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FACTORS EFFECTING TOOL LIFE:The life of tool is affected by the following factors: 1. Cutting speed. 2. Feed and depth of cut. 3. Tool geometry. 4. Tool material. 5. Work material. 6. Nature of cutting. 7. Rigidity of machine tool and work. 8. Use of cutting fluids. EFFECT OF CUTTING SPEED:-

Out of the above factors, the maximum effect on the tool life is of cutting speed. The tool life varies as the cutting speed, i.e., the higher the cutting speed the smaller the tool life.Genrally, the reduction in the tool life corresponding to an increase in cutting speed is parabolic, as shown in fig 5.30.Based on the pioneer work of F.W. Taylor , the relationship between cutting speed and the tool life can be expressed as: VTn = C V = cutting speed (m/min.) T = Tool life (minutes) n = An exponent whose value largely depends upon the material of the tool , called tool life index. Upto a certain limit ,its value Is also influenced by some other variables like tool material, cutting conditions etc. C = A constant called machining constant, which is numerically equal to the cutting speed in meters per min. that would give a tool life of one minute.

For all the practical purposes the average values of exponent “n” for common tool materials can be taken as: n = 0.1 to 0.15 for high speed steel tools. = 0.2 to 0.5 for cemented carbide tools = 0.6 to 1.0 for ceramic tools. For assessing “t” the tools are operated at different cutting speeds to failure and the results recorded. If the relation ship between different cutting speeds and the corresponding tool life is plotted on a log-log graph, st. lines are obtained as shown in fig 5.31, which reveal the tool life decrease with increase in cutting speed.

FEED AND DEPTH OF CUT: Feed rate and the depth of cut are the other important cutting variables which also affect the tool life appreciably an increase in the feed rate and the depth of cut has a similar effect, .i.e. reduction in the tool life, as experienced by the tool when cutting speed is increased. However, the effect on not so severe as produced in the latter case. The different variables, cutting speed, tool life, feed rate and depth of cut, are inter-related as given in the following formula:

V= Cutting speed in m/min T= Tool life in minutes f = Feed rate in mm/min t = Depth of cut in mm. For a given tool life the relationship among other variables As given in the following formula:

C = A constant. The exponents “a” & “b” depend upon the mechanical property of the work piece material. From the above expression cutting speed “V” will decrease if “f” & “t” will increase. TOOL GEOMETERY: Many geometrical parameters (tool angles) of a cutting tool influence its performance and life. For, the RAKE ANGLE, if it is increased in a positive direction the cutting force and the amount of heat generated is reduced. Obviously, this should help increase the life of cutting tool. But, if it is very large the cutting edge is weakened and also its capacity to conduct heat is reduced .Thus for a large no. of increase in rake angle reduction in mechanical strength of tool & hence reduces tool life. For RELIEF ANGLES or CLEARANCE ANGLE as they help to prevent rubbing of tool flank against the machined work surface. As they help in lowering the amount of heat generated and, therefore, increasing in tool life. For CUTTING EDGE ANGLES ,the FRONT OR END CUTTING EDGE ANGLES effects the tool wear up to certain optimum value an increase in this angle permits the use of higher speeds without an adverse effect on tool life but after this it reduce the tool life gradually and a larger angle increase the tool life. TOOL MATERIAL: Tool life also depends upon the nature of tool material .We therefore conclude that the higher the hot hardness and toughness in the tool material the longer the tool life. WORK MATERIAL: The microstructure of the work material plays a significant role because it directly affects the hardness of the work material. The increase in cutting temperature and power consumption vary directly as the hardness of work piece material, so higher the hardness of the work piece greater will be the tool wear and so shorter will be the tool life. Adverse effects on tool life are also experienced in machining of the pure metals because of the tendency to stick to the tool face, especially at higher temp. This results in more friction and a high amount of wear on the tool and therefore shorter tool life. NATURE OF CUTTING: Tool life is also affected by nature of cutting, i.e., whether it is continuous or intermittent. In the latter case the tool is subjected to repeated impact loading and may give way much earlier than expected until it is made substantially strong and tough .In continuous cutting m a similar tool will have a relatively longer life.