## 3 Results and Discussion
### Fast Fourier Transform (FFT) Analysis
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \(v_{c}\) [m/min] & \multicolumn{10}{c}{**Frequencies [Hz]**} \\ \hline
400 & 265 & 530 & 795 & 1060 & 1325 & 1590 & 1855 & 2120 & 2385 & 2650 & 2915 & 3180 & 3445 & 3710 & 3975 & 4240 & 4505 \\
600 & 398 & 796 & 1194 & 1592 & 1990 & 2388 & 2786 & 3184 & 3582 & 3980 & 4378 & 4776 & & \\
800 & 531 & 1062 & 1593 & 2124 & 2655 & 3186 & 3717 & 4248 & 4779 & & & & \\
1000 & 663 & 1326 & 1989 & 2652 & 3315 & 3978 & 4641 & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tooth passing frequencies and harmonics. Figure 18: FFT for a feed of (**a**) 0.05 mm/tooth, (**b**) 0.1 mm/tooth, (**c**) 0.15 mm/tooth, (**d**) 0.2 mm/tooth, (**e**) 0.25 mm/tooth, and (**f**) 0.3 mm/tooth, applying the hydraulic toolholder. non-harmonics frequencies could also be seen, but their value was several times lower. With increasing cutting speed, the magnitude of the vibration also increased and was the highest of all the toolholders used, especially at the highest cutting speed. From the conducted analysis, it can be considered that the obtained results corresponded fairly well with the predictions that could be made from the SLDs (Figure 1). When the milling process was carried out at cutting speeds that were in the unstable machining area, there was a temporary increase in the vibration magnitude. However, if the assumed technological parameters were in the stable machining area, increasing the cutting speed caused a gradual growth in vibration amplitude. The rotational speed did not change when the feed per tooth was changed, and therefore the tooth passing frequency was approximately 531 Hz for all cases. When using the ER toolholder (Figure 16), the amplitude value on the tooth passing frequencies was much lower than the harmonics, similarly as for cutting speed changing. In most cases, a gradual extinction of the subsequent harmonics could be observed as the frequency increased. Amplitudes at frequencies that are not multiples of the tooth passing frequencies can also be seen in the charts, but their values were much lower than the harmonics, indicating relatively good process stability. There was also an increase in the vibration magnitude as the feed increased, but the change was very slight within the range \(f_{z}\) = 0.15-0.30 mm/tooth. In contrast to the other toolholder types, when using the shrinkfit toolholder (Figure 17), the peak heights at the tooth passing frequencies were comparable to the harmonics. In most cases, the highest amplitudes occurred at the frequency corresponding to the second harmonic, but in the further frequency range, the vibration spectrum became extinct. Amplitudes at frequencies other than harmonics can also be seen on the chart. These were only slightly lower than the harmonics, which is probably related to the very low values of the vibration amplitude, which were several times lower than when using the ER and hydraulic toolholders. In addition, the change in feed had no great effect on machining stability, as the change in vibration magnitude due to an increase in feed was negligible. When using a hydraulic toolholder (Figure 18), the tooth passing frequencies were also again lower than the initial harmonics. The largest peaks appeared at the second harmonic for a feed \(f_{z}\) = 0.05-0.20 mm/tooth and the third harmonic for a feed \(f_{z}\) = 0.25-0.30 mm/tooth, indicating a certain tendency for vibration occurrence. However, the amplitudes gradually decreased in the further range. A renewed increase in vibration magnitude occurred above 4 kHz, but this was relatively small, especially at higher feed values. The recorded amplitude values were the highest of all the toolholders used, rising to the highest values at the highest feed per tooth. The research conducted so far mainly concerned milling using straight tool paths. The influence of the type of toolholder during the milling of magnesium alloys was previously analysed for AZ91D [34] and AZ31B [35].