## 3 Results
### Surface Damage Analysis  
Figure 6 shows SEM images of the inner borehole wall conditions of AA2024, AA6061, and AA5083 for holes drilled under dry conditions during the multi-spindle simultaneous drilling process. It was observed from the SEM micrographs that the damage and distortion found in the holes increased with the increase in spindle speed (\(n\)) and the feed (\(f\)). This is speculated to be due to the increase in vibration level, which influenced the geometrical hole tolerances. Additionally, it can be attributed to the increased thermal softening of the work material because of the rise in cutting temperatures with the increase of \(n\) and deformations due to an increase in the \(f\)[27].  
Figure 4: Effect of cutting parameters on hole cylindricity.  
Figure 6 also revealed the formation of feed marks, deformation marks due to chips evacuation, adhesive debris, smearing, and plastic flow on the surface of aluminium alloys. Smearing is a thin layer of plastic deformation that forms at the surface of the workpiece. It is often difficult to recognize and mainly occurs in materials with ductile natures [28]. The smearing in this instance appeared due to the rise in temperature because of increased \(n\) and \(f\). The random chip debris on some of the surfaces of the hole walls was also caused by the generation of material smearing [27]. Moreover, the deformation marks might be either due to the rubbing action of the chips that collided with the hole walls during evacuation [3]. The feed marks occurred as a result of an increase in feed. Furthermore,  
Figure 5: Effect of cutting parameters on hole perpendicularity.  
Figure 6: SEM of the hole drilled at 3025 rpm and 0.14 mm/rev.  
there were also chances of material side flow near the regions of feed marks where the plastically deformed material was plugughed aside due to the high thrust force and cutting temperature when the tool was used under dry cutting conditions [29]. Interestingly, no visible cracks were found on the surface under all cutting conditions. However, the damage and distortion in the hole of AA5083 were found more often than those seen in AA2024 and AA6061. A likely explanation for this might be the poor thermal conductivity of the work material, which enhances the heat accumulation in the primary deformation zone [30]. In addition, drilling at a higher \(n\) and \(f\) showed plastic flow within the hole walls of AA5083, which might also be due to its ductility and low silicon contents.  
Figure 7 shows the top and bottom edges of the holes of all the alloys studied in this work. The results showed that the holes at both the top and bottom of the hole for all the three alloys (AA5083, AA6061, and AA2024) increased with an increase of \(n\) and \(f\). The results also showed that the hole edges of all the alloys were found to be more uniform around the edges, while the bottom sides had irregular appearances and heights. Therefore, the holes at the entrance were more deteriorated at the bottoms than the tops, except for AA5083, where most deterioration occurred at the lower \(n\) and \(f\). Other details can be found in Appendix A. Additionally, a detailed discussion of the results can be found in the succeeding section.