Hi Steve, so the big step that you're kind of missing here is that when you project a curve onto a surface, you often don't want to get rid of the surface and try to recreate it from scratch, instead you want to actually make use of the surface itself too, like for instance by trimming the surface by that curve.

Trying to recreate the surface that goes through a non-planar projected curve is going to be really difficult - but you don't need to recreate it, because you've already got it there since it was what you projected on to in the first place. You just need to cut it using the Trim command.

But also another thing is that several commands like both booleans and trim already have curve projection built into them so you don't need to do them as a separate manual step like it looks like you've done here.

It looks like the final result that you want is an extrusion with rounded ends.

A good way to create that is actually by a totally different process than projecting curves and then trying to create surfaces to fill them in, you can instead build an extrusion and then cut the extrusion to produce the rounded ends.

So in your case here, you'd start with 2 planar curves like this:



Extrude those out to make your base shape:



Now position some spheres to use for cutting it up:




Select all 3 objects, and then use the Construct > Boolean > Merge command, that will slice up all the objects and leave all the results behind, then you just delete the parts that you don't want to keep. After you delete the pieces of the sphere you'll see your extrusion was sliced up like this:



So then delete the outer pieces and only keep this inside one which has the shape you want:




Usually this kind of process of building an extended solid base shape and then cutting it up is the best way to approach things rather than trying to build individual projected curves and then trying to fill those in.


You want to kind of take a look at your object and try to think if it has some kind of central underlying element to it which is then cut off at a certain spot. In your case it's exactly that - you want to have a base shape that is an extrusion but that then has cut off rounded ends. So usually with NURBS modeling you want to actually build in that exact method, build the extrusion and then cut it as shown above.

- Michael