The Limitations of Direct Threading
Tapping threads directly into 3D printed holes with a hand tap works for one-off prototyping, but plastic threads strip after 3-5 tightening cycles. Furthermore, small printed threads (M2 to M3) suffer from poor layer adhesion on thread crests, shearing off when subjected to vibration.
For repeatable, production-grade assemblies that will be opened for battery replacement or maintenance, brass heat-set threaded inserts are the gold standard.
Boss Geometry Rules of Thumb
A heat-set insert relies on surrounding molten plastic flowing into its knurled herringbone exterior. If the boss wall is too thin, the boss cracks during insertion. If the hole is too small, excess plastic overflows into the internal threads.
- Hole Diameter: Match the minor diameter of the insert shank (typically specified by manufacturer drawing, usually +0.1 mm over shank bottom).
- Boss Wall Thickness: Boss outer radius should equal at least 2x the hole radius. For an M3 insert (hole diameter ~4.0 mm), provide at least 2.0-2.5 mm of solid surrounding plastic.
- Lead-in Chamfer: Add an 8-degree chamfer at the mouth of the hole to locate the insert perpendicular before heating.
- Depth Clearance: Extend the hole depth at least 1.0 mm deeper than the insert length so molten plastic does not pool and block the screw tip.
Proper Insertion Technique
Use a temperature-controlled soldering iron fitted with a flat brass insert tip set to the glass transition temperature of the polymer (around 220 °C for PLA, 240 °C for PETG, 260 °C for ABS). Apply gentle, vertical downward pressure. Let heat conduct into the brass and melt the plastic naturally-never force it in mechanically.
