Heat-set inserts give 3D-printed parts strong, reusable metal threads — far better than screwing directly into plastic. The key to a solid insert is the boss hole: too big and it spins, too small and the boss cracks. This tool gives the right hole for your insert and material.
How it works
A heat-set insert grips by melting the plastic around it. When you press a hot insert into a slightly undersized hole, the displaced molten plastic flows into the insert’s external knurls and freezes there, locking it in place.
The rule is simple: the boss hole is a little smaller than the insert’s knurl (major) outer diameter. How much smaller depends on the plastic:
- PLA flows freely when hot, so it takes the smallest hole.
- PETG and ABS flow a little less, so they get marginally more clearance.
- Nylon is tough and resists flow, so it needs the most room to avoid cracking the boss.
The tool also gives the hole depth (insert length plus a small margin), the recommended boss wall so the surrounding plastic doesn’t split, and a small top chamfer to help the insert start square.
Typical hole diameter guidance by thread size
For reference, the general relationships for M-series inserts look like this (exact values depend on the specific insert brand — Voron, CNC Kitchen, McMaster, Ruthex, etc., all vary slightly):
| Thread | Insert OD range | Typical hole in PLA | Typical hole in PETG/ABS |
|---|---|---|---|
| M2 | 3.2–3.5 mm | ~3.0–3.1 mm | ~3.1–3.2 mm |
| M2.5 | 3.8–4.2 mm | ~3.6–3.8 mm | ~3.7–3.9 mm |
| M3 | 4.2–4.6 mm | ~4.0–4.2 mm | ~4.1–4.3 mm |
| M4 | 5.4–5.7 mm | ~5.1–5.3 mm | ~5.2–5.4 mm |
| M5 | 6.4–7.0 mm | ~6.1–6.4 mm | ~6.2–6.5 mm |
| M6 | 8.0–8.5 mm | ~7.7–8.0 mm | ~7.8–8.1 mm |
These are illustrative ranges only. Always check the datasheet for your specific insert brand, test with a sacrificial print, and verify pull-out strength on critical parts.
The importance of boss wall thickness
Thin boss walls are the most common failure mode for heat-set inserts. As the hot insert displaces plastic outward, a thin wall can split or balloon. The general guidance:
- Boss outer diameter should be at least 2× to 3× the hole diameter
- For M3 in a structural part, a boss outer diameter of at least 8–9 mm is typical
- Increasing wall thickness from 1 mm to 2 mm dramatically improves pull-out and torque resistance
On tight parts where the boss cannot be large, consider switching to a shorter insert with a wider-than-standard outer diameter (barrel-style inserts) or move to threaded brass nuts captured in the print geometry instead.
Soldering iron temperature by material
| Material | Recommended iron temperature |
|---|---|
| PLA | 190–210 °C |
| PETG | 220–240 °C |
| ABS | 230–250 °C |
| Nylon | 240–260 °C |
Too cold: the insert won’t sink fully and will sit proud. Too hot: the plastic chars, weakens the surrounding material, and may cause the boss to discolour and lose strength. Use a dedicated insert tip if your iron supports them — the flat barrel tip keeps the insert square and protects the thread interior.
Tips and notes
- Press inserts in with a soldering iron tip matched to the insert thread; let the part cool fully before loading the screw.
- Keep the insert square to the hole — a tilted insert sits proud and won’t seat the screw properly. A small alignment jig or a printed guide boss helps.
- For load-bearing or pull-out-critical parts, increase boss wall thickness and use inserts with more aggressive knurling.
- If an insert spins after cooling, the hole was too large — reprint with the recommended diameter rather than relying on glue.
- A small 45° chamfer at the top of the hole guides the insert in and helps keep it perpendicular during the first few seconds of pressing before the plastic has softened fully.