I'm trying to model a 3/4" steel stringers for an elliptical spiral stair. The goal is to do this as a sheet metal part that will allow me to flatten the pattern for cutting. The stinger rotates only about 3/4 of an ellipse, so it does not intersect with itself. First, I created a sheet metal part from a sketch of a "constructed" ellipse, giving me a partial elliptical tube with 3/4" walls. This part would flatten out. Then I did a swept cut to shape the slanted top edge of the part. I tried to flatten the part at this point and got an error. The "constructed" ellipse sketch had 6 arcs in it, 5 of the resulting bends would flatten, but the last bend returned an error and would not flatten.

Any ideas?