3d-cad-skill

name: 3d-cad-skill description: Create and iteratively refine parametric 3D CAD models through progressive refinement with inspectable feedback. Use when the task involves OpenSCAD, build123d, STL/STEP/3MF output, fixture/enclosure/adapter design, or debugging shape accuracy from renders or screenshots. Covers both generation and editing as a unified workflow.

3D CAD Skill — Progressive Refinement

Use this skill for parametric 3D modeling where geometry must be correct, manufacturable, and easy to revise. Design is inherently iterative — this skill treats generation and editing as a single unified loop, not separate tasks.

When To Use It

Trigger this skill when the user asks for:

• A 3D printable or CAD-ready part
• OpenSCAD or build123d model generation
• Edits to an existing parametric model
• Iterative refinement of a model through multiple rounds of feedback
• Shape debugging from screenshots, renders, or exported meshes
• Precise dimensions, fit checks, tolerances, wall thicknesses, or hole placement

Core Rule

Do not trust mental visualization of 3D geometry. Write code, render or export a view, inspect the result, then revise. One change at a time.

If the environment supports screenshots or image inspection, use them after every meaningful geometry change. If not, inspect through deterministic evidence: multi-angle orthographic projections, section cuts, bounding-box checks, and explicit dimension calculations.

Prefer multi-turn interactive refinement over attempting a perfect one-shot model. Evidence shows that iterative refinement with inspection between turns dramatically improves success rates for both novices and experts.

Progressive Refinement Philosophy

CAD design is not a single-step task. The workflow below treats every model as a draft that improves through structured iteration:

• Generation is the first edit — creating a model from scratch is just the initial step in a refinement chain
• Each turn produces inspectable evidence — never advance without verifying the current state
• Qualitative and quantitative checking are complementary — visual inspection catches proportion errors; numerical checks catch dimensional errors
• Parameter consistency is proactive — validate that related parameters are mutually consistent before rendering (e.g., inner radius < outer radius, wall thickness > 0)
• Multi-turn dialogue reduces error — breaking complex designs into a sequence of simpler refinements produces better results than one-shot generation

Workflow

1. Capture The Design Brief

Before modeling, extract:

• Target output: stl, step, 3mf, source code only, or all of them
• Modeling stack: prefer the user's requested tool; otherwise default to the simplest proven option
• Critical dimensions and units
• Mating parts, clearances, print orientation, and strength constraints
• Whether the goal is appearance, fit, mechanism, or manufacturability

If requirements are incomplete, make explicit assumptions and keep the first version simple.

2. Structured Reasoning (Before Writing Code)

Before generating or editing geometry, reason through four steps explicitly:

1. Intent Understanding — Restate what the user wants in spatial terms. What should the model look like? What are the key features and their relationships?
2. Modeling Analysis — Identify which primitives, boolean operations, and transforms are needed. Which parts are base bodies, which are cutouts, which are additive features?
3. Parameter Computation — Compute all critical dimensions, positions, and transforms. Check that parameters are mutually consistent (no impossible geometry).
4. Position Identification — For edits, identify exactly which parts of the existing model need to change and what the edit sequence should be.

3. Build The Smallest Correct Parametric Skeleton

Start with a coarse model that proves overall proportions and alignment:

• Define top-level parameters first.
• Separate base body, cutouts, mounts, and cosmetic details.
• Keep transforms and coordinate frames explicit and minimal.
• Delay fillets, chamfers, text embossing, and decorative features until base geometry is correct.
• Validate parameter consistency before rendering: check that holes are smaller than the bodies containing them, clearances are positive, and wall thicknesses are printable.

Structure code so a later pass can adjust one concern without rewriting the whole model.

4. Render And Inspect (Multi-View + Dual Modality)

After each meaningful change, inspect the result systematically from multiple angles.

Qualitative inspection (visual):

• Check silhouette and proportions from at least 3 orthogonal views (front, side, top) plus an isometric view
• Check symmetry and centering across all major axes
• Check whether holes, cutouts, and mating faces are centered and aligned
• Check whether the model matches the user's stated intent, not just whether the code runs

Quantitative inspection (numerical):

• Verify critical dimensions against the design brief with explicit measurements
• Check bounding box dimensions
• Validate hole diameters, wall thicknesses, and clearance distances
• Confirm that parameter relationships are logically consistent

Fabrication risk:

• Check printability: unsupported spans, wall thickness, trapped voids, and impossible overhangs
• Check for sharp internal corners where a radius is likely needed
• Identify thin or fragile members

Read references/geometry-iteration.md for the full inspection loop and diagnostic patterns.

5. Fix One Geometric Issue At A Time

When something looks wrong:

• Name the defect precisely (not "it looks wrong" but "the left mounting hole is offset +2mm in X")
• Identify the smallest parameter or operation likely causing it
• Change one thing
• Re-render and compare against the previous result
• If the user provides feedback, treat it as an editing instruction — localize the affected geometry and apply the minimal edit

Avoid broad rewrites unless the part architecture is clearly wrong.

6. Deliverables

Default deliverables:

• The source model
• A short list of exposed parameters
• Assumptions and unresolved risks
• Export instructions if the environment cannot generate meshes directly

When useful, also provide:

• Recommended print orientation
• Suggested tolerance ranges
• Notes on likely failure points or reinforcement options

Quality Bar

The final result should be:

• Parametric rather than hard-coded — critical dimensions are named variables
• Geometrically valid — code executes without errors, booleans produce manifold geometry
• Internally consistent — no contradictory parameters (e.g., cutout larger than its parent body)
• Easy to inspect and revise — operations are grouped by concern, transforms are explicit
• Dimensionally explicit — key measurements are stated, not buried in expressions
• Realistic for the intended fabrication method — wall thicknesses, clearances, and overhangs are appropriate
• Honest about assumptions — unverified fit, estimated dimensions, and design risks are flagged