## 3 Results  
The results from the experiments and measurements are presented in the form of bar charts, with the standard deviation marked as a measure of the scatter of the measurement results. 2D roughness parameters, including their mean values after milling, are given in Figures 5-12, while after brushing is shown in Figures 13 and 14. For the sake of 3D roughness parameter analysis (Figures 15 and 16), surface roughness maps were created, one for each machining parameter, hence these charts do not contain a standard deviation.  
Figure 5(a) reports the results for RSm, i.e., the mean width of the profile elements. In this case, the increase in the trochoidal step, \(\mathrm{s_{tr}}\), affects the value of RSm. The highest values of this roughness parameter were observed for \(\mathrm{s_{tr}}\) = 10% (for AZ31 0.27 mm and AZ91D 0.41 mm, respectively). The results obtained at \(\mathrm{s_{tr}}\) 5%, 20%, and 25% were the lowest and thus may be considered as the most favorable from the analyzed viewpoint. Figure 5(b) shows the results for Ra, which oscillated at approximately 1.5 to 2.0 \(\mathrm{\SIUnitSymbolMicro m}\). From the data presented in Figure 5(b), it can be seen that increasing the trochoidal step leads to the emergence of different relations depending on the type of workpiece material. In the case of the AZ91D alloy, the surface roughness increases (comparing \(\mathrm{s_{tr}}\) = 5% and \(\mathrm{s_{tr}}\) = 30%, Rz: from 7.1 to 11.6 \(\mathrm{\SIUnitSymbolMicro m}\)), whereas in AZ31 alloy, the roughness parameters change only to a slight extent.  
Figure 6: Effect of the trochoidal step, \(\mathrm{s_{tr}}\): (**a**) RSm and (**b**) Ra, Rz (\(\mathrm{v_{c}}\) = 800 m/min, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured at the end face of the specimen.  
Figure 7: Effect of the trochoidal step, \(\mathrm{s_{tr}}\): (**a**) Rz and (**b**) RSm (\(\mathrm{v_{c}}\) = 800 m/min, \(\mathrm{f_{z}}\) = 0.15 mm/tooth, \(\mathrm{a_{p}}\) = 6 mm) measured on the lateral face of the specimen.  
As the trochoidal step, \(\mathrm{s_{tr}}\), increases, the values of the Rz and RSm parameters change. The lowest values of both parameters were observed for \(\mathrm{s_{tr}}\) = 5%. In the case of the Rz parameter, the least favorable \(\mathrm{s_{tr}}\) is 15% while for RSm, the \(\mathrm{s_{tr}}\) ranges from 20 to 30%. The least favorable conditions of machining, the 15% trochoidal step, indicate an unstable machining area. Similar relationships are often found for other indicators, e.g., for the radial depth of cut \(\mathrm{a_{e}}\), the stable areas were determined for \(\mathrm{a_{e}}>75\%\) d and \(\mathrm{a_{e}}<25\%\) d (d is the tool diameter). Therefore, with respect to Rz, the stable machining area is \(\mathrm{s_{tr}}\) = 5% and \(\mathrm{s_{tr}}\) = 20-30%. In addition, increasing the \(\mathrm{s_{tr}}\) results in an increase in the amount of workpiece material subtracted per one trochoidal step (increase in the diameter of the area of the cut). The higher extraction rate leads to the rise in the values of the horizontal surface roughness parameter, Rsm (mean width of the profile elements).  
Figure 8 shows the values of Rku and Rsk. In both cases, for most changes in the range of \(\mathrm{s_{tr}}\), the parameters, Rku and Rsk, assume positive values (the exception being the Rsk parameter for alloy AZ31 at \(\mathrm{s_{tr}}\) = 5% and alloy AZ91D at \(\mathrm{s_{tr}}\) = 30%). Positive values of both parameters may indicate more rounded peaks of micro-irregularities, an increased friction coefficient, and higher corrosion resistance.