Thread Insert Calculator — Heat-Set Insert Sizing for 3D Prints

A heat-set insert is a brass threaded insert that is pressed into a 3D printed part using a soldering iron. The heat melts the surrounding plastic which flows into the knurled outer surface, creating a strong threaded connection. They are far stronger than threading directly into plastic.

The hole should be slightly smaller than the insert outer diameter so the brass melts into and grips the surrounding plastic. Using the Ruthex / CNC Kitchen standard sizes, an M2 insert needs a 3.2mm hole, M2.5 and M3 both use 4.0mm, M4 5.6mm, M5 6.4mm and M6 8.0mm. This calculator gives the correct size for each insert type and lets you fine-tune it with a tolerance factor.

PETG and ABS provide the strongest insert hold due to their higher glass transition temperatures and better flow around the insert knurling. PLA works but is more brittle. All materials benefit from printing the boss with 100% infill or extra perimeters.

Set the soldering iron roughly 10-20°C above your print temperature so the brass melts the plastic cleanly: about 225°C for PLA, 245°C for PETG, 265°C for ABS and 255°C for ASA. Press slowly and straight, seating the insert flush. A dedicated insert tip for your soldering iron gives the most consistent results.