Mechanical Load Identification for Spontaneous Tool Failure Monitoring

Sammanfattning: The problem of spontaneous cutting tool failure, such as tool chipping and breakage, is becoming more and more important in the manufacturing technology. The sponataneous tool failure is usually associated with the stresses subjected on the tool during a metal cutting process. When the maximum stress acting in a cutting tool exceeds its critical stress, which is usually determined by tool geometry and tool material, the spontaneous failure will occur. Therefore, identification of the state of stress in a cutting tool is crucial both for monitoring the spontaneous tool failure and for predicting the risk of the tool failure in a cutting process. This thesis presents a method for the identification of maximum mechanical stresses acting on a cutting tool, identification of the relavent cutting load parameters, and analysis of failure probability of the cutting tool in continuous and intermittent cutting. The method for the identification of maximum mechanical stresses in a cutting tool consists of four steps: estimation of the contact load on the tool faces, calculation of the maximum related stresses with the FEM, modelling of maximum related stresses with an artificial neural network and, finally, identification of the maximum principal stress, and maximum effective stress, with the use of measured cutting forces or cutting parameters. The method for identification of load parameters is mainly based on the equivalent chip thickness and the cutting force model. As an example of this method, the development of a model for the identification of the maximum mechanical stresses acting on a DNMA150608 type insert has also been reported in this thesis. In addition, cutting experiments, under both continuous and intermittent cutting conditions have been conducted in this study, intending to identify the maximum mechanical stresses which may cause spontaneous tool failure under practical cutting conditions. In continuous cut, the strength of the cutting tool is mainly determined by its maximum effective stress. At low or medium feed in the steady state of continuous cut the dominant form of tool deterioration is tool wear rather than spontaneous failure. In intermittent cutting, however, the probability of spontaneous tool failure is much higher than in the continuous cutting. Tool chipping and breakage occur in intermittent cutting at feed rates considerably smaller than in continuous cutting. The high probability of spontaneous tool failure in intermittent cut is primarily attributed to the high principal stress induced in the entry and exit phases of the cut. In the entry phase of the cut, an unbalanced load in the tool face, due to a steep rise in the primary cutting force and small contact area between chip and tool face, will induce peak principal stress on the rake face which is responsible for tool breakage. The probability of spontaneous tool failure conforms with the Weibull distribution. The risk of spontaneous tool failure can be predicted with an known distribution function.