Key Takeaway: When building SolidWorks assemblies from layout sketches, keeping subassemblies in the “Rigid” state rather than “Flexible” prevents unexpected component movement, reduces rebuild time, and avoids mate errors that plague complex multi-level assemblies.
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The Rigid vs Flexible Problem
SolidWorks offers two states for subassemblies within a parent assembly: Rigid and Flexible. Rigid treats the subassembly as a single solid part — all internal mates are preserved, but the components cannot move relative to each other within the parent. Flexible allows internal motion (pistons, hinges, linkages) to propagate up to the parent assembly.
While Flexible seems useful, it comes with significant costs: increased rebuild time, increased RAM usage, increased chance of mate errors, and unpredictable behavior when opening the assembly on different computers or SolidWorks versions. For layout sketches — where you are defining the overall envelope and position relationships — Rigid is almost always the right choice.
Why Keep Subassemblies Rigid in Layout Sketches
1. Performance
A Flexible subassembly forces SolidWorks to solve all mates in the subassembly every time the parent assembly rebuilds. In a machine with 5-10 subassemblies, each containing 10-50 mates, this multiplies the solver workload by the number of Flexible subassemblies. Rigid subassemblies solve once and cache the result.
2. Stability
Flexible subassemblies are notorious for “exploding” — mates that worked perfectly in the subassembly context fail when the subassembly is made Flexible within the parent. This happens because the parent assembly’s coordinate system and reference geometry differ from the subassembly’s context. Rigid subassemblies never have this issue.
3. Layout Sketch Integrity
Layout sketches define the spatial relationship between major assemblies — where the motor mounts relative to the frame, how the conveyor aligns with the worktable. These relationships should be controlled by layout sketch dimensions and mates, not by the internal flexibility of individual subassemblies.
Best Practice Workflow
- Design subassemblies independently — Build and test each subassembly in its own window. Verify motion and fit before inserting into the parent.
- Insert as Rigid — When bringing subassemblies into the top-level assembly, accept the default Rigid state.
- Mate using layout references — In the top-level assembly, mate subassembly origins, planes, or layout sketch entities — not individual component faces.
- Use Flexible only when needed — The rare exceptions where Flexible is justified are: interference checking of moving components, cable/hose routing validation, and animated presentations for stakeholders.
Frequently Asked Questions
Can I switch a subassembly between Rigid and Flexible without losing mates?
Yes. Right-click the subassembly in the FeatureManager tree, select “Properties,” and toggle “Flexible.” Mates are preserved. However, Flexible mates that fail to resolve in the parent may require manual fixing.
Does the Rigid setting affect BOM output?
No. The BOM lists individual components regardless of Rigid/Flexible state. Rigid simply controls how the subassembly behaves visually and computationally in the parent.
Should I use layout sketches with blocks instead?
For very large assemblies (500+ components), layout sketches with blocks offer even better performance than Rigid subassemblies. However, blocks lack the parametric intelligence of fully-mated subassemblies and are harder to maintain through design changes.
Related Reading
External Resources
Sources: SolidWorks Documentation, Field Experience
