Printed versus tapped threads in FDM parts

Printed threads and tapped threads solve the same problem, a screw needs somewhere to bite, but they behave very differently and have different size limits. Confusing the two is one of the most common design errors we see on incoming CAD files, usually a printed M3 internal thread that was never going to survive a screwdriver.

This guide separates the two approaches clearly: what printed threads can and cannot do, where tapping takes over, and how to prepare the hole so a tap actually cuts a clean thread instead of tearing the plastic.

Printed internal and external threads

A printed thread is generated as native geometry in the CAD model and comes out of the printer already formed, no secondary operation. This works reasonably well for external threads, such as a bottle-style cap, from about M8 upward, because the male thread form is a series of outward bumps that the extruder can trace with normal perimeter and top layers.

Internal printed threads are harder. The nozzle has to trace a concave helical groove, and at small diameters the groove width falls below what a 0.4 mm nozzle can resolve without the perimeters merging into a rough approximation of a thread rather than a true one. This is why internal threads below M6 rarely hold torque reliably, and why we recommend an insert or a tapped hole for anything smaller.

Why anything below M6 internal rarely works printed

The physical reason is straightforward. A standard metric thread has a pitch and a crest width that scale with diameter. At M6 the pitch is 1.0 mm and the crest is wide enough for two or three passes of a 0.4 mm nozzle to build a recognisable thread flank. At M3 the pitch drops to 0.5 mm, and the crest width becomes comparable to the nozzle diameter itself, so the printer cannot draw a clean flank, only a rounded approximation that grips poorly and wears out after one or two screw insertions.

Layer height compounds the problem for vertical threads. A helix climbing at 0.5 mm per turn crosses only two or three layers at 0.2 mm layer height, so the thread profile is built from a handful of stepped rings rather than a smooth ramp, which further degrades flank contact.

Thread profile and clearance modelling

Where a printed thread does make sense, model it with clearance rather than a nominal thread profile. Add roughly 0.15 to 0.2 mm of radial clearance between mating printed threads to compensate for the typical ±0.2 mm dimensional tolerance on PLA, PETG and carbon-filled grades, or ±0.3 mm on ABS, ASA and PC. Without that clearance the two halves either will not turn together or bind after a quarter turn.

We also recommend rounding the thread crest slightly instead of using a sharp V profile, since a sharp crest is the first feature to be lost when the nozzle cannot resolve fine detail. A trapezoidal or slightly flattened crest profile prints more consistently and still functions as a mechanical thread for low-load applications like caps and covers.

Tapping into printed material

Tapping means printing a plain, straight pilot hole and cutting the thread into it after printing with a standard metal tap, exactly as you would in a machined plastic or aluminium part. This produces a much cleaner and stronger thread than a printed one because the tap forms continuous flanks regardless of layer lines, and it works reliably from about M5 upward in most of our engineering filaments.

Getting the pilot hole diameter right matters more in printed plastic than in machined metal, because FDM holes tend to print slightly undersize on the inside due to how perimeters close on a circle. We typically compensate the CAD hole diameter by 0.1 to 0.3 mm depending on material and hole size, and confirm the final fit against standard tap drill charts before cutting.

Hole preparation checklist

1. Model the pilot hole as a straight cylinder, no printed thread geometry inside it.
2. Add a small chamfer at the hole entrance so the tap starts square and does not chip the first layers.
3. Orient the part so the hole axis is vertical if possible, for a rounder, more consistent bore.
4. Confirm hole diameter against the tap drill size for the target thread, adjusted for FDM shrinkage.
5. Cut the thread slowly with cutting oil or wax appropriate to the plastic, backing off every half turn to clear chips.

Thread reinforcement geometry

Whether a thread is printed or tapped, it only performs as well as the material around it. A thin wall around a threaded hole will crack under torque regardless of thread quality. We recommend a minimum of 2.5 to 3 mm of solid wall around any tapped or printed thread, increasing to 3 to 4 mm for threads that will see repeated tightening or vibration.

Adding a boss, a raised cylindrical wall around the hole, is usually cheaper in material and print time than thickening the whole surrounding wall, and it lets you concentrate strength exactly where the thread needs it. We size bosses at roughly 2 to 2.5 times the thread's nominal diameter in outer wall diameter as a starting point, then adjust based on the load case.