Modelling threads

A thread is just a helical groove wrapped around a cylinder, and modelling that helix as real geometry is expensive — it turns a clean cylinder into hundreds of tiny faces that slow every regen and bloat every export. Kapy sidesteps that. When you add a thread in the editor you get a fast visual overlay, not a sculpted helix. The thread stays cosmetic and cheap while you design, and only becomes real geometry the moment you export. This article covers how that workflow behaves, and the harder question underneath it: whether you should print the thread at all.

Cosmetic now, baked on export

Drop a thread feature onto a hole or a stud and you'll see the threads rendered as an overlay — the right pitch, the right depth, oriented the right way — but the underlying solid is still a plain cylinder. Nothing in your model gets heavier. You can move it, change its diameter, pattern it, and the part stays as quick to regenerate as it was before.

The real geometry appears at export, and the form depends on the target:

• STL — the helix is cut into the mesh with a boolean (manifold-3d). You get a watertight printable thread, baked at the moment of export.
• STEP — short threads are emitted as a true B-Rep helical thread, so the file carries proper CAD geometry another tool can read.

The upshot: model with the thread feature freely, and let the cost land once, at the end, instead of carrying it through every edit.

Should you even print the thread?

Here is the part most people learn the hard way. A printed thread is built from stacked layers, and the fine ridges of a small thread land right at the limit of what a 0.4 mm nozzle and 0.2 mm layers can resolve. Below roughly M6 the crests come out rounded and weak, and the first time you torque a screw into them they strip.

So treat printed threads as a large-feature trick:

• M6 and up — printable and useful. Bottle caps, jar lids, big adjustment knobs, tripod-style mounts. The coarse pitch prints cleanly.
• M2 to M5 — don't print the thread. Use a heat-set insert or tap the hole instead. Both give you real metal threads in a printed boss. See Designing for heat-set inserts.

If a printed thread is borderline, ask what happens when it strips: a knob you hand-tighten is fine, a structural fastener is not.

Pitch and clearance for printed threads

When you do print a thread, give it room. A nominal thread pair (a bolt and nut at exact size) will bind because the printed surface is rougher and slightly oversized. Offset the diameters so they slide.

A coarse, rounded profile has more meat per turn, so a stripped crest here and there doesn't kill the thread. Sharp metric V-threads, scaled down to print, are mostly gaps.

Orient the axis vertical

Threads print best with the cylinder axis vertical. Each turn of the helix then climbs gently layer by layer, and every layer rests on the one below — the same self-supporting logic as any gentle slope.

Lay the thread on its side and the helix becomes a series of overhangs and bridges wrapping the cylinder. The underside crests sag, the thread goes out of round, and the fit you tuned on screen is gone. If a part has a threaded feature you care about, rotate it so that feature points up, then worry about the rest of the orientation second.