CAD Generation, Inspection, and Validation
Overview
This skill enables AI agents to create or modify parametric CAD models from natural-language requirements, generate validated STEP/STP artifacts, inspect geometry references, and return checked outputs. STEP is treated as the primary CAD artifact. STL, 3MF, and native GLB are secondary export workflows.
Requirements
Dependencies
Install the required packages before using this skill:
pip install -e ~/.claude/plugins/marketplaces/text-to-cad/skills/cad/scripts/packages/cadpy
This will install:
- build123d (parametric CAD modeling)
- cadquery-ocp (OpenCASCADE Python bindings)
- All required dependencies
Tools
From the CAD skill directory, the launcher commands are:
python scripts/step ... # STEP generation, GLB/topology artifacts, mesh sidecars
python scripts/inspect ... # refs, measure, align, frame, diff
python scripts/snapshot ... # PNG/GIF visual review packets
When to Use This Skill
Use this skill when the user asks for:
- CAD files, STEP/STP files, build123d source
- Mechanical parts, assemblies, enclosures, brackets, fixtures
- Holes, counterbores, countersinks, slots, pockets, bosses, standoffs, ribs, fillets, chamfers, shells
- Reference images or 2D technical drawings of a part to reproduce
- STL, 3MF, or native GLB output from CAD geometry
- Geometry measurements, alignment checks, or validation
Do NOT use this skill for:
- Render-only concept art
- CAM toolpaths
- Engineering certification
- FEA conclusions
- Architectural BIM
- Freehand illustration
Default Assumptions
Use these defaults unless the user specifies otherwise:
- Units: millimeters
- Origin: center of the main part or assembly
- Base plane: XY
- Up/extrusion axis: positive Z
- Output geometry: closed, positive-volume solids
- Small plastic enclosure wall: 2.0-3.0 mm when unspecified
- Cosmetic fillet: 1.0-3.0 mm when safe for local geometry
- M3/M4/M5 normal clearance holes: 3.4/4.5/5.5 mm unless another standard is requested
Modeling Workflow
1. Classify the Task
Determine the task type:
- New part from scratch
- New assembly
- Source modification
- Direct STEP/STP inspection
- Reference selection
- Measurement/alignment check
- Snapshot review
- Secondary output request (STL/3MF/GLB)
2. Write a Natural-Language CAD Brief
Extract from the user's request:
- Dimensions and units
- Coordinate convention
- Feature intent
- Output paths
- Assumptions
- Validation targets
3. Plan Before Coding
Define before editing:
- Parameters (named variables for dimensions)
- Intent labels
- Source paths
- Expected bounding boxes
- Mating/positioning datums
4. Author build123d Python Source
Create a Python file with gen_step() function:
from build123d import *
# Parameters as named variables
outer_diameter = 80.0
thickness = 20.0
hole_diameter = 30.0
def make_part() -> Solid:
"""Create the part geometry"""
with BuildPart() as part:
# Base solid
Cylinder(radius=outer_diameter / 2, height=thickness)
# Subtractive features
with Locations((0, 0, thickness / 2)):
Hole(radius=hole_diameter / 2)
# Fillets/chamfers last
chamfer(part.edges(), 1.0)
return part.part
def gen_step():
"""Generate STEP file - required function signature"""
return make_part()
5. Generate STEP File
Run the step generation script:
python ~/.claude/plugins/marketplaces/text-to-cad/skills/cad/scripts/step path/to/your_model.py
This generates:
- .step file (primary CAD artifact)
- .glb file (for viewer)
- Topology artifacts
6. Validate Geometry
Run baseline inspection:
python ~/.claude/plugins/marketplaces/text-to-cad/skills/cad/scripts/inspect refs path/to/model.step --facts --planes --positioning
This returns:
- Face/edge/vertex counts
- Bounding box dimensions
- Major plane identification
- Positioning information
7. Generate Snapshot
Create visual verification:
python ~/.claude/plugins/marketplaces/text-to-cad/skills/cad/scripts/snapshot --input path/to/model.step --output path/to/output.png --appearance workbench
Modeling Best Practices
Design Strategy
- Choose construction that makes spec dimensions direct parameters
- Order operations: Base solid -> Major additions -> Subtractive features -> Shell -> Through holes -> Fillets/chamfers last
- Overshoot boolean tools: Extend cutting tools 1mm past entry/exit faces
- Sanity-check proportions: Compare expected vs actual bounding box
Parameters First
Always use named variables for dimensions:
# Good - named parameters
outer_diameter = 80.0
thickness = 20.0
# Bad - magic numbers
Cylinder(radius=40, height=20)
Coordinate System
Declare the convention:
Origin: center of primary part
XY: main base/sketch plane
+Z: up/extrusion direction
Common Failure Modes
Avoid these issues:
- Fillet radius larger than local edge geometry
- Open sketch profile produces invalid face
- Face selector changes after boolean or fillet
- Part origin is arbitrary
- Coincident or coplanar tool/target faces
Fillet/Chamfer Tips
- Fillets are the most failure-prone operation
- Postpone fillets to the end of modeling
- If chamfer fails, try smaller radius or different edge selection
- Select edges carefully after boolean operations change topology
Assembly Workflow
For assemblies with multiple parts:
from build123d import *
from cadpy.assembly import AssemblyHelper, label_shape
def make_assembly():
asm = AssemblyHelper("my_assembly")
base = asm.add(make_base(), "base")
lid = asm.add(make_lid(), "lid")
# Add hardware with labels
screw = label_shape(
Cylinder(radius=2.0, height=8.0),
"m4_screw",
"front_left"
)
return asm.compound
Export Workflows
Primary: STEP
STEP is the primary CAD artifact. Always generate STEP first.
Secondary: STL/3MF/GLB
For 3D printing or web visualization:
python scripts/step path/to/model.py --stl --3mf
DXF (2D Drawings)
For 2D technical drawings, use the DXF skill separately.
Validation Checklist
Before delivering CAD files:
- [ ] STEP file exists and is non-empty
- [ ] Bounding box matches expected dimensions
- [ ] All holes are through-holes where specified
- [ ] Fillets/chamfers applied correctly
- [ ] No self-intersecting geometry
- [ ] Snapshot visually reviewed
- [ ] Inspection passes without errors
Code Style Guidelines
- Write minimal, concise Python code
- Use meaningful parameter names
- Comment non-obvious geometry decisions
- Keep functions short (<50 lines)
- Separate parameter definition from geometry creation
Example: Circular Flange
from build123d import *
from math import cos, sin, radians
# Flange parameters
outer_diameter = 80.0
thickness = 20.0
center_hole_diameter = 30.0
bolt_circle_diameter = 60.0
bolt_hole_count = 6
bolt_hole_diameter = 8.0
chamfer_size = 1.0
bolt_angles = [i * 360 / bolt_hole_count for i in range(bolt_hole_count)]
def make_flange() -> Solid:
with BuildPart() as part:
# Base cylinder
Cylinder(radius=outer_diameter / 2, height=thickness)
# Chamfer outer edges
chamfer(part.edges(), chamfer_size)
# Center hole
with Locations((0, 0, thickness / 2)):
Hole(radius=center_hole_diameter / 2)
# Bolt holes
for angle in bolt_angles:
x = (bolt_circle_diameter / 2) * cos(radians(angle))
y = (bolt_circle_diameter / 2) * sin(radians(angle))
with Locations((x, y, thickness / 2)):
Hole(radius=bolt_hole_diameter / 2)
return part.part
def gen_step():
return make_flange()
Generate and validate:
python scripts/step flange.py
python scripts/inspect refs flange.step --facts --planes --positioning
python scripts/snapshot --input flange.step --output flange.png
Troubleshooting
Chamfer/Fillet Fails
- Try smaller radius value
- Select specific edges instead of all edges
- Apply chamfer before boolean operations if possible
- Check for coincident faces
Invalid Geometry
- Ensure all sketches are closed profiles
- Verify boolean tools overshoot target faces
- Check for self-intersecting edges
- Validate parameter proportions
Script Errors
- Verify cadpy package is installed
- Check Python file encoding (use UTF-8)
- Ensure
gen_step()function exists - Validate build123d imports
