Is a ʼBoolean Unionʼ the correct way to merge two solids in Rhino?
Short Answer
Yes — in Rhino 3D, BooleanUnion is the correct and most common professional way to merge two closed solids into one polysurface. It works best when the objects clearly intersect and are valid closed volumes. Limitation: it can fail on bad geometry, tiny gaps, or non-intersecting solids.
What You Need to Know Before
Warning: BooleanUnion depends on clean, intersecting closed solids. A very common hidden failure is when faces only touch tangentially or edges are slightly open, causing Rhino to return separate objects or a failed boolean with no merged result.
How to Merge Two Solids in Rhino
Command: BooleanUnion
Shortcut:
_BooleanUnion(common command alias:BoolUnionif customized)Quick Steps:
- Select the two closed solids in the viewport, then run BooleanUnion from the command line or Solid menu.
- Press Enter to confirm the selection and let Rhino calculate the intersecting volumes.
- Check that the result is one closed polysurface; if needed, use Properties or What to verify it is closed.
Variables & Settings
Key Setting: Model Absolute Tolerance
Expert Setting: Rhino’s boolean operations are heavily affected by Absolute tolerance in Document Properties > Units. If tolerance is too loose, intersections may be inaccurate; if too tight, the boolean may fail or become unreliable on imported geometry.
Why it Fails
- Cause 1 (Geometry): One or both objects are open polysurfaces, invalid solids, or do not truly intersect as closed volumes.
- Cause 2 (layers/Locks): One object is on a locked layer or is locked individually, preventing proper selection or editing.
- Cause 3 (Command/Logic): The solids only touch at an edge or face without a clean overlapping volume, so BooleanUnion has no reliable volume merge to compute.
Quick Fix & Best Practice
- Quick Fix: Run ShowEdges to find naked edges, repair the solid, and then retry BooleanUnion.
- Manager’s Verdict: Use BooleanUnion as the default method for merging production solids in Rhino, but avoid it on dirty imported geometry until you verify closed volumes and tolerance settings.
FAQ
Can BooleanUnion work on open surfaces?
No, it is intended for closed solids or valid polysurfaces with a true boolean volume result.
What if Rhino does not merge the two solids?
Check whether the objects actually intersect and confirm both are closed before retrying.
Is Join the same as BooleanUnion in Rhino?
No, Join connects touching surfaces or edges, while BooleanUnion merges overlapping solid volumes.
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