Surface integrity remains a crucial factor in ensuring the durability and functionality of machined parts, especially in large-scale production settings. This study investigates, through controlled experiments, the effects of three cooling and lubrication methods—dry cutting, flood cooling, and minimum quantity lubrication (MQL)—in combination with key machining variables on average surface roughness (Ra) during the turning of free-machining SAE 1215 MS steel. Tests were conducted on a Tsugami BO12 CNC lathe, employing a Taguchi L9 orthogonal array to systematically vary three levels of cutting speed (V c ), feed rate (f), and depth of cut (t). Statistical analysis used analysis of variance (ANOVA) and signal-to-noise (S/N) ratios to interpret the results. The ANOVA revealed feed rate as the primary factor influencing surface roughness, accounting for 50.4%–54.5% of the total variation, with cutting speed ranking second (20.5%–23.7%) and depth of cut ranking third (13.8%–17.6%). Cross-method comparisons showed that no single cooling technique was universally superior; MQL was most effective at a moderate speed of 94.20 m/min, but dry cutting delivered the best finish at a higher speed of 113.04 m/min—an unexpected benefit. The best surface finish achieved was 0.713 μm, recorded under dry conditions at 113.04 m/min, with a feed of 0.06 mm/rev and a depth of cut of 1.5 mm. This performance results from a significant interaction between V c and t, exclusive to dry machining, in which a greater depth of cut—usually seen as a disadvantage—becomes advantageous at high speeds, highlighting the complex role of process interactions in optimizing surface quality and underscoring the nuanced role of process interactions in optimizing surface quality.
Pham et al. (2026) studied this question.