Fillet

What the operation changes#

A fillet trims the adjoining faces and inserts transition geometry between them. Autodesk Fusion Help describes an exterior round as removing material and an interior fillet as adding material. Both uses replace the original sharp edge with new faces and boundaries in the B-Rep.

That topology change matters to downstream sketches, drawings, CAM selections, and features that reference the resulting edges. A successful rebuild does not guarantee that those references remain stable after another revision.

Radius and cross-section controls#

Fillets are often constant-radius circular transitions, but that is only one construction. Fusion documents constant, chord-length, variable, and asymmetric sizing plus tangent or curvature continuity. Onshape's Fillet tool also supports partial and variable fillets, asymmetric values, and circular, conic, or curvature-controlled cross sections.

A variable fillet changes size along the edge. A partial fillet rounds only a bounded section. An asymmetric fillet consumes different amounts of its two adjoining faces. These controls should express the intended shape, not merely force an invalid constant-radius feature to rebuild.

Why fillets fail#

The surrounding geometry limits the transition. A requested radius may run into a nearby wall, exhaust a narrow face, collide with another round, or become impossible at a multi-edge corner. Imported bodies can add split edges and sliver faces that make the construction harder.

The diagnostic response is to isolate the first failing edge and inspect its local faces and endpoints. Reducing the radius can identify a space limit, but the largest value that rebuilds is not automatically an engineering requirement.

Relation to face blends#

An edge fillet starts from an edge or edge chain. A face blend starts from selected face sets and can form a transition even when the existing edge is unsuitable or absent. Both create blend geometry, but their inputs and controls differ.