What is a built-up edge?

A built-up edge is a layer of workpiece material that adheres to the cutting edge and rake face under pressure and heat, and work-hardens there. The layer takes over part of the cutting and changes the effective geometry of the edge. It grows, breaks off periodically and forms again; fragments stay on the workpiece or, as they break away, take a small piece of the cutting edge with them.

What does a built-up edge depend on?

A built-up edge forms above all with ductile materials that adhere easily, such as soft aluminium, low-carbon steel, austenitic stainless steel and copper, and at low to moderate cutting speeds. There the temperature at the edge is too low for the adhered layer to soften and be carried away with the chip. Above that range the effect usually disappears. A sharp edge with a positive rake angle, a polished rake face, a coating with little affinity for the workpiece material and good cooling or lubrication reduce the risk. Which cutting speed is enough depends on material and tool; the tool supplier gives a working range for it.

How does a built-up edge work in practice?

A built-up edge can add cost in three ways: a rough, torn surface that can call for an extra finishing pass or operation, tools that chip sooner, and dimensions that drift during the batch. At small diameters and low spindle speeds, as in drilling, tapping and turning near the centre, the cutting speed is low and the effect forms more easily. Taken into account in the choice of tool and cutting data, it can usually be prevented; the effect then sits in the cutting data and so in the machining time.

What is a built-up edge often confused with?

A built-up edge is not ordinary wear: wear removes material from the tool, a built-up edge adds material to it, although its breaking away can cause adhesive wear and chipping. Nor is it a burr: a burr sits on the workpiece, the built-up edge on the cutting edge.