Mechanisms for Tool wear

The failure of the tool is taking place in the following modes:

1. Failure through plastic deformation: During machining operation, when the tool tip exceeds the hot hardness temperature of the tool material, the tool loses its hardness considerably and the tool tip will get deformed plastically called plastic deformation failure. This is mainly due to the wrong selection of the process parameters or the wrong selection of the tool material. However, this type of failure is considered an abnormal tool failure.

2. Mechanical breakage failure of the tool: During the machining operation, the tool tip will suddenly chip away, which is called a mechanical failure. This is due to the impact loads on the tool material, excessive plastic deformation, transient thermal stresses, localized cooling, etc. The mechanical impact may be caused by intermittent cutting or due to the presence of blow holes or machining of rigid inclusions, etc. Thermal stress is caused due to sharp temperature gradients within the tool and between the insert and the tool holder. Localized cooling may result from an incorrect application of the cutting fluid. This is also considered as an abnormal failure of the tool.

3. Failure through gradual wear: As the machining continues, the tip of the tool will wear out slowly, and when the wear becomes considerable, the tool can’t perform the machining satisfactorily, which is called gradual wear failure of the tool. The gradual wear failure of the tool is taking place due to

a) Crater wear: The wear on the rake face of a tool, like a crater, is called crater wear. The reason for crater wear is

• The presence of friction at the chip tool interface.
• The abrasive action of microchips or powdered particles at the chip tool interface.
• The abrasive action of fragments of the built-up edge in the chip tool interface.
• Diffusion wear occurs when, due to the atomic attraction between the tool and work, the atoms of the tool material are diffused and deposited over the work.

b) Flank wear: The wear taking on the face is called flank wear. The reasons for flank wear are:

• The presence of friction at the tool-work interface.
• The abrasive action of microchips or powdered particles at the tool work interface.
• Diffusion wear occurs when atoms of the tool material are diffused and deposited over the workpiece due to the atomic attraction between the tool and the workpiece.