Module fillet 

Constant-radius fillets, where the rolling ball’s envelope is closed-form.

A ball of constant radius rolling along a straight edge between two planar faces stays tangent to both, and its envelope is exactly a cylinder; the same ball rolling around the rim where a cylindrical wall meets its perpendicular cap traces a torus. These are the blends whose new surface costs nothing, and both are built the same way: the chamfer’s wedge with the bevel exchanged for the envelope (legs running along the faces to the tangency lines, and the envelope between them, every pcurve exact). The boolean then does what it did for the chamfer: the legs melt into the solid’s own faces by same-domain resolution, and the envelope stays as the blend.

Functions

fillet_edge

Round a convex edge of a solid with a constant-radius blend tangent to both of its faces.

fillet_edge_variable

Round a straight convex edge with a blend whose radius runs linearly from start_radius at the edge’s start to end_radius at its end.

fillet_edges

Round a chain of edges in one call, each blend applied in turn and every later edge resolved through the history the earlier blends wrote.