Using screws and nuts with PLA and PETG
I'm making face plates for a ventilation system and I need to use regular metal screws to fasten the face plate to the "back plate" that goes on the other side of the drywall. Here's the rough sketch I drew on a white board:
Just for reference, of course, the two sides of the face plate and the two sides of the back plate go all around the hole, so it's one face plate and one back plate. The issue I'm having is that I want the nuts in place, on or in the back plate, so when I slide the back plate through the hole and have it on the other side of the drywall and inside the wall itself, the nuts will stay in place and not fall off or turn with the screws. I've seen a number of printed parts that allow a slot for nuts so they can be slid in place. I like that, but my problem is that those slots are always perpendicular to the "print bottom" of a piece. For reasons I don't want to go into here, the part of the back plate that has to go on the print bed (which is labeled in the pic) is parallel to the nut, so any slot I make for a nut will result in either an overhang or bridge inside the back plate. That would mean support material, which would be tough to clean out of a small area.
I'm sure this is the kind of thing a lot of people have run into with printing in the past: Finding just the right place and method to fit a nut into a printed object.
First, I'm open to suggestions - I'm hoping someone has a brilliant idea I haven't even thought of! But if that's not the case, I have a few thoughts I'd like to get feedback on. First, I could use something like a metal screw:
This is a simple solution, but I'm concerned that over-tightening it or having to remove it and replacing it a few times might lead to the threads basically stripping the printed hole it goes into, to the point where it no longer works and I have to print an entirely new back plate. What are your experiences with using this kind of screw on PLA and PETG? Does it strip the hole after a few uses so it no longer holds things in place?
Another idea I had is to pause the printer at a specific level, after it's printed holes in the back plate for nuts, then drop the nuts in place, then continue printing. I have yet to work with any prints where I pause the printer. Are there issues I need to be aware of for pausing a print? Can the printer pause the print and sit for a while until I drop the nuts in and resume? I have a number of these to print and it can be frustrating if I have to stay in the shop and wait for a pause, so it's not a preferred method.
And the last thought I had was to print a nut bracket or holder, like this:
I could put the nut into the holder and glue it to the back of the back plate. Of course the glue has to be strong enough to hold the nut in place when I tighten the screws. (I have a tendency not to trust glue to be but so strong, so I know sometimes I'm underestimating the strength of some glues.) I also thought I might be able to design this so the holder has some snaps on it that would go through holes in the back plate, but that also seems like a lot of extra design and printing work and that there should be a simpler solution.
So what do you do to hold nuts in place in your printed projects?
RE: Using screws and nuts with PLA and PETG
Those slots for something like a square nut when toleranced up do bridge if designed correctly and do not require support. There is zero clean out. You just screw through the single layer bridge into the nut. I use m3 square nuts like are used to assemble the mk3 printers all the time.
RE: Using screws and nuts with PLA and PETG
I'm not clear what you mean by "toleranced up."
So the bridging hasn't been a problem? That's good to know and makes it easier than the other solutions I was planning.
RE:
I suggest doing some bridging tests to see how your printer behaves in printing those. In general most printers should be able to print bridges which allows using M6 nuts without any supports at least.
Use brigading test models or just make an example box with a nut hole in it and see how it goes.
I've done something like this with PETG and up to M8
See my GitHub and printables.com for some 3d stuff that you may like.
RE:
By toleranced up I meant getting the sizing and print tolerances good for YOUR printer. You want the slot for the square nut (whatever sized one you decide on) to be loose enough to take nut manufacturing variance into account, not all are the same especially if you buy a cheaper batch later from china as I learned to my detriment but thats a different story 🙂
As we all know bridging does vary so you want the slot high enough that any slight droop doesn't stop the nut going in.
I use a 'cutter' shaped like so -
This is used as a boolean negative to make the required holes in my part geometry. Note the thin gaps between the slot for the square nut and the hole for the screw. Those are sized at 0.21, so just above my usual layer height. This means I get a single bridge layer on the slot if the screw is perpendicular to the print platter. If horizontal to the platter on final printing then if using classic there's no perimeter over the hole although arachne does put a thin one.
The single layer is easy to just screw your fastener through, although I usually poke through with a small screwdriver before hand just to make sure the nut is positioned correctly.
It is a bit of trial and error getting the final size sorted out but you just print some small test parts to get it nice and once done the cutter can be used again and again.
You can design for bridging using normal hex nuts too so that support isn't needed.
RE: Using screws and nuts with PLA and PETG
I suggest doing some bridging tests to see how your printer behaves in printing those. In general most printers should be able to print bridges which allows using M6 nuts without any supports at least.
I'll be doing tests - but it always helps to get a bit of info from those with experience so I'm not just starting blindly on tests. So thanks!
By toleranced up I meant getting the sizing and print tolerances good for YOUR printer.
Okay, got it. Wasn't sure just what you meant.
The single layer is easy to just screw your fastener through, although I usually poke through with a small screwdriver before hand just to make sure the nut is positioned correctly.
That's a good idea - not something I was thinking of at all. Thank you!
RE:
Below print is printed perpendicular to the print bed using Prusament PETG on Prusa Mini+.
Notice, though, this what you see is not how it was printed - you can see layers vertically from left to right, so this was actually printed rotated 90 degrees, which helps to not to have bridges but slopes and horizontal elements become vertical, and it is easier to print than bridges.
I can put M5 with a nut in it, and for reference I showed nut dimension.
I believe if the hole was rotated 45 degrees it would be printed without issues.
For quick testing printer capabilities I recommend https://www.printables.com/model/61996-nano-all-in-one-3d-printer-test
Something around 5mm from the bottom are different length bridging tests.
See my GitHub and printables.com for some 3d stuff that you may like.
RE: Using screws and nuts with PLA and PETG
For normal hex nuts where the recess is printed perpendicular to the bed I found this older thread https://forum.prusa3d.com/forum/prusaslicer/slicer-settings-for-screw-counterbores/ in it Joan shows the nut recess design that allows it to be printed without a sacrificial layer although that is also an option.
I've used that method too and it works well.
RE: Using screws and nuts with PLA and PETG
Back to the original question...
You could design in a hexagonal pocket around the hole for the screw, in the slicer tell the print to stop at the top of that pocket, manually install the nuts in the pocket and then allow the print to continue to cover the nut.
Or you could use heat set inserts, which would be my choice
RE: Using screws and nuts with PLA and PETG
You could design in a hexagonal pocket around the hole for the screw, in the slicer tell the print to stop at the top of that pocket, manually install the nuts in the pocket and then allow the print to continue to cover the nut.
I'm considering that, but it's a low ranked option based on my point about it in my original post.
RE: Using screws and nuts with PLA and PETG
You stated that you will possibly screwing and unscrewing these: if that is the case a possible solution would be to use heat inserts and regular bolts. Prusa resells them on their web-store. There a a lot of threads in various forums on how to use etc..
Hope it helps
St George
It's also possible to print the internal threads - ok for light useage and takes away the whole issue. I have gone as small as M3 but bigger is easier.
Cheerio,
RE:
Agreed, that is part of the solution space. It all depends on what is needed --- requirements. If this was just a one time thing screw in place once then then all that would be needed is an under-sized hole (like the predrill for wood screws size hole) and at the other extreme would be the heat threaded inserts. Now in between would be light usage would be printed threads. Depends on user requirements and what they are willing to "spend" in time, effort, and money.
Its a solution space.
St George
RE: Using screws and nuts with PLA and PETG
It's also possible to print the internal threads - ok for light useage and takes away the whole issue. I have gone as small as M3 but bigger is easier.
Cheerio,
I've also printed my own screws and used threads on larger items, like 100mm conduit. I seriously considered printing my own screws for this. While the wall plates won't be carrying huge loads at all, I decided against using printed screws.
I considered printing threads, but one thing I wonder about: If you use a metal screw and overtighten it, won't that start to mess up the printed threads?
RE: Using screws and nuts with PLA and PETG
Printed threads will survive 3-4 screw changes, then they are gone. If you want to unscrew/reattach regularly, go with heat-set inserts or nuts in the part.
RE: Using screws and nuts with PLA and PETG
Printed threads will survive 3-4 screw changes, then they are gone. If you want to unscrew/reattach regularly, go with heat-set inserts or nuts in the part.
Yeah, I figured that. I'm finding that I can use printed threads if I'm screwing something printed into them - but that's also with larger threads that seem to be stronger. I'm not going to rely on printed threads for a metal screw.
At first I was going to avoid heat set inserts, but that's because I misunderstood them and thought they were set into the part while the printer is paused. That's not practical for me, since it means a lot of sitting there waiting for the printer to pause, but now that I see how heat set inserts are done, that sounds like they might be a really good solution for me.
In about 24 hours, since I posted this question, though, I've had some ideas that could radically change the design of this project, so at this point, I'm still paying attention to all suggestions and keeping them in mind. (And also keeping them in mind for future projects.)
RE: Using screws and nuts with PLA and PETG
If the forces used are not high, then you could design your holes and tap the threads into the material. Add a couple of extra walls where the screws go. You can also use self tappers into plastic as well. Many piece of equipment is assembled that way.
In your second image, the nut should pull against printed material. Or you need to make sure it is glued in place.
I prefer inserts myself.
RE: Using screws and nuts with PLA and PETG
If the forces used are not high, then you could design your holes and tap the threads into the material.
When I can, I prefer to do it that way, since it means I do all the work in design, then I print and it's done, but I think I'll have to use another solution for this.
You can also use self tappers into plastic as well. Many piece of equipment is assembled that way.
I thought about that, but one thing I've found, in designing and building our current house and designing and overseeing the renovations on our old pig barn, and adapting the barn space later, is that I find it necessary to redo things later because when I use a space, I find things don't work as I imagined. So I want something I can take apart and redo if needed. I thought about self tapping screws, but my guess is that I can use them 2-3 times on printed material before the plastic in the hole is stripped away by the metal threads. (And overtightening, by accident, can also do that.)
I prefer inserts myself.
I'm leaning that way at this point. I've ordered some and I need to print out just a flat sheet with holes so I can practice putting inserts in. I think once I get the hang of it, and can do it easily, I'll be much more likely to use those in the future.
RE: Using screws and nuts with PLA and PETG
I have used self tapping machine screws for stuff at work. There are a series of self tapping screws for plastic with a different thread design.
If you use long enough screws, the chance of pulling out and damaging the threads will lessen. The greater number of threads will prevent tear out. I wouldn't print the threads but use a tap to get better quality. Also thicker walls where the threads are going. I am still learning.