One of the most difficult operation in Surface Design in CATIA V5 is to keep tangency in situation like the following.

It's necessary to design a flange (bending/forming on sheet metal) which the guide is a slot profile on curved surfaces. First times was impossible to give a thickness to the surface because the flange was not tangent to the slot profile.

If the slot is on two or more surfaces, a simple sweep does not ensure the tangency. 

In a situation like that, I always do this kind of operations:

- Parallel Curve (Slot profile)

- Projection of Parallel Curve (On upper plane)

- Line (or profile) to connect slot profile and parallel curve

- Sweep (with two guide curves)

- Edge Fillet on edge (Bending Radius) 

- Other optional Splits

Did u find in something similar? Please share your experiences!