## 3 Results and Discussion  
Table 2 presents the measured surface roughness values of the test pieces machined using different parameters, including variable and constant factors. The surface roughness was measured using a surface roughness measuring device. Based on the data in the table, graphs were drawn. Upon examining the values in Table 2, it was observed that the surface roughness increased with an increase in the feed. This finding has practical implications for manufacturing processes, as it suggests that controlling the feed can help maintain surface quality. The surface roughness values decreased when the cutting-edge angle (CEA) of the cutting tools grew. The numbers given in the symbols in which the parameters are given in Table 2 (such as 1, 2, and 3, respectively) explain the experiments performed when they matched similar number parameters. When the surface roughness is examined according to the spindle speeds, it is seen that the surface roughness gets lower values with increasing spindle speed, with some exceptions. Surface roughness data obtained from experimental studies using different parameters must be reliable. When the data obtained from experimental studies are trustworthy, a correct evaluation can be made. For this purpose, since the number of variables in this study is high, the measured parameters were grouped according to a particular feed, cutting-edge angle, and spindle speed constants, as in Table 2. The average of the roughness values in each group and the deviations from the average were examined. This reliability examination showed that all data ranged from \(-2\sigma\)\(<\) u \(<\) 2\(\sigma\) and below. Since the confidence interval of \(-3\sigma\)\(<\) u \(<\) 3\(\sigma\), which is generally accepted for engineering studies, is sufficient, it is seen that all surface roughness values measured in this study are reliable in terms of making an evaluation. Figure 4 shows the results for the sample, which was machined on a lathe. In Figure 4, the sample was machined on a lathe using a cutting-edge angle of 70\({}^{\circ}\) and a depth of cut of 0.5 mm. This study examined the effects of varying feed on surface roughness. The graph shows that the lowest average surface roughness recorded was 2.282 \(\upmu\)m, achieved with a feed of 0.028 mm/rev at a spindle speed of 630 rpm. In contrast, the highest average surface roughness recorded was 4.745 \(\upmu\)m, which occurred at a feed of 0.155 mm/rev and a spindle speed of 1250 rpm. In the graph, the average surface roughness measurement Ra1 corresponds to the value obtained at a spindle speed of n1 rpm. Ra2 and Ra3 represent similar measurements at different spindle speeds. To achieve optimal average surface roughness, the most suitable operating parameters with a 70\({}^{\circ}\) cutting-edge angle are identified as a spindle speed of 630 rpm and a feed of 0.028 mm/rev, resulting in a Ra value of 2.282 \(\upmu\)m. The higher average surface roughness values observed at spindle speed n3, compared to n1 and n2, can be attributed to inappropriate processing parameters, which likely increased machine vibrations. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{} & \multicolumn{1}{c}{**Average Surface Roughness Values Ra (\(\upmu\)m)**} & & & & & & & \\ \hline
**Feed** & & & & & & & & & & \\ \hline
**(mm/rev)** & **n1** & **n2** & **n3** & **n1** & **n2** & **n3** & **n1** & **n2** & **n3** \\ \hline
0.028 & 2.513 & 2.282 & 2.711 & 2.336 & 2.101 & 1.785 & 1.228 & 1.639 & 1.784 \\ \hline
0.088 & 3.363 & 3.108 & 3.357 & 2.530 & 2.180 & 1.810 & 1.868 & 1.667 & 1.757 \\ \hline
0.155 & 3.573 & 3.558 & 4.745 & 2.956 & 2.888 & 2.680 & 2.457 & 2.214 & 2.057 \\ \hline CEA (\({}^{\circ}\)) & & 70\({}^{\circ}\) & & & 80\({}^{\circ}\) & & & 90\({}^{\circ}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average surface roughness variation in machining with feed, spindle speed, and cutting-edge angle variables.