[Solved] MK4/S - Prints failing, under-extrusion, no extrusion, clicking extruder  

MK4/S - Prints failing, under-extrusion, no extrusion, clicking extruder

Alright, try #3. I get a generic error page when submitting the first and second time and I do not have time to write it all up nicely a third time, so here's the quick and dirty version. (Seems like an auto-logout "feature" that occurs before I can finish typing...)

TL;DR: Failed bearing(s) in Nextruder stepper, replaced bearings (or could replace stepper entirely) and problem went away.

Over the past couple of months, this printer had gone from flawless prints every time to failing every print, most often within the base layer or within the first few layers. The extruder has a clicking noise which correlates with filament not extruding and the print subsequently failing. If left to continue, the filament will grind and become "stuck" as the gear can no longer push or pull it. This will also lead to the MMU3 being unable to retract the filament as it does not have enough tension to overcome the extruder gear past the ground portion. After many hours of disassembly / cleaning / reassembly, I believe I found the issue in my particular case. 

Primary Symptoms

• Appears to skip layers occasionally
  • As condition worsened over time, this very quickly became spaghetti rather than missed layers
• Grinding filament
  • As condition worsened over time, grinding became less common
• Little/no extrusion
• Clicking extruder
• Filament stops feeding into extruder
• Condition worsens over time

Testing / Diagnostics

• Multiple cold pulls, no change
• New nozzle, no change
• Temperature of nozzle is normal
  • Using clear PETG to test, which goes from milky at 240C to fully clear at 250C, results retained with set temperatures during symptoms
• Removed PG cover, planet gears do not move when clicking noise occurs
  • Appears that the shaft on stepper is stalling, causing the "click"
  • Internals appear clean and free of debris
  • Not recommended to run printer with cover removed for long periods
    • I used washer stacks to temporarily replace the cover to keep the bronze ring tight and help retain the gears
• Checked cabling, no issue
• Tried different filaments, opened and new, no change
  • PLA seems to lessen symptoms
  • PETG is worse
• Adjusted idler arm tension
  • Increased tension worsens symptoms
  • Decreasing tension lessens symptoms, but does not resolve
  • Holding idler by hand with light/moderate tension seems to resolve the issue, not a permanent solution
• Removed PG assembly, spun stepper shaft by hand
  • Rough spot / increased force needed at certain point(s) in rotation
  • Mostly smooth otherwise
  • Planet gears normal, minimal wear
• Replaced PG assembly with new, no change
• Removed and disassembled E-axis stepper
  • Shaft is in good condition, minimal wear
  • Rotor laminated disc on shaft side has some rubbing
  • One small blob of dark debris, seems like a ball of greasy dust, does not seem to be an issue
  • Shaft side bearing feels rough, loose internals, additional noise when free spinning
  • Rear side bearing feels tight, smoother

I ordered some identically sized bearings from McMaster-Carr (6153K15, trade name 625-2Z, 5x16x5mm shielded) and installed in E-axis stepper. Reassembled Nextruder as normal and performed Gears Calibration. Problem is resolved, began a 3-hour print in PETG which completed without issue and has no defects. 

The initial cause was likely a single event where I had a tangled spool which broke the PTFE guide clip from the MMU3 "array" of filament spool holders, quickly binding everything in the line and putting excessive strain on the extruder stepper during a print. It only took about 20 seconds from hearing the clip snap off to stopping the print, but that seems to have been enough to give this result. 

Side note, there were High Precision bearings available in the same size, but the cost of the bearings together are more than the cost of a new stepper straight from Prusa. I leave that decision up to you if you have a similar problem and yield similar testing / diagnostics.