Simulated and experimental analysis on serrated chip formation for hard milling process

Ideally, hard milling is a specialized machining process that allows machine shops to mill parts that have a hardness higher than 50HRC. Currently, with the imminent advancement of high-performance cutting tools and high-speed machine tools, hard milling process has become more popular in the fields of manufacturing high precision automotive and molding components. The process itself is ecofriendly, of high-throughput, cost effective and achieves the desired surface quality. However, the process suffers from several shortfalls, namely: the onset of serrated chip ties up with cutting forces, tool-chip interface temperature and surface quality as well as dynamic behavior of cutting system. A focus on serrated chip formation reveals that much experimental work has been carried out in a bid to comprehend the chip formation mechanism and its associated physical phenomena.

A plethora of literature exists where each work depicts a different approach focused at comprehending various shortfalls of the cutting process. Unfortunately, very limited studies have investigated the milling process with focus being the formation mechanism of serrated chip.

Therefore, deep understanding and optimizing of cutting parameters is of great interest; thus, it is imperative to explore the serrated chip formation with respect to physical aspects encountered in milling processes. In this view, a group of researchers from the School of Mechanical Engineering at Shandong University: Binxun Li, Qing Zhang and Luli Li and led by Professor Song Zhang proposed to concurrently simulate by finite element method and experimentally investigate the effects of cutting speed on chip characteristics. Their main objective concerned the serrated chip formation accompanying micro-cracks and physical phenomena which was achieved using finite element method (FEM). Their work is currently published in Journal of Manufacturing Processes.

To begin with, the research team proposed a hard-milling simulation model based on Johnson-Cook visco-plastic constitutive material model combined with Johnson Cook damage accounting for shear localization. Secondly, the researchers validated the proposed FE model using acquired experimental results.

The authors reported that the predicted results were in good agreement with experimental results regrading chip morphology and cutting forces. Further, the physical phenomena governing the formation of serrated segment and micro-cracks were discussed and clarified deeply through FE analysis. Additionally, the localized plastic strain state on the machined surface associated with serrated segment formation was observed and identified.

In summary, the study by Professor Song Zhang and his research group introduced a simplified 2D FE model to investigate the serrated chip formation concerning the occurrence of micro-cracks and variation of physical phenomenon during hard milling of AISI H13 steel. To validate their model, the researchers carried out hard milling experiments. Overall, in an interview with Advances in Engineering, Professor Song Zhang, the lead author emphasized that their work, on one hand could deepen the understanding of the formation mechanism of serrated segment; while still, the results obtained could offer guidance for cutting parameters optimization and desired machined surface integrity control.