Fillet: A fillet is a rounded transition between two faces of a part, used to remove sharp corners, reduce stress concentration and make a part easier to machine, mould or print.
A fillet replaces a sharp corner with a curve of a set radius. On an inside corner it adds material; on an outside corner (where it is sometimes called a round) it removes material. A sloped flat cut across a corner instead of a curve is a chamfer.
Fillets matter to manufacturing in different ways for each process:
- CNC milling: an end mill is round, so it always leaves a radius in an inside vertical corner. A design with sharp inside corners either cannot be milled as drawn or needs EDM or extra operations. A corner radius slightly larger than the cutter lets it turn the corner smoothly.
- Moulding: generous fillets help plastic flow and reduce stress and sink marks.
- 3D printing: fillets at the base of tall features reduce stress and cracking.
Example: a pocket drawn with a 3 mm inside corner radius can be cut with a 6 mm cutter in one pass. Draw it with a 1 mm radius and the shop needs a 2 mm cutter, which cuts much more slowly, so machining time and the price rise.
A frequent confusion is assuming that sharp inside corners are “free” because CAD draws them easily. On a milled part, they are usually the opposite. Read design for CNC machining for typical radius rules, or see draft angle for the related moulding rule.
Go deeper: Design for CNC machining: the rules that cut cost