Introduction.

According to the first law of thermodynamics, when work is transformed into heat, the quantity of heat produced is equivalent to the quantity of work. Heat generated, through conversion of mechanical energy. Three distinct sources of heat in metal cutting are given below:

1] The shear zone, 1, where the primary plastic or shear deformation takes place.

2] The chip tool interface, 2, where secondary plastic deformation due to friction between the heated chip and tool takes place.

3] The work-tool interface, 3, at flanks where frictional rubbing occurs.

For example, in a typical study of machining mild steel at 30 m/min at about 750 deg of cutting temperature at tool-chip interface, the distribution of tool energy developed at the shear zone is as follows:

Energy at chip – 60 percent.

Energy to work piece – 30 percent.

Energy to tool – 10 percent.

The rate of energy consumption during orthogonal cutting is given by

$W_c = F_c V_c$

Where $F_c$ = Cutting force, N

$V_c$ = Cutting speed, m/min

When a material is deformed clastically, the energy used is stored in the material as strain energy and no heat is generated. However, when a material is deformed plastically almost all the energy used is converted into heat. In metal cutting, the material is subjected to extremely high strains and elastic deformation forms a very small proportion of the total deformation. Hence all the energy is assumed to be converted into heat. Thus,

$Q = F_c V_c / J$

where J is mechanical equivalent of heat.

The cutting energy is converted into heat in two principal regions of plastic deformation.

• The shear zone or primary deformation zone AB.

• Secondary deformation zone BC.

If, as is common in most practical situation, the cutting tool is not perfectly sharp, a third heat source BD would be present due to friction between the tool and the newly machined surface. However, unless the tool is severely wom, the heat generated at this source will be small and hence could be neglected.

The temperature distribution in the workplace, in this instance the chip zone, as seen in typical experimental study, is given in figure 2. As point X in the material moves towards the cutting tool. It approaches and passes through the primary deformation zone, and is heated till it leaves the zone, being carried away within the chip. However, point Y passes through both deformation zones and continues to get heated till leaves the region of secondary deformation. It is then cooled as the heat is conducted into the body of chip, and eventually the chip achieves a uniform temperature throughout. The maximum temperature thus occurs along the tool face some distance from the cutting edge. The point Z, that remains in the work piece, is heated as it passes below the tool cutting edge, by conduction of heat from the primary deformation zone. Some heat is removed from the secondary deformation zone by conduction into the body of the tool.

The heat generated is shared by the chip, cutting tool and the blank. The apportionment of sharing that heat depends upon the configuration, size and thermal conductivity of the tool – work material and the cutting condition. Figure 3. Shows that, maximum amount of heat is carried away by the flowing chip. From 10 to 20% of the total heat goes into the tool and some heat is absorbed in the blank.