*2.3. Impact Tests*

Impulse excitation was used to determine the natural frequencies of the tool under different conditions. Impacts were carried out with an impact hammer (KISTLER 9722A2000) (Kistler, Winterthur, Switzerland) with steel tip (9902A) (Kistler, Winterthur, Switzerland) Its maximum frequency was 9.3 kHz and its maximum force was 11 kN. These characteristics were deemed suitable as, according to a previous study [30], the natural frequencies of a similar tool were lower than 5 kHz.

Impacts were performed at three tool points in the vertical direction (points I1, I2, and I3) and vibrations were measured at three points (P1, P2, and P3) in three directions, as seen in Figure 4. The conditions under which the impacts were carried out were as follows:


**Figure 4.** Lateral view of the impact points on the tool.

In total, 15 impact tests were performed. The vibration assistance was not activated during the impact tests as a previous work [31] demonstrated that the ultrasonic vibration of the assistance does not affect a tool's natural frequency.

The acquisition and further analysis of the vibration signals were performed with a 3053-B-120 analyzer (Brüel & Kjær, Nærum, Denmark) and the PULSE Reflex software (version 2.3), respectively. Ten channels were defined: three for each triaxial accelerometer and one for the impact hammer.
