Hi Pilou,

> And other thing: how do make for know that some
> curves or volumes are overlaped?

For surfaces, you can see that there is an adjacent face there because it will be a shaded-in area rather than an empty area.

So for example in your original model you posted, you had a surface here:



You can see that there is a surface there due to the shaded filled in area rather than an empty area... I deleted those surfaces in the updated file that I posted, so that it was easier for you to select the edge of the surface you wanted to blend with.

If you have a curve overlapping a surface edge, you can tell that by moving the mouse over top of it with the surface not selected - normally to select an edge you must select the surface first. If you see something that looks like an edge being highlighted when you move the mouse over it, but you have not selected the surface, then that is how you can see that there is a generator curve there.

- Michael