## 3 Machinability Analysis
### Machining Forces  
To evaluate the mechanical interaction between the tool and the workpiece during machining, the three main components of the cutting force were measured: the passive force (_Fx_), the cutting force (_Fy_), and the feed force (_Fz_). Based on the measured values, the resultant (_R_) was calculated by combining the three orthogonal components. This resultant force reflects the overall magnitude of the mechanical effort required during the turning operation. The results obtained for each individual force component, along with the calculated resultant force, are presented in Figure 4 as a function of cutting parameters for different final diameters. The _Fx_ values (Figure 4a) showed a progressive decrease with increasing _vc_, regardless of \(f\) and \(D\), reflecting the influence of temperature on the dynamic behavior between the tool and the material. As temperature increases, PLA-CF exceeds its _Tg_, leading to a softening of the polymeric matrix that increases the plasticity of the material, and therefore, the mechanical shear strength is reduced. The \(f\) effect was less pronounced. In general, slight decreases in _Fx_ can be observed when increasing \(f\), especially at lower _vc_. Although an increase in undeformed chip thickness might suggest a rise in cutting resistance, this trend was not reflected in the passive force, likely due to the compensatory thermal effects that dominate under these conditions. A gradual decrease in _Fx_ can be noted as \(D\) decreased from 18 mm to 14 mm. This behavior remained consistent with the temperature reduction observed in the cutting process. The _Fy_ component (Figure 4b) exhibited a clear sensitivity to both _vc_ and \(f\). Among all the force components, _Fy_ showed the highest variation in response to changes in process parameters, reflecting its strong dependence on chip thickness and material removal mechanics. The most significant trend corresponds to the \(f\) increase, increasing the value of _Fy_. This behavior is consistent with classical machining theory, where a greater chip thickness has required more energy for material separation, increasing cutting force. The \(v_{c}\) influence on \(Fy\) is more complex. At low \(f\) values (0.05 mm/rev), an increase in \(v_{c}\) led to a notable reduction in \(Fy\), confirming that thermal softening of the PLA matrix plays a key role in reducing cutting resistance. However, at higher \(f\) (0.20 mm/rev), the effect of temperature was less important, with the mechanical load being dominant. This indicates that the interaction between thermal and mechanical effects must be considered together in the force analysis. Regarding specimen final diameter, the difference in \(Fy\) is less noticeable than for the cutting parameters. A general decrease can be observed as \(D\) decreased, particularly at lower \(v_{c}\). This fact is attributed to reduced engagement and the increase in thermal dissipation but also points to a reduced structural resistance of the material under high cutting demands. The \(Fz\) component (Figure 4c) showed moderate values compared with \(Fy\) but showed consistent trends in relation to the cutting parameters and specimen geometry. Contrary to the behavior typically observed in metallic materials, increasing \(f\) did not result in a significant increase in \(Fz\). In several cases, even a slight reduction was observed due to the thermomechanical behavior of PLA-CF. Higher \(f\) values increase the chip thickness and material removal rate, which can raise the overall heat generation. However, due to the shorter contact time per revolution of the cutting tool with the material and the  
Figure 4: Forces measured during the cutting process: (**a**) passive forces, (**b**) cutting forces, (**c**) feed forces, and (**d**) resultant. improved chip evacuation, localized thermal buildup can be reduced, allowing the material to deform more easily in the feed direction. Therefore, the axial resistance does not increase proportionally with the feed rate and, in some cases, decreases [54].