I am trying to generate an approximately 0.1" thick shell model of a freeform part with many smooth contoured curves. The part has no planar surfaces and no symetry. I have modeled the outside surface of the part as a set of cross section sketches. Each sketch is a closed spline. I can do a surface loft (actually a series of surface lofts, since the number of cross section sketches is quite large), and get a quite good model of the outside surface. The problem is trying to do a surface offset to get the inside surface of the model. In some cases I can offset the outer surface loft just fine. In other cases I cannot offset it because the outer surface has a radius of cuvature of less than .1". In these cases I generate a modified set of outer surface cross section contours with a larger radius of curvature, generate a new loft, and offset that to get the inner surface. But in some cases the surface offsets don't seem to be working properly.

For example, in one case I have generated a surface loft using five sketches as profiles. This loft measures approximately 0.47 inched high. When I do a surface offset, the preview shows the inner surface just fine. The offset generates without error, but the inner offset surface ends up being a VERY thin band measuring as little as .01 iches high! This band is near the bottom of the outer surface loft that I had offset. I can make the outer surface loft taller by including one or more additional profiles, but the generated inner surface offset remains exactly the same. Some lofts offset just fine, but this one will not. What gives?

Maybe someone can tell me a better way to generate my model? Maybe someone can help me figure out why the surface offset does not seem to be generated properly?