Screw thread generator

1. Pick what to make: a bolt and nut, a bolt or a nut alone, a threaded rod, a jar with a screw-on lid, or the fit test.
2. Choose the size. The standard metric sizes (M3 to M30) use the ISO pitch, hex size and nut height, so a printed nut fits a real spanner and a printed bolt fits a real nut. Or set your own diameter and pitch.
3. Set the gap between the threads, then download the STL, a 3MF, or each part on its own.

Printed plastic spreads a little, so printed threads need a gap to turn. The gap is shared between the two parts: the bolt is made a little thinner and the nut a little wider. 0.3 mm suits most well-tuned printers.

The surest way to find your number is the fit test: one bolt and four nuts with gaps of 0.1, 0.2, 0.3 and 0.4 mm, marked with one to four notches. Print it with your usual settings and use the gap of the nut that turns smoothly by hand without wobbling.

• Bolts stand up, head down, as they're laid out: the thread then prints as smooth layers all round. Lying down, it comes out flat on one side.
• Size matters: M8 and bigger print well on a 0.4 mm nozzle. M3 to M5 work with fine layers (0.12 mm) and slow outer walls, but they won't be strong.
• Strength: three or four walls and 30% infill or more. Plastic threads hold well for jars, knobs, clamps and jigs, but not for loads a steel bolt would take.
• Jars: the lid prints top down, as laid out, so its thread needs no supports. The ribs round the lid give grip.

The pitch is the distance from one ridge of the thread to the next: M10 has a 1.5 mm pitch, so one turn moves the nut 1.5 mm. All threads here are right-handed: clockwise tightens. Jars use a coarser, shallower thread, which closes in fewer turns and forgives more.

Need a hole for a screw in a model you already have? Cut it with Merge and cut STL. For a box with a lift-off or sliding lid, use the box generator.