What is a ʼSub-Assemblyʼ in SolidWorks?

ByCADbrief Staff

Short Answer

A sub-assembly in SolidWorks is an assembly file placed inside a higher-level assembly to group related components and manage motion, mates, and bom structure more efficiently. The most common professional method is creating it with Form New Subassembly Here in the FeatureManager tree. Limitation: nested sub-assemblies can complicate mate solving and top-level edits.

What You Need to Know Before

Warning: If you choose the wrong sub-assembly solve behavior, parts may stop moving as expected at the top level, especially in mechanisms with in-context mates. A common failure is locking motion because the sub-assembly is solved as rigid instead of flexible.

How to Create a Sub-Assembly in SolidWorks

  • Command: Form New Subassembly Here

  • Shortcut: None by default

  • Quick Steps:

    1. In the FeatureManager design tree, Ctrl-select the components you want to group, then right-click in the assembly tree.
    2. Click Form New Subassembly Here from the right-click menu and name the new assembly file.
    3. If needed, right-click the new sub-assembly, choose Component Properties, and set it to Solve as Flexible when internal motion must work at the top level.

Variables & Settings

  • Key Setting: Solve as Flexible

  • Expert Setting: This option controls whether mates inside the sub-assembly can still move when inserted into the parent assembly. Use Flexible for mechanisms, cylinders, linkages, or sliding groups; keep it rigid for stable hardware groups to improve performance.

Why it Fails

  • Cause 1 (Geometry): Internal mates inside the sub-assembly conflict or over-define movement, so components do not behave correctly after grouping.
  • Cause 2 (layers/Locks): Components may be fixed or constrained by external references in the parent assembly, preventing expected repositioning after conversion to a sub-assembly.
  • Cause 3 (Command/Logic): The sub-assembly is solved as rigid, so internal motion does not propagate to the top-level assembly.

Quick Fix & Best Practice

  • Quick Fix: Right-click the sub-assembly, open Component Properties, and switch Solve as to Flexible if internal movement is required.
  • Manager’s Verdict: Use sub-assemblies to organize repeatable hardware sets, mechanisms, and purchased modules, but avoid excessive nesting because it slows troubleshooting and makes assembly logic harder to manage.

FAQ

Can a sub-assembly move inside a top-level assembly?

Yes, if it is set to Solve as Flexible.

Is a sub-assembly a separate SolidWorks file?

Yes, it is saved as its own .SLDASM file.

Does a sub-assembly affect the BOM?

Yes, depending on BOM settings, it can appear as a single item or show its child components.

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