Rhino 3D Tip: BooleanUnion Best Practices for Watertight Solids

BooleanUnion is the fastest way to combine multiple closed solids into a single, watertight part, simplifying edits, fillets, and downstream documentation. Here’s how to get predictable, production-ready results every time.

When to use it

• Fuse intersecting bodies to create one clean solid for fabrication or printing.
• Enable continuous edge operations (FilletEdge/ChamferEdge) across former part boundaries.
• Reduce object count, simplify layer management, and prepare assemblies for export.

Preflight checklist (do this before BooleanUnion)

• Verify they’re solids: use SelOpenPolysrf and ShowEdges (set to Naked/Non-manifold) to fix leaks first. Volume should report a value (not “unknown”).
• Confirm tolerances: set Absolute tolerance appropriate to model scale (e.g., 0.001–0.01 units for cm/mm work). Update file units/tolerances before modeling large unions.
• Ensure real overlap: parts should intersect volumetrically—not just touch along an edge/face. Slightly extend or nudge parts to create a clear intersection if needed.
• Preview intersections: run Intersect on candidates; you should see continuous, closed intersection curves without tiny gaps or zigzags.

Recommended workflow

1. Duplicate the inputs to a “_BU_backup” layer so you can revert if necessary.
2. Run BooleanUnion and select all target solids at once (Rhino handles multi-body unions well).
3. Inspect the result with ShowEdges. If any naked edges appear, Undo and improve inputs before retrying.
4. Cleanup: run MergeAllFaces to collapse coplanar patches; consider SimplifyCrv on any problematic trims (via manual split/join if needed).

Troubleshooting fast

• Union fails or creates holes
  • Check inputs with SelBadObjects and Audit3dmFile. Repair or rebuild suspect faces.
  • Use Intersect, then Split each solid by the intersection curves; delete interior faces; Join to finish manually.
• Edge-to-edge or face-to-face contact with no volume overlap
  • Introduce a small, intentional overlap, then retry. Perfectly coplanar faces often stall booleans.
• Tiny sliver surfaces after union
  • Refine the input geometry (clean profiles, Rebuild where appropriate) and re-run. Use MergeAllFaces post-union.
• Working with SubD or meshes
  • Use SubDBooleanUnion for SubD objects and MeshBooleanUnion for meshes to avoid NURBS conversion surprises.

Pro moves

• Set a command alias (e.g., “bu” → BooleanUnion) for speed.
• Use Selection Filter (Polysurfaces) to avoid grabbing curves or meshes accidentally.
• After a successful union, run FilletEdge/ChamferEdge across the new, continuous solid for smooth transitions.
• Document with Make2D or export as STEP for CAD exchange once the model is clean and watertight.

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