- Surfaces of revolution follow ISO 10303-42 strictly in reader and writer: the normal is d/d(angle) x d/d(profile), the opposite of truck's revolution, as Fusion reads it (fixes Fusion inverting fillet sphere corners and the dimple). step_orientation_errors flags faces whose loop runs clockwise round their normal; engine, oracle and kernel round trips assert it. - Revolved faces with analytic profiles are written as native PLANE, CYLINDRICAL/CONICAL/SPHERICAL/TOROIDAL surfaces, and the pieces truck splits a revolution into are merged back into one face per surface (with a seam edge for full turns), so face counts match other systems. - A horn torus (tight sweep: tube touching its axis) is written as one rational B-spline face whose loop passes the pole twice, with truck's zero-length pole edges collapsed, instead of a degenerate surface of revolution; the reader starts such figure-eight loops off the pole (Fusion's own part 50 now imports with the right volume). Co-Authored-By: Claude Opus 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL