name: cad
description: > Expert CAD assistant for engineers and designers. Use this skill whenever the user needs: help with computer-aided design, 3D modeling, parametric design, assembly modeling, or generating manufacturing outputs. Includes both software usage and design methodology. trigger: Any engineering design problem involving CAD - from concept to manufacturing. license: MIT compatibility: opencode metadata: audience: engineers category: engineering
Computer-Aided Design — Principles and Practice
Covers: 3D Modeling · Parametric Design · Assemblies · Manufacturing · Standards · Best Practices
CAD Fundamentals
What is CAD?
Computer-Aided Design: creation, modification, analysis, and optimization of designs digitally.
CAD Benefits
- Faster iteration
- Easy versioning
- Simulation integration
- Manufacturing automation
- Collaboration
2D vs 3D
2D: Drawings, schematics 3D: Parts, assemblies, visualization
Direct vs Parametric
Direct modeling: Push/pull faces, edit geometry directly.
Parametric modeling: Dimensions and relations drive geometry.
- Changes propagate automatically
- Design intent preserved
History-Based Modeling
Feature tree records all operations:
- Easy to edit
- Non-destructive
Feature-Based Modeling
Common operations as features:
- Extrude, sweep, loft
- Hole, fillet, chamfer
- Shell, rib, draft
Feature Order
Dependent features after dependencies:
- Chronological tree
- Rollback to edit
Design Intent
How model responds to changes:
- Dimensions vs. relations
- Equations link dimensions
Sketches
2D profiles for 3D features:
- Constrain geometry
- Define relationships
Sketch Constraints
| Constraint | Symbol | Function |
|---|---|---|
| Coincident | ● | Points/curves same |
| Collinear | — | Lines on same line |
| Parallel | ∥ | Lines never meet |
| Perpendicular | ⟂ | 90° between |
| Concentric | ◎ | Same center |
| Equal | = | Same length/radius |
| Horizontal | ↔ | Line horizontal |
| Vertical | ↕ | Line vertical |
Underdefined
Degrees of freedom remain:
- May cause issues
- Add constraints
Fully Defined
All DOF constrained:
- Blue (under) → Black (fully)
- Stable model
Overdefined
Conflicting constraints:
- Must resolve
- Delete or edit
Dimensions
- Reference (driven)
- Driven (from geometry)
- Driving (controls geometry)
3D Feature Operations
Extrude
Project sketch along direction:
- Distance or through all
- Taper angle
- Direction (one/both)
Revolve
Rotate sketch around axis:
- Full 360 or angle
- Axis selection
Sweep
Extrude along path:
- Guide curves
- Smooth transitions
Loft
Blend between profiles:
- Guide curves help
- Can be tricky
Swept Blend
Combined sweep and loft:
- Profile along path
- Guide at sections
Helix
Spiral path:
- Pitch, turns
- Cone angle option
Variable Section Sweep
Profile varies along path:
- Multiple sections
- Guide curves essential
Thin Wall
Hollow part:
- Default inward offset
- Thickness varies possible
Shell
Hollow interior:
- Faces to remove
- Default thickness
Rib
Structural support:
- Profile → thickness
- Draft for molding
Draft
Taper for mold release:
- Faces to draft
- Pull direction
Hole
Standard cutouts:
- Simple, counterbore, countersink
- Spotface, thread
Fillets and Rounds
Smooth edges:
- Variable radius
- Edge vs. face fillet
Chamfer
Beveled edges:
- Distance or two distances
- Angle and distance
Offset
Duplicate face/edge:
- Distance
- Direction
Mirror
Symmetry:
- Plane selection
- Features or bodies
Pattern
Linear or circular:
- Instances
- Spacing
- Geometry seed
Assembly Design
Bottom-Up Assembly
Create parts, then assemble:
- Standard approach
- Clean geometry
Top-Down Assembly
Design in context:
- Layout first
- Parts reference skeleton
Assembly Constraints
| Constraint | Aligns |
|---|---|
| Mate | Faces, opposite direction |
| Flush | Faces, same direction |
| Tangent | Surface to curve |
| Insert | Axial insertion |
| Parallel | Two items parallel |
| Distance | Specified gap |
Degrees of Freedom
Each unconstrained part has 6 DOF:
- 3 translation
- 3 rotation
Smart Components
Auto-assemble:
- Define connections
- Apply automatically
Subassemblies
Group related parts:
- Hierarchical
- Simplifies top levels
Design Tables
Excel-driven configs:
- Multiple variations
- Part numbers
Large Assembly Tips
- Simplified parts
- Hide features
- Large design review
- Speedpak
Interference Detection
Check for collisions:
- Static analysis
- Dynamic (motion)
Clearance Analysis
Verify gaps:
- Clearance circles
- Report issues
Bill of Materials (BOM)
Parts list:
- Item number
- Description
- Quantity
- Part number
Auto-BOM
From model:
- Update automatically
- Add custom properties
Design Checks
- Mass properties
- Center of mass
- Moment of inertia
Surface Modeling
B-Spline
Basis spline:
- Control points
- Knot vector
Bezier
Polynomial curve:
- Control points
- Endpoints on curve
NURBS
Non-uniform rational B-spline:
- Weights on points
- Most flexible
Blend Surface
Transition between:
- G1 (tangent)
- G2 (curvature)
Fill Surface
Fill gap:
- Constraints at edges
- Continuity options
Loft Surface
Profile to profile:
- Guide curves
- Start/end constraints
Swept Surface
Profile along path:
- Can twist
- Guide rails
Extend Surface
Continue surface:
- Distance or to point
- Extend type
Offset Surface
Parallel surface:
- Distance
- Both sides
Knit Surface
Join surfaces:
- Create solid
- Fill gaps
Boundary Surface
From boundary edges:
- Complex topology
Style Spline
Smooth through points:
- Handles for control
- Curvature continuous
Rendering and Visualization
PhotoView 360
Realistic rendering:
- Materials
- Lighting
- Environment
Material Assignment
Physical properties:
- Color, texture
- Reflectivity
- Roughness
Lighting
- Ambient
- Directional
- Point
- Spot
Ray Tracing
Photo-realistic:
- Reflections
- Refractions
- Shadows
Animation
Show design:
- Exploded view
- Motion study
- Rendering sequence
Virtual Reality
Immersive review:
- Walk through
- Experience scale
3D PDFs
Portable:
- Embed in PDF
- Annotate
Augmented Reality
On mobile:
- Overlay in reality
- Check fit
Manufacturing Outputs
STEP
Standard exchange:
- Neutral format
- Most complete
IGES
Older standard:
- Geometry only
- May lose features
STL
Stereolithography:
- Triangle mesh
- 3D printing standard
ACIS
SAT format:
- SolidWorks native
- Feature data
Parasolid
XT:
- Neutral solid
- Industry standard
3D PDF
Publish design:
- Embed viewer
- Annotate
DXF/DWG
2D drawings:
- AutoCAD format
- For fabrication
IDF
PCB integration:
- Board outline
- Mount holes
cgr
Lightweight:
- No geometry detail
- Viewing only
Render Image
PNG, JPEG, TIFF:
- Marketing
- Documentation
Animation Video
MP4, AVI:
- Presentations
- Manuals
Simulation Integration
Finite Element Analysis (FEA)
Structural analysis:
- Meshing
- Loads, constraints
- Results visualization
Mesh Types
| Type | Description |
|---|---|
| Solid | Tetrahedra |
| Shell | Triangles/qu |
| Beam | Line elements |
Mesh Quality
- Aspect ratio
- Jacobian
- Element quality
Loads
- Force
- Pressure
- Gravity
- Thermal
Boundary Conditions
- Fixed
- Displacement
- Roller
Thermal Analysis
Heat transfer:
- Conduction
- Convection
- Radiation
Fluid Flow
CFD:
- Flow paths
- Pressure drop
Modal Analysis
Vibration:
- Natural frequencies
- Mode shapes
Optimization
Improve design:
- Topology
- Shape
- Size
Design Study
Parameters vary:
- What-if analysis
- Optimization
Simulation Types
- Static
- Dynamic
- Fatigue
- Buckling
Design for Manufacturability
Design for Injection Molding
- Draft angles
- Uniform wall thickness
- Radii at corners
- Undercuts need slides
Design for Machining
- Allow for tool access
- Standard drill sizes
- Chamfers for edges
Design for Sheet Metal
- Bend allowances
- K-factor
- Relief cuts
Design for Casting
- Draft
- Uniform sections
- Rounded fillets
Tolerancing
GD&T (Geometric Dimensioning and Tolerancing):
- Position
- Profile
- Flatness
- Perpendicularity
GD&T Symbols
| Symbol | Meaning |
|---|---|
| ⌒ | Position |
| ⌐ | Profile |
| □ | Flatness |
| ⟂ | Perpendicularity |
| ∥ | Parallelism |
| ○ | Circularity |
| ⬭ | Cylindricity |
Tolerance Stack Analysis
Cumulative variation:
- Worst case
- Statistical
Cost Optimization
Minimize material:
- Hollow sections
- Shape optimization
Rapid Prototyping
3D printing:
- Orientation
- Supports
- Materials
Parametric Best Practices
Sketch Best Practices
- Fully constrain
- Use relations
- Name dimensions
- Reuse sketches
Feature Best Practices
- Draft first
- Simple features
- Mirror when possible
- Use patterns
Assembly Best Practices
- Use subassemblies
- Smart components
- Layout skeleton
- Top-down where useful
Design Intent
- Define intent early
- Use equations
- Global variables
- Configuration
Large Assembly Best Practices
- Speedpak
- Hide details
- Load components as needed
- Defeature for analysis
Performance Tips
- Lightweight components
- Suppress features
- Rebuild only when needed
- Graphics quality
Collaboration
- Design library
- Design tables
- Pack and go
- PDM integration
Version Control
- Revision control
- Backup
- Release process
- ECOs
Standards
- Company standards
- Industry standards
- ISO, ASME Y14.5
- Drawing templates
Documentation
- Title block
- Views
- Dimensions
- Notes
Common Errors to Avoid
- Unconstrained sketches causing issues
- Too many features making rebuild slow
- Not using design intent properly
- Missing draft on molded parts
- Forgetting tolerance stack-up
- Over-constrained models
- Not checking interference
- Saving in wrong format
- Ignoring simulation results
- Not considering manufacturing in design
- Complex lofts causing issues
- Not testing in context
Key References
- SolidWorks Official Training
- Autodesk Inventor Official Training
- Mastering SolidWorks by Parrish
- GD&T Application and Interpretation by Krulikowski