<![CDATA[The surface quality of lathe-machined workpieces is primarily governed by three cutting parameters: spindle speed, feed rate, and depth of cut, as the resulting surface roughness directly determines the functional quality of the machined component. Empirical observations indicate that achieving a favourable surface roughness in the turning process depends on the appropriate selection of cutting tool parameters, since the cutting tool constitutes one of the principal elements of the turning system alongside the machine tool and the workpiece itself, and exerts a significant influence on the resulting surface finish. This study aims to determine the surface roughness values and roughness grades of St.42 steel under varying spindle speeds and depths of cut during the turning process. Experiments were performed using a PL-1000 G lathe machine with ST 42 steel specimens, and surface roughness was measured using a Mitutoyo SJ-310 roughness tester. The results indicate that the optimum surface roughness value (Ra) at a spindle speed of 1020 rpm and a depth of cut of 0.2 mm was 3.573 µm, corresponding to roughness grade N8. At a spindle speed of 1800 rpm and a depth of cut of 0.4 mm, the optimum Ra value was 1.678 µm, corresponding to roughness grade N7. Meanwhile, at a spindle speed of 550 rpm and a depth of cut of 0.2 mm, the Ra value obtained was 3.925 µm, corresponding to roughness grade N8. Overall, the average surface roughness values obtained, in terms of Ra and roughness grade (N7 and N8), indicate that the machined surfaces fall within the fine-finish category for the turning process.]]>
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