**3. Cutting tools**

Cutting tools in terms of the tool geometry and tool material are important factors that need to be determined before conducting drilling operations because they can affect the tool wear, tool life and hence surface finish of the Al 7075. Twist drill, as shown in **Figure 1** (the image of the tool was captured by the authors), is the common type of drill that has been used, especially in drilling Al 7075 because it has helical grooves to facilitate the chip evacuation along with the flutes. The helix angle is the angle that forms between the leading edge of the drill and drill axis [5]. The efficiency of the drilling operation can be improved by using a drill with a low helix angle since the tool strength is improved by reducing the helix angle. For drilling Al 7075, the common helix angle that has been used is 30°. The point angle, which is the angle form between cutting lips is also important to ensure drilling efficiency. The standard point angle that is normally used for general purpose drilling is 118°.

The selection of drill material is important because it determines the toughness, hardness, wear-resistance and thermal resistance of the drill during the drilling process. The drill materials that have been used in drilling Al 7075 are commonly made from high-speed steels (HSS) and tungsten carbide (WC). It has been stated that the HSS drill is often used due to the availability in a wide range, low cost and hardness; however, the HSS drill does not resist high cutting temperature well [6]. To manufacture high-performance components, tungsten carbide drills are preferable as they usually produce better hole quality due to their ability to resist high cutting temperature and maintain their toughness compared to HSS drills. Less material adhesion, less BUE, low tool wear and improved chip formation were reported when using carbide drills during drilling Al 7075 compared to HSS drills [7]**.** This is supported by previous research [8] which found that tungsten carbide drill has 20% longer tool life than HSS drill, and it also produces high-quality holes with good dimensional accuracy within a range of 6.04 to 6.12 mm when drilling with 6 mm drill diameter in dry condition. Therefore, a tungsten carbide drill is generally recommended for drilling Al 7075.

**Figure 1.** *Typical cutting tool for drilling aluminum alloys.*
