Surface roughness of ST 40 steel under spindle-speed variation in CNC XTRA-420 turning

Abstract

This study analyzed the surface-roughness trend of ST 40 steel at three spindle speeds during CNC XTRA-420 turning. Nine specimens were distributed equally among spindle speeds of 1,200, 1,400, and 1,600 rpm, with three replicates at each level. The remaining machining parameters followed the experimental matrix: a DNMG insert with a 0.8 mm nose radius, a nominal feed of 0.10 mm/rev, a 1 mm depth of cut, and water-based coolant. Surface roughness was expressed as arithmetic average roughness (Ra) and measured using a surface roughness tester. The mean Ra values at 1,200, 1,400, and 1,600 rpm were 2.426, 2.397, and 2.171 µm, respectively. Increasing spindle speed from 1,200 to 1,600 rpm reduced the mean Ra by 10.5%. However, one-way ANOVA did not show a statistically significant difference among the three groups, F(2,6)=1.648 and p=0.269. Therefore, 1,600 rpm produced the lowest observed mean within the tested range, but the result should be interpreted as a descriptive trend rather than conclusive evidence of an effect. Further work should increase replication and measure tool condition, vibration, cutting force, and cutting temperature