Based on the discussion given above, it is found that the form of wear of CBN tool in cutting hardened AISI 4340 alloy steels varies with cutting speed. Its wear mechanisms can be summarized as follows:
(1) When the cutting temperature is comparatively low (i.e. at low speed cutting), there are high cutting forces because the degree of the softening of work material is not significant. Hence, the tool face would be scratched by the hard carbide particles of work material and the binder of the CBN is removed. The main type of wear is abrasion, and there are scratched marks on tool face.
(2) The softening of work material becomes significant as cut- ting speed is increased. This, in turn, causes cutting forces to reduce. Abrasion is not important any more. The diffusion and oxidation would be more pronounced due to higher cutting temperature. A layer with oxidized com- pounds exhibiting a lower melting point is generated. This layer can protect cutting tool from wear. Hence, tool wear is reduced and tool life is prolonged.
(3) When there is very high cutting temperature, the sticking layer is worn away due to a great friction force. In addition, the bond between the CBN particles of the cutting tool is weakened owing to severe diffusion of work and tool mate- rials. Hence, CBN particles are plucked out of the tool face. Subsequently, cutting edge is weakened, and flank wear is increased.
Based on the discussion given above, it is found that the form of wear of CBN tool in cutting hardened AISI 4340 alloy steels varies with cutting speed. Its wear mechanisms can be summarized as follows: (1) When the cutting temperature is comparatively low (i.e. at low speed cutting), there are high cutting forces because the degree of the softening of work material is not significant. Hence, the tool face would be scratched by the hard carbide particles of work material and the binder of the CBN is removed. The main type of wear is abrasion, and there are scratched marks on tool face. (2) The softening of work material becomes significant as cut- ting speed is increased. This, in turn, causes cutting forces to reduce. Abrasion is not important any more. The diffusion and oxidation would be more pronounced due to higher cutting temperature. A layer with oxidized com- pounds exhibiting a lower melting point is generated. This layer can protect cutting tool from wear. Hence, tool wear is reduced and tool life is prolonged. (3) When there is very high cutting temperature, the sticking layer is worn away due to a great friction force. In addition, the bond between the CBN particles of the cutting tool is weakened owing to severe diffusion of work and tool mate- rials. Hence, CBN particles are plucked out of the tool face. Subsequently, cutting edge is weakened, and flank wear is increased.
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