Threaded inserts: the easy way to put screws in a printed part
How to size the hole for an M3 brass insert, how to fit it with a soldering iron, and why tapping plastic directly does not last.
4 min read
If your part will be opened more than two or three times, you need inserts. A screw tapped straight into printed plastic cuts the layers every time it goes in; by the fourth or fifth time it spins free. A brass insert turns that point into a metal thread good for hundreds of assemblies, and costs pennies.
What hole to leave
| Insert | Hole diameter | Depth |
|---|---|---|
| M2 | 3.2 mm | 4.0 mm |
| M3 | 4.2 mm | 5.7 mm |
| M4 | 5.6 mm | 7.0 mm |
How to fit one
- 1Set the iron to 200-220 °C. Hotter than that scorches the plastic around it.
- 2Rest the insert in the hole and bring the tip down on it, square.
- 3Let it sink under its own weight. Do not push: it goes in crooked.
- 4Lift the iron when the top face is flush and let it cool for a minute.
Leave at least 2 mm of wall around the insert. Too close to an edge and the molten plastic pushes outwards, raising a bulge on the outside face.