Fillet

A fillet is a rounding of an internal corner or the junction between two surfaces. It transforms a sharp interior angle into a smooth, curved transition, reducing stress concentration and improving both strength and appearance.

How it works

A sharp internal corner concentrates stress — when a part is loaded, the stress at a sharp corner can be many times higher than in the surrounding material. A fillet distributes this stress over a larger area by replacing the sharp angle with a curve. The radius of the fillet determines how much the stress is reduced: a larger radius means more reduction.

Why fillets matter

• Strength: In castings, forgings, and welded joints, fillets at internal corners dramatically improve fatigue life and load-bearing capacity.
• Manufacturing: In casting, molten metal flows more smoothly into rounded corners than sharp ones, reducing voids and defects. In machining, a fillet radius matches the tool’s natural cutting profile.
• Aesthetics: Rounded inside corners look finished and intentional. Furniture, consumer products, and architectural details all use fillets.
• Cleanliness: Rounded corners are easier to clean — bacteria and grime do not accumulate in them as easily as in sharp corners. This is why food-grade and pharmaceutical equipment specifies fillet radii at internal junctions.

Fillet vs. chamfer

A fillet is a rounded internal corner (concave). A chamfer is a flat, bevelled external edge (typically 45°). Both serve to relieve stress and improve the transition between surfaces, but they have different profiles and are applied to different types of corners.