## 3 Results and Discussion  
### Analysis of Thrust Force  
The thrust force used in drilling processes is one of the main components of force generation affected by the drilling parameters, workpiece mechanical properties, number of drill holes, tool wear, drilling operation and machine tool structure [27]. Previously, the thrust force generated during the one-shot single drilling process has been widely studied [3; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Therefore, this study represents the thrust force generated during multi-hole simultaneous drilling operations. Figure 2 shows the thrust force obtained from the uncoated carbide and TiN and TiCN coated carbide drills. It shows that thrust force was found to increase with the increases in the feed where reasons might be attributed to the increase in uncut chip thickness [37; 38; 39] and the rapid rubbing action of the tool and workpiece at high feed, which might have increased the heat energy being absorbed by the tool, which caused the tool wear [40]. Figure 2 also shows a decrease in thrust force with an increase in the spindle speed.  
Figure 1: Details of the equipment.  
According to [PERSON] [41], the increase in temperature at high spindle speed affects the strength of the workpiece and causes the plastic deformation of the workpiece, which leads to a reduction in thrust force. Further, at the highest spindle speed selected in this study, a slight increase in the thrust force was observed at the high feed, which might be due to the increase in the number of holes, which promotes tool wear due to high friction at the interface of the tool-workpiece and subsequent high-force generation [42]. However, in general, the impact of spindle speed was not significant on the thrust force.  
It was also observed that the uncoated carbide drills produce a lower thrust force than the TiN and TiN coated drills, regardless of the drilling parameters. However, the reduction in the thrust force using uncoated drills was minimal and did not exceed 5% compared to that generated from using the coated tools. The lower thrust force from uncoated drills was mostly noted at the low spindle speeds. At a spindle speed of 3025 rpm and increasing feed, there are negligible differences between the thrust force generated by uncoated and TiN coated drills and somewhat lower thrust force values from TiN coated drills. This could imply that TiN coated tools might show a better performance than the other two tools when machining at higher spindle speeds [6]. The highest impact on the thrust force was due to feed following the drill type while no significant impact was found from the spindle speed, or the linear interactions between the studied parameters as shown in ANOVA results given in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Source** & **DF** & **Seq SS** & **Adj SS** & **Adj MS** & **F-Value** & **\(p\)-Value** & **Contribution** \\ \hline Model & 18 & 11,25,285 & 1,125,285 & 62,516 & 671.61 & 0 & 99.93\% \\ Linear & 6 & 1,120,666 & 1,120,666 & 186,778 & 2006.57 & 0 & 99.52\% \\ \(n\) & 2 & 663 & 663 & 332 & 3.56 & 0.078 & 0.06\% \\ \(f\) & 2 & 1,117,200 & 1,117,200 & 558,600 & 6001.1 & 0 & 99.22\% \\ \(Dt\) & 2 & 2803 & 2803 & 1401 & 15.06 & 0.002 & 0.25\% \\
2-Way Interactions & 12 & 4619 & 4619 & 385 & 4.14 & 0.026 & 0.41\% \\ \(n\times f\) & 4 & 2131 & 2131 & 533 & 5.72 & 0.018 & 0.19\% \\ \(n\times Dt\) & 4 & 1363 & 1363 & 341 & 3.66 & 0.056 & 0.12\% \\ \(f\times Dt\) & 4 & 1125 & 1125 & 281 & 3.02 & 0.086 & 0.10\% \\ Error & 8 & 745 & 745 & 93 & - & - &