It is very difficult to predict accurately the cutting forces encountered in metal cutting operation due to large number of variable involved, still, some estimate of cutting forces is desirable to enable decide cutting speeds to be within capability of machine tool, to prevent unacceptable deflections, etc.
We will study how the cutting forces are influenced by the following most important variables like cutting speed and cutting fluid, and effective rake.
At high speeds, the forces can be predicted quite accurately due to absence of built up edge. It has been found that cutting fluids at high cutting speed do not influence cutting forces appreciably. However at lower speeds, the cutting forces can vary in an unpredictable manner due to formation of built up edge. The cutting forces can be reduced considerably at slow speeds (below 0.7 m/sec.) by using suitable cutting fluid which inhibits built-up edge.
The cutting force reduces considerably with increase in the effective rake in the positive direction in oblique cutting at normal cutting speeds. This is due to reduction in the shear plane area as the rake increases (which increases shear angle). Thus to reduce the hardness, + ve rake is provided on HSS tools depending on cutting force, and material being cut.
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For harder materials, lower rake is selected (0-5° for hard alloy steels, hard cast iron, 10-15° for medium carbon steels), and for softer materials, larger rake is selected (20- 25° for free cutting steels and 30-40° for aluminium and soft alloys).
Rake angle also affects the rate of flank wear. High rakes, if used on hard materials would produce rapid tool wear. Thus to a larger extent rake angle is decided by considerations of tool life and not by consideration of cutting forces. In the case of tungsten carbide and ceramic tools, + ve rake, though it results in decrease in cutting forces, can’t be tolerated due to inherent brittleness of these materials.
For these, – ve rake is provided but high cutting speeds can be adopted due to higher resistance to thermal softening of these materials. As the cutting speed is increased with ceramic tools having – ve rake angle, the area of shear plane also decreases.
The cutting forces at high speeds are not much affected by increase in rake angle (but considerably affected at lower speed) due to closer approach to adiabatic conditions, (the heat conduction in the chip and workpiece being reduced at high speeds) which result in higher strain rates and higher thermal softening effect of the work material.
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Tool Life Equation:
Although the cutting speed is the most significant variable affecting the tool life, depth and feed also have influence on it.
The tool life equation in such a case can be generalized as:
(d being depth of cut in mm and x = 0.15, and f is tool feed rate in mm/rev and y = 0.6, n ≃ 0.15)
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Criteria for Judging the End of Tool Life:
Usually the tool life is judged by flank wear on tool.
However tool life can also be judged by sudden increase of radial cutting force (increase by ≃ 10 %), surface roughness, bright band on machined surface of steel or dark spot on cast iron, noise, particularly in drilling and by measuring radioactivity of chip.
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Factors Affecting Tool Life:
Tool life is affected by speed, feed rate, depth of cut, rake angle—(there is optimum rake angle for maximum tool life), side cutting edge angle (increasing it increases tool life), nose radius, relief angle, formation of built up edge, materials of tool and workpiece, rigidity of machine tool, coolant, etc.
Factors Responsible for Poor Surface Finish:
Built up edge, chatter/vibrations, inaccuracies in tool movements, defects in structure of workpiece material, discontinuous chips in case of brittle materials, cutting ductile materials at low speed, chip flow marks; etc.