*3.2. Cutting Load*

Figure 5 allows for the observation of the cutting loads according to each selected operational condition. The significant evolution of these loads can be noticed for each selection of cutting parameters due to the considerable wear rates promoted by the low machinability of the alloy. The initial force values, which were minimum due to the fact of insignificant tool wear, are represented by "min.", whereas "max." illustrates the last passage load values, which were maximum due to the fact of tool degradation. The average force values over the entire test are represented by "avg.". Overall, the highest values were achieved by the passive force component (Fp) followed by the cutting force (Fc) and feed force (Ff), in descending order, given that when the depth of cut is smaller than the nose radius of the tool, the passive or radial component of the tool forces is found to be most dominant [24,25,29,35,36]. Moreover, passive force had a large evolution over time, often amounting to triple and even quadruple the initial (i.e., virgin tool) value, when reaching the end of the tool's life. Monitoring the evolution of this force can provide relevant information on the tool wear evolution. The force appears to be highly sensitive to the evolution of the tool wear and degradation of the tool edge surface, which is a tendency also observed by other researchers [24,35,37]. In addition, the passive force seems to be associated to the ploughing force, which is caused by the flow of material ploughed by the clearance face of the tool due to the elastic deformation of the workpiece material under the clearance surface [35].

The first pass cutting forces for each tool, cutting speed, and feed are shown in Figure 6 for each distinct cutting tool. This first pass values are important, since at this point the cutting tools have the most similar wear conditions, enabling a more suitable comparison of the cutting forces. As expected, higher cutting forces were achieved when machining with higher feeds for both tools, given that maintaining the depth of cut and increasing the feed results in a higher chip cross-sectional area along with higher cutting forces. It is important to note the high stability of the cutting forces in the steady-state regime (standard deviation < 5 N). When varying the feed from 0.1 mm/rev to 0.2 mm/rev and cutting with the TiC binder inserts, this resulted in an average increase of 52.1% in the cutting forces, while under the same conditions, an increase of 55.5% was observed when machining with the TiN binder inserts. The cutting forces proved to have a slightly decreasing tendency when increasing the cutting speed. As the cutting speed increased, there was a larger energy input in the machining process, which combined with the lower heat diffusion (near-adiabatic process), promoted a temperature increase that resulted in thermal softening of the workpiece material. For the tests, it was not possible to perceive a

significant difference in the cutting forces among the distinct CBN binders. Regardless of the employed cutting insert, the measured load values oscillated at approximately 120 N when machining with a feed of 0.2 mm/rev and 80 N with a feed of 0.1 mm/rev.

**Figure 5.** Cutting load evolution according to the selected cutting parameters and depending on the first (min.) and last (max.) cutting tool passages: (**a**) TiC binder cutting tool and *f* = 0.1 mm/rev; (**b**) TiC binder cutting tool and *f* = 0.2 mm/rev; (**c**) TiN binder cutting tool and *f* = 0.1 mm/rev; (**d**) TiN binder cutting tool and *f* = 0.1 mm/rev.

**Figure 6.** Measured cutting forces on the initial passage of each tool for each tested configuration of cutting parameters: (**a**) TiC binder cutting tool; (**b**) TiN binder cutting tool.

The measurement of the first pass cutting forces enabled the calculation of the specific cutting pressure, *Kc*, which is a highly relevant parameter used for the evaluation of a material's machinability and enables an estimate of the cutting force for scenarios with distinct operational conditions. The specific cutting pressure can be defined as the ratio between the main cutting force and cross-sectional area of an undeformed chip, *A*0. The cross-sectional area of the undeformed chip can be calculated using Equation (1), and the specific cutting pressure, *Kc*, can be calculated using Equation (2).

$$A = a\_p \cdot f \tag{1}$$

$$K\_c = \frac{F\_C}{A} \tag{2}$$

Figure 7 exhibits the evolution of the specific cutting pressure according to the feed conditions. Typically, *Kc* exponentially increases for a smaller chip section while stabilizing when increasing the chip section, which is a phenomenon associated with size effects in metal cutting, as widely observed by researchers [35,38,39]. The occurrence of defects in metals, such as grain boundaries or missing atoms, is generally acknowledged, and the chance of encountering such stress-reducing defects increases for larger sizes of removed material [39]. In addition, the occurrence of ploughing for increasingly smaller feed rates explains the size effect. When the uncut chip thickness becomes smaller or similar to the tool edge radius, the cutting loads did not decrease proportionally due to the considerable relative contribution of workpiece deformation (compression), leading to higher *Kc* values.

**Figure 7.** Specific cutting pressure values considering new and used tools and their comparison with values in the literature for similar cutting insert geometries.

The turning tests comprised two different chip cross-sectional areas, with the highest values of cutting pressure being achieved for the lowest feed, which corresponded to the smaller chip section, as explained by the size effect in the metal cutting. Values between 4000 and 5500 N/mm<sup>2</sup> were obtained for a feed of 0.2 and 0.1 mm/rev, respectively, at a cutting speed of 300 m/min and depth of cut of 0.15 mm. These are typical values for Inconel 718 turning processes with negative rake angles (commonly used for Ni-based alloys), hence, inducing high values of specific cutting forces. Furthermore, Cantero et al. [35] obtained similar *Kc* values when turning Inconel 718 (45 HRC) with commercially available KB5625 and CBN170 PCBN inserts. These inserts have a medium CBN content and ceramic binder, with a honed edge preparation (25 μm) similar to the tested tools in this work. Whereas in Figure 7 the specific cutting pressure labeled with "min." corresponds to the

first passage of the longitudinal turning (i.e., virgin cutting insert), "max." corresponds to the last passage of that insert. Significantly higher specific cutting pressures (>30%) occur when reaching the tool's end of life, proving that the wear affects the cutting power, leading to a less efficient machining along with a higher surface roughness of the part, thus affecting the process sustainability and overall quality of the generated surface.
