## 3 Machinability Analysis
### Machining Forces
Regarding the influence of \(v_{c}\), a general decrease in Fz can be observed as \(v_{c}\) increased, particularly at lower \(f\). This trend reflects the thermal softening of PLA-CF as the temperature exceeds its \(Tg\), which reduces the axial strength of the material. As for the specimen diameter, the effect was less clear. Although \(Fz\) tends to decrease slightly as \(D\) decreases, especially at low \(v_{c}\) and \(f\), some exceptions were observed. This variability is due to the interaction between the reduction in contact area and the increase in slenderness, which may result in a reduction or a slight increase in axial loads depending on the cutting conditions [55]. The resultant cutting force (\(R\)) was obtained by vectorial combination of the three orthogonal components measured during the process. Therefore, \(R\) reflects the combined effect \(Fx\), \(Fy\), and \(Fz\) forces. Its evolution across the different cutting conditions offers an integrated view of the overall mechanical load acting on the tool during the turning operation. In general, the \(R\) results revealed that \(Fy\) had the strongest influence, as it consistently represented the largest contribution. Variations in \(Fx\) and \(Fz\), had a comparatively smaller impact, particularly at high \(v_{c}\), where their values were significantly reduced. As \(D\) decreased, \(R\) followed a gradual downward trend, in line with the reductions observed in all three components. This further confirms that the calculated \(R\) is a reliable indicator of the overall force environment in the cutting zone. Its dependence on \(Fy\) also highlights the relevance of controlling the cutting load directly, especially in processes involving reinforced thermoplastics, where surface quality and dimensional stability are strongly affected by the interaction between the tool and the material. As part of the experimental plan, this work aimed to assess the machining capacity of PLA-CF parts by gradually reducing the final diameter of the specimens during the turning process. The objective was to identify the geometric and mechanical limits beyond which the structural integrity of the material could no longer withstand the loads induced by the cutting process. During these trials, it has been found that machining specimens down to 12 mm in diameter consistently led to catastrophic failure before the operation could be completed. In all cases, fracture occurred along the layer interfaces, highlighting the critical role of interlaminar bonding in the structural behavior of the sample. The failures were not caused by an increase in cutting force magnitude, but rather by the reduction in cross-sectional resistance. Considering the resultant cutting force in the XY plane (\(Rxy\)), calculated from \(Fx\) and \(Fy\) (Equation (1)), this value tends to decrease with diameter. Nevertheless, due to the cross-sectional area decreased, the calculated shear stress (\(\sigma\)) increased (Equation (2)), as can be observed in Table 4. Therefore, based on the values obtained in this study, a shear stress of 0.237 MPa can be considered a practical upper limit for the interlaminar shear strength of PLA-CF specimens manufactured by FFF. This value reflects the maximum resistance the material can offer between layers before failing under combined mechanical and thermal loads during the turning process. \[Rxy=\sqrt{Fx^{2}+Fy^{2}} \tag{1}\]  
\[\sigma=\frac{Rxy}{\pi\cdot D^{2}} \tag{2}\]