Hi Mike,

> But what should I do, if I have a model with an open end
> and I want to f.e. close that opening when it is not planar?
> How should I rebuild the edges of that opening?

Usually the easiest method is to initially build it with a planar end, and then use a boolean to slice the end off with a 2D curve to make the final non-planar ending shape.

That general strategy of building something that is initially extended further than your final shape and then cutting it off tends to be a good method to use for a lot of situations.

So that would looks something like this:











So note there that you don't initially worry about the final non-planar end shape at first - just get a simple shape constructed first and then put in some details by slicing that shape up.

That's a lot easier approach than if you tried to for example model the non-planar shape all at the beginning and then have to worry about how to fill it in.

With a good NURBS solid modeling workflow some of the edges in your final shape should not be drawn directly by you but instead should be the result of some kinds of boolean operations. It's not good to try and just draw every single edge including 3D warped edges right from the start and then try to patch them in - that's frequently what people with a poly modeling background seem to end up doing though so kind of watch out for that.

- Michael