blog.draftsperson.net

the art of technical drawing

blog.draftsperson.net

HELIX3D LISP: Draw a Rising Helix as a 3D Polyline

HELIX3D draws a helix that climbs as it turns, as an ordinary 3D polyline.

What SpiralPath does

Why, when AutoCAD has had a HELIX command since 2007 that asks for much the same numbers? Because HELIX makes a HELIX object — a spline underneath — and there are things you cannot usefully do with one. You cannot offset it, you cannot get at its vertices, and exploding it gives a spline rather than segments.

A polyline you can offset, trim, edit vertex by vertex, and hand to a machine as a toolpath. That is the whole reason to keep this.

How to use it

  1. Load the routine with APPLOAD, or put it on your support file search path.
  2. Type HELIX3D.
  3. Pick the start point and give the radius, the rise per turn and the number of turns.
  4. For a flat spiral, use SPIRAL in ShopShapes instead — it already draws one as a polyline.

How it works

The helix is generated point by point and written as a 3D polyline, starting where you told it to.

π comes from AutoLISP, not from a typed constant. Typing it by hand as 3.141596235 — where the real value is 3.141592653, so the fifth and sixth decimals are wrong — is enough that a spiral of any length drifts off where it should be and never quite closes. AutoLISP has pi built in.

The error handler is installed and the old one saved properly. A routine that finishes by setting the session error handler from a variable that was never given a value sets it to nil — and after running it once, every later error in any routine reports as a bare AutoCAD message with no cleanup at all.

Notes and limits

  • The result is a real polyline, editable and offsettable, unlike a HELIX object.
  • For a flat spiral use SPIRAL in ShopShapes — there is no sense having two.
  • More segments means a smoother helix and a heavier object; pick to suit what it is for.
  • One undo group covers the helix.

Download and details

NameSpiralPath — a rising helix as an ordinary 3D polyline
DownloadSpiralPath.lsp — 6.2 KB, plain AutoLISP source. Downloads directly; no zip to unpack.
TypeAutoCAD AutoLISP routine
CommandHELIX3D
Requiresnothing beyond a CAD platform with AutoLISP.
Source size6.2 KB, 161 lines
Error handlingYes — a *error* handler restores every system variable it changed, on cancel as well as on error.
UndoThe whole operation is wrapped in a single undo group, so one U reverses all of it.
CompatibilityAutoCAD (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.
AuthorYZ, August 2026
LicenceFree to use, supplied “as is” with no warranty.

References