Summary: When working with multiple fillet radii, we start with the largest values first and progress in order, to the smallest fadius, 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.

Download Rhino 3DM file

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.