Analysis of droplet characteristics and cooling lubrication effects in MQL milling of 316L stainless steel

The infiltration and cooling effect of lubricant droplets in the cutting zone largely depend on setting minimum quantity lubricant (MQL) system parameters appropriately. The study determines the optimal spray angle and spray distance relying on the geometric position relationship between the nozzle and the overall cutting zone. A numerical model that combines computational fluid dynamics (CFD) and discrete phase model (DPM) is presented to simulate droplet transport process for dynamic milling and one-tooth milling by flow field distribution and droplet characteristics. The influences of different spray distances and spray angles on the infiltration process and cutting tool temperature are analyzed. The experiments aimed at verifying the results of the analysis are conducted by measuring cutting force, cutting temperature, surface roughness, surface microtopography and tool wear. The result indicates that the spray distance has a significant impact on the jet divergence, and insufficient lubrication or excessive lubrication is effectively avoided with the optimal cutting distance. Spraying in the direction of cutting-out promotes the accessibility of the lubricant and greater concentration of droplets in the cutting zone, especially later in the cutting process, which results in increased infiltration, reduced friction, and improved heat dissipation. Moreover, experiments demonstate that the optimal state of jetting in the direction of cutting-out is 46 °C lower than that of dry cutting, the surface roughness is reduced by 19.8%, and the wear amount is reduced by roughly 34%. This study has a better understanding of the setting of spray distance and spray angle in MQL milling and the development of equipment, which provides theoretical guidance for the optimization of MQL system parameters in practical applications.