## 4 Geometrical Analysis
### Macrogeometrical Deviations  
Figure 9 shows the values of roundness (_RON_), straightness (_STR_), and cylindricity (_CYL_) deviations obtained for each of the machined specimens as a function of the cutting parameters and final diameter. To compare the impacts of machining, the figure also includes reference values obtained from two specimens manufactured directly by FFF without any post-processing. The first reference specimen was printed with a layer thickness of 0.05 mm, representing a high-quality FFF configuration. The second reference specimen was produced with a layer thickness of 0.30 mm, which significantly reduces printing time but results in lower surface and geometric quality. These two cases provide a useful benchmark to evaluate the improvement achieved through subtractive post-processing and the sensitivity of the form deviations to both machining parameters and initial part quality. The _RON_ (Figure 9a) analysis reveals significant differences depending on both the machining parameters and the diameter of the machined specimens. In general, _RON_ errors tend to decrease with increasing \(v_{c}\). Specimens machined at \(v_{c}\) = 200 m/min consistently show lower _RON_ values compared with those machined at \(v_{c}\) = 50 m/min, indicating a more stable cutting process and better surface generation at higher speeds. Figure 9: Macrogeometrical deviations obtained in turned samples as a function of \(v_{c}\), \(f\), and \(D\) and printed samples by FFF as a function of layer thickness: (**a**) roundness, (**b**) straightness, and (**c**) cylindricity. The \(f\) effect is also noticeable. Higher \(f\) values are associated with slightly higher \(RON\) values, likely due to the increased cutting forces and reduced tool engagement time per revolution, which can amplify tool vibration or surface disruption. This effect is more pronounced in the slenderest specimens, especially those with \(D\) = 14 mm, where the structural flexibility makes \(RON\) more sensitive to dynamic effects during machining. Comparing across different diameters, the \(RON\) deviation increases as the diameter decreases. The \(D\) = 18 mm specimens generally exhibit the lowest \(RON\) values, while \(D\) = 14 mm specimens present the highest, confirming once again the influence of slenderness and rigidity on the achievable geometric quality. This is consistent with the deformation and dimensional deviation analyses discussed earlier. Finally, when these values are compared with those of the reference FFF specimens, it becomes evident that the machining process has improved them. While the high-quality printed specimen exhibits relatively good \(RON\) values, it is still surpassed by the values obtained through turning operation. In contrast, the low-quality FFF shows considerably worse \(RON\), further validating the need for post-processing in applications requiring precise macrogeometric control. The analysis of \(STR\) deviation (Figure 9b) further reinforces the influence of machining parameters and specimen geometry on the macrogeometric quality of turned PLA-CF parts. As with the trend observed in \(RON\), \(STR\) values also tend to improve with increasing \(v_{c}\). In most cases, the \(STR\) deviation is significantly lower at \(v_{c}\) = 200 m/min, reflecting a more stable interaction between the tool and the workpiece in the axial direction of the part. With respect to \(f\), higher values generally result in larger \(STR\) deviations, especially in specimens with smaller diameters. This is attributed to the increased cutting forces and mechanical vibrations induced by the higher material removal rate, which can lead to microscopic deviations from the ideal cylindrical axis, particularly when the stiffness of the part is insufficient to resist flexural excitation. In terms of diameter, there is a clear relationship between specimen rigidity and straightness deviation. Larger diameters show lower \(STR\) values, indicating greater resistance to axial bending and deflection during turning.