Fillets in CNC Machining

A fillet is a rounded arc that blends two surfaces together where they would otherwise meet at a sharp edge. The clearest everyday example is the curved transition where a stiffening rib rises out of the base of a machined housing.

Fillets appear in two places: at internal corners, where the arc curves inward, and along external edges, where it rounds off a sharp corner. Fillet size is called out as a radius, so on a drawing dimensioned in millimeters, "R3" means a 3 mm radius.

What Matters in Design and Machining

Internal radii drive tool selection. An end mill is a rotating tool, so wherever two vertical walls meet inside a pocket, the cutter inevitably leaves behind a radius matching its own. Specifying a tighter corner radius forces the shop to switch to a smaller-diameter cutter, and slender cutters are less rigid: they deflect and chatter, and the problem gets worse as pocket depth increases. The usual consequences are more passes, longer cycle times, and greater difficulty holding dimensional accuracy and surface finish.

Not every fillet comes for free. The radius between a pocket wall and its floor, and any rounding along an external edge, will not appear on its own during ordinary milling. Each one calls for a shaped cutter or an extra machining operation. From the machinist's point of view, then, where a fillet sits matters as much as how big it is. Rounding every edge on a part drives up cost; when the goal is simply to eliminate a sharp exposed edge, a chamfer is usually the easier answer.

At a load-bearing internal corner, a well-chosen fillet reduces stress concentration — the sharp buildup of stress within a very small area — which helps the part resist cracking under repeated loading. On components that get handled frequently, rounded external edges also make the part more comfortable to grip. That said, adding a fillet is not an automatic guarantee of strength: whether it helps depends on its size and position relative to the part's overall geometry, the material, and the actual loading conditions.

Details Designers Can Overlook

Give the cutter room to work. Avoid unnecessarily tight radii in deep pockets. Where function allows, specify generous internal corner radii and agree on suitable values with your machinist early. Letting the tool sweep through the corner along a curved path — rather than squeezing into a corner sized exactly to the cutter diameter — noticeably improves cutting conditions.

Check how mating parts fit. An internal fillet can stop a square-cornered component from seating fully. The fix is either to modify the corner of the mating part or to add a corner relief: a small recess cut into the corner that clears the interference. Verify this on the assembled geometry, not on individual parts in isolation.

Standardize radii where you can. Reusing the same fillet sizes across a part reduces tool changes. At the same time, clearly identify function-critical radii and their tolerances on the drawing, and make sure those callouts stay consistent with the CAD model.