Threads
Use Thread to cut a helical thread onto a cylinder. Select one cylindrical face and press T. Unlike everything else in this section it does not consume a sketch, because it reads the cylinder you point at and works out the geometry from there.
What the face tells it
From that one face alone, Kapy takes:
- the axis, from the cylinder itself,
- the radius,
- whether the thread is external or internal, by probing which side of the face is solid,
- and the length, which is the whole face.
That last one matters. A thread always covers its entire face, and there is no start offset and no length field. To thread only part of a bolt's shank, split the shank into two faces first, then thread one of them.
| Field | What it means |
|---|---|
| Pitch | Tooth to tooth, along the axis, in millimetres |
| Depth | Crest to root, measured radially |
| Flank angle | Half-angle of the tooth from the perpendicular, 0° to 60°. 30° is the ISO profile, 0° is a square thread, and anything above 45° draws a printability warning |
| Direction | Right or left hand |
| Starts | How many independent threads wind side by side. The tooth-to-tooth distance stays the pitch; each thread advances pitch × starts per turn |
The rules it enforces
Depth and flank angle fight each other, and the tool draws the line where the crests would vanish. Steeper flanks eat the flat on top of each tooth, so the steeper the flank, the shallower the thread can be. At 30° the depth can reach about ¾ of the pitch, and at 45° it is about ⅖ of it. A square thread, flank 0°, has no such limit at all.
On an external thread the depth also has to be less than the radius, or there would be nothing left in the middle.
The other hard limit is the number of turns. A thread that would need more than sixty turns per start is refused, so give a longer thread a coarser pitch, or more starts.
Two warnings appear without blocking anything. A flank angle above 45° is flagged as hard to print, and a lead (pitch × starts) longer than the face itself is flagged as not fitting a single turn.
Cosmetic, then real
For about a third of a second after each edit, the thread you see is a cosmetic ribbon, a decoration wrapped around the cylinder so the viewport keeps up with your typing. Then the editor quietly computes the real cut geometry and shows you that instead. At rest, what is on screen is the real thing.
Exports differ in how they get there:
| Output | How |
|---|---|
| STL | A mesh boolean. A thread whose helix degenerates to zero turns is dropped silently and that body exports smooth. Any other failure aborts the export |
| STEP | A true solid cut, one thread at a time. If a cut cannot be made cleanly the whole export fails rather than shipping a smooth part |
| Slicing | Real geometry, the same as STL |
| Thumbnails and previews | The cosmetic ribbon |
That asymmetry cuts both ways. An STL that comes out smooth where you expected threads has lost a degenerate one without saying so. A STEP export that refuses to finish has usually met a thread it could not cut, so try a coarser pitch, a shallower depth, or fewer turns.
See also
- Helical Sweep: a thread modelled as real geometry
- Working with Several Bodies: what a hidden body does to an export