The failure of the tool is taking place in the following modes:
1. Failure through plastic deformation: During machining operation, when the temperature of the tip of the tool is greater than the hot hardness temperature of the tool material, the tool loses its hardness considerably and the tooltip will get deformed plastically called as plastic deformation failure. This is mainly due to the wrong selection of the process parameters or the wrong selection of the tool material. But this type of failure is considered as an abnormal failure of the tool.
2. Mechanical breakage failure of the tool: During machining operation suddenly the tool tip will get chipping away called as a mechanical failure of the tool. This is due to the impact loads acting on to the tool material, excessive plastic deformation, transient thermal stresses and localized cooling etc. The mechanical impact may be caused by intermittent cutting or due to the presence of blow holes or machining of hard inclusions etc. thermal stress are 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 be wearing out slowly and when the wear becomes considerable, the tool can’t perform the machining satisfactorily called gradual wear failure of the tool. The gradual wear failure of the tool is taking place due to
a) Crater wear: The wear taking place on the rake face of a tool like a crater called the crater wear. The reason for crater wear are
- The presence of friction at the chip tool interface.
- the abrasive action of microchips or powdered particles present at the chip tool interface.
- The abrasive action of fragments of built up edge present in the chip tool interface.
- The diffusion wear, due to the atomic attraction between the tool and work the atoms of the tool material will get diffused and deposited over the work called as diffusion wear.
b) Flank wear: The wear taking on the flank 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 present at the tool work interface.
- The diffusion wear, due to the atomic attraction between the tool and work the atoms of the tool material will get diffused and deposited over the workpiece called as diffusion wear.
