Due to continuous use, the tool starts giving unsatisfactory results. This involves poor surface finish, high cutting forces, increased power requirements and loss of dimensional accuracy. This happens due to tool wear. The tool will not give satisfactory results until it is changed or reground. The reasons for tool wear are
High temperature between tool and work piece
High friction
Hardness of cutting tool
Cutting speed and feed
Cutting fluid
The tool wears are of following types
Flank wear – It is caused due to abrasion. It depends upon the hardness of contacting surfaces. It occurs at high cutting speed and low feed. It occurs between newly generated surface and tool flank
Crater wear – The edges are weakened due to excessive wear and high temperature in tool chip interface. This occurs due to friction between the rake surface and chip flowing over it. The crater occurs on the rake face.
Plastic deformation – Due to high temperature and pressure, plastic deformation of edge or flank impression leads to poor chip control.
Built up edge (BUE) – The cutting edge is welded to the work piece material due to low cutting speed and low feed. It can also occur due to negative cutting geometry. It is a time dependent mechanism which disappears at high cutting speed.