Hi Joe, I think I see what you're doing there - I was a little confused at first, but now I see that you've got one set of rails slightly to the inside of the side surfaces edges.

Don't use those rails - instead use the actual edge curves of the surface as the rail curves for your top sweep piece. That's what I did for the attached version, which will now join.

It's good idea in general to set up the rails so that they are attached right to the end point of a profile, that way when you sweep using those rails the resulting surface will coast right along that rail edge.

It is ok to use rails that are not exactly touching the endpoints of the profiles for certain situations, but having them exactly touching will guarantee more accuracy. When the endpoint is off of the rail it will cause the ends to wiggle just a slight amount more as the profile is swept along the rails. As you have seen it doesn't take very much wiggling to stop things from joining.

Here you are seeing some of the finicky parts of NURBS modeling which can take some time to get used to - it is sort of a package deal - NURBS modeling provides for more accuracy in a lot of things (like being able to cut accurate holes in things), but also it sort of depends on accurate results for stitching parts together as well.

- Michael