Rhino 3D Tip: Rhino Mirror Best Practices for Clean Seams

Mirror is powerful for symmetry, but careless use can introduce seams, shading issues, and downstream failures. Use it deliberately and finish the seam correctly.

• Choose the right symmetry plane
  • Decide early whether symmetry is World-based (Top/Front/Right) or component-based with a custom CPlane. Set a Named CPlane for repeatability.
  • Keep a locked “centerline” curve or reference surface for consistent picks and quality checks.
  • If you work in assemblies, mirror in the part file, not in the master, to avoid confusion with transforms and blocks.
• Model only half, but design for a clean seam
  • Aim edges and isocurves perpendicular to the symmetry plane so the mirror joins cleanly.
  • Minimize micro-surfaces at the centerline; simplify with Rebuild or MergeAllFaces on the half before mirroring.
  • For curves, keep the centerline edge straight and planar; use SetPt or SimplifyCrv if needed.
• Execute Mirror with precision
  • Use object snaps (Mid, Cen, Perp) for the two mirror-plane picks. Turn on SmartTrack to lock a true perpendicular read.
  • Set Copy=Yes so your original half remains intact; keep it on a separate layer until the seam is validated.
  • Mirroring blocks/instances is fine, but verify orientation and normals after export to other CADs.
• Finish the seam properly (NURBS)
  • Join the halves and run ShowEdges (Naked/Non-manifold) to catch gaps early.
  • Use MatchSrf across the seam (G1 Tangent or G2 Curvature) on key surfaces for reflection-quality continuity.
  • Align directions with Dir; Flip if normals oppose and UnifyNormals for meshes.
  • If capping fails, check tolerance; tighten Absolute tolerance slightly and rerun Cap or rebuild the rim curves.
• SubD workflows
  • Prefer live symmetry with Reflect (Rhino 8) to maintain a dynamic, edit-safe centerline without duplicate edits.
  • If you mirror a finished SubD, stitch the seam cleanly and inspect edge flow before subdividing further.
• Rendering and texturing checks
  • Mirroring can flip UVs; verify materials and decals. Reapply mapping or use MatchMapping across the seam if textures appear reversed.
  • For meshes, Weld and UnifyMeshNormals to remove a visible hard crease along the centerline.
• Avoid common pitfalls
  • Don’t “double-mirror” or mirror groups that already contain mirrored parts—use SelDup to remove unintended copies.
  • Keep tolerances appropriate to scale; over-loose tolerances mask gaps, over-tight tolerances block joins.
  • Fillets across a raw mirror seam often fail; ensure tangent/curvature continuity first, then fillet or BlendSrf.
• Speed tips
  • Create a macro/alias that sets the CPlane and toggles Copy=Yes before Mirror for consistency.
  • Save a “Symmetry Check” named view aligned to the plane to quickly review highlights and zebra stripes.

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