Hi Pilou,

> Or blue lines must stop must just at the vertical of the internal bottom point?

If I understand correctly, you want the blue lines and the red lines to intersect near the midpoint of where the fillet will be going.

You make that portion come to a sharp edge between the 2 surfaces, don't attempt to draw each piece already perfectly round and meeting smoothly in that spot, make it come to a sharp intersection point and then use Fillet to smooth out the sharp edge.

It does not matter exactly how far the 2 pieces overlap past one another, just make them go some small distance past each other so that they fully intersect and can trim each other.

Just look at the previous file I posted for an example...

- Michael