<![CDATA[The CNC Turning machining process is one of the widely used manufacturing processes due to its ability to produce components with high tolerance and precision, one example being the stepped shaft. Surface roughness and Material Removal Rate (MRR) are two important parameters that determine the quality and productivity of the machining process, particularly for S45C steel, which is widely applied in machine components such as gears, shafts, and others. However, the simultaneous optimization of these two responses remains a challenge, as they exhibit conflicting characteristics. This study aimed to determine the optimal process parameters for CNC Turning of S45C steel using a CNMG 120408 insert, and to investigate the effect of process parameters (spindle speed, feed rate, and depth of cut) on MRR and surface roughness. The method used was the Taguchi method with an L9(3³) orthogonal array, combined with Grey Relational Analysis (GRA) to transform the multi-response problem into a single Grey Relational Grade (GRG) value. The results showed that the optimal parameter combination consisted of a spindle speed of 1528 RPM, a feed rate of 0.252 mm/rev, and a depth of cut of 0.15 mm, yielding a confirmation test GRG value of 0.8137, higher than the predicted value of 0.7018. Meanwhile, the most influential machining process parameter was depth of cut, contributing the largest proportion at 36.396%.]]>
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