Printed threads are the weakest link in most 3D printed assemblies. A screw driven straight into FDM plastic pulls out across the layer lines within a handful of cycles, and a modeled thread finer than roughly M6 rarely survives the layer resolution. The fix is to stop asking the plastic to be the fastener. Four strategies cover almost every case: heat-set brass inserts, tapped holes, thread-forming screws into bosses, and through-bolts with captured nuts. This guide covers when to reach for each, how to size the hole, and how to design a boss that does not split.
Heat-set brass inserts: the default for reusable threads
Heat-set inserts are knurled brass bushings pressed into an undersized hole with a soldering iron. The heat melts a thin skin of polymer that flows into the knurls and re-solidifies, locking the insert with far more pull-out and torque resistance than plastic threads. They are the right answer any time a joint is assembled and disassembled more than once.
| Thread size | Boss hole diameter | Min boss OD | Notes |
|---|---|---|---|
| M2 | ~3.2 mm | ~5 mm | Small; easy to over-melt, go gentle |
| M2.5 | ~3.5 mm | ~5.5 mm | Common in electronics enclosures |
| M3 | ~4.0 mm | ~6 mm | The workhorse size |
| M4 | ~5.6 mm | ~8 mm | Good load capacity |
| M5 | ~6.4 mm | ~9 mm | For higher clamp loads |
Install with the iron set 20–40°C above the polymer's melt or print temperature. Press the insert in square, let it seat flush without forcing molten plastic out, then hold light pressure until it cools so it does not lift back out.
The boss hole must be sized to the insert, not the screw. Too tight and the insert sits proud or splits the boss; too loose and it spins under torque. Start from the manufacturer's recommended hole and keep at least one insert-diameter of wall around it.
Tapped holes and printed threads
You can cut a thread directly into printed plastic with a standard tap, or model the thread into the part. Both work within limits:
- Tapped holes suit infrequent assembly. Cut slowly, back the tap out to clear chips, and expect fewer reinstalls than metal — the threads strip if over-torqued.
- Printed threads should be coarse and large. Below about M6 the layer resolution cannot resolve a clean thread form, so the flanks come out ragged and weak. Reserve modeled threads for large, coarse, low-cycle features like a knob or a bottle-cap.
- Thread engagement in plastic needs to be longer than in metal. Aim for at least 1.5–2× the thread diameter of engaged length so the load spreads over more turns.
Thread-forming screws into bosses
For high-volume, cost-sensitive assemblies, a thread-forming screw driven into a plain boss is often the cheapest joint. Thread-forming (not thread-cutting) screws displace material rather than remove it, so they suit ductile plastics without creating chips that jam the joint.
- Pilot hole roughly 0.8× the screw's major diameter (0.75–0.85 is the usual window); too tight splits the boss, too loose strips.
- Boss OD around 2× the screw major diameter so there is enough wall to resist the hoop stress of forming.
- Add a lead-in chamfer at the boss mouth to start the screw straight.
- Watch the drive-to-strip torque window — forming plastics leaves only a narrow margin between the torque that forms the thread and the torque that strips it. A clutch or torque-limiting driver is worth it on a line.
Don't reuse a thread-forming screw in the same hole for repeated assembly — it tends to cut a fresh, weaker thread each time. If a joint will be opened more than a few times, spend the extra and use a heat-set insert instead.
Through-bolts, captured nuts, and clamp load
When a joint has to carry real load, take the plastic threads out of the load path entirely. Run a bolt clear through the part into a nut on the far side, or capture a nut or T-nut in a hexagonal pocket modeled into the part.
- Size the hex pocket about 0.2–0.4 mm oversize (see the tolerances guide) so the nut drops in but cannot spin.
- Under high clamp load, plastic creeps and the joint loosens over time. A metal compression limiter — a tube the bolt passes through — lets you torque to metal-on-metal without crushing the plastic.
- Spread the load with a large washer or a molded flange; a small bolt head sinks into soft plastic and relaxes.
| Method | Hole prep | Reusability | Relative strength | Best for |
|---|---|---|---|---|
| Heat-set insert | Undersized boss hole | High | High | Reusable metal threads in plastic |
| Tapped hole | Drill + tap | Low | Medium | Infrequent, light-duty assembly |
| Printed thread | Modeled in CAD | Low | Low | Coarse threads ≥ M6, knobs and caps |
| Thread-forming screw | Pilot hole ~0.8×D | Low | Medium | High-volume, single-assembly |
| Through-bolt + nut | Clearance + hex pocket | High | Highest | Structural, high-clamp joints |
Designing the boss
Whatever the method, the boss makes or breaks the joint. A boss is a thick, tall feature, and left alone it wants to sink and warp:
- Fillet the base where the boss meets the wall to kill the stress riser that cracks it off.
- Support tall bosses with gusset ribs, but keep rib thickness to roughly 0.5–0.6× the wall so the ribs do not leave sink marks.
- Keep the boss wall close to the part wall and core out solid bosses so they do not become a thick blob that warps.
- On FDM, orient the boss so fastener load does not pull straight across the layers.
Inserts, bosses, and fastener load paths are a full module in the Pro FDM course, with the pull-out math, boss geometry, and worked assemblies that keep printed parts bolted together.
Take the FDM course See Pro pricingFastening checklist
- Pick the strategy by reuse and load: insert for reusable, through-bolt for structural, thread-forming for one-time high-volume.
- Size the hole to the insert or screw spec, not by eye — confirm on the datasheet.
- Keep at least one fastener-diameter of wall around the hole, and fillet the boss base.
- Reserve modeled threads for coarse features M6 and larger.
- Add compression limiters or captured nuts wherever clamp load is high.
- Torque with a limiter and prove pull-out on a printed coupon before release.