A COMPARATIVE STUDY OF NEW CONCEPTS OF. TOOL LIFE ASSESSMENT

Abstract

Assessment of tool life is made using various criteria of tool failure, such as flank wear width, crater wear depth, total failure, cutting force, dimensional accuracy, surface finish etc. Out of these the conventional criteria of flank wear width and crater wear depth are widely used. These have certain disadvan-tages. Recently new criteria for tool life assessment viz., specific tool wear rate, tool wear weight (volume) and maximum machining ratio have been proposed to overcome some of these dis-advantages. Specific tool wear rate, which is defined as the volume of tool material worn away per unit area of contact per unit length of rubbing distance, has the advantage that it is a dimensionless quantity, applicable to both flank and crater wear and can be found out experimentally without wearing the tool to its failure. Tool wear weight found by direct weighing of tool before and after use or by indirect method using radioisotopes has the advantage that it assesses real damage to the tool. Machining ratio defined as the ratio of volume of material remov-ed to the volume of tool wear, is dimensionless and is based on combined wear of flank, crater, chip notching, primary and secondary grooving. Review of literature reveals that enough work for testing the suitability of these new criteria for tool life assessment under varying cutting conditions has not been done so far. It was therefore, decided to further investigate into these criteria and compare them with the widely used conventional criteria of flank wear width. The work embodied in the thesis presents the results of experimental investigations and their analysis on (1) speCific tool wear rate, (ii) tool wear weight, (iii) machining ratio, (iv) flank wear and (v) crater wear with different cutting condi-tions. Cutting experiments were performed using throw away cemented .carbide inserts and mild steel work material. Measure-ments of flank and crater wear parameters and wear weight were made using a microscope, optical sectioning microscope and a sensitive electronic balance respectively. Response surface methodology was used for designing the experiments. Central composite second order rotatable design was adopted and the levels of the cutting variables were selected as per the desigried plan for 3-x variables. Mathematical models of tool life, specific wear rate, tool wear weight and machining ratio were developed using this statistical technique. The effects of cutting variables on these models were also studied. In flank wear studies, the critical point i.e., the point beyond which rapid growth of wear takes place and cutting tool fails very soon after reaching this point, was experimentally determined. This point was also located by Zore.vf s method. The points were then compared. Using this criterion of total failure, a tool life model has been developed and tested. An empirical relation between specific wear rate and tool life has been verified based on the statistical analysis. specific wear rate model in terms of cutting variables has been developed and tested. l'Uol life based on this criterion has also been compared with the conventional criterion of flank wear width. -iv- The investigations in tool wear weight include the deriva-tion of a theoretical wear model based on flank and crater wear and development of a relationship between actual wear weight 'W' and theoretical wear weight 'WM' using least square technique. A mathematital wear weight model has also been developed and tested using the statistical technique. Tool life based on wear weight criterion has been compered with the flank wear width criterion.