A helical groove on a cylinder or cone used for fastening or translation, with FDM-specific design rules for minimum size, engagement length, and post-processing to achieve reliable function.
ISO metric threads: M designation = nominal diameter (mm), pitch in mm. External thread = bolt; internal thread = nut. Engagement length for adequate strength: ≥ 1.5× diameter in plastic. FDM minimum reliable thread: M5 or larger; M3–M4 are marginal and often stripped. Thread-forming (self-tapping) screws work better in FDM than machine screws.
Best practice: design undersized hole, tap after printing. Or use heat-set brass inserts (M2–M8). For printed external threads, add 0.3–0.5 mm clearance to mate with steel. Print threads vertically (axis horizontal) for best geometry. For very small screws (M2–M3) always use heat-set inserts.
FDM plastic threads strip at low torque: always torque-limit screws in printed parts. Layer lines create stress concentrations at thread roots. Anisotropic shrinkage distorts thread geometry. Never rely on printed plastic threads for structural load paths.
Related terms: Press Fit, Assembly Clearance, Hole Sizing for 3D Printing, Tolerance
| Field | engineering |
|---|---|
| Also called | printed thread, metric thread, thread engagement |
| Source | EDi Brain: rules-core |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert