## 4 Discussion  
Hole quality is assessed by factors such as cutting parameters, tool materials and coatings, tool geometry, the vibration of the drill, the chatter or drilling temperature, etc. [31]. Generally, in this study, the impact of \(n\) was found more than \(f\), especially in the case of deviation of hole size and cylindricity error. According to [PERSON] et al. [7], the deviation of hole size due to the high \(n\) was expected due to the increase in the drilling vibration, which might impact hole geometrical features. Moreover, there were chances of increasing temperatures, especially at high spindle speeds, which could have influenced the hole accuracy and surface finish. However, it is worth noting that the ANOVA analysis indicated that the \(n\) and alloy type and their interaction were significant on the hole size and circularity. Meanwhile, ANOVA results showed that both \(n\) and \(f\) did not have an impact on the hole perpendicularity, which means that different aluminium alloys will have different circularities under the same cutting parameters. Furthermore, other parameters which were not taken into account in this study could have a strong influence on perpendicularity, such as vibration, hole location, or machine dynamics [23]. Additionally, according to [PERSON] et al. [12], such variations could have resulted from errors in measurement by CMM, as even after cleaning the surface, there might be chances of debris potentially adhered to the surface that could have affected the measurement. In addition to the drilling parameters, ease of machining was affected by the work material properties like hardness, tensile properties, chemical composition, microstructure,  
Figure 7: SEM of top- and bottom-hole edges drilled at 3025 rpm and 0.14 mm/rev. and work material production. Therefore, the above results showed that the surface finish of AA6061 was better than those obtained during the drilling of AA2024, following AA5083. This might be related to the high percentage composition of silicon in AA6061 compared with AA2024 and AA5083, as previous research concluded that a high silicon content might result in a good surface finish [32; 33]. Furthermore, according to [PERSON] [34], the higher value of elongation is related to the amount of plastic deformation, which is governed by the ductility of the material. Therefore, it might also be possible that the highest percentage of elongation affected the surface finish of AA2024. Overall, the worst surface finish was generated in the drilling of AA5083, which was attributed to its low machinability and low silicon contents [16]. Furthermore, according to [PERSON] [35], hardness is an essential characteristic of materials, affecting the surface finish, and it is worth noting that AA5083 has the lowest hardness compared to the two alloys. Hardness tends to form high built-up edges, and hence, a poor surface finish. Additionally, the above results indicated that the poor machinability and larger thermal coefficient of AA5083 compared to AA2024 and AA6061 caused the more oversized and irregular pattern that formed around the top and the bottom of the hole edges [16]. This finding also confirms our previous result where the surface roughness of AA6061 was found to be the lowest, following AA2024 and AA5083. Furthermore, the highest build-up edge was formed on the tools used for drilling of AA5083, due to long and thick chips tangled around the tools [16]. The above SEM image results also illustrated that the surface finish at the top region was affected more than at the bottom of the hole. This is also in agreement with [PERSON] et al. [7], who suggested that the vibration may have maximum values when the drill touches the workpiece. The spring back phenomenon can occur because of the workpiece material's elastic deformation during the drilling tool movement. However, in this study, most of the drilling parameters, including the hole surface finish at the bottom, were highly affected.