Printed threads / threaded joints

Theory

Printed threads work for coarse metric sizes (≥M6 in FDM, ≥M3 in SLS). Finer pitches cannot be faithfully reproduced due to layer resolution and shrinkage. Tapping is preferred for M3–M5 in FDM: drill 0.2 mm undersize, tap with standard tap, apply cutting oil for ABS and Nylon.

Application

For printed threads: add 0.3–0.5 mm clearance on the thread outer diameter in the mating hole. Orient thread axis vertically (Z) in FDM for best thread form. For tapped holes: print the minor diameter hole, tap immediately after print while plastic is still slightly warm. Use heat-set inserts for M3 and below.

Common mistakes

Horizontal FDM threads (axis in XY) have stepped profiles from layer lines and engage poorly. Never tap printed threads more than 3–4 cycles: plastic wears fast. Use metal inserts for structural bolted joints. SLA threads are brittle: resin cracks at the root on first engagement under load.

Related terms: Heat-set threaded insert, Press-fit joint, Clearance for moving / mating parts