Threads in 3D printing: when to print them and when not to

When a printed thread works

A thread is a helical groove with a fixed pitch. For printing it suits larger diameters, roughly M8 and up, and a coarse pitch, where the profile holds well in layers. Jar lids, bottle caps or large nuts come out well.

On small metric threads such as M3 or M4 the profile is only a few layers in size. The plastic is flexible and the layers are not exact, so the thread falls apart or breaks the first time you tighten it.

How to design a printed thread

• Clearance. Leave 0.2 to 0.4 mm between the outer and inner thread, see tolerances.
• Orientation. Keep the thread axis vertical, so the layers wrap around the axis. A horizontal thread splits along the layers when tightened.
• Profile. Choose a coarser pitch or a trapezoidal profile. A sharp triangular thread prints badly.
• Chamfer. Lead the start of the thread in with a chamfer so it starts easily.

Better alternatives for small threads

If you need to join two parts with a screw, do not design a thread into the plastic, design holes for an ordinary screw. A screw cuts its own way into a smooth hole slightly narrower than its diameter. For frequent screwing it is better to use a heat-set insert or to place a nut in a hexagonal pocket.

Holes usually print slightly narrower than designed, so test the right diameter on a single piece.

What this means for your order

We print the thread in your model the way you design it. We do not tap or repair threads after printing. FDM printing is not as precise as machining, so test the thread on one piece first and only then order a bigger batch.