Existing small radius
Summary: When working with multiple fillet radii, we start with the largest values first and progress in order to the smallest radius, to avoid running a fillet off the edge of a surface it is following. This tutorial shows a technique to use when an existing smaller fillet is complicating things.
Small to big
 | The existing fillet is 3mm. We will add a 4mm filet around the top edge. |
 | Run FilletEdge with 4mm. It will do what it can and fail. |
 | Extract the two 4mm surfaces, and the 3mm surface. |
 | Split the red surface by isocurve where it intersects the edge of the 4mm filet surface. |
 | Delete the top piece of the 3mm surface. |
 | Untrim the fillet surfaces. |
 | Use FilletSrf to make the curved transition surface. |
 | Join all the surfaces to the polysurface, and cap to replace the missing top surface. |