## 3 Machinability Analysis
### Chip Observation  
To further understand the material behavior during the machining process, chip morphology was analyzed by capturing Stereo Optical Microscope (SOM) images of the chips generated under different cutting conditions. The resulting images are shown in Figure 6 and correspond with the cutting parameters' combination.  
At low \(v_{c}\) (50 m/min) and low \(f\) (0.05 mm/rev) (Figure 6a), the chip exhibits a continuous ribbon shape with moderate curvature and a relatively smooth surface. This morphology reflects a stable mechanical chip formation process with limited thermal influ  
Figure 6: Chip morphology obtained under different cutting parameters: (**a**) \(v_{c}\) = 50 m/min and \(f\) = 0.05 mm/rev; (**b**) \(v_{c}\) = 50 m/min and \(f\) = 0.20 mm/rev; (**c**) \(v_{c}\) = 200 m/min and \(f\) = 0.05 mm/rev; and (**d**) \(v_{c}\) = 200 m/min and \(f\) = 0.20 mm/rev.  
ence. The material remains in a solid state, and plastic deformation dominates, consistent with the higher cutting forces previously recorded for this condition.  
When \(f\) is increased to 0.20 mm/rev (Figure 6b) at the same \(v_{c}\), the chip remains continuous, but it becomes noticeably thicker and slightly rougher. The morphology indicates an increase in chip volume, as expected with higher material removal, while still maintaining structural integrity due to the limited temperature rise at low \(v_{c}\). No significant fragmentation or thermal degradation is observed, and the chip formation remains mechanically controlled.  
In contrast, the chip generated at \(v_{c}\) = 200 m/min and \(f\) = 0.05 mm/rev (Figure 6c) shows an irregular structure with signs of local delamination, tearing, and partial fusion. The morphology suggests that the cutting temperature exceeded \(T_{S}\), leading to excessive softening of the PLA matrix and a loss of cohesion in the chip. This unstable morphology confirms the previously discussed reduction in cutting forces under this condition and highlights the dominance of thermal effects during chip formation at high \(v_{c}\) and low \(f\). The tendency of the chips to adhere or collapse also contributes to the surface irregularities observed on the machined part when compressed air is not applied.  
At higher \(v_{c}\) and \(f\), the chip becomes more regular again, despite being thicker and slightly rougher. The increased chip volume facilitates faster evacuation from the cutting zone, which helps reduce localized thermal accumulation and prevents the fusion effects seen in the previous condition. The surface of the chip appears matte and granular, indicating a balance between thermal softening and mechanical shear, resulting in a stable chip formation process.  
Compared with chip morphologies typically observed in the machining of metallic materials, the chips generated from PLA-CF exhibit markedly different characteristics due to the thermoplastic nature of the polymer matrix. In metals, chip formation often results in segmented, curled, or spiral shapes, particularly under high \(v_{c}\) and \(f\), due to a periodic shear localization and the ductile fracture mechanisms that govern metallic plastic deformation [56]. In contrast, the PLA-CF chips are predominantly continuous, even at high feed rates, and did not show clear segmentation or spiral formation, and their morphology is strongly influenced by thermal effects. These differences emphasize the need for a specialized interpretation of chip morphology in polymer.