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Prusa Bolts and Nuts - Best Practices  

New Member
Prusa Bolts and Nuts - Best Practices

I'm about to assemble my first machine and one issue that crops up repeatedly in the online assembly comments is problems with bolts and nuts.  This one issue seems to span just about any step that includes the need for a nut or bolt.  So much so that I'm surprised that it doesn't have it's own step in the Introduction section of the online manual.

  I installed door hardware for over 5 years and one thing I learned is how quickly the 30 second process of installing a bolt can turn into an hour long journey to hell by a bolt that has suddenly seized during the tightening process.  Everything is running smooth and then a bolt seizes.  It won't go in any further, it won't come back out.  You, my friend, are screwed! plan on spending the next half hour trying to get that stupid bolt loose and hope that you don't damage other parts in the process. So below are some suggestions I've learned from my experience. 

I've never assembled a printer before, so I welcome the opinion of others, especially anyone who has experience building these machines.  Perhaps any ideas that receive a common consensus can be added to the Introduction portion of the manual.

1. Test every bolt/nut combo prior to use.  Multiple times I read about nuts pressed into parts followed by the assembler having an issue with that bolt/nut combo.  We're not sure what the problem is, could be the nut, could be the bolt, maybe the nut is one size and the bolt another. Simply checking to see if a bolt and nut work well together prior to installing them will prevent loss of time and potential damage to parts.

2.  Use a lubricant on the treads of the bolt.  In my experience,  lubricating a bolt will almost always prevent or overcome an issue where the bolt seizes on a nut.  You may not need it on every bolt, but if you feel some resistance then testing the bolt/nut combo (see item #1 above) then a little lubricant usually solves the problem.  Again, I have never put a printer together so I would love to hear from the community regarding the use of lubricants in general and specifically what type of lubricant would work best.  My opinion would be to use something light like a WD-40 or Tap Magic Aluminum.

3. Use a thread adhesive such as Loctite to eliminate the chance of bolts loosing over time from vibration.  I think I would really only use this as a last resort.  The Purple Loctite takes 20 foot-pounds of pressure to break free.  That sounds like a lot for the little bolts that are used in assembly.  Popular opinion is that the screws supplied are easily damaged.  I would worry that bolt heads would be stripped or the bolts themselves would shear before the threads would unlock if adhesive was used during assembly.  I only mention it here because Prusa has recommended the use of thread adhesive in cases where the threading on the female side is sloppy.  Again, I seek the wisdom of the crowd.

4. The Prusa pro tip of using a bolt/nut combo to press a nut into place may increase it's chance of seizing.  See Thread Galling for more info.  I would suggest using a c-clamp to press the nut into place, or use a lubricant on the bolt if the nut can not be seated another way.  This may or may not be an issue in assembling these machines, I just know that the issue does exist and it's never a good idea to use the bolt/nut to draw parts together if other options exist. 

5.  Okay, you screwed up.  Despite very precaution, you ended up with a bolt seized to a nut or aluminum extrusion.  Heat generally helps but it will most likely melt or discolor anything near it, so you probably can't go that route.  Be gentle, try your best not to strip out the head of the bolt.  Most bolt heads are stripped by not pushing hard enough along the axis of the bolt while twisting around the  axis of the bolt.  Still can't get it loose?  I have had success drilling into the bolt head using a bit slightly less than the diameter of the screw.  When I say diameter of the screw, I mean without the thread.  So if you think of a bolt have two diameters, one described by the outermost extent of the threads and one described by the innermost extent of the threads, I'm talking about the second diameter, the one that would exist if filed off the threads and were left with a smooth cylinder.  So grab a drill bit slightly smaller and go straight through the center of the bolt head.  The bolt head should break free and run up the drill bit since there isn't enough metal left to keep it attached to the threads.  You need to keep drilling till you reach the back of the nut, at which point everything should break loose.  The thread should just be strips of metal that twist free of the nut due to the torque from the drill.  This method should be only used after you have exhausted every other option.  There is no guarantee  that it will not damage surrounding parts or even work at all.  I just know that it works like a charm to remove stripped flat headed screws in door hinges.

That's all I got right now, feel free to correct or add to these tips.   Also, I couldn't fins a search feature on the forum so I'm not sure if this topic has been already addressed.

Posted : 15/12/2019 6:28 pm
Noble Member
RE: Prusa Bolts and Nuts - Best Practices

I don't think you have seen many posts on this because problems are few and far between with the Prusa kits.

Your points are all good, but the M3 / M4 hardware used by Prusa seems to have Class 2 tolerances.

I had no problems in my kit assembly with any of the fasteners.

The embedded nuts, thin square, are DIN 562. They are used a lot in Thinkiverse addons. If you need more - be careful to get the correct ones.

Posted : 16/12/2019 12:41 am
New Member
Topic starter answered:
RE: Prusa Bolts and Nuts - Best Practices

Actually, issues involving bolts and nuts are the single most frequent problem I have seen mentioned when reading through comments in the online manual.  It's probably only a small percentage of people who are having problems, yet it seems to be by far the most common issue I have seen, so I wouldn't say it's completely trivial.  It was only after reading comment after comment that I began to wonder why the manual didn't give a bit more info on how to avoid problems with the bolt and nuts.  Simply running the nut onto the bolt before using it in assembly would most likely avoid 90% of the problems, yet no where is this mentioned in the manual.  Instead, you get this:

"Yup, striped thread. No easy way to get this out now. Can’t unscrew it.

Randy Andrews - May 14

Hello, try to grab the screw with pliers and pull it out. Or push it from the other side.

Martin L. - Official Prusa CS - May 14"

Check out the comments and I think you would agree that not many people have issues with their build, but when they do, it's usually going to be something to do with a bolt or a nut.

And than you for the information about what nuts to buy.  I'm a true noob when it comes to 3d printing so any advice I can get is helpful.



Posted : 16/12/2019 1:19 am
Dave Avery
Honorable Member
RE: Prusa Bolts and Nuts - Best Practices

other than straight up cross threading i don't think you will see many fastener problems. given that everything is M3 they pretty much just work. no need to lubricate then and in most cases no need for threadlocker. i've never seen an M3 gall it's threads.

Posted : 16/12/2019 5:37 pm