Brass Heat-Set Inserts
Durable metal threads in 3D-printed parts — choosing the right insert, designing the boss, and installing without cracking the plastic.
Printed threads strip after a few assembly cycles. Even at 100 % infill, plastic threads cut by a bolt deform under tension and loosen over time. Brass heat-set inserts solve this permanently: the insert is pressed into the plastic with a heated soldering iron, the knurled outer surface mechanically locks into the surrounding material, and you are left with true metal threads that can be torqued and re-assembled hundreds of times.
In a cockpit build, panels are repeatedly removed for wiring access and component changes. Every fastener location that gets cycled more than two or three times benefits from an insert. The cost — a few cents per insert — is negligible compared to reprinting a panel because printed threads stripped.
Not all brass inserts are the same. The knurl pattern and flange shape affect pull-out strength and installation behaviour.
| Type | Knurl pattern | Best for | Notes |
|---|---|---|---|
| Standard heat-set (M2–M5) | Diagonal diamond knurl | General panel and enclosure work | Widely available; fine for PLA+, PETG, ABS |
| Voron-style (flanged) | Straight + diagonal knurl with collar | Structural joints, lid hinges, high-cycle assemblies | Flange sits flush on the surface and prevents the insert sinking too deep; preferred for Voron 3D printer builds and high-quality cockpit work |
| Press-fit (non-heat) | Aggressive barb knurl | Resin prints where heat would crack the part | Pressed in cold with a vice or thumb; lower pull-out strength than heat-set — reinforce with a drop of CA glue if needed |
| Ultrasonic (production) | Any | Production volumes with an ultrasonic welder | Not relevant for home builds — listed for completeness |
The hole in your CAD model should be 0.2–0.3 mm smaller than the insert outer diameter (OD). The displaced plastic flows into the knurls and locks the insert — too loose and it spins; too tight and the boss cracks.
| Thread | Insert OD (typical) | CAD hole ⌀ | Insert length (short / standard) | Common cockpit use |
|---|---|---|---|---|
| M2 | 3.2 mm | 3.0 mm | 3 mm / 4 mm | PCB standoffs, small sub-panel fixings, display mounts |
| M2.5 | 3.8 mm | 3.5 mm | 4 mm / 5 mm | ESP32 / Raspberry Pi enclosure lids, thin overlay panels |
| M3 | 4.6 mm | 4.2–4.3 mm | 4 mm / 6 mm | Most common. Panel-to-frame fixings, MCP/FCU face plates, overhead sections |
| M4 | 5.6 mm | 5.2 mm | 6 mm / 8 mm | High-load joints: gear lever pivot, throttle quadrant mounts |
| M5 | 7.0 mm | 6.7 mm | 8 mm / 10 mm | Structural frame bolts, seat-rail style tracks |
The cylindrical boss that surrounds the insert hole is the most critical design element. An undersized or thin-walled boss will crack during installation or strip under load.
| Design rule | Reason |
|---|---|
| Boss outer wall ≥ 2× insert OD | The insert displaces plastic outward as it sinks — thin walls split. For M3 (OD 4.6 mm) the boss OD should be at least 9.2 mm |
| Boss height = insert length + 0.5 mm | Gives the bolt tip clearance at the bottom and prevents it punching through. Without clearance, tightening the bolt bottoms out against the boss floor and stresses the insert |
| 0.5 mm entry chamfer on the hole | Centres the insert as it begins to sink and prevents tearing the top surface of the boss |
| 4+ perimeters on boss walls | Dense perimeter walls give the knurls more plastic to grip — infill alone is not sufficient |
| Orient boss vertically in slicer | Perimeters wrap continuously around a vertical boss. A horizontal boss relies on layer-to-layer adhesion which is significantly weaker |
| No support material inside the hole | Supports leave rough surfaces that cause the insert to sink unevenly. Design the hole to print support-free (vertical orientation achieves this automatically) |
A standard soldering iron works but a dedicated insert tip produces more consistent results. The tip fits inside the insert thread and presses directly on the top face, allowing even heat transfer without risk of slipping off sideways.
| Tool | Pros | Cons |
|---|---|---|
| Dedicated heat-set tip (e.g. Hakko, Weller) | Precise depth control; tip fits the insert ID; clean installs | Tip set costs extra; brand-specific fitting |
| Flat / chisel soldering tip | Works with any iron; no extra purchase | Can slip off the insert; harder to keep perpendicular |
| Heat gun (not recommended) | No contact required | Heats the entire part unevenly; insert sinks crooked and surrounding surface deforms — avoid for precision work |
Temperature by material
| Filament | Iron temperature | Sink time (approx) |
|---|---|---|
| PLA / PLA+ | 180–200 °C | 3–6 s |
| PETG | 220–240 °C | 3–5 s |
| ABS / ASA | 240–260 °C | 4–7 s |
| PA (Nylon) | 250–270 °C | 4–8 s |
| Step | Detail |
|---|---|
| 1. Set iron to correct temperature | Allow the iron to fully stabilise — a cold tip stalls halfway and the insert sinks crooked |
| 2. Rest insert over hole | Place the insert flat on the chamfered hole entry — gravity holds it in position, no need to press |
| 3. Apply tip with steady downward pressure | Place the iron tip on top of the insert and press slowly and evenly. The insert should descend smoothly at a constant rate. If it resists, the iron is too cool; if it drops too fast, reduce temperature |
| 4. Stop flush or 0.1 mm proud | Remove the iron when the insert top reaches the surface. A flanged (Voron-style) insert self-stops when the collar seats. Standard inserts need more attention — err slightly proud rather than too deep |
| 5. Flatten immediately | Before the plastic re-solidifies (within 2–3 s), press a flat metal plate or the flat jaw of a calliper firmly over the insert for 5–10 s. This corrects any tilt and gives a clean flush surface |
| 6. Let cool fully before threading | Wait at least 30 s before running a bolt in. Threading into a partially cooled insert can shift it before the plastic has locked |
| 7. Test with a bolt | Thread in by hand first. Smooth engagement with no wobble indicates a good install |
| Problem | Likely cause | Fix |
|---|---|---|
| Insert sinks crooked | Tip not perpendicular; iron too cold so one side melted first; no entry chamfer to self-centre the insert | Reheat and press the high side down while still warm; add a 0.5 mm chamfer to the CAD model for next print; use a dedicated insert tip |
| Boss cracks during installation | Wall too thin; hole too tight (insert OD − hole > 0.4 mm); iron too hot | Increase boss OD in CAD; use 0.2–0.3 mm clearance; lower temperature and slow down |
| Insert sinks too deep | Iron too hot; too much pressure; standard (non-flanged) insert with no depth reference | Switch to Voron-style flanged inserts which self-stop; reduce temperature; work more slowly |
| Insert spins when bolt is tightened | Hole too large; insert OD too small for filament chosen; not enough wall material | Measure insert OD with calipers and adjust CAD hole; add a drop of CA glue around the insert base before re-heating; increase wall count in slicer |
| Plastic bubbles or foams around insert | Iron too hot; moisture in filament (common with Nylon) | Reduce temperature; dry the filament before printing |
| Insert proud of surface (cannot sit flat) | Boss too shallow; insert bottomed out before reaching flush | Increase boss height in CAD by insert length + 1 mm; or carefully reheat and press lower with the flat plate |
Cured resin is brittle and will crack if you apply heat-set inserts the normal way. Two alternatives work:
| Method | How | Strength |
|---|---|---|
| Press-fit insert + CA glue | Print hole 0.1 mm smaller than insert OD. Apply a drop of thin CA glue into the hole, press the insert in with a vice or thumb, hold for 30 s. Wipe excess immediately | Moderate — sufficient for lids and non-structural panels |
| Through-hole nut trap | Design a hexagonal pocket in the resin part to capture a standard M3 hex nut. Fill with CA glue before inserting the nut. No heat required | Good — nut cannot rotate; works well for thin resin bezels |