## 3 Results and Discussion  
### Evaluation of Dynamics and Surface Roughness  
During the face milling experiments with various radial immersions (defined by the \(a^{\prime}\) factor), the number of active teeth was contained within the range \(z_{c}=1.77\div 2.11\). This indicates that in the milling process, 1 to 3 teeth, depending on tool rotational angle and selected radial immersion, are engaged simultaneously. The non-integer values of \(z_{c}\) denote that in the milling process, the number of teeth engaged in the cutting is variable, which in its turn causes variations in maximal chip cross-sections per the consecutive teeth, and thus the changes in cutting force peak values per teeth (see Figure 3a-c). On the other hand, the variations in cutting force peak values can be also induced by the geometrical errors of the machine-to-loidholder-tool system (manifesting in axial run out), which contributes to the changes in real values of axial depths of cut per consecutive tooth. Figure 3a-c show also that independently on selected radial immersion, the feed \(F_{y}\), and feed normal force \(F_{x}\) components' absolute values are significantly higher than thrust force \(F_{y}\) values. It should be noted that \(F_{y}\) and \(F_{x}\) components are directly correlated with milling kinematics and the chip decohesion process. On the other hand, the thrust force is only marginally dependent on chip formation mechanisms, and its value is strictly correlated with the rubbing and ploughing mechanisms occurring between the tool's flank face and the machined surface. Thus, the potential growth of thrust force could be attributed to the relatively high friction coefficient between the workpiece and the tool flank face, as well as to the progressing tool wear. All face milling tests were carried out with constant axial depths of cut \(d\) and milling width \(B\). Thus, the cutting force values should be comparable, independent of selected radial immersions. Nevertheless, some differences in cutting force absolute values are found between the tested \(a^{\prime}\) factors. Figure 3 shows that absolute values of cutting force components in the case of milling with \(a^{\prime}\) = 0 mm are lower than those reached during machining with \(a^{\prime}\) = 6.3 mm and \(a^{\prime}\) = 12.3 mm. These differences can be explained by the various initial uncut chip thicknesses \(a_{i}(\psi_{1})\) found for different \(a^{\prime}\) values. The \(a_{i}(\psi_{1})\) corresponds to the uncut chip thickness during the initial contact of the tooth with the workpiece. The growth of \(a^{\prime}\) values induces the increase of \(a_{i}(\psi_{1})\). When the \(a^{\prime}\) = 0 mm, the \(a_{i}(\psi_{1})\) = 0 mm, and the growth of chip cross section (and thus cutting forces) is gradual. However, in the case of \(a^{\prime}\) = 6.3 mm, the \(a_{i}(\psi_{1})\) = 0.06 mm, and when \(a^{\prime}\) = 12.3 mm, the \(a_{i}(\psi_{1})\) = 0.08 mm. This reveals that initiation of chip formation during machining with higher \(a^{\prime}\) values occurs at relatively high uncut chip thicknesses, and thus the impact of the cutting tooth on the workpiece can cause some forced vibrations, contributing to the growth of cutting force dynamic components. These relations can be also found for the root mean square (RMS) values of \(F_{x}\) force in the function of \(a^{\prime}\) (Figure 4a), i.e., the growth of \(a^{\prime}\) factor leads to the increase in force. This means that the \(F_{x}\) force component is strongly affected by milling dynamics. In order to further analyze these findings, the RMS values of accelerations of vibrations in