L 18.1 Heat in Metal Cutting

A considerable amount of heat is generated in the process of metal cutting the three reasons where heat is developed in the machining process: 

  1. The Primary Shear Zone  

It is the region in which actual plastic deformation of the metal occurs during machining. 

Due to this deformation, heat is generated. A portion of this heat is carried away by the Chip, due to which its temperature is raised. The rest of the heat is retained by the workpiece. It is known as Primary Deformation Zone. 

  1. Tool-Chip Interface 

As the chip slides upwards along the face of the tool friction occurs between their surfaces, due to which heat is generated. A part of this heat is carried by the Chip, which further raises the temperature of the chip, and the rest transferred to the tool and the coolant. This area is known as Secondary Deformation Zone. 

It is reckoned that the amount of heat generated due to friction increases with the increase in cutting speed. However, it is not appreciably affected by the increase in depth of cut. When the feed rate is increased the amount of frictional heat generated is relatively low. But, in that case, the surface finish obtained is inferior. 

  1. Tool-workpiece Interface 

That portion of the tool flank which rubs against the work surface is another source of heat generation due to friction. This heat is also shared by the tool, workpiece and the coolant used. It is more pronounced when the tool is not sufficiently sharp. 

Fig. 18.1 Regions of Heat in Metal Cutting 

  • 80 to 85% of heat is generated in the shear zone 

  • 15 to 20% of heat is generated in the chip tool interface 

  • 1 to 3% heat is generated in the tool-workpiece interface zone 

  • 80% of the heat generated is carried away by the chip 

  • 15 to 20% heat flow into the tool 

  • 5% heat flow into the workpiece 

The temperature at the chip deformation zone will be high enough to influence the tool were and friction during the cutting process. As the temperature increases the hardness of the two decreases and it became more plastic thereby the tool fails by plastic deformation, becomes blunt and lose its geometry.