Fillet

A fillet is a rounded transition between two intersecting surfaces or edges, defined by a radius R. In CNC machining, an internal fillet is concave and an external fillet or round is convex. Fillets replace sharp corners to reduce stress, improve edge safety, and create smoother load transfer. Drawings specify them with radius callouts such as R0.5 or R1.

On the shop floor, fillets are normally generated during the milling pass rather than as a separate finishing step. When a pocket calls out R0.5, the programmed cutter radius decides whether that geometry is reachable. If the specified radius is smaller than the effective cutter radius, the corner will not clean out, forcing a tool change, smaller cutter, or redesign. External fillets and edge rounds are more forgiving because the tool approaches from an open side. In millwork, the same rounded transition appears as a smooth corner break that improves handling and reduces chip-prone edges. Drawings specify fillets with radius callouts such as R1 or R2, and CAM generates the toolpath directly from that callout. The limiting factor is almost always accessibility: deep internal corners constrain tool diameter, while external edges can be blended with a corner-rounding tool or ball end mill.

• ⚠️Tool-radius mismatch on internal corners: A pocket that specifies a tight internal fillet cannot be cut when the programmed cutter radius is too large, so material remains in the corner and mating parts fail to seat.
• ⚠️Fillet misread as chamfer: Confusing a curved radius with a flat bevel leads to the wrong edge break, causing assembly interference, uneven appearance, and a dimensionally invalid feature on the machined part.
• ⚠️Too-small radius on a critical corner: A tight fillet that barely rounds the edge does not meaningfully reduce stress concentration, so cyclic loading can still crack, chip, or fatigue the transition.