Considerable heat is generated at the cutting edge of the tool due to friction between tool and work, and the plastic shearing of metal in the form of chips, when the tool is machining metal on a machine tool. The heat is evolved at three zones. A, B and C shown in Fig. 22.27.
In zone A (shear zone), maximum heat is generated because of the plastic deformation of metal, and practically all of this heat is carried away by the chip as machining is rapid and continuous process. A very minor portion of this heat (5-10%) is conducted to workpiece.
In zone B, known as friction zone, the heat is generated mainly due to friction between moving chip and tool face and partly due to secondary deformation of the built up edge. In zone C, known as work-tool contact zone, the heat is generated due to burnishing friction and the heat in this zone goes on increasing with time as the wear land on the tool develops and goes on increasing.
The direction of maximum heat flow from these zones to chip of workpiece is indicated by the arrows in Fig. 22.27, of course, some heat always flows in other directions also.
It will be noted that each of these three zones leads to rise of temperature at the tool chip interface and it is found that the maximum temperature occurs slightly away from the cutting edge, and not at the cutting edge. This temperature plays a major role in the formation of crater on the tool face and leads to failure of tool by softening and thermal stresses.
The temperature at the chip-tool interface can be determined either by tool work thermocouple or by calorimetric set up.
The ratio (Qc/Q) is indication of percentage of heat that can be dissipated by the chips. This ratio is fairly independent of cutting speed except at very low speed (of the order of 400°C) but the temperature of the tool rake face or tool-chip interface increases with increase in cutting speed and is about twice the temperature of the chips.
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The distribution of heat in chips, tool and work versus cutting speed is shown in Fig. 22.28 and it is found that distribution of heat in chips, workpiece and tool is in the ratio of 80: 10: 10, when cutting with carbide cutters at speeds above 30 m.p.m.
The various factors which lead to maximum tool temperature are cutting speed, feed, properties of material etc. These machining variables affect the size of shear zone and chip tool contact length and thereby, the area over which heat is distributed. Shorter length of contact of chip with tool results in severe temperature rise.
Cutting temperature depends upon several factors like workpiece and tool material, cutting conditions, cutting fluid and tool geometry. If a material has high tensile strength and hardness, more energy is required for chip formation and more heat is generated. If thermal conductivity is high then temperature developed will be lower. Cutting temperature is also dependent on cutting condition (in order of seriousness), cutting speed, feed, depth of cut.
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At very high speeds the cutting fluid is not able to reach tool-chip interface and as such cutting fluid does not affect the chip-tool interface temperature. The speed at which the cutting fluid becomes ineffective decreases as the depth of cut is increased. Temperature variation of 20°C only has been noted for rake angle change from – 10° to + 30°. It increases with increase in approach angle and radius of tool.
Factors affecting Temperature:
The various factors influencing cutting temperature are:
(i) Workpiece and Tool Material:
Tensile strength and hardness of workpiece material have considerable influence on cutting temperature. Materials with higher thermal conductivity produce lower temperature than tools with lower conductivity.
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(ii) Cutting Conditions:
The cutting speed has predominant effect on the cutting temperature. Feed has little effect, and depth of cut the least.
(iii) Cutting Fluid:
At high speeds, such as employed for carbides, cutting fluid has negligible effect on tool-chip interface temperature. The fluid is carried away by the outward flowing chip more rapidly than it could be forced between the tool and the chip.
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(iv) Tool Geometry:
While rake angle has only a slight influence on the temperature, it increases considerably with increase in approach angle.