Minimum Printable Thread

Theory

Thread geometry requires accurate reproduction of small helical features. FDM layer lines create a stair-step approximation of the thread profile, which weakens the engagement length. The larger the thread pitch and diameter, the more helical surfaces are visible per layer and the stronger the engagement. SLS uses fine PA12 powder (50–80 µm particle size) and achieves much better thread definition. SLA can print very fine threads (M2+) but resin brittleness limits torque capacity. DMLS metal threads are accurate but post-machining is usually required for any precision fastening.

Application

FDM: design M6 or larger threads for direct use; use a thread tap on M3–M5 printed holes; press or solder heat-set inserts for M2–M3 with high cycle count. SLS: design M3–M4 threads into the print; verify with first article inspection. SLA: avoid threaded features: use captured nut pockets instead. DMLS: machine all critical threaded holes after build.

Common mistakes

Printed FDM threads strip after 3–5 cycles under full torque. Never design a structural joint relying on printed FDM threads alone. In SLS, thread tolerances vary with orientation: axial threads (axis parallel to Z) are more accurate than radial. Resin SLA threads can crack if over-tightened due to low impact strength.

Related terms: Minimum Hole Diameter, Minimum Wall Thickness