Modeling tool wear progression by using mixed effects modeling technique when end-milling AISI 4340 steel

Semi-dry and dry machining is being adopted by the metal cutting industry worldwide to reduce the harmful effects of traditional metal cutting fluids and the cost associated with procurement, use, and disposal of these fluids. This research focuses on end-milling of AISI 4340 steel with multi-layer physical vapor deposition (PVD) coated carbide inserts under semi-dry and dry cutting conditions and proposes a mixed effects model for the analysis of the longitudinal data obtained from a designed experiment. This modeling approach considers unobserved heterogeneity during machining and proposes a tool wear progression model that has a higher power of detecting effects of significant factors than traditional regression models. One such source of variation is work piece hardness that was observed within and across test blocks. From the wear progression model developed, the lowest initial flank wear values are obtained at a cutting speed of 183 m/min, a feed rate of 0.10 mm/rev under semi-dry cutting conditions. A higher rate of wear progression and lower tool life was observed at the higher cutting speed level of 229 m/min. Cutting speed has the most significant effect on flank wear progression in this study. Depth of cut on the other hand did not show any significant effect on tool wear when compared to cutting speed, feed and cutting conditions. The wear mechanism of the insert was analyzed using electron dispersive X-ray (EDX) and environmental scanning electron microscope (ESEM) images. From this analysis, diffusion wear was confirmed under both semi-dry and dry machining conditions. It is expected that the proposed model can reduce the number of repetitions in tool wear modeling experiment for tool manufacturers leading to substantial cost savings.