## 3 Results
#### 3.1.2 Effect of the Cutting Speed, \(\mathrm{v_{c}}\)  
(a) The End Face of the Specim  
Figure 9 presents the results of the kurtosis and skewness measurements. It shows that the slope factor of the profile is positive, which indicates the increase in the friction coefficient (due to more rounded surface irregularities). In contrast, the profile asymmetry factor in the majority of cases takes negative values. Therefore, it can be concluded that the peaks and valleys are non-symmetrical in distribution, and the corrosion resistance is lower.  
Figure 10a, which shows the recorded values of RSm, gives a clear indication that the increase in the \(\mathrm{v_{c}}\) value impacts the RSm parameter value. The highest values of RSm were observed for \(\mathrm{v_{c}}\) = 400 m/min. The results obtained in the 600-1200 m/min range of \(\mathrm{v_{c}}\) produced the most beneficial  
Figure 8: Effect of the trochoidal step \(\mathrm{s_{tr}}\): (**a**) Rku and (**b**) Rsk (\(\mathrm{v_{c}}\) = 800 m/min, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured on the lateral face of the specimen.  
Figure 9: Effect of the cutting speed, \(\mathrm{v_{c}}\): (**a**) Rku and (**b**) Rsk (\(\mathrm{s_{tr}}\) = 15%, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured at the end face of the specimen.  
conditions of machining, with the lowest RSm. In addition, for \(\mathrm{v_{c}}\) = 1000 m/min, the RSm parameter values were: 0.115 (AZ31) and 0.2 mm (AZ91D). Figure 10b, on the other hand, shows that the lowest values of Ra were in the region of 1 \(\mathrm{\SIUnitSymbolMicro m}\). It can also be seen that as the cutting speed, \(\mathrm{v_{c}}\), increases, the Rz surface roughness parameter exhibits a tendency to drop. The lowest values of the Rz parameters are observed in the range of \(\mathrm{v_{c}}\) = 1000-1200 m/min. This highlights that machining at high cutting speeds (HSM) is beneficial for the condition of the surface. The lowest Rz values were 5.45 \(\mathrm{\SIUnitSymbolMicro m}\) for the AZ31 alloy and 5.05 \(\mathrm{\SIUnitSymbolMicro m}\) for the AZ91D alloy (at \(\mathrm{v_{c}}\) = 1000 m/min). The observed drop in the values of Ra and Rz could be the result of the machining speed entering the range of HSM, in which the cutting forces and the surface roughness are known to decrease.  
(b) The Lateral Face of the Specim  
Figure 11 reports the values of the Rz and RSm parameter at the changing cutting speed, \(\mathrm{v_{c}}\), measured at the end face of the specimens. From Figure 11, it can be seen that with the increase in \(\mathrm{v_{c}}\), the values of the Rz and RSm parameters change. An increase in these parameters causes an increase in the analyzed machinability indices. The lowest values of both parameters were observed for \(\mathrm{v_{c}}\) = 400 m/min. In the case of the Rz parameter, the least favorable machining scenario is \(\mathrm{v_{c}}\) = 800 m/min, and in the case of the RSm, it is the entire 800 to 1200 m/min range of \(\mathrm{v_{c}}\). From the observations, it can be seen that the lowest values of the surface roughness parameters (Rz and Rsm) on the lateral face of the specimen are obtained when the process is executed at lower speeds and following the trochoidal tool path.  
Figure 11: Effect of the cutting speed, \(\mathrm{v_{c}}\): (**a**) Rz and (**b**) RSm (\(\mathrm{s_{tr}}\) = 15%, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured on the lateral face of the specimen.  
Figure 10: Effect of the cutting speed, \(\mathrm{v_{c}}\): (**a**) RSm and (**b**) Ra, Rz (\(\mathrm{s_{tr}}\) = 15%, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured at the end face of the specimen.  
Figure 12 presents the values of Rku and Rsk. In both cases, the parameters, Rku and Rsk, take positive values, which may indicate more rounded peaks of micro-irregularities, thus a higher coefficient of friction, as well as higher corrosion resistance.