Thermal aspects of machining

THERMAL ASPECTS OF MACHINING

In the metal cutting process, the energy dissipated at the cutting edge is converted into heat. This heat influences a tool wear on cutting tools and it develops friction between cutting edge of the tool and chip interface. The continuous heat accumulation takes place on tools and it leads to plastic deformation on cutting edges. If the elastic deformation occurs during machining, the stored energy will be released when the machining process gets completed and there will be no heat generation at the tool-chip interface. But, the accumulation of heat is continuous. So, the plastic deformation takes place and it leads to convert the whole energy into heat. Finally, the energy used for machining is stored in the material or workpiece as strain energy. Therefore, the heat is generated in three regions such as shear zone, tool-work interface region and tool-work interface region.

(i) Shear zone:

The zone which is affected by the energy required to shear the chip or to separate the chip and work is called shear zone. So, the energy required to shear the chip is the source of heat. Nearly, 80-85% of heat is generated in this region.

(ii) Chip-tool interface region:

In this region, the energy required to overcome the friction completely is the source of heat. Here, some plastic deformation also takes place. The heat generation is in the range of 15-20%.

(iii) Tool-work interface region:

In this region, the energy required to overcome the rubbing friction between flank face of the tool and workpiece is the source of heat. In this region, the heat generation is in the range of 1-3%.

Various regions of heat generation in the metal cutting process such as shear zone, tool- work interface region and tool-work interface region are illustrated Figure 1.25. The above mentioned three zones lead to increase the temperature at the chip-tool interface. It affects the life of tool by forming the crater on the tool face. Finally, the tool damages due to softening and thermal stresses.

The tool temperature increases due to the following factors such as

(i) Cutting speed

(ii) Feed

(iii) Properties of tool materials etc.