build123d CAD

Parametric 3D CAD via build123d. This skill provides Python scripts for generating and measuring 3D solids. Run them with the exec tool.

Setup (first time only)

cd {baseDir}
uv venv --python 3.12
uv pip install build123d

Commands

All scripts use the venv Python at {baseDir}/.venv/bin/python. All output JSON to stdout.

Generate — export STEP/STL/SVG

{baseDir}/.venv/bin/python {baseDir}/scripts/cad_generate.py \
  --script 'from build123d import *
with BuildPart() as result:
    Box(100, 60, 40)' \
  --format step \
  --filename my_box

Output: { "success": true, "artifact_path": "...", "format": "step", "file_size_bytes": N, "bounding_box_mm": {...} }

Measure — dimensions, volume, mass

{baseDir}/.venv/bin/python {baseDir}/scripts/cad_measure.py \
  --script 'from build123d import *
with BuildPart() as result:
    Cylinder(10, 50)'

Output: { "success": true, "bounding_box_mm": {...}, "volume_mm3": N, "surface_area_mm2": N, "center_of_mass_mm": {...}, "face_count": N, "edge_count": N }

Section — 2D cross-section SVG

{baseDir}/.venv/bin/python {baseDir}/scripts/cad_section.py \
  --script '...' \
  --plane XY \
  --offset 5.0

Output: { "success": true, "artifact_path": "...", "plane": "XY", "offset_mm": 5.0 }

API Reference — build123d cheatsheet

{baseDir}/.venv/bin/python {baseDir}/scripts/cad_api.py

Output: JSON with primitives, operations, selectors, export functions. Call this first if you need to learn what's available.

Validate — interference, clearance, swept-volume collision

Check an assembly of multiple parts for interference, minimum clearance, and moving-part collisions.

{baseDir}/.venv/bin/python {baseDir}/scripts/cad_validate.py \
  --script '...' \
  --mode full \
  --min-clearance 1.0

Modes: static (Boolean intersection between all pairs), clearance (bounding box gap), sweep (rotate moving parts through angular range, check collisions), full (all three, default).

The script must define parts = {"name": solid, ...} dict. Optionally define sweeps list for moving parts:

from build123d import *

with BuildPart() as enclosure:
    Box(100, 80, 50)
    offset(amount=-3, openings=enclosure.faces().sort_by(Axis.Z)[-1:])

with BuildPart() as pcb:
    with Locations((0, 0, 5)):
        Box(60, 40, 2)

with BuildPart() as servo_arm:
    with Locations((30, 0, 25)):
        Box(5, 20, 3)

parts = {
    "enclosure": enclosure.part,
    "pcb": pcb.part,
    "servo_arm": servo_arm.part,
}

sweeps = [
    {
        "name": "servo_arm",
        "axis_origin": (30, 0, 25),
        "axis_direction": (0, 1, 0),
        "angle_start": -45,
        "angle_end": 45,
        "angle_step": 5,
    },
]

Output: { "verdict": "PASS|WARN|FAIL", "static_interference": {...}, "clearance": {...}, "swept_volume": {...} }

• PASS — no interference, clearance OK
• WARN — clearance below threshold but no hard collision
• FAIL — parts intersect or moving parts collide during sweep

Swept volumes are exported to ~/.openclaw/workspace/cad-output/swept_.step for visualization.

Script Format

All single-part scripts must assign the final solid to result via a BuildPart context:

from build123d import *

with BuildPart() as result:
    Box(100, 60, 40)
    fillet(result.edges().filter_by(Axis.Z), radius=5)
    with Locations((0, 0, 40)):
        CounterBoreHole(radius=5, counter_bore_radius=8, counter_bore_depth=3, depth=40)

For validation scripts, define parts dict (and optionally sweeps list) instead of result.

All dimensions in millimeters. Exported files go to ~/.openclaw/workspace/cad-output/.

Workflow

1. If unsure about build123d API, run cad_api.py first for the cheatsheet.
2. Write a parameterized script (no magic numbers).
3. Run cad_measure.py to verify dimensions before exporting.
4. For multi-part assemblies: run cad_validate.py --mode full before exporting. Fix any FAIL/WARN before proceeding.
5. Run cad_generate.py with the desired format (step for CAD, stl for 3D printing).
6. For clearance visualization, run cad_section.py at relevant planes.
7. Report artifact paths and validation verdict.

Design Rules

• Parameterize all dimensions for reusability.
• Add fillets to stress concentrations (min 1mm for plastic, 0.5mm for metal).
• Include mounting features (bosses, standoffs, screw posts) where applicable.
• Specify material and process assumptions in comments.
• Output both the script and the exported file.