Rhino 3D Tip: Controlling Twist in Sweep2 Surfaces

Sweep 2 is powerful for shaping surfaces between two rails, but twists can sneak in. Here’s how to anticipate and control them for predictable, production‑ready results.

When to prefer Sweep 2

• Use when a surface must follow two guiding boundaries and keep cross‑sections consistent between them.
• Ideal for hulls, transitions, and fairings where one rail alone can’t capture shape change.
• Avoid if rails cross, reverse direction, or differ wildly in curvature; consider NetworkSrf in those cases.

Prep your curves (prevention beats repair)

• Continuity: Make rails smooth (G1+). Remove kinks or rebuild with Rebuild/RefitCrv for cleaner parameterization.
• Direction: Use Dir to ensure both rail arrows run the same way. Mismatched arrows are twist culprit #1.
• Section order and seams: Pick section curves in a consistent order along both rails. For closed profiles, align seams using CrvSeam before sweeping.
• Density: Fewer, higher‑quality sections beat many messy ones. Add sections only where shape genuinely changes.

Command workflow and options that matter

• Start Sweep2 → select rails in a left‑to‑right (or consistent) order → pick sections in the same spatial order.
• AdjustSeam: In preview, click the small arrows on closed sections to align all seams to the same side of the rails.
• AddSlash: Insert slashes across the sweep when you see corkscrew behavior. Place slashes near rapid rail banking or where sections “flip.” Keep them sparse and strategic.
• MaintainHeight=Yes: Preserves section dimensions across the sweep—critical for manufacturing intent and wall thickness control.
• Cross‑section control: Use Refit with a tight tolerance (e.g., 0.01–0.1 units) for accuracy, or Rebuild with a moderate point count for stability and lighter surfaces.
• SimpleSweep: Try it when rails/sections are already clean; it can produce leaner surfaces with fewer spans.

Fast twist‑fix checklist

• Preview corkscrews? Flip a rail with Dir.
• Random buckles? Align closed‑curve seams (CrvSeam) and re‑pick sections in a consistent order.
• Local twist only? Add 1–2 AddSlash markers exactly where rotation occurs.
• Still unstable? Add an intermediate section at the problematic station and reduce rotation between consecutive sections.
• Rail chaos? Rebuild rails to even parameterization; remove tiny spans and micro‑features.

Quality control and finishing

• Inspect with Zebra and CurvatureGraph to verify flow; use GCon to check curve continuity feeding the sweep.
• Match edges with adjacent surfaces using MatchSrf (G1/G2 as needed). Keep isocurves aligned for editability.
• Trim and ShrinkTrimmedSrf to simplify, then Join to ensure watertightness.
• RecordHistory: Turn on before sweeping so edits to rails/sections update the surface; lock curves on a separate layer to prevent accidental drifts.

Pro tips

• Place sections normal to the average rail direction at each station to reduce hidden rotation.
• Keep rail curvature monotonic where possible; abrupt inflections commonly trigger twists.
• If banking is intentional, distribute it across multiple stations rather than a single dramatic flip.

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