something Changed

Thanks for the response.  I don't suspect any sort of parameters like that as the issue, as prior to the last few months, the only issue I had was occasionally with adhesion to the bed, with ~550 m of filament and 19 days of print time.  I don't know if that is a lot or a little, but this didn't start happening until probably the las 5-10m of printing.  The exact same gcode file that I am having issues with now previously printed perfectly.  So something changed, I need to find what it is.

RE: MK3 Extruder Skipping

After dozens of hours spent trying to get to the bottom of this issue, multiple wasted prints then replacing the PTFE tube, the heatsink, even the whole hotend to a Phaetus I found the skipping extruder was caused by the faulty extruder motor. The knowledgebase could highlight this as a possible cause to save many wasted hours and $$$.

TL;DR - PRUSA EXTRUDER MOTOR CAN BE FAULTY AND YOU WON'T NOTICE IT RIGHT AWAY

Here's how to identify if your Prusa MK3S  extruder motor is faulty:

STEP 1:

Preload some PLA and using Cura or Pronterface run the following:

M83 (relative extruder)

M109 S200 (heat the nozzle to 200C)

G1 E60 F200 (extrude 60mm using 200mm/min speed)

G1 E60 F300

G1 E60 F400

etc.

at some point you might get the skipping noises coming from the extruder motor.

My faulty motor started skipping at around F250-F350. I believe a properly functioning motor can do F450-F800 (depending on the hotend/temp etc.)

So if you're getting skipping noises around F200/F300 it's a good indication that your extruder motor might be faulty.

STEP 2:

To confirm that it is in fact the motor's fault swap the extruder motor with the X axis motor.

This can be done relatively easily to verify the problem:

1. Loosen the X-axis belt tensioner (it's between the frame and the X motor)

2. Unscrew the X-axis motor, unscrew the pulley

3. Pull the X-carriage to the left so the X-axis motor can reach the extruder

3. Disasemble the extruder motor, unscrew the Bondtech gear and put it in the same position on the X-axis motor

4. Mount the X-axis motor in the extruder and make sure the gears are aligned with the filament

5. Swap the connectors on the Einsy board (put the X-axis connector inside the Extruder socket)

6. Run the procedure described in STEP 1. You should be able to run the extruder with at least F450-F500 speeds. I was able to run with F800 with no clicking.

7. If confirmed that this is in fact the extruder motor replace it witw a new motor. Put the X-axis motor back in place and order a new extruder motor from Prusa shop. It'll come with the right cable and connectors attached.

Hope this helps someone. I wish I knew it before wasting my time and money on this issue.

RE: MK3 Extruder Skipping

I totally agree.

When I had the issue, Prusa support did not mention this as a possibility and seemed to dismiss as implausable my sugestion that the motor might be at fault.

I have since fitted a cooling fan to keep the motor cool and i have a backup motor... just in case.