Fillet

A fillet is a rounded, curved surface that replaces a sharp internal or external edge, specified by a radius. Fillets reduce stress concentration at corners, which matters especially for fatigue-loaded parts, and are often required at internal pocket corners simply because a milling tool cannot produce a sharp internal corner at all.

Why internal and external fillets are not equally simple

An external fillet is usually a straightforward, fast operation; an internal fillet radius, on the other hand, directly constrains which tool diameters can be used in an adjacent pocket or slot, tying the fillet callout to broader tooling decisions.

Where it appears on the drawing

As an explicit radius callout on a specific edge, or implied as the minimum radius a pocket or slot's internal corners must carry given standard tooling.

Common mistakes

Specifying an internal fillet radius smaller than any practical tool can produce in that pocket's depth, which forces either a design change or a much slower, more fragile tooling strategy.

Is a fillet the same as a radius?

In casual use yes; "fillet" specifically refers to the rounded surface, while "radius" is the dimension describing its size.

Why do internal pocket corners always have some fillet?

Because a rotating end mill is round, so it cannot cut a perfectly sharp internal corner; some minimum radius, matching the tool, is unavoidable.

Does a fillet always help reduce stress?

Generally yes for external corners; for internal corners it's often driven by tooling reality first, with stress reduction as a secondary benefit.