Wide or stripped holes in print bed; can't fully tighten bed screws
 
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Wide or stripped holes in print bed; can't fully tighten bed screws  

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Chairbot
(@chairbot)
Eminent Member
Wide or stripped holes in print bed; can't fully tighten bed screws

Hello,

After both installing the print bed and performing the Nyloc mod, then reverting back to the standard installation per the assembly instructions, I have found that I am unable to tighten to of the screws completely, causing them to just spin in place rather than catch and hold like the others. It seems like the holes on the board are wider than the screws themselves, either through defect or my work on them, and the screws just spin in place after I'm able to tighten them down just a little bit.

This interferes with both bed leveling and the Nylock mod because the screws will never stop turning when I rotate them counterclockwise to tighten.

I'm not sure if this is why my bed is uneven after trying both methods, but is there any solution to repairing these so that I can fully tighten and adjust the don't have to replace the entire print bed?

Posted : 24/11/2022 12:32 pm
Netpackrat
(@netpackrat)
Estimable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

A couple options:

https://www.partsbuilt.com/y-axis-carriage-for-prusa-mk3-s-ldo/

https://www.amazon.com/Helical-Helicoil-Chainsaw-Stainless-Restoring/dp/B08R63S6YY

Posted : 24/11/2022 1:40 pm
RandyM9 liked
Chairbot
(@chairbot)
Eminent Member
Topic starter answered:
RE:

I was wondering if it could be the Y-axis carriage, but my thought went to the holes in the print bed because they seem to be hollowed out to different degrees, making me think that the issue was closer to the head of the screw rather than lower into the holes, especially with the difference between the lower middle and lower right screws and how far into the holes they can go with everything is tightened at the same rate using the method described in the assembly guide:

 

Posted : 24/11/2022 1:54 pm
RandyM9
(@randym9)
Reputable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

The threads are in the carriage, not in the bed, so any difference in clearance in the holes in the bed doesn’t have any effect on getting the screws to tighten, assuming the screws are getting full purchase on the carriage.

Both of @netpackrat ‘s suggestions are good ones. I wouldn’t have even thought to use helicoils but that’s actually brilliant. You get to maintain the use of stock M3 screws and you get improved holding strength and less chance of stripping out the holes again at some future time.

I would have said to check if an M4 screw head would recess into the heat bed and then drill the holes out oversized and tap for an M4. But I think you have two better solutions already.

Cheers

Posted : 27/11/2022 5:58 am
Netpackrat
(@netpackrat)
Estimable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

 

Posted by: @randym9

Both of @netpackrat ‘s suggestions are good ones. I wouldn’t have even thought to use helicoils but that’s actually brilliant. You get to maintain the use of stock M3 screws and you get improved holding strength and less chance of stripping out the holes again at some future time.

Thanks.  Where I work, we have guys who are good with hydraulics, guys who are good with wiring, others who are just great nuts and bolts mechanics.  I'm the guy they call when somebody snaps off or strips out a screw, so I have done quite a few thread repairs.

That said, the OP may have a point in that it's entirely possible he has a problem with one or more of the holes in the heat bed, which is basically a composite material, and a lot softer than the screws.  If I wanted to know if the issue were with the carriage or the bed, I would remove the heat bed from the carriage, and get an m3 socket or button head screw.  Then I would put that screw into each of the suspect threaded holes through one of the bed spacers, and see if it will come tight, or if the threads in the carriage are stripped.  If you can get a BHCS or SHCS to tighten on a spacer in a carriage hole that was giving you trouble, then you will know to look to your heat bed. 

Posted : 27/11/2022 10:48 am
Neophyl
(@neophyl)
Famed Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

It may also be a difference in the screws themselves.  I've had batches of countersunk M3 screws delivered where the heads were fractionally larger diameter than previous orders.  This resulted in them not fitting in to the countersunk recesses in my parts.  If thats the case on the heatbed then the heads will potentially be proud of the surface and could have issues if that stops them from reaching the threads on the frame properly.  Just something else to rule out.

Posted : 27/11/2022 11:05 am
RandyM9
(@randym9)
Reputable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

That said, the OP may have a point in that it's entirely possible he has a problem with one or more of the holes in the heat bed, which is basically a composite material, and a lot softer than the screws.  If I wanted to know if the issue were with the carriage or the bed, I would remove the heat bed from the carriage, and get an m3 socket or button head screw.  Then I would put that screw into each of the suspect threaded holes through one of the bed spacers, and see if it will come tight, or if the threads in the carriage are stripped.  If you can get a BHCS or SHCS to tighten on a spacer in a carriage hole that was giving you trouble, then you will know to look to your heat bed.

Point taken, I assumed (I know, I know…) it would be fairly obvious if the screws were not catching the countersinks in the heatbed and if they’re properly threading into the carriage beneath, they should still pull tight at some point.

Your suggestion of removing the heatbed and examining the threads with the spacers and screws is a good one. 

@neophyl ‘s point re the uneven countersinks in the heatbed is valid and I’ve observed the same effect with my printer. I don’t worry much about it, as long as the screws snug-up properly.  My concern was if an oversized screw would fit the recesses without sitting proud of the heatbed surface.

Let us know how you make out.

Cheers

Posted : 27/11/2022 9:18 pm
Chairbot
(@chairbot)
Eminent Member
Topic starter answered:
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

 

Posted by: @randym9

That said, the OP may have a point in that it's entirely possible he has a problem with one or more of the holes in the heat bed, which is basically a composite material, and a lot softer than the screws.  If I wanted to know if the issue were with the carriage or the bed, I would remove the heat bed from the carriage, and get an m3 socket or button head screw.  Then I would put that screw into each of the suspect threaded holes through one of the bed spacers, and see if it will come tight, or if the threads in the carriage are stripped.  If you can get a BHCS or SHCS to tighten on a spacer in a carriage hole that was giving you trouble, then you will know to look to your heat bed.

Point taken, I assumed (I know, I know…) it would be fairly obvious if the screws were not catching the countersinks in the heatbed and if they’re properly threading into the carriage beneath, they should still pull tight at some point.

Your suggestion of removing the heatbed and examining the threads with the spacers and screws is a good one. 

@neophyl ‘s point re the uneven countersinks in the heatbed is valid and I’ve observed the same effect with my printer. I don’t worry much about it, as long as the screws snug-up properly.  My concern was if an oversized screw would fit the recesses without sitting proud of the heatbed surface.

Let us know how you make out.

Cheers

I'm new to the lingo, so please pardon me if I don't fully understand, but, to summarize: if the holes in the heatbed are oversized and an M4 screw of the same length (M4x12b / M4x14b?)  doesn't solve the issue, I should try removing the heat bed, getting an M3 socket, and attempt to secure each M3 screw through the heatbed holes and spacers into the socket.

If one or more screws fails to secure into the socket, then I know the issue lies with the screw(s). If I'm able to secure all screws into the socket using this method, then there is a good chance the countersinks in the carriage are stripped and I need to replace it.

Is this correct?

Posted : 28/11/2022 1:51 pm
Chairbot
(@chairbot)
Eminent Member
Topic starter answered:
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

 

Posted by: @netpackrat

A couple options:

https://www.partsbuilt.com/y-axis-carriage-for-prusa-mk3-s-ldo/

https://www.amazon.com/Helical-Helicoil-Chainsaw-Stainless-Restoring/dp/B08R63S6YY

Apologies for the near-immediate double-post, but if the troubleshooting outlined above (testing the screws into an M3 socket) does indeed point to damaged or stripped countersinks in the carriage itself, I can install these into the carriage to see if that resolves the issue before replacing the entire carriage?

Posted : 28/11/2022 2:07 pm
Netpackrat
(@netpackrat)
Estimable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

Just to clarify, what I suggested was getting either a socket head screw (or a button head screw), and installing in each of the carriage holes with a spacer, but without the heatbed to eliminate the bed from the equation.  Essentially just a screw with a flat shoulder at the head instead of a countersink.

Anyway, if the bed ends up being the problem, yeah a larger M4 screw could be a fix, BUT you will have to drill and tap the carriage for the larger M4 sized screw.  Later if you decide to replace the heat bed, not sure if the carriage could be heli-coiled back to m3 or if you would have to replace it.  Depends on the relative sizes of the M4 and the outer thread of the m3 helicoil which I haven't looked into.  A second potential problem with going to the M4 screw, is the possibility that the head will be too big and stand proud of the bed (which would give the steel sheet a high spot).  But you should be able to check that before actually drilling and tapping the carriage.

If the threads in the carriage itself are stripped, yes you can install the helicoils like I linked but you will need to have the ability to drill and tap the carriage, and you'll need to be able to do so straight.  The way the inserts work, is you drill out the stripped out threads, and then run a special threading tap (which is included with the helicoil kit) that's a unique size for the helicoils, then you can thread the helicoils into the holes.

If done right, the result will actually be superior to the original threaded holes because you'll now have stainless steel threads instead of aluminum which are far less likely to be stripped out, and they do get messed up later on, you can usually just pull out the old helicoil and put in a new one.  In aviation we use them a lot as standard fitment where fasteners are threaded into aluminum components, because it's stronger and a more durable setup than threading directly into the aluminum.  OTOH, it's probably a lot easier to just replace the carriage than bother with the inserts.

Posted : 28/11/2022 9:28 pm
RandyM9 liked
RandyM9
(@randym9)
Reputable Member
RE: Wide or stripped holes in print bed; can't fully tighten bed screws

+1 - What @netpackrat said.

I tried to post earlier but my answer got corrupted and I got disconnected.

Prusa may still be running their free worldwide shipping so you can buy a new Y carriage and save the damaged one (if it’s damaged) to experiment with.

https://www.prusa3d.com/product/y-carriage-mk3-s/

Cheers

Posted : 29/11/2022 4:09 am
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