## 2 Materials and Methods  
The alloys of aluminium used in this study were AA5083, AA6061, and AA2024, which are the commonly used alloys in automotive, marine, and aerospace industries,respectively [16]. The chemical compositions in wt.% and the properties of aluminium alloys studied in this work are given in Table 1.  
A manual milling machine model (5 KS, 5 KV) that has a maximum spindle speed of 3450 rpm was used for the drilling operations, and a SUNHER poly-drill head type, MH30/13, was used for the multi-hole simultaneous drilling process. Spindle speeds of 1007, 2015, and 3025 rpm and feeds of 0.04, 0.08, and 0.14 mm/rev were selected based on the limitations of the available manual milling machine and previous literature on drilling aluminium alloys in the multi-spindle drilling process [16; 17; 18; 19; 20]. All the drilling experiments were conducted under dry conditions due to environmental concerns and the cost associated with the cutting fluid and its disposal [21; 22]. The drill bits used were 6 mm twist drill uncoated carbide. The details of tool geometry are given in Table 2.  
The hole's geometrical tolerances, such as hole size, circularity, perpendicularity, and cylindricity, were measured at the top and the bottom of each hole using a coordinate measuring machine (CMM), type Mitutoyo Crysta-Apex S776, equipped with a Renishaw PH10 MQ, fitted with an AM2 adjustment module and an SP25M scanning probe system as shown in Figure 1.  
A 2 mm diameter ruby stylus was used for the measurements, first taking five points on the top surface of the sample to generate a base plane. Afterwards, three points were used to locate each hole before taking the geometric data by scanning the inside surface and taking points at a 0.1 mm pitch. A Gauss filter was applied to these points. Two measurement passes were performed inside each hole, 1 mm below the top surface of the sample and 1 mm above the bottom surface. With \(\mathcal{O}\)6 mm holes, this resulted in between 95 and 97 points per measurement. Circles were generated from this data using the Maximum Inscribed Circle for each measurement. Roundness calculations were done internally by MCOSMOS. The hole size was also measured by scanning circles at the top and bottom of the bore with a \(\mathcal{O}\)2 mm ruby stylus. Points were taken every 0.1 mm, and the circle was calculated using the Maximum Inscribed Circle of the resulting point cloud, and the diameter of the circle was then taken. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multicolumn{4}{c}{**Composition in wt.\%**} & \multicolumn{3}{c}{**Properties**} & \multicolumn{3}{c}{**Aluminium Alloy**} \\ \cline{2-7}  & **AA2024** & **AA6061** & **AA5083** & & **AA2024** & **AA6061** & **AA5083** \\ \hline Mg & 1.2–1.8 & 0.8–1.2 & 4.0–4.9 & Machinability & 70\% & 50\% & 30\% \\ Si & 0.5 & 0.4–0.8 & 0.4 & Shear strength (MPa) & 283 & 207 & 190 \\ Mn & 0.3–0.9 & 0.15 & 0.4–1.0 & Ultimate tensile strength (MPa) & 483 & 310 & 317 \\ Ti & 0.15 & 0.15 & 0.15 & Hardness, Vickers & 137 & 107 & 96 \\ Zn & 0.25 & 0.25 & 0.25 & Elongation break & 18\% & 12\% & 16\% \\ Cu & 3.8–4.9 & 0.15–0.4 & 0.1 & Modulus of elasticity (GPa) & 73.1 & 68.9 & 71 \\ Fe & 0.5 & 0.7 & 0.4 & Hardness, Brinell & 120 & 95 & 85 \\ Cr & 0.1 & 0.04–0.35 & 0.05–0.25 & Thermal conductivity (W/m-K) & 121 & 167 & 117 \\  & & & & The average coefficient of thermal & & & \\ Al & Balance & Balance & Balance & expansion (Coefficient from 20 to 100 \({}^{\circ}\)C) (\(\mu\)m/m-\({}^{\circ}\)C) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Details of aluminium alloys used in the current study [16]. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Specification/Description**} \\ \hline Type & Twist drill \\ Material & Uncoated carbide \\ Number of flutes & 02 \\ Point angle & 140\({}^{\circ}\) \\ Helix angle & 30\({}^{\circ}\) \\ Drill diameter & 6 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Details of drill bits.