Investigation of Surface Roughness in Nanofluid MQL-Assisted Hard Milling of Hardened 9CrSi Steel on a CNC Vertical Machining Center

Hard milling is widely used as an alternative to grinding for machining hardened steels while maintaining high surface quality. This study investigates the influence of cutting parameters on surface roughness during CNC hard milling of hardened 9CrSi steel (56–60 HRC) under CuO nanofluid-assisted minimum quantity lubrication (MQL) conditions. Experiments were conducted on a Mazak VCS-530C vertical machining center using a nano-composite coated carbide end mill. A total of 45 experimental runs were performed by varying cutting speed (Vc), feed per tooth (fz), and depth of cut (d). The experimental data were analyzed using the Taguchi signal-to-noise ratio and analysis of variance (ANOVA). The results indicated that feed per tooth was the most influential factor affecting surface roughness, with a contribution of 44.36%, followed by cutting speed (42.22%) and depth of cut (6.74%). The minimum surface roughness of 0.21 μm was achieved at Vc = 150 m/min, fz = 0.02 mm/tooth, and d = 0.10 mm. The findings confirm the effectiveness of CuO nanofluid MQL in improving surface quality during hard milling of hardened 9CrSi steel.