## 3 Results  
### Analysis of Hole Size Analysis and Circularity Error  
The evaluation of hole size is one of the essential output parameters of the drilling process, especially during dry drilling, which is associated with higher cutting temperatures at the tool-chip interface due to increased friction and clogging of the chips over the flutes of drill bits [7]. Figure 2 shows the hole size of AA2024, AA6061, and AA5083 at different spindle speeds and feeds. The results showed that the holes obtained in AA6061 were less deviated from the nominal size than AA2024 or AA5083. The hole size for AA6061 ranged between 5.9990 and 5.9696 mm, while this range increased to 6.0034-6.0863 mm for AA2024 and 6.0333-6.0941 mm for AA5083. Certain industries, such as aerospace industries, demand tight hole tolerances in metallic structures, as tight as \(\pm\)12 microns and as relaxed as \(\pm\)30 microns (H7 to H9 hole tolerance fit) [3]. Cutting tool manufacturers state that an acceptable hole size in aluminium alloys can range between \(\pm\)20 and \(\pm\)40 microns [23]. Other industries, such as in automotive industries, have more relaxed tolerances due to a less critical impact on the integrity of the vehicle structural components. This means that most of the hole size data reported in this study for AA2024 and AA6061 alloys fell within the limits of allowable hole tolerances in aerospace and automotive structures. For the AA5083 alloy, although not used in aerospace applications, it appears that the deviation in hole size exceeded the maximum recommended in aeronautical structures. Similarly, hole size deviation in the AA6061 alloy reported in this study was significantly lower than that reported in a recent study [24]. The only difference in the tools used in both studies was that the previous study used coated tools (TiN, TiAlN, and TiN/TiAlN), while the current study used uncoated tools. The coating layer thickness, despite being less than several microns, might have had an impact on increasing the hole size deviation. Therefore, it can be said that drilling an AA6061 alloy using uncoated carbide drills might yield a hole size closer to the nominal drill diameter. The results show that, in general, hole size tended to increase with the increase of the spindle speed (\(n\)) and the feed (\(f\)), especially for the AA6061 and AA5083 alloys. This could be attributed to the increased vibratory displacement with the increase of the \(n\) and the \(f\), which occurs during the initial contact between the chisel edge and the workpiece and triggers dynamic instability, causing higher hole size deviations [7; 25]. However, it is evident from Table 3 that the impact of \(f\) on the hole deviation was found to be insignificant,  
Figure 1: Hole geometry measurement setup using CMM. with a percentage contribution of 2.74%. Overall, the hole deviation was affected more by the type of alloy, with a percentage contribution of 51.75%. The influence of \(n\) was found to be 9.71%, while the two-way interaction of spindle speed and alloy type significantly impacted hole size with a contribution of 24.81%. Figure 3 shows the circularity error of the hole at different drilling parameters during multi-hole simultaneous drilling for all the three alloys selected in this study. It is illustrated in Figure 3 that drilling in AA2024 showed less hole circularity error than AA5083 and AA6061 alloys, irrespective of the drilling parameters. AA5083 showed a higher circularity error than that obtained in the drilling of AA6061; however, at the \(n\) of 3025 rpm, the circularity error increased compared to AA6061. This means that AA5083 performs better when machined at the lower \(n\), while the AA6061 alloy shows lower hole circularity error when machined at a higher \(n\). This could mainly be attributed to the higher magnesium content in the AA5083 alloy compared to the AA6061 alloy.