THREAD draws a 60° V thread along a centreline you specify — the detailed representation, with the actual crests and roots, rather than the two parallel lines of the simplified convention.
What ScrewThread does
For a drawing where the thread form matters: a detail of a screw, a tapped hole in section, a leadscrew, or anything being made from the drawing rather than bought to a standard.
How to use it
- Load the routine with
APPLOAD, or put it on your support file search path. - Type
THREAD. - Give the diameter and length.
- Give either the pitch or the number of threads per unit — whichever your data is in.
- Choose right or left hand.
How it works
60° included, which is the ISO metric and the Unified form both. The pitch is the distance from one crest to the next.
The crest and root are flat, not sharp. A real thread is truncated at both, and a sharp V is a form that cannot be cut. The flat is an eighth of the pitch at the crest and a quarter at the root, which is close enough to the ISO profile to read correctly at any drawing scale.
Left hand is a proper option. Getting it by typing a negative diameter works — a negative radius makes polar go the other way — and is an undocumented trick nobody would guess and nobody should have to.
Notes and limits
- A thread too short to fit one turn is handled rather than stopping on a bad argument.
- Give pitch or threads per unit, whichever you have.
- This is the detailed convention. For production drawings the simplified two-line form is usually what standards ask for.
- One undo group, so a cancelled thread leaves nothing behind.
Download and details
| Name | ScrewThread — draw a screw thread in section, with real crests and roots |
| Download | ScrewThread.lsp — 10.7 KB, plain AutoLISP source. Downloads directly; no zip to unpack. |
| Type | AutoCAD AutoLISP routine |
| Command | THREAD |
| Requires | nothing beyond a CAD platform with AutoLISP. |
| Source size | 10.7 KB, 219 lines |
| Error handling | Yes — a *error* handler restores every system variable it changed, on cancel as well as on error. |
| Undo | The whole operation is wrapped in a single undo group, so one U reverses all of it. |
| Compatibility | AutoCAD (any release with AutoLISP), BricsCAD, ZWCAD, GstarCAD, ProgeCAD and other IntelliCAD-based platforms. AutoCAD LT needs a LISP enabler. No .NET, no ObjectARX, no installer. |
| Author | YZ, August 2026 |
| Licence | Free to use, supplied “as is” with no warranty. |
References
- MACHFEAT LISP — tapped holes with the thread envelope, in section
- ISO metric tap and die chart — pitches and drill sizes
- SPURGEAR LISP — the other computed profile in this library
- More free AutoLISP routines on this site
- Wisey’s Steel Shapes — free structural steel section drawing program