ENHANCEMENT OF TOOL-LIFE OF HARD TURNING PROCESS VIA CRYOGENIC COOLANT AND MICROPATTERNED INSERT

Abstract

High hardness and high strength of materials are essential for advanced engineering applications to guarantee the safety and durability of the manufactured products. The hard turning process enables the machining of hardened materials which makes it one of the most widely used operation in automobile and heavy machinery industries. However, the problem with the hard turning is extreme machining conditions such as high cutting force and temperature generation leading to accelerated tool wear. Cubic boron nitride (CBN) is the essential insert of the hard turning operations. However, due to the higher production cost of CBN cutting tool, its performance should be maximized to achieve the higher machining and economic efficiency. The cryogenic liquids were mainly applied to machining of difficult-to-cut materials. The cryogenic liquids such as liquid nitrogen and carbon dioxide have been used as the cutting fluids to remove the heat generated in the cutting zone. Furthermore, some researchers have found that the textured (or patterned) functional surfaces can improve the frictional and tribological conditions between the two contacting surfaces. Hence, a proper development of the textured surfaces on the cutting tool can enhance the machining performances in the hard turning process. This dissertation presents a framework for the development of a numerical model and experimental investigations for enhancing the performance of the hard turning process using a cryogenic coolant and micropatterned tool. The first of this research presents the developed cutting model based on the modified Oxley’s theory. The cooling effects of cryogenic coolant were included in the model by implementing the proper heat transfer coefficient. Thermal effects generated in the primary and secondary zones were also modeled using the moving heat source technique. The model provides the predictions of cutting force, temperatures and tool wear, which were validated by experimental works. It was found that the use of LN2 coolant can reduce the effect of thermal softening in secondary deformation zone, which in turn increases the cutting forces. However, the cryogenic cooling mainly contributed the decrease in the diffusive and abrasive wear mechanisms to the improvement of tool life. The tool wear of CBN cutting tool was reduced by 20~34% in cryogenic cooling condition as compared to dry condition. Furthermore, finite element method (FEM) simulations were used to analyze the various pattern geometries and dimensions of micropatterned inserts. The simulation results showed that the micopatterned tool can decrease the stress distribution, force, and friction in the tool-chip interfaces. When the perpendicular and parallel type micropatterned tool having 100 𝜇m edge distance, 100 𝜇m pitch size and 50 𝜇m in height was used, the force and the friction was reduced by 6% and 25%, respectively. The experiments were conducted using micropatterned tool for the hard turning process and the variations in the chip morphology, cutting force, friction, and tool wear were analyzed. The results showed that the micropatterned tool was able to reduce the forces by 18.7%, friction by 34.3% and tool wear by 11.4% within the given ranges of experimental conditions. The model and the experimental findings of this research can be beneficial for the enhancing efficiency of the hard turning processes used in different industries. The techniques developed in this work can be further extended to see its applicability in other cutting operations such as drilling and milling.