Rhino 3D Tip: True Intersections for Accurate, Repeatable Trim Curves

When accuracy and repeatability matter, generate your trim curves from true intersections rather than sketching or guessing.

Choose the right tool for the intersection:

• Intersect: Quick, all-in-one intersection of selected objects (surfaces, polysurfaces, curves, meshes).
• IntersectTwoSets: Faster and cleaner on large scenes—explicitly pick Set A vs Set B to avoid unintended hits.
• Section: Create intersection curves with a plane (great for consistent cutlines through assemblies).
• Contour: Produce multiple evenly spaced section curves for patterns, ribs, or inspection.
• MeshIntersect: Use when one or both objects are meshes to get reliable mesh-based trim curves.

Prepare geometry and tolerances first:

• Set AbsoluteTolerance appropriately (e.g., 0.001–0.01 for most mm workflows). Too loose creates gaps; too tight fragments curves.
• Simplify and stabilize input: ShrinkTrimmedSrf, MergeAllFaces, and RebuildEdges on suspect polysurfaces.
• Manage periodic seams (cylinders, tori): MoveCrvSeam or adjust surface seams to avoid split intersection curves at seam locations.

Create intersection curves with intent:

• Lock or hide non-target objects to reduce candidate intersections and speed computation.
• With IntersectTwoSets, use NamedSelections for repeatable, controlled set picks.
• For planar cutlines on freeform parts, combine Section with Align=World or CPlane for consistent orientation.

Make curves “trim-ready” before cutting:

• Clean up: SelDup to remove duplicates, Join fragmented segments, SimplifyCrv to reduce unnecessary kinks.
• FitCrv with a tight tolerance if you must smooth overly faceted results—avoid overfitting that shifts edges off the surface.
• Find and fix tiny slivers: SelShortCrv and MergeCrv to reduce micro-segments that break Trim.
• If the curve was created off-surface (e.g., from Section), use Project or Pull to the target surface; prefer Pull for double-curved targets.
• Use CurveBoolean on planar faces to quickly build closed, watertight trimming loops.

Trim or split with confidence:

• Trim with ExtendCuttingCurves=Yes to meet edges cleanly; or Split when you want both sides preserved.
• For solids, use Split and then Cap to restore watertightness where applicable.

Validate the result:

• ShowEdges (Naked/NonManifold) to confirm a clean boundary after trimming.
• Analyze continuity where you will later fillet or blend; resolve gaps now to avoid Boolean failures later.

Speed and reliability tips:

• Filter your selections (SelectionFilter) and disable unnecessary Osnaps to reduce accidental picks.
• Create a quick macro alias for intersection cleanup, e.g.: IntersectTwoSets; SelLast; Join; SimplifyCrv

Need licenses, plugins, or expert advice? Explore Rhino solutions at NOVEDGE. For rendering or visualization workflows that pair with precise trims, check options like V-Ray for Rhino at NOVEDGE. Stay current with updates and training resources via the NOVEDGE Blog.

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