If you want to connect two printed parts with a screw, this is a type of a zero-cost thread that you should seriously consider using. It’s super easy to design into your parts, provides strength that’s more than good enough for almost anything you could use it for, and it’s ready to use straight off the printer – no setting inserts or cutting threads.
The best part is: It’s just a fancy hole, and solving problems with free details is something that 3D printers are really good at.
I use standard machine screws all the time with my prints, but as good as this method is, there is no one-size-fits-all solution. In this video, I’ll give you an overview of the options you have, show you the easiest ways to use them in your own designs, and then rate each one based on ease of use, strength, and durability, so that you know which option is the best choice for the kind of project you’re working on.
Straight Hole
We’re going to start simple with the humble straight hole. This is arguably the easiest to design, but in practice, it’s the hardest one to get right. Any CAD suite has a hole tool, that you can plop down anywhere and then tell it what exact hole you want.
The small M3 thread is my most used type of screw, and to get that to bite into an unthreaded hole requires super tight tolerances from your printer. If the hole is just slightly too large, the threads won’t get enough engagement and you’ll just strip it out. If the hole is too small, the screw will need to stretch out the entire printed perimeter, which makes it really tough to thread, and if you go too fast, you risk melting the plastic.
On horizontal holes, you can even split apart the layers, so overall, I’m giving this option two stars for ease of use. If you get it perfect, this sort of connection can be pretty strong, but in practice, it’s a coin toss every time, and because it’s not repeatable, it also gets two stars for strength and reliability. Would not recommend.
Now, a quick note about using coarser thread screws made for plastic, or wood screws – In my opinion, those are actually not a great fit for 3D prints. They have a deeper thread engagement, so they’ll bury themselves deeper into the plastic, and that works for injection molded parts, but because we’re printing with perimeters and a mostly hollow part behind that, that also means the coarser screw will cut through more of the load-bearing shell of your print. Also, these are just a lot harder to buy than standard machine screws.
Tapped Hole
Next, straight holes that you tap to size. Super easy to design, you look up the center bore size for your thread, use the hole too, print it, and then use a hand tap or machine tap to cut a perfect thread into that hole. I don’t have the patience to do that by hand, so I will always use a drill, but that makes it easy to melt out the thread, especially on PLA or when you have deeper holes that create a lot of shavings that you have to clear out.
I printed these air filter holders in PETG, they’re individual segments that are connected with a bunch of M3 screws, and even though I was using ethanol to cool everything, I still blew out about a third of the threads. But if you do get it right, you’ll get a thread that makes full contact with the screw, and even though you did cut away some of the perimeters, it’s still going to be a plenty strong connection that won’t easily wear out over time, either. If you need a stronger connection, create a longer thread for more engagement, or use a bigger thread size. Three stars for ease of use, four for strength, and three for durability.
A variation of these is printed threads, but unless you’re printing in resin, I find they only print well enough for sizes M6 (¼”) and larger, and those are just not used that often for connecting simple printed parts. But at least in Fusion, modeled threads are super easy to add with the hole tool, in Onshape, it’s not baked in, but you can use a third-party addon to get the same functionality.
Self-Forming Thread
Next, the new one, let’s call this one a self-forming thread. Now, I’m probably not the first one to come up with this idea, and it’s a pretty logical approach. I needed something for the mid-frame of the sensor box that would require no specialized tools and easily print on any 3D printer, and in function, this is very similar to the straight untapped hole, but it reduces the cross-section that the screw has to deform down to three individual ridges. Because the overall hole is a little larger than the screw, these ridges can be designed to have a larger overlap with the screw threads, because now the deformed material has somewhere to go instead of the screw having to stretch out the entire hole.
Plus, if you print it vertically, the way that your slicer generates perimeters means it’ll actually add more materials behind those ridges, so you get the full perimeter thickness all the way around, and if you print it horizontally, as long as you orient one ridge straight down, it’s going to print just fine. I’ll give these five stars for ease of use, and because they’re super predictable, they’ll get four for strength, and lastly, three for durability.
Here’s how to add these to your parts:
Of course, you could model these from scratch every time you want to use them, but it’s much simpler to model the negative space, the cutout, once and then import that into the thing you’re currently working on.
I did these in Onshape, but the idea is the same in Fusion. Once you’re ready to add the self-forming thread features to your actual part, you create the reference points where you want them, then use the derive feature to import the geometry to your workspace. Onshape uses these mate connectors as references, and I found they snap to your target points best if you create a small hole first instead of just a sketch point. Then, use the boolean tool to subtract each of the negative space bodies from your part and that’s ready to print. You can even take this a step further if you commonly use M3x10 screws and add the cutout geometry for the through-hole part, too. So if you have a box with a lid, import it once, place it at the point where the thread needs to start, then cut the box and the lid with the same negative space part and you’re always going to have both halves mate up perfectly with the right amount of thread engagement.
You can find these designs here.
Metal Threaded Inserts
Moving on to metal threaded inserts, there is quite a selection here. I’ve done a video where I compared a whole bunch of different inserts, and what they all do really well is to keep threads from wearing out. On the filterbox, I ended up using the self-forming threads for the connections between the segments, those will only be tightened once, but I opted for heat-set inserts for the lids, as these will need to come off every time I change or check the filters.
A metal-on-metal connection is hard to beat for durability. Heat-set inserts are easy to design for, you just need a plain hole, but this time, the tolerances are much more forgiving as you’re melting the entire thing.
For setting them straight and accurately, you need a good amount of patience, a soldering iron, ideally with the proper setting tips, and some people even build little linear axis guides to keep their soldering irons straight, so overall, it’s quite a bit of post-processing time you have to spend on every single threaded hole you want to use.
Something like these thread repair inserts are a little harder to find and less standardized, but they do provide the same strength as a heat-set insert, and for the larger sizes, if you model their external thread into your print, they also don’t disturb your printed perimeters.
For absolute maximum strength, nothing so far has beaten prong nuts. For these to fail, you’d have to tear them through your printed part instead of just pulling them out of a hole, but they do require that you design your part around them, as these are quite large and flared, and you’ll need access to the back side of where you want your threaded hole to end up. Just like for the thread repair inserts, designing for these can be made a little easier by modeling the negative space once, and then importing and cutting that out of your final part.
So overall, these metal inserts all fall off for ease of use in one way or another, but they provide between “very good” and “unbeatable” strength, and their threads are impossible to strip or wear out.
Inserted Nuts
Lastly, inserted nuts. These are typically used by creating a slot at a 90° angle from your screw hole, and then sliding them in.
Hex nuts will often tilt while pushing them in or start spinning in the printed part when you’re tightening them, so usually, square nuts are the preferred choice here. Designing for them is fairly easy, simply create a cutout that is a little larger than the nut itself, and extend that to the nearest edge of your part. The center hole for the screw is the standard clearance size for the thread all the way through, and because this is all simple geometry, the slot will come out very usable in pretty much any print orientation.
In the finished part, you then simply slide your nuts in all the way, and fasten the screws through them. Now, this can be a bit finicky as there’s nothing that keeps the nut aligned to the screw thread, so I’ll use a small hex key to push it in, but in general, this is pretty quick and easy. Four stars for ease of use, but I’ve seen plenty of parts crack in the weak spot that the added slot creates, so I can only give it three stars for strength. But being a metal-on-metal thread and having the ability to easily replace the nut if you ever damage it earns this a five-star durability rating.
I think it’s great that we have all these options now, and it just makes designing functional 3D prints so much more viable. What I’ll personally be using for low- to medium-strength applications is the self-forming threads up to M5 and printed threads beyond that, and for anything that needs more strength than that, either heat-set inserts because they’re easy to do in CAD, or the thread repair inserts if you have parts with lots of threaded connections.
A big shoutout to everyone who’s supporting the channel on YouTube Memberships and Patreon, thanks for watching, keep on making, and I’ll see you all in the next one!
Onshape file for the M3 tapless threads
CNC Kitchen heat-set inserts (use code tom5 for 5% off!)
Self-tapping inserts on Amazon
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