What is a heat-set thread insert?

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.

What hole size do I need for heat-set inserts?

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.

Which material is best for heat-set inserts?

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.

What temperature should I use to install inserts?

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.

3DPrintCalcs

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

Calculate hole diameter, depth, wall thickness, installation temperature and pull-out strength for heat-set inserts.

Insert TypeMetric thread size of the heat-set insert (e.g. M3 = M3 screw thread)

Number of Inserts

How many inserts this part needs (used for your bill of materials)

MaterialFilament the boss is printed in — sets the install temperature and pull-out estimate

How We Calculate This

This calculator provides hole dimensions and installation parameters for standard heat-set brass inserts in 3D printed parts.

How it works

Hole diameter uses the Ruthex / CNC Kitchen drill size for each insert (slightly under the insert outer diameter so the brass melts in and grips). Hole depth is the insert length plus a 1mm buffer so displaced plastic has somewhere to go in a blind hole. Boss diameter keeps at least ~1.6mm of wall (or 60% of the insert outer diameter) around the insert so the boss does not crack — this is roughly the widely cited "boss outer diameter ≥ 2× insert outer diameter" rule.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify settings with test prints before committing to final prints.