Hi ed, thanks for posting the sweep file, I got it from your post in the other thread (here http://moi3d.com/forum/index.php?webtag=MOI&msg=4737.148).

So the part that's a bit unusual about your case here is that you've got 2 curves that are totally to one side of the sweep rail here:



Then the middle one is hanging completely off the other side of the rail:



The way 1-rail sweep works is that the profiles basically morph from one profile into the other profile as they step along the rail - by making the profiles basically go through an extended morph because they're on totally opposite sides of the rail from each other that will sort of introduce more movement in the profiles and make things harder to control.

To get more control over the shape in this particular case you probably won't actually use a scaling rail, but instead use a 2 rail sweep so that the profiles can be more simply positioned all inside the 2 rails - that would be some kind of curve structure more like this:





With the rails positioned like that you are then more directly controlling the outside edges of the generated surface by the shape of that rail curve, rather than having the outside edges being generated second-hand by the result of transitioning profile curves.

In fact you can probably focus more on just having 1 profile and the 2 rails like this:



Does that give you the kind of control that you are looking for in the generated shape?

3DM file also attached.

I got these particular rail curves by rotating your other one a couple of times, using Orient line to line and Revolve axis a couple of times.

- Michael