**2. Materials and Methods**

### *2.1. Experimental Setup*

In order to characterize the dynamic behavior of the machine-tool-part setup, two types of tests were performed: impact tests under static condition; and vibrations, forces, and acoustic emission monitoring under running (dynamic) conditions of the burnishing system. The ball burnishing tool used was a prototype designed and patented by the authors [29]. It can be used in both NVABB and VABB processes. The frequency of its ultrasonic vibration is 40 kHz. All tests were performed in a PINACHO SE 200 × 1000 mm CNC lathe (Pinacho CNC, Guipuzcoa, Spain). The specimens were fixed between a selfcentering three-jaw chuck plate and the point.

Different burnishing parameters were taken into account in this study: specimen material (C45 steel, EN 10020:2000, and GJL250 grey cast iron, EN 1560:2011), burnishing force (90 N and 270 N) and ultrasonic vibration assistance (yes or no). The specimens were previously machined. Table 1 presents their initial and final dimensions, the cutting parameters, and the measured roughness. No cutting fluids were used in the machining.


**Table 1.** Cutting parameters and specimen dimensions (initial and final).

Different accelerometers were used. Three triaxial accelerometers were installed in the burnishing tool in order to study its vibrating behavior. Two were mounted in the frontal part near the burnishing ball (positions P1 and P2 of Figure 1a), and the third was mounted in the opposite part (position P3 of Figure 1b). The characteristics of these accelerometers are presented in Table 2. The measurement directions of the accelerometers correspond to the burnishing feed (X), vertical direction (Y), and direction of the burnishing force (Z).

**Figure 1.** Triaxial accelerometers installed in the tool: (**a**) frontal view; (**b**) rear view.

**Table 2.** Accelerometers used for the measurements.


Three monoaxial accelerometers were installed in the lathe bed in order to determine the vibrational transmissibility from the machine during the burnishing process. Figure 2 depicts these accelerometers and their measurement directions: A (axial according to the specimen rotation), V (vertical), and H (horizontal). The characteristics of these accelerometers are presented in Table 2.

The monitoring criteria were selected taking the following into account:


**Figure 2.** Accelerometers mounted on the lathe bed.

### *2.2. Burnishing Force Monitoring*

The compressive deformation of the spring installed inside the tool-holder is linearly related to the force transmitted by the burnishing tool to the target surface [20]. In turning processes, it is linearly related to the penetration of the tool in the direction of the depth of the pass. The nominal burnishing forces of the tests were 90 N and 270 N for the steel specimen and only 90 N for the grey cast iron, as 270 N is an excessive load for this material.

The force was monitored by a KITSLER 9129AA dynamometer (Kistler, Winterthur, Switzerland) adapted to the lathe holder where the tool was mounted, as seen in Figure 1a. The force signal was conditioned by a KITSLER 5070A12100 amplifier (Kistler, Winterthur, Switzerland) Burnishing forces were acquired in impact tests under static conditions (lathe turret stopped) and in measurements under operating conditions (lathe turret moving) of the burnishing system (Figure 3).

**Figure 3.** Average, maximum, and minimum forces recorded during the tests.
