Fillet restrictions

• The fillet constructor works with two surfaces only.
• The surfaces must intersect.
• If trimming, the top and bottom boundary curves of the fillet must form a valid trimmed surface in the respective surfaces. See Trimming restrictions for the rules of valid trimming curves.
• Fillet creation does not consider trimmed portions of a surface so if you fillet a trimmed surface, the fillet extends for the complete length of the original surface. You can either trim the fillet away after it has been created or use an intersection curve to limit where the fillet runs.
• The input surfaces must be smooth with no sharp corners.
• The fillet radius should be less than the smallest radius of the intersection curve. If it is larger, the fillet may overlap itself.

See Surface design hints for additional recommendations.