STUDY OF THE FACTORS CONTRIBUTING 'TO TOOL FAILURE IN INTERRUPTED CUTTING

Abstract

In metal cutting operations, one often comes across situations involving intermittent cuts. Unlike in continuous machining, an intermittent cutting process is characterised by cyclic impact and thermal loading of the tool. The cutting tool subjected to an intermittent cut either experiences mechanical failure due to chipping, wear or fatigue failure due to cyclic thermal and mechanical stresses. The review of literature reveals that thermal and mechanical phenomena play a significant role in determining the tool life of harder materials, such as cemented carbides, ceramics, etc., during intermittent cutting. The work presented in this thesis reports the experimental and analy-tical results pertaining to tool performance and failure in intermittent cutting. In the past, for interrupted cutting a number of analytical models for computing the temperature distribution within the interior of the tool body have been suggested. These are based on the assumption that the tool rake surface is subjected to time dependent heating during cutting and cooling during idling. The laws of heating and cooling assumed for this purpose are not realistic enough and hence the results thus derived are somewhat erroneous. During interrupted turning of slotted bars the tool would enter the workpiece gradually with the undeformed chip thickness increasing from zero to full value over a finite period of time. On this account the build up of temperature at the tool-chip interface would not be. instantaneous. A similar situation would also occur during the exit of the tool fram the workpiece. Thus the rectangular heat flux model as suggested by Wu and Mayer is not a true representation of the actual situation. The author has, therefore, assumed that the heat flux acting over the tool face during the cutting period would have a trapezoidal shape. This implies finite heating rate at the tool face during the tool entry, steady-state cutting followed by finite cooling rate and steady-state cooling during the tool exit. Based on the trapezoidal heat flux model, the temperature distribution and thermal stresses induced in the tool have been computed. A knowledge of the thermal stress distribution within the tool bedy could enable to predict the onset of cracking.