## 4 Results and Discussion  
### Investigation of the Inputs' Influence on the Responses  
Figure 6 illustrates the combined influence of the input parameters on the responses. In general, the surface suggests that a strong effect is present for all combinations. Specifically, Figure 6a depicts the combined effect of cutting speed with feed, which is the strongest of all. It is evident that higher feed rates coupled with a cutting speed of over 250 m/min are responsible for increased surface roughness values. This finding is supported by a previous work of the authors for similar conditions [27], where surface roughness was maxed at values beyond 250 m/min. Figure 6b shows how the cutting speed interacts with the depth of cut. It was found that the worst surfaces were generated between 1 mm and 1.5 mm at maxed cutting speed. Contrarily, 0.5 mm depths and low cutting speeds favored quality. Similarly, low feeds close to 0.6 mm/rev and low depths of cut contributed towards better surface quality, as shown in Figure 6c. On the other hand, higher depths always lead to surfaces of low quality. Higher depths of cut were also reported to negatively affect surface roughness when machining pure PETG [44]. In general, a similar magnitude of surface roughness was determined by works on finishing laser machining [45], as well as finishing milling [46] of 3D-printed CFRP components. Regarding the effects on the generated cutting temperature, Figure 6d indicates that the combination of cutting speed and feed has the most considerable effect, leading to high temperature values when feeds of over 0.1 mm/rev are combined with high levels of cutting speed. The lowest temperature was identified at 0.05 mm/rev feed and 115 m/min cutting speed. As expected, higher speeds and faster tool translations generated more friction and heat, ultimately leading to an increase in temperature. [PERSON] et al. [36] reported similar findings regarding the rise in temperature during CFRP composite machining. Continuing, Figure 6e,f confirm that cutting speed is the most influential parameter. In contrast, the depth of cut does not affect the temperature significantly, as opposed to its behavior in terms of the surface roughness. Finally, when combined with feed, it presents a slightly more influential pattern. In general, the machining of carbon-fiber-reinforced PETG was determined to be more difficult than pure polymer. [PERSON] et al. [47] reported constant porosity when PETG testing specimens were machined, irrespective of the printing orientation angle. In contrast,  
Figure 6: The surface plots of the responses in relation to the input combinations: Vc–f for Ra (**a**), Vc-ap for Ra (**b**), ap–f for Ra (**c**), Vc–f for T (**d**), Vc-ap for T (**e**), and ap–f for T (**f**). a previous work of the authors [13] identified the 3D-printed CFRP materials as challenging when machined, especially at high feeds and depths, reporting defects at the layer bonding that affect porosity. Despite this issue, the 3D-printed CFRP tends to generate superior surfaces compared to pure polymer when machined at a certain range of conditions. According to [PERSON] [48], the carbon fibers, in this case, strengthen material uniformity and structural stability. The results reported in the aforementioned study indicate that milling PETG generates surfaces with roughness values between 5 \(\upmu\)m and 8 \(\upmu\)m, with feed rate being the dominant parameter. The similarity with this study lies in the feed rate being the most influential factor, whereas the generated surface roughness for CF-PETG is lower, with values below 4 \(\upmu\)m. This fact supports the benefits that derive from the addition of carbon fibers in the polymer matrix. Furthermore, another study [49] investigating the dry turning of pure PETG exhibited remarkably higher values of surface roughness, at the range of 30 \(\upmu\)m, indicating that 3D-printed carbon-fiber-reinforced polymers produce superior surfaces during machining.