Effects of cutting tool surface tecturing on tribological conditions in machining

Abstract

A large variety of metalworking fluids, such as neat oils, oil-water emulsions, water based synthetic coolants, aerosols, etc., are commonly employed in the metal cutting industry to mitigate the severe tribological conditions typically encountered in machining processes. However, the difficulty in handling these cutting fluids, and the associated economic costs, environmental impacts, and operator exposure to health hazards are major issues of concern. The necessity to reduce the economic, societal and environmental impacts associated with machining gave rise to the concept of sustainable machining. Dry cutting, or cutting without any coolants/lubricants, near-dry or minimal quantity lubrication (MQL) machining, gas assisted machining, and cryogenic machining are some important sustainable alternatives being explored. However, most studies did not consider the surface texture of the cutting tool, which plays a significant role in determining the tribological conditions during machining. From the available literature it is concluded that appropriate texturing of cutting tools’ rake and flank surfaces may potentially be exploited for tribological benefit in machining. In the absence of conventional metalworking fluids surface texturing of cutting tools might help to improve the access of gaseous reactants present in the machining environment to the undersurface of the chip, as well as the tool-workpiece contact zone, which in turn reduces the extent and severity of tool-chip/tool-workpiece contact for overall machining performance improvement, i.e., reduction in cutting forces, decrease in cutting temperatures, consequent improvement in machined surface integrity and tool life, etc. Though surface texturing of the cutting tool has the potential to reduce overall friction in machining inadequate scientific explanation is available for the tribological mechanisms at the tool-chip and tool-workpiece interfaces to enable scientific development of surface textured cutting tools. Therefore, a detailed tribological study is required for understanding the tribomechanisms for surface texture cutting to enhance sustainable machining processes, namely; dry, minimum quantity lubrication (MQL), gas-assisted machining. Hence, the role played by the surface texture of the cutting tool material needs to be scientifically investigated, and this is the focus of the proposed doctoral research work. It is hypothesized that textures on cutting tool surfaces play a significant role in reducing contact intensities at the tool-chip and tool-workpiece interfaces by enhancing the action of gaseous reactants present in the machining environment through provision of appropriate microcapillary networks, and thus eliminate/minimize the need for conventional cutting fluids. In the initial phase of this work, a numerical model is developed to estimate tool-chip friction in dry orthogonal cutting considering both the mechanical effects of the roughness features on the cutting tool’s rake surface as well as the chemical effects of the environment on the interfacial film strength coefficient at each asperity on the tool surface along with merchant’s approach. The model is validated with experimental data for ferrous material i.e., AISI 1045. In the second phase of this study, to observe the influence of surface texture parameters when machining for non-ferrous material, the orthogonal cutting experiments were conducted under dry, gas-assisted and minimum quantity lubrication machining environment. Before conducting the experiments, parallel to cutting edge and perpendicular to cutting edge linear textures were generated on the rake and flank surfaces of the cutting tool via grinding process. Further, these tools were sub categorised on the basis of surface roughness values. These uncoated textured tools were used to perform orthogonal machining on aluminium alloy. i.e., Al6061-T6. In the final phase of this study, the initial developed numerical model was updated using Challen and Oxley approach coupled with Oxley’s machining model for ferrous material. For non-ferrous material, numerical model was proposed using Challen and Oxley approach coupled with Oxley’s machining model and Johnson Cook approach to estimate the coefficient of friction at tool-chip interface. The trends shown by estimated machining forces are validated with the experimentally measured machining forces. In summary, numerical models are developed to estimate tool-chip friction in dry orthogonal cutting for ferrous and non-ferrous materials. Moreover, the influence of rake and flank surface textured cutting tools on machining performance of aluminium alloy Al-6061-T6 is also studied in detail. Further, several avenues of future research are highlighted.