*2.4. Vibration Monitoring during the Burnishing Process*

Different burnishing tests with different forces were performed in order to characterize the machine-tool-specimen setup during the burnishing process. The burnishing forces used were 90 N and 270 N for the C45 steel and 90 N for the GLJ250 grey cast iron. The second variable was the presence (VABB) or absence (NVABB) of ultrasonic vibration in the tool. The burnishing speed was 2.33 m/min and the feed was 0.15 mm/revolution. The burnished length of each test was 10 mm, except for some tests that were carried out without feed for 3 min.

The layout of these tests are presented in Figure 5 for both the C45 and GJL250 specimens.

**Figure 5.** Burnishing test areas.

From these measurements, an ODS of the tool was obtained. ODS is a vibration analysis tool that allows for the determination of the deflection of a component or structure under real operating conditions. Vibration time histories are recorded under operating conditions and, by applying the Fourier transform to these recordings, vibration level versus frequency is determined at different points. A system's wire frame model can then be animated in order to demonstrate the movement at each measured point and at each frequency [32].

### *2.5. Acoustic Emission Measurements during the Burnishing Process*

As previously explained, acoustic emission waves are high-frequency waves (in the ultrasonic frequency band) generated when any kind of damage is produced in a material. During manufacturing processes, different acoustic emission signals are usually emitted by machined parts as a consequence of the damage produced in them. For the burnishing process presented in this paper, the eventual presence of acoustic emission events was explored as previous studies have validated the application of this this technique to the characterization of the process itself.

To this end, a Vallen acoustic emission sensor, model VS700-D (Vallen Systeme, Wolfratshausen, Germany), was installed in the tool holder. A Vallen preamplifier, model AEP4, a Vallen AMSY5 acoustic emission system (Vallen Systeme, Wolfratshausen, Germany), and Vallen acquisition software (Vallen Systeme, Wolfratshausen, Germany) were used for conditioning and recording of the acoustic emission signals. The sampling frequency of the acquisition was 625,000 samples/s.
