Fillet

Theory

A fillet replaces a sharp edge with a smooth arc of defined radius r. Internal fillets (concave) reduce stress concentrations at interior corners. External fillets (convex rounds) improve aesthetics and ergonomics. In CAD, fillets are applied as a feature after base geometry is established. The fillet radius must not exceed the thickness of adjacent faces, or the feature will fail.

Application

For 3D printing, internal fillets of r≥1mm at base corners dramatically reduce stress cracking and delamination in FDM parts. They also improve layer adhesion by reducing sharp stress risers. External rounds improve surface feel and reduce sharp edges on final parts. Fillets should be the last features added in a parametric model: adding them early causes frequent feature tree failures.

Common mistakes

Fillet radius larger than the smallest adjacent face width causes failure. Applying fillets before other features (holes, pockets) can create geometry conflicts. Mesh-based fillet tools (Blender Bevel modifier) produce approximate results: use CAD solid filleting for precise radius control. Overly large fillets can create near-zero-wall regions that print as single-extrusion walls.

Related terms: Chamfer, Draft Angle, Wall Thickness