SECTION 2.5
The Built-Up Edge

Another phenomenon we must consider is the built-up edge. As the cutting tool continues to cut into the work piece, the tool face and flank are chemically cleaned as is the metal which is being rubbed against these surfaces. Investigations have shown that freshly machined surfaces brought into rubbing contact with each other, in the absence of any contaminant, undergo metal to metal seizure at the point of actual contact.

In most cases, it is virtually impossible to prevent some amount of seizure between the chip and the tool face. Unless surfaces are perfectly flat, contact is made along the high spots over only a fraction of the total area. As the chip passes over the tool face, cutting forces give rise to extremely high unit pressures, sufficient to form pressure welds. If these welds are stronger than the ultimate shear strength of the material, that portion of the chip which is welded to the tool shears off as the chip is displaced and becomes what is called a built-up edge.

The built-up edge is a transient wedge-shaped mass of metal which is usually harder than the work piece itself. Actual hardness varies with the work-hardening characteristics of the metal being machined. A built-up edge can be helpful or detrimental to the tool. If the built-up edge is large and flat along the tool face, it decreases the effective face angle which makes it necessary to exert more power to cut the metal and, therefore, causes more heat to be generated. If, on the other hand, the built-up edge is small and sharp, it increases the effective face angle and lowers the required force, generating less heat. In addition, the built-up edge helps protect the cutting edge of the tool. The tool merely supplies the foundation for the false cutting edge which is actually doing the work.

It is interesting to note that the built-up edge provides the reason why some coolants work better than others. The correct coolant introduces the correct contaminant to prevent the clean metal to metal contact. One of the functions of the cutting fluid is to optimize the amount of this built-up edge for a given face angle.