Tool Life Distributions—Part 4: Minor Phases in Work Material and Multiple-Injury Tool Failure

Abstract

Distributed tool life under production machining conditions results in the need for unplanned tool changes. In the case of large volume or automated production systems, such production interruptions invariably lead to higher manufacturing costs. When the distribution in tool life is known, logical operating strategies can be devised to minimize the costs associated with unforeseen production interruptions. To facilitate this, analytical models for tool life have been developed and presented in the first two parts of this paper. These stochastic models portray tool failure as resulting from injuries due to damage producing encounters in the course of machining. In Part 3 of this paper, a physically consistent model for damage producing encounters which result in tool fracture has been identified and validated for single-injury tool failure. The case of multiple-injury failure is considered here with emphasis on the tool life scatter due to the variations in minor phase content of the work material (nonsulphide, nonmetallic inclusion content). The role and significance of the oxygen-rich nonmetallics to tool wear and machinability in unalloyed carbon steels is examined. It is shown that given a steel, the chemistry and volume fraction of oxygen-rich nonmetallics in it may well determine the tool life (machinability) and tool life scatter. If this be the case, details of the steel making process can be varied to limit and control the detrimental effects of the oxygen-rich, nonmetallic phases to the tool life. Some such techniques that allow machinability enhancement by steel making process modifications are discussed to illustrate the validity of the concepts postulated here. The analysis suggests that the tool life (or machinability) can be improved by limiting the frequency of damaging encounters. But since the minor phase is dispersed and the encounters are stochastic, the tool life improvement will have to be accompanied by an increase in scatter in agreement with previously reported results.