openscad-cad — parametric design: sketch + dimensions → printable STL

The DESIGN stage of the print pipeline. Sibling skill 3dprinter (same 3dprinting/ category) handles everything AFTER an STL exists (slice, verify, upload, print on a Flashforge AD5X). This skill handles everything BEFORE: turning a sketch, photo, or spoken description with dimensions into a parametrically-coded, verified, printable STL.

Trigger conditions

1. User says "design / make / create an object" (or "can you model X?") and supplies a sketch, annotated photo, or verbal dimensions.
2. User hands over a rough sketch / photo with arrows + measurements and expects a dimensional model back.
3. Iteration on an existing model: "make it 4 mm wider", "add a hole here".
4. A downloaded STL needs dimensioning edits (re-model it here instead of mesh-sculpting; then continue in 3dprinter / multipart-stl-plates).

Tooling

• OpenSCAD — code-driven parametric CAD: the model is text, dimensions are named variables, STL export is CLI-only. Ideal for agent-driven design (chat → .scad → STL), and every model is git-versionable. Install on Debian/Ubuntu:

  sudo apt-get install -y openscad   # universe repo; verify with `openscad --version`

  On other distros use the official binary from https://openscad.org/downloads.html.
• CadQuery / build123d (Python, OCCT kernel) — optional. Only reach for it when geometry is genuinely computed (threads, patterns, math-defined curves) and OpenSCAD becomes painful. pip install cadquery pulls ~1 GB of OCCT wheels — ask the user before installing.
• Preview renders — stl_preview.py from sibling skill 3dprinter (scripts/) renders iso/front/side/top PNGs with no GPU deps.

Workflow

1. Gather the spec (do not skip)

Ask until every dimension that affects printability is known. Default to a markdown table of named dimensions so the user can correct one value and the rest of the flow stays honest:

| dim | value (mm) |
|-----|-----------|
| width | 40 |
| depth  | 25 |
| height | 12 |
| wall   | 2   |
| hole Ø | 5   |

When the user supplies a photo of a hand sketch, confirm units (cm vs mm is the classic 10× disaster) and which measurements are critical fits.

2. Author the parametric model

Start from templates/parametric-part.scad. Rules:

• All dimensions as named variables at the top — never literals scattered in geometry. Iteration = edit variable + re-render.
• One module per logical feature; assemble in a top-level union()/difference().
• $fn = 64 for smooth curves; drop to 6–12 only for stylized/low-poly effects or when triangle count explodes (many holes).
• Add assert() statements for sanity bounds (e.g. wall ≥ 0.8 mm, overall ≤ bed envelope).
• Comments: one line per feature naming the dimension it uses.

3. Render to STL (CLI, headless)

openscad -o out.stl --export-format binstl part.scad
# Override a dimension from chat without editing the file:
openscad -o out.stl -D 'width=44' -D 'depth=28' --export-format binstl part.scad

Binary STL is the default format for slicers and for the 3dprinter skill. If exporting ASCII for diffing, add --export-format asciistl.

4. Verify before showing the user

• Manifoldness: OpenSCAD's stderr prints Simple: yes/no after rendering (CGAL). Simple: no means non-manifold — fix the CSG before slicing.
• Well-formed file: assert the binary size:

  import struct
  d = open('out.stl','rb').read(); n = struct.unpack('<I', d[80:84])[0]
  assert len(d) == 84 + 50*n  # header(80) + count(4) + n × 50-byte records

• Envelope check: dims must fit the printer bed with ≥ 5 mm margins all around (Orca rejects plates that hug an edge — see multipart-stl-plates). Default constraint in the template is a 220 mm-cube bed (Flashforge AD5X).
• Visual: stl_preview.py out.stl preview → PNGs; inspect them. Vision models MISREAD raw meshes — trust the OpenSCAD source and section analysis over a vision model's guess when they disagree.
• Confirm final dims from the model (e.g. re-measure with a mesh reader) match the agreed table before presenting.

5. Hand off

Save the STL under the workspace models dir and hand to sibling 3dprinter for channels-confirm + slice + print.

FDM printability constraints (0.4 mm nozzle reference)

Pitfalls (learned)

• Units: OpenSCAD has NO units — everything is mm by convention. A part modeled in inches is 25.4× too big; sanity-check envelope after first render.
• Boolean slivers: difference() leaves zero-thickness walls when the cutter touches the outer surface — offset the cutter by ≥ 0.1 mm and never leave a shared-face boolean.
• High $fn everywhere: hundreds of smooth holes → multi-MB STL and slow slicing. Reserve 64 for visible curves; use $fn=6–12 cylinders inside hidden cavities.
• Render report is the truth: check Simple: yes and the stderr triangle summary BEFORE previewing — CGAL refuses/silently fixes some degenerate CSG.
• Naive chamfers bloat the part: a bottom-edge chamfer built with minkowski() expands the footprint by the chamfer radius (40×25 declared became 40.5×25.5). Keep the box true to size and let the slicer's elephant-foot compensation handle layer-1 squish.

See also

• Sibling 3dprinter — slice → verify → print pipeline (OrcaSlicer headless, Flashforge LAN API, material channels).
• Sibling multipart-stl-plates — multi-shell STLs, plate packing, binary STL read/write, Orca plate-rejection rc codes.
• templates/parametric-part.scad — starter skeleton with named dims, asserts, and a printability pre-flight.