Heat-Set Insert Jig Grants Threads to 3D Prints

FDM 3D prints might be coming home this holiday as seasonal ornaments, but with a few tweaks, they may even stand up to the tests of the real world as functional prototypes. Heat-Set inserts are one such tweak that we can drop into a print, and [Kurt] spares no expense at laying down a guide to get us comfortable with these parts. Here, he’s created a drill press adapter and modified his soldering iron to form an insert jig to start melting these parts into his next project.

Heat-set inserts grant us proper screw threads in any thermoplastic. Simply heat them up, stake them into your part, let cool, and: voila–a screw thread that’s firmly embedded into our part. We can load these inserts with clumsy hand tools, but why fumble and bumble with a hot soldering iron when we can adapt our drill press to do most of the tricky fixturing for us? That’s exactly what [Kurt] did here. With a 3D-printed adaptor, he’s letting his drill press (turned off!) hold the soldering iron so that he can use the lever to slowly stake the part into the 3D print. Finally, for no additional charge, [Kurt] turned down his soldering tip to mate cleanly into the insert for a cleaner removal.

We’ve seen adapters like this one before, but it’s never too often to get a reminder of the structural bonus that these parts can add to our 3D prints.

28 thoughts on “Heat-Set Insert Jig Grants Threads to 3D Prints

  1. I’ll point these out again: https://www.harborfreight.com/metric-drill-tap-deburr-bit-set-95529.html as a great way to deal with threads. Make a hole about the size you would drill out, then use those, and 3D printed part with threads. (I do recommend using more than normal perimeters.)

    They are on of those Harbor Freight gems: cheap, not right for the normal (metal) use, but awesome for something else. They are great on plastic however, I’ve been using a 3D printed battery holder for some 18650 cells probably 4 years now with screws that make contact using them, and can safe it, by being unscrewed so, that’s probably thousands of times being rotated, and the threads are still great.

    Seriously, if you plan on ever trying to use them in metal: don’t. Get a real drill bit and tap (and not Harbor Freight taps *shudder*), because they will break even on aluminum.

    1. I got the full set at HF last time it was on sale because it had the handles, but yeah I agree 100%. I wouldn’t trust this stuff for metal applications, but it’s worked great for threading printed holes.

  2. If you’re not planning on repeatedly disassembling your prints, then tapping threads or self-tapping screws work great for printed parts. Where inserts like this really shine are things like battery doors, and calibration panels.

      1. It might be your choice of screw — you shoudn’t require anything but a simple hole of the correct diameter. I routinely use these screws designed specifically for plastic: https://www.mcmaster.com/#97349a100/=1a5s2zm They work extremely well. For this size (#4), I specify the hole to be 2.0 mm for PETG and 2.2 mm for PLA. With a 0.4 mm nozzle on my printer they come out about 0.2 mm smaller than that.

        For repeated insertions, you do need to remember the old technique of inserting the screw in the previously-threaded hole, then slowly turn it backward to find the ‘click’ to seat into the previous thread. If you just ham-fistedly drive it in and start a new thread you’ll chew the material up, just like you will in any other soft material.

      2. I’ve recently used some self-tapping screws in a 3d printed object. I am building a Mendel90, mostly as designed by Nophead. Mostly he uses nuts but there are a few places with self-tapping screws. Anyway.. something I noticed were prism shaped holes. Check out the stls where he prints D-connector shells and you will see he uses some funny prism shaped holes for self tapping screws.

        Maybe it was unintentional or a slicer artifact or something but I was thinking that might be on purpose. Maybe the sides of the triangle are narrow to give the threads something to bite into while the points extend just past the threads to give displaced material a place to flow to. I don’t know if that is what he intended but it certainly seemed to work!

      3. With ABS, at least, your print shouldn’t have any trouble with a screw digging into the plastic. Just make sure the printing layers are perpendicular to the screw shaft to prevent splitting. If you’re worried, put a drop of acetone (or some PLA solvent) in the hole and let it absorb and evaporate to fuse it all tight.

    1. “Where inserts like this really shine are things like battery doors”

      I so miss the days when I could change a device’s battery without any tools whatsoever. Screws should be for removal when one has real electronics or mechanical work to do, not every day usage. Thanks a whole lot for ruining our battery doors Europe!

  3. So, a little bit of insider information here.

    First, make sure insertion force is low, and the insert is heated well. You can apply excessive pressure to get the insert recessed, and the resulting part will not be able to take the same applied forces before failing.

    Second, get an insert which is properly threaded. If your insert is designed poorly (Chinese knock-offs), the outside knurl/pattern will match insertion rotational direction. This is a big no-no for applied forces.

    If you’re willing to buy in relative bulk, try Spirol. If nothing else, they can provide some data sheets of mild relevance (not a plug, but my experience is with their product).

    Next, don’t go cheap here. Steel inserts may be a bit cheaper, but you’d better have a powerful iron. The brass inserts are more expensive, but a couple of bucks isn’t anything huge.

    Finally, let these suckers cool after application. You’d be surprised how hot they stay, and how hot they get. I’ve seen liquid plastic ooze around an insert when somebody had the setting way too high.

    Take all of the above with a grain of salt for a 3D print. I work with injected plastic components, which are much less forgiving given their application. The principal is sound for about 140 lbs of pull force on the larger inserts, so do be careful.

  4. I usually tap my printed parts using a long screw in a battery drill. Just screw it in really fast and the friction will melt the plastic, forming a surprisingly good thread. For stuff that is routinely disassembled I use nut traps.

  5. I guess it really depends on how often or how much of these inserts you really need. For the few thread inserts i actually used, the manual use of a soldering iron did just fine. Most of the time i actually manage to just use the right type of screw (thermoplastic threads, not standard metric screws) or work with nut traps.

      1. thanks!, I have looked everywhere, but only found chinese ones, I’m not sure if those are good if you pull, but the ones that shoved up in the search now are the ones I’m looking for, great!

  6. Oh I should point out that the construction methods of all soldering irons might not be compatible with having a large end force exerted on them. So if you do this yourself, let the heat do the work. If you make one for a multiuser shop, expect it to break weekly.

  7. I concur with using heat-set threaded inserts as much as you can for proper 3D printed parts. Once you get them, you will never want to use nut traps.

    And you don’t need to cut your soldering iron tip. When you buy your threaded inserts, just buy an inserting tip for your soldering iron that fits the inserts.

    A nice thing about threaded inserts (vs nut traps) is you can install them even if you didn’t design it into the model. As long as you have a full or near-full in-fill, you can just drill a hole and put the insert in. They’re also WAY more forgiving of hole size than nut traps and way less annoying generally. They’re also usually stiffer without backlash (which is important for some uses).

    In a lot of ways, heat-set threaded inserts are the difference between yoda heads and actual, functional parts. There’s a reason why 3D printers like the Lulzbot Taz use heat-set inserts in the body instead of tapping the plastic or using nut traps everywhere (although they still use a few nut traps).

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