## 2 Materials and Methods  
The material used in this study was a polylactic acid (PLA) matrix composite that had been reinforced with short carbon fibers (CFs). The selection of carbon fiber provides better mechanical properties in comparison with unmodified PLA, including increased stiffness, strength, and dimensional stability. Additionally, the PLA matrix possesses the advantage of a biodegradable material. The initial sample geometry for this study was a solid cylindrical specimen designed for further processing by turning operations, the geometry of which can be related to rotational components of mechanisms such as a drive shaft. The initial geometry of the specimens was a solid cylinder with an outer diameter of 20 mm and a total length of 200 mm. These dimensions were selected to represent a slender part configuration, where the length-to-diameter ratio introduces a structural limitation during machining. This setup increases the risk of deflection or fracture under cutting forces, thus allowing the evaluation of machinability under mechanically demanding conditions. The sample was manufactured by additive manufacturing using FFF technology. A specific printing strategy was adopted in which the part was built in a vertical orientation so that the material was deposited layer by layer in a direction perpendicular to the axis of revolution. This configuration was chosen for two main reasons. First, it eliminates the need for support structures, simplifying the manufacturing process and minimizing the risk of surface defects caused by contact with the support material. Second, this orientation allows the subsequent turning operation to interact directly with the interlayer planes, which are often the weakest regions of FFF-printed components, allowing for an assessment of any post-processing limitations. In addition, the chosen geometry results in a length-to-diameter (\(L/D\)) ratio, which can be a limiting factor due to bending deformation that can occur during machining, especially at higher cutting loads. This feature is expected to influence the machinability and surface quality of the final part, so it was also taken into account for the evaluation of the feasibility of turning as a post-processing operation for PLA-CF parts manufactured with FFF. The specimens were manufactured using a Raise3D Pro2 FFF printer (Raise3D Technologies, Inc. Irvine, CA, USA). The slicing and process control were performed using the ideaMaker software (Version 4.3.1). The manufacturing conditions applied in the printing process can be observed in Table 1.  
The samples were manufactured using two different layer heights. One sample was printed with a layer height of 0.05 mm, considering a finer layer resolution because it contributes to improving dimensional accuracy and surface quality in FFF-printed components [41]. However, this high resolution of the specimen means an increase in manufacturing time, being in this case necessary a time of 24 h and 35 min, reducing the manufacturing efficiency in a high number of parts. Therefore, to explore the use of machining as a post-processing strategy to improve surface characteristics and reduce printing time, a set of \begin{table}
\begin{tabular}{c c} \hline
**Manufacturing Setting** & **Value** \\ \hline Extrusion temperature & 210 \({}^{\circ}\)C \\ Heated bed temperature & 80 \({}^{\circ}\)C \\ Extrusion speed & 70 mm/s \\ Layer thickness & 0.30–0.05 mm \\ Infill density & 100\% \\ Nozzle diameter & 0.40 mm \\ \hline \end{tabular}
\end{table} 
Table 1: FFF manufacturing settings for PLA-CF specimens. samples was fabricated using a thicker layer height of 0.30 mm. This adjustment resulted in a substantial reduction in fabrication time, reducing it to 4 h and 9 min. Once the specimens were manufactured by FFF, they were subjected to a longitudinal turning operation in order to evaluate the feasibility of machining as a post-processing strategy. The turning operation was carried out on an EMCO EMCOTURN E45 CNC lathe(EMCO GmbH, Hallein, Austria).