Loft Fails After Guide Rail Edit
You are building a transition duct as a loft between a circular inlet of 100 mm diameter and a rectangular outlet of 120 x 80 mm, with a 300 mm offset between their centers. A single guide rail was sketched as a spline through three points and the loft succeeded. You then added a second guide rail to control the side profile. The loft now fails with a self-intersection error, and the preview shows a pinch near the midpoint. The outlet profile is a closed rectangle with four corner fillets of 10 mm. You have a review in one hour and the duct must remain a single solid.
1. Why does adding the second guide rail cause a self-intersection, and what conditions must guide rails satisfy?
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A loft guide rail must touch every section at a corresponding point and must not cross another rail or itself between sections. With one rail the surface had freedom to find a smooth path. The second rail was sketched independently, so its midpoint likely lies on the opposite side of the first rail's path or outside the region the sections can span, causing the two rails to cross near the midpoint. Where rails cross, the loft surface folds through itself, producing the pinch and the self-intersection error.
Rails also need consistent start and end attachment: each rail should intersect each section profile at a pierce point, and the order of attachment must match across sections. A rail that attaches to a fillet arc on one section and a straight edge on the other introduces a twist. Check rail-to-rail clearance at several stations, confirm pierce points exist on both sections, and verify the rails do not reverse direction relative to each other.
2. Give a recovery plan that keeps a single solid and is achievable within the hour.
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First, isolate the problem: suppress the second rail and confirm the loft still builds, which proves the rail is the cause. Then inspect the second rail against the first at the midpoint and at quarter points; adjust its middle point so the two rails maintain separation and do not cross. Ensure both rails have pierce constraints to both sections in matching order. If the rails still fight, reduce to one rail plus a centerline or use a rail plus a twist-controlled loft.
If time runs short, a robust fallback is to build the duct as two lofts or a loft plus a boundary patch and then combine into one solid, but only if the seam can be made tangent; otherwise keep the single loft with one rail and control the side profile with a section at the midpoint. Add a mid-section profile to steer the shape instead of a second rail. Validate with a section analysis and check the solid is watertight before the review.