MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
What could prevent simple unthreading of the M3x40 Extruder-idler tension screw/spring unit from the M3n nut mounted in the printed Extruder-idler bracket on a 13 month-old MK3S w/ an est 1,200+ print hours? The task SEEMS like it should be quick and trouble-free, but instead appears to be very close to the point where drilling out the head or nut of the tensioner assy and perhaps replacing the E-idler brkt becomes necessary.
I am asking on behalf of the owner. I had no part in assembly of the machine.
From examination of text and photos in Step 19 and Steps 27-32 of the following PRUSA document
"3. E-axis assembly"
https://help.prusa3d.com/en/guide/3-e-axis-assembly_55136
I have come to the belief that:
- the tension screw/spring unit fits into a deeply counter-sunk hole in the printed Hotend Assembly.
- The spring seats against the shoulder of the Hotend's counter-sunk hole
- the end of the tension screw passes out the right side of the Hotend's hole into the cavity for the
Extruder-idler bracket
- When the Extruder-idler bracket is in place, the end of the tension screw passes through the inboard side
of the screw hole in the Extruder-idler bracket to engage with the M3n nut mounted in the molded hex pocket
on the outboard side.
- The M3n nut appears to have a conventional thread with no provisions for 'locking' (ie: not a NyLock/Stover nut)
On examination, about 1 - 1-1/2 threads of the M3x40 were protruding out from the M3n nut. It was NOT possible to turn the M3x40 CCW, but it WAS possible to turn it CW perhaps 1/3 turn (no filament in place) and then 1/3 CCW back to the initial position.
The hex socket on the M3x40 is damaged/deformed from the owner's efforts. The torque required for the movement described above has further disfigured the socket.
I noticed what I thought might be a small amount of metal oxide on the outwardly sloped lower side of the cavity in the Hotend Assembly while peering up at the bottom gap between the Hotend & Extruder-idler pieces. The owner later suggested that what I saw might be filament residue.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Ok. Let me rephrase the question.
On YOUR MK3S, can YOU disengage the threads of the M3x40 Extruder-idler Tension Screw from the M3n nut captured in the Extruder-idler bracket without significant torque requirements or other difficulty?
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
If the adjusting screw is damaged to the point of no removal, I would do this.
Print a new extruder-idler plate. if possible. You're gonna need one after the next step.
Remove the extruder cover and the pivot screw for the idler plate. This will now give a little space at the top so you can then grip the idler plate firmly with a good pair of pliers.
Bend the pliers away from the extruder (to the right) to SNAP off the idler plate. The adjusting screw can now slide out enough to grab the head and nut to unscrew with 2 pliers. Or cut the screw. The screw will bend a bit. Just my work around.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Thank you for the suggestion.
Does your tension screw turn easily out of the nut ?
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Yes, it does. Smooth as silk.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
There could be potential damage to the extruder housing in this repair. As you apply pressure to snap the idler, the screw/spring will bear down on the housing. The housing is much more dense than the idler plate so it should be OK.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Yes, it does. Smooth as silk.
Thank you.
With no prior involvement, the Extruder Head/Hotend component assembly was (literally) a black box to me. Caution, born of prior experience with concealed "Gotchas !" or booby-traps unwittingly or intentionally engineered into products, is a guiding principle.
Interestingly, the owner told me the E-Head arrived fully assembled from Prusa. The tension screw was threaded into the nut and has never been removed.
Present Working Hypothesis: cross-threaded at Prusa factory.
There could be potential damage to the extruder housing in this repair. As you apply pressure to snap the idler, the screw/spring will bear down on the housing. The housing is much more dense than the idler plate so it should be OK.
Yes.
I appreciate any and all attempts to assist with problem resolution. However, I do not anticipate resorting to breaking the idler bracket.
I accept that sufficient damage to the idler bracket might occur from methods other than intentional breakage that the bracket may not be salvageable, but I will try hard to prevent rendering the part useless. While the owner has another filament printer, the Prusa's idler bracket appears to require closer tolerances for compatibility w/ the existing idler shaft and extrusion housing than, say, a basic cover plate.
Thank you again for your ideas and for taking the time to inspect the screw/nut on your machine. I appreciate the support and opportunity to bounce thoughts back and forth.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Interestingly, the owner told me the E-Head arrived fully assembled from Prusa. The tension screw was threaded into the nut and has never been removed.
Present Working Hypothesis: cross-threaded at Prusa factory.
I retract the remarks above.
Either I misunderstood or the owner is now contradicting what was said earlier. In any event, I'll stick to posting what I KNOW from first-hand experience in future.
In a separate matter. Thank you to Prusa for providing surplus fasteners - the owner has produced replacements for the tension screw and nut from hardware that remained after the initial build. (At least I assume these were surplus ...)
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
One more work around idea. Slide the extruder all the way to the right side, X positive. Get a hand drill ad a 3mm bit or a #31. carefully support the extruder and lightly, carefully align the bit on the socket of the cap screw. Drill into the socket until the head becomes free. This would be a two person job. see photo below. (if it loads)
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
How much of the screw is sticking out of the nut on the tension door? If the end of the screw is not out very far you should be able to use and allen wrench and push the screw into the spring, while holding the door closed, and push the nut out of the plastic. With the nut out of the plastic you can grab it with a pair of pliers and try to un-thread it, or break it off if it's badly cross threaded. I just tried it on mine with the end of the screw flush with the door and the nut came out past the surface of the door several mm.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
If the end of the screw is not out very far you should be able to use an allen wrench to push the screw into the spring, while holding the door closed, and push the nut out of the plastic.
I just tried it on mine with the end of the screw flush with the door and the nut came out past the surface of the door several mm.
Excellent !
More later, probably w/ photos, but this is to say that a variation on your suggestion proved to be a WINNER !!
With the tension-screw out of the way and the Extruder-idler bracket in hand, further trouble is revealed.
The shaft that the Bondtech and bearings runs on is protruding out from one of the two supports molded into the E-I bracket. An attempt to push it further into place is met w/ resistance. Examination of the opposite molded support exposes bore obstruction from print residue that was unaddressed on initial assembly.
A study of the bracket / shaft / Bondtech relationship seems to indicate shaft engagement was so shallow on the obstructed side that the shaft / Bondtech operated in single-shear in the bracket.
A look at the shaft shows asymmetrical burnished bearing wear tracks. The wear tracks on the shaft look like they have been coloured blue, as if from heat. There are one or more longitudinal dark lines. There is fine dark grit. A brief preliminary look at the bearings shows dark grit.
No evidence of lubrication, but then while reading the "3. E-axis assembly" document I noticed w/ interest that no lubrication was called for during assy.
Work on repairs is stopped for the day.
I recommended that the owner order replacements for the shaft, bearings and probably the Bondtech, also.
Although the existing parts are damaged, I intend to clean them well, lightly polish the burnishing, clear the print swarf and securely locate the shaft in the center of the molded supports using end plugs or something else suitable.
QUESTIONS:
- Are the bearings intended to run dry on the shaft?
- Is there an argument AGAINST lubrication?
- Are there opinions about petroleum grease, synthetic grease, dry-powder (graphite) lubrication?
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Glad you got it opened up. Sounds like that extruder assembly ran into multiple cases of bad luck, cross threading of the M3 screw and the 3D printed door had clean up issues.
I can't think of a reason against lubrication, but it is a very low RPM application. Now you're making me want to take mine apart and check it to see if there is wear, and now that I've thought of it I'll have to do it...
One reason I like building from a kit is you can double check all the parts, fix issues with parts fitting together, etc., basically spend much longer time building it than a person on an assembly line would.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Caveat on lubrication, if you do lubricate it has to be something that would stay in place, not flow onto the outside of the gear and possibly get onto the filament. So a tiny amount of some very tacky grease would be the only thing I would use, or nothing, if everything is aligned well there shouldn't be a problem, they do run these things in the print farm making the parts all day every day for years.
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
Details and photos yet to come, but a quick few remarks:
Printer is back together. It is running well, according to owner.
- Extruder-idler pocket in the Hotend Assembly was vacuumed
- 'Bondtech' idler, bearings and shaft were cleaned w/ Isopropyl Alcohol using Q-Tips
and closely inspected for Q-Tip fibers.
- Shaft was polished lightly by hand w/ Autosol Metal Polish on a cloth to reduce bluing,
remove linear streaks of residue and smooth variations. Wiped afterward w/ IA to
remove traces of polish.
- Bearings installed dry
PRUSA offers a kit consisting of a Bondtech, a pair of bearings and a shaft for US$ 50.
The owner will be placing an order for this kit and other items soon.
Anomalies noted:
- Significant flaking of nickel/chrome plating from inside the bore of the Bondtech idler
(this is the running surface for the bearings.)
- Original nut removed from tension screw does NOT appear to be cross-threaded.
- Original nut is ATTRACTED by a magnet. Replacement nut, replacement tension screw
and original tension screw all are NOT ATTRACTED by a magnet.
These fasteners are stainless steel. I suspect thread galling occurred between the original
tension screw and nut. When a SS nut galls to a SS screw/bolt, it is game over as far as
any hope of removal by unthreading.
- I plan to split the nut in hopes to separate it from the screw so I can examine further.
- I intend to apply a small daub of 'Nickel Anti-Seize' lubricant to the new
screw/nut as a cautionary measure as soon as the present print job ends.
Regarding thoughts about inspecting the Bondtech, its bearings and shaft condition/orientation
in the bracket on your machine, I vote in favour. If the tension screw moves freely in the nut,
disassembly is a cinch. The owner tells me that adjusting tension on the filament after reassembly
is a non-issue.
I'll be particularly interested in hearing what you see when you shine a flashlight up one end of
the Bondtech and look down the other. Plating the ID of a bore is more troublesome than plating
the exterior. The finish left by machining of the Bondtech is comparatively rough, this doesn't
instill confidence that the plating process is considered critical. Maybe flaking plating in the bearing
bore of the Bondtech is widespread?
RE: MK3S: Removal of Stuck M3x40 Extruder-idler Tension Screw
I did take mine apart to inspect it, I've only got 30 to 40 hours of printing on mine so wasn't expecting any wear. And did it before reading the post above so didn't shine a flashlight inside the bondtech gear.
But I did see some very tiny flakes of metal on the bearings, I put the bearings on the shaft and rolled them over my finger back and forth and more tiny specks came out, leaving what looked like glitter on my finger. It did this until nothing new came out of them. The specs were almost microscopic, small enough to fit in the groves of my fingerprints.
Putting it back together I did put a very small amount of tacky red lithium grease on the shaft and inside the gear with a Qtip.