Most CAD models do not fail at creation. They fail on the third change request, when a sketch relation snaps to a face that no longer exists and forty downstream features roll back into error. Change-safety is a design decision made long before the first extrude.
Why rebuild errors are a manufacturing problem
A broken feature tree is rarely just an inconvenience. It delays the drawing package, invalidates the CAM setup, and forces the shop to work from a stale revision. On production programs the cost of a fragile model is measured in machine hours, not modelling hours.
The ten rules
1. Start with a layout sketch, not a feature
A single fully-defined layout sketch carries the interfaces of the part: mounting centres, envelope, and datum structure. Every later feature references that sketch instead of referencing each other.
2. Reference datums, never faces
Faces are transient. Planes, axes, and origin geometry are permanent. The single largest source of rebuild errors in production models is a sketch plane placed on a filleted or trimmed face.
3. Name the features you will look for later
- 1Rename any feature another engineer will need to find: interfaces, seals, fastener patterns.
- 2Leave routine fillets and chamfers with default names — noise dilutes signal.
- 3Prefix configuration-driving features so they sort together in the tree.
4. Keep cosmetic features at the bottom of the tree
Fillets, chamfers, draft, and appearance work belong in a final block. Suppress that block and the model becomes a clean CAM body in one click.
5. Drive dimensions from a single equation set
wall_min = max(0.8 mm, 0.12 × D_nominal)
6. Constrain sketches completely — always
7. Model for the process you will actually use
A cast part is modelled from the parting line out. A machined part is modelled as stock minus operations. Modelling against the process is how impossible internal corners end up on a drawing.
Process-driven modelling constraints
| Process | Min. wall | Internal radius | Draft | Typical tolerance |
|---|---|---|---|---|
| 3-axis milling | 1.0 mm | ≥ 1.6 mm (tool radius) | 0° | ±0.05 mm |
| 5-axis milling | 0.8 mm | ≥ 1.0 mm | 0° | ±0.025 mm |
| Sheet metal | 0.6 mm | 1 × thickness bend | 0° | ±0.2 mm |
| Injection moulding | 1.5 mm | ≥ 0.5 mm | 1–2° | ±0.1 mm |
| Investment casting | 2.5 mm | ≥ 3.0 mm | 1.5° | ±0.3 mm |
8. Separate the master model from the deliverable
The master carries intent. Derived configurations carry deliverables: simplified CAM body, FEA body, render body, and the drawing body. One source, many outputs.
release/
01_master/ bracket_master.sldprt # intent, equations, full tree
02_derived/ bracket_cam.step # cosmetics suppressed
bracket_fea.x_t # defeatured
03_drawings/ bracket_rev_c.pdf
04_manufacture/ bracket_op10.nc
bracket_flat.dxf
README.txt revision + tolerance notes9. Tolerance the drawing, not the model
10. Rebuild, roll back, and break it on purpose
Before release, drive every key parameter ±20% and force a full rebuild. If nothing errors and no geometry inverts, the model is change-safe.
Pre-release model checklist
- Force rebuild passes with zero errors and zero warnings
- All sketches fully defined
- No external references to unreleased files
- Mass properties within 3% of the estimate
- Material and density assigned
- Derived CAM body regenerates
- Drawing datums exist as physical features
- Parameters swept ±20% without failure
A model is finished when someone else can change it safely — not when it looks right.
Seeing it on the floor
Interactive model viewer
Rotate, section, and measure the released bracket directly in the browser.
Interactive viewer coming soon
Frequently asked
Does this work outside SolidWorks?
Yes. The rules are history-tree agnostic — they apply equally in Fusion 360, Inventor, Onshape, and NX. Only the terminology changes.
How strict should the ±20% sweep be?
Sweep the parameters a customer can realistically change. Envelope, hole pattern pitch, and wall thickness matter; a cosmetic fillet does not.
Should the CAM body live in the same file?
Use a derived configuration when the geometry difference is small, and a separate derived file when the shop needs an immutable snapshot.
