Comparison of the performance of the internally cooled tool in closed circuit against standard PCBN tools in turning AISI D6 hardened

Abstract

Machining is a widely used manufacturing process that involves removing material from a workpiece. During this process, more than 95 % of the mechanical energy from the machine tool's electrical motor is converted into heat. This can cause the cutting tool's temperature to soar to extremely high levels, reaching up to 1200 °C or even higher, especially when working with hardened steels. To cool down the machining region, cutting fluids are commonly employed in industries. However, these fluids are not considered sustainable because they pose environmental pollution risks, are costly, challenging to dispose of properly, and can also be hazardous to operators' health. Moreover, in some applications, cutting fluids are not recommended. Considering these issues, this research aims to propose and test a more sustainable approach. The method involves using a closed-circuit system where a mixture of water and ethylene glycol circulates internally in polycrystalline cubic boron nitride (PCBN) tools. These tools are employed in the turning operation of AISI D6 hardened steel and are compared with conventional dry machining. The study utilizes a design of experiment (DOE) and analysis of variance (ANOVA) to plan and analyze the results with statistical reliability. Key output variables considered include cutting temperature, tool life, cutting forces, and surface roughness. The implementation of the internally cooled tool (ICT) system has shown significant benefits. It notably reduces the temperature of the tool's rake face, as measured using a thermographic camera. Despite not being the main variable affecting the maximum temperature, the cooling system plays a crucial role in achieving these improvements. The ICT system also substantially increases the tool life in comparison to conventional dry machining across all tested cutting conditions. While the feed rate remains the most influential factor affecting cutting forces due to increased cutting area, the cooling system also plays a significant role. The ICT system, with reduced temperatures, performs better in this regard. As for surface roughness, the condition of the tool edge emerges as the sole significant factor. Surprisingly, the worn tool provides better surface finishing due to the presence of crater wear. With conventional dry machining, higher cutting temperatures lead to increased tool wear, primarily caused by abrasion and attrition.