## 3 Machinability Analysis
### Temperature  
Figure 3 shows the average temperature recorded throughout the entire cutting length during the turning operation for each cutting parameter and reflects the thermal behavior of the process. Results exhibit a strong dependency on the cutting speed (\(v_{c}\)), which proved  
to be the most influential parameter in the generation of heat within the cutting zone. As \(v_{c}\) increased, a significant temperature increase can be observed, independently of the feed rate (_f_) or final workpiece diameter (_D_). This effect is attributed to the increase in the relative velocity between the cutting edge and the material, which intensifies the shear strain rate and frictional interactions between the tool and the material. Although chip removal acts as a primary mechanism of heat dissipation during cutting, the \(v_{c}\) increase leads to greater energy input per unit time, which exceeds the capacity of the chip to evacuate the generated heat [48]. Moreover, the low thermal conductivity of PLA-CF restricts heat transfer into the chip and surrounding material, resulting in localized thermal accumulation at the tool-material interface. These thermal conditions promote elevated plastic deformation and increased conversion of mechanical energy into heat, particularly in thermoplastic composites such as PLA-CF, where the low thermal conductivity intensifies the formation of localized temperature peaks [49]. In addition to \(v_{c}\), the \(D\) reduction also contributes to the observed thermal behavior. Although \(v_{c}\) was held constant, the decrease in diameter required a proportional increase in spindle speed (_S_) to maintain the \(v_{c}\) value. This rise in rotational velocity further enhanced friction and thermal accumulation, particularly during the final passes on smaller diameters, where the highest temperature values were recorded. Regarding feed rate (_f_), its influence was less pronounced than that of \(v_{c}\). Temperature tends to decrease as \(f\) increases from 0.05 to 0.20 mm/rev. This is attributed to the shorter contact time between the tool and each point on the workpiece, which limits localized heat input and allows faster chip evacuation. Although this trend may differ from what is commonly observed in metals, it is consistent with the thermoplastic behavior of PLA-CF, and was confirmed by low standard deviation values across the temperature data, indicating stable thermal response. Together, these observations highlight the critical role of cutting speed in thermal generation, while also revealing the mitigating effect of increased feed rate under otherwise identical machining conditions. Therefore, elevated \(v_{c}\) values establish a thermal condition that must be carefully managed to avoid the softening or degradation of the printed material during post-processing. A critical thermal threshold in the machining of PLA-CF is the material glass transition temperature (_Tg_), typically situated between 60 and 65 \({}^{\circ}\)C [50]. In the present study, the lowest temperature recorded during the turning operations was 82.38 \({}^{\circ}\)C, which clearly exceeds the _Tg_ range. This implies that the material enters a softened state during machining, leading to reduced mechanical stiffness and increased surface tackiness. This condition favored the adherence of chips to the freshly machined surface, particularly in the absence  
Figure 3: Cutting temperature in the turning process. of forced chip evacuation. This fact led to compressed air jets being used during the process. Its function was dual: on the one hand, it contributed to reducing the temperature in the cutting zone by enhancing convective cooling; on the other hand, it facilitated chip evacuation, preventing the accumulation and fusion of thermally softened material on the surface. Despite this intervention, the cutting temperature remained above _Tg_, confirming the necessity of additional thermal management strategies when machining PLA-based composites.