## 3 Results and Discussion
### Evaluation of the Hole Quality
#### 3.2.2 Surface Roughness  
The surface roughness has a key role in the evaluation of machining performance of a workpiece ([PERSON] et al., 2018). Figure 6 indicates that \(R_{a}\) was affected by the drilling parameters, irrespective of the coatings of the  
Figure 5: Hole images from TiN coated carbide drills.  
drills. Increase in both spindle speed and feed resulted in increased \(R_{a}\); however, the spindle speed was found to be more effective in increasing the \(R_{a}\) compared to feed. This might be due to the increase in the ductility of the workpiece due to the rise in temperature at high spindle speed [43]. Additionally, the possibility of high vibration at high speeds would have caused the rough surface roughness [11]. Regarding the high feed, the increase in chip thickness could be the source of an increase in \(R_{a}\)[37].  
Figure 6 also shows that the uncoated carbide tool gave lower values of \(R_{a}\) than the TiN and TiCN coated drills. This is likely attributed due to the built-up edge (BUE) because of the higher chemical affinity of aluminium for coatings due to the constant release of particles on the tool's surface, which results in high thrust force and increased \(R_{a}\)[10]. However, the TiCN coated drill performed better at a high spindle speed of 3025 rpm. According to [PERSON] et al. [6], the reason for the lower \(R_{a}\) of holes drilled using coated tools at high spindle speed is because the coating provides a thermal barrier from high temperature therefore reducing the diffusion wear process at high cutting speed for dry drilling of Al2024. Additionally, the \(R_{a}\) obtained during multi-spindle drilling performance of TiN of Al2024 was higher than those measured from TiCN, regardless of the drilling parameters. The reason behind this is expected to be due to the low hardness values and low friction coefficient of TiN compared to TiCN [24]. The hardness of TiN is 23 GPa while that of TiCN is 27 GPa [24]. Previous studies have shown that TiCN coated tools provide better tool life than TiN coated tools due to their higher hardness [44]. Moreover, the addition of carbon adds more hardness and makes the TiCN coating more resistant to adhesions, making TiCN coated tools somewhat more effective in reducing \(R_{a}\) at higher speeds and feeds [45]. Additionally, the coefficient of friction of TiN and TiCN is 0.4 and 0.2, respectively [24], and coatings with a low friction coefficient resulted in low cutting loads and less tool wear; hence, small values of \(R_{a}\) can be expected. This is because the low friction coatings can reduce the tendency to stick and pick up material from the surface used in cutting and forming tools [46]. Table 3 shows the percentage contribution from ANOVA, which indicates that the spindle speed was the most influential drilling parameter on the \(R_{a}\), following by the feed and the drill type, which includes the uncoated and coated drills. The linear interaction of the input parameters had a negligible contribution on \(R_{a}\) of less than 1.5%.  
Figure 6: Average surface roughness (\(\mu\)m).  
Therefore, from the ANOVA results, it can be concluded that the use of coated drills does not provide any significant contribution to reducing thrust force or \(R_{a}\) when drilling Al2024 alloy. However, this might be only true for the range of studied feeds and speeds. A broader study that looks into the performance of the studied coatings at higher speeds and feeds is recommended.