A built-up edge is a phenomenon in which some of the material removed during cutting welds itself (adheres) to the cutting edge.
It takes its name from the way it looks — as if a new edge had formed on the tool.
It is a troublesome phenomenon that degrades machining quality.
A Closer Look
When a built-up edge forms, the shape of the cutting edge changes, degrading the dimensional accuracy and surface roughness of the machined part.
Because the built-up edge repeatedly forms and breaks away, the machined surface characteristically looks unstable and rough.
Built-up edges form most readily with materials such as aluminum and at low cutting speeds (lower edge temperatures).
Countermeasures include raising the cutting speed, cooling the edge with a suitable cutting fluid or high-pressure coolant, and choosing coated tools that resist built-up edge formation.