Cutting Force Analysis

Cutting forces developed during machining can provide valuable information regarding the tools' wear. To acquire these cutting forces, a 4-component KISTLER 9171A dynamometer was used, coupled to a KISTLER 5697A1 data acquisition system, which allowed to record the cutting forces developed in X, Y and Z axes, as well as the developed torque (Mz), through appropriate software supplied with the dynamometer. The equipment was coupled to the spindle of the CNC machining center, having then the appropriate clamping system for placing the tool. The acquisition rate of the developed forces was selected according to the spindle's rotation speed, and the force of all the cutting edges of the tool at each rotation was registered. The results were collected and analyzed, enabling the identification of any increase or abnormal behavior of the cutting forces that could be caused by wear phenomena on the tool.

### Machining Test Parameters

With the determined cutting lengths, the wear mechanisms that these tools suffer during machining could be evaluated. The main concern was to identify the early wear phenomena as well as its evolution under these cutting conditions, and not to determine the lifespan of the tools. The determined parameters for each test can be observed in Table 4, including the sample reference used for each of the tested tools.

Three tools were used for each of the conditions presented above. The use of lubricant during the machining tests caused difficulties in the analysis of the tools' surfaces. Thus, all the tools underwent one ultrasonic bath using acetone for 5 min to remove the presence of lubricant on the areas where SEM analysis was to be performed. This bath had a short duration to prevent the eventual removal of adhered material or sections of coating (near detachment). The surface roughness of the material was also assessed after each machining test.

### 2.2.3. Surface Roughness Test

Surface roughness evaluation is very important in the machining process, as it is closely related to machining performance, tool wear and process stability. In this work, the material's machined surface roughness was characterized in two directions: radial and tangential (machining direction). To assess this parameter, surface roughness tests were conducted using a MAHR PERTHOMETER M2 profilometer, following the standard procedure described in DIN EN ISO 4288/ASME b461. Each test covered a length of 5.6 mm, corresponding to seven segments of the cut-off value (0.8 mm), with the first and last value being ignored due to the acceleration and deceleration of the probe arm. For the surface roughness analysis, the following parameters were considered: arithmetic mean roughness (Ra) and the maximum roughness (Rmax). The R profile was also analyzed to identify any sharp peaks in the surface roughness values and other phenomena.

#### 2.2.4. Tool Wear Analysis

After machining, all the tools were subjected to SEM analyses to access the amount of wear that these tools sustained and to identify the wear mechanisms developed on the various tools during machining. To access the wear, the values for flank wear were measured (VB) according to the ISO 8688-2:1986 standard [52]. Furthermore, the values of VB that were presented were the mean values for each tool for each of the test conditions.

A reference for the tool analysis was created and can be observed in Figure 1; both the rake face and clearance face were analyzed, identifying the wear mechanisms that were present and measuring the flank wear (VB) on the clearance face of the tested tools.

The numerations observed in Figure 1 were adopted in the identification of the SEM images, with the rake face and clearance face being identified with RF and CF, respectively, identifying the edge under analysis as presented in Figure 1. Additionally, since three tools were used for each of the test conditions, an identification number (ranging from 01 to 03) was added at the end of the image reference.

**Figure 1.** Reference used for the SEM analysis of the end-mills with (**a**) two cutting edges, and (**b**) four cutting edges.
