Rhino 3D Tip: Diagnose and Repair Invalid Geometry with Check and ShowEdges

Diagnose and fix problematic geometry early with Rhino’s Check and ShowEdges. These two analysis tools quickly reveal invalid objects, tiny gaps, and non‑manifold conditions that undermine joins, booleans, offsets, and clean exports.

When to run them

• After importing STEP/IGES/SAT from other CAD platforms.
• Before Boolean operations, Cap, OffsetSrf, or Shell.
• Any time Join fails, Make2D looks messy, or STL export produces holes.

Check: what it tells you

• Validity: confirms if a curve, surface, or polysurface is a “valid” Rhino object.
• Topology flags: bad trims, self‑intersections, or non‑manifold edges.
• Join tolerances: reports near‑miss edges that are outside current document tolerance.

ShowEdges: where the problems are

• Naked edges: highlight open borders that prevent a closed solid.
• Non‑manifold edges: reveal edges shared by more than two faces (common cause of boolean failures).
• Boundary and seam edges: useful for inspecting continuity and parameterization.
• Tip: Use the Select option in ShowEdges to capture problem edges, then zoom and isolate for repair.

Fast fix playbook

• Tiny gaps between edges
  • Preferred: improve the underlying geometry so edges actually meet. Try MatchSrf (Position/Tangency), ExtendSrf to close small gaps, or rebuild and refit boundary curves (Rebuild, FitCrv, SimplifyCrv), then re‑Trim/Join.
  • Last resort: JoinEdge can force a join if the gap is within tolerance, but do this sparingly and only after confirming surfaces truly coincide.
• Messy trims or oversized underlying surfaces
  • Untrim then re‑Trim with clean, simplified curves.
  • ShrinkTrimmedSrf to reduce hidden surface extents and stabilize edges.
• Sliver faces and micro features
  • MergeAllFaces to simplify coplanar or tangent patches where possible.
  • RebuildEdges to re‑fit edge representations to the current tolerance.
• Open solids that should cap
  • ShowEdges → Naked edges: ensure the open border is planar, then use Cap on planar openings.
  • Non‑planar openings: create a well‑formed closing surface via Sweep/Loft/BlendSrf, then Join.
• Non‑manifold conditions
  • Explode, address overlapping or branching faces, remove duplicates, then Join back into a clean solid.

Tolerance and prevention

• Set absolute tolerance appropriately before modeling (e.g., 0.01 mm in millimeters; 0.0001 m in meters). Avoid changing mid‑project.
• Enable CheckNewObjects to automatically validate as you build.
• Maintain clean input curves: consistent degree, fair curvature, and no micro‑segments.
• After heavy edits, re‑run ShowEdges and Check to confirm integrity before downstream operations or exports (DWG, STEP, STL).

Practical workflow

• Run Check on suspect parts, then ShowEdges with Naked and Non‑manifold toggled.
• Repair with Untrim/Trim, MatchSrf, ExtendSrf, RebuildEdges, and MergeAllFaces as needed.
• Verify closure with ShowEdges; only then proceed to Booleans, Cap, OffsetSrf, or export.

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