**7. Hole quality**

Ensuring that the holes are produced with good quality as required is important for component assembly hence the product functionality and service life. The drilled hole quality can be measured and assessed in terms of the hole diameter, roundness, machined surface roughness and burr formation.

#### **7.1 Hole diameter and roundness**

The diameter of drilled holes has been reported to increase when the cutting speed, feed rate and point angle increase [25]. The increase in cutting speed and feed rate leads to the increase in the oversized hole because of the vibrations that are induced at higher speed and feed rate [10, 26]. HSS drills have been reported to produce undersized holes compared to carbide drills [26]. Therefore, carbide drills are recommended for drilling the Al alloy to produce holes within the required tolerance. The authors conducted a drilling experiment on Al 7075-T6 (thickness of 13 mm) using 6.5 mm diameter carbide tools with two different feed rates of 0.01

**Figure 8.**

*The effect of feed rate on hole diameter in drilling Al 7075-T6 (heat-treated) at a constant cutting speed of 120 m/min and 6.5 mm diameter carbide cutting tools.*

and 0.1 mm/rev at a constant cutting speed of 120 m/min. The drilled hole diameters were measured and results are shown in **Figure 8**. Increasing hole numbers resulted in increasing hole diameter, which is likely caused by the increased chip adhesion on the cutting tool. Generally, the use of a higher feed rate of 0.1 mm/rev resulted in more accurate hole diameters closer to the tool nominal diameter of 6.5 mm.

In addition, hole roundness is also an aspect that determines the drilled hole quality. The roundness of the hole is defined as how closely the hole matches a perfect circle, which can be determined by measuring the consistency of the hole diameters at various orientations. The hole roundness error has been reported to be significantly influenced by the feed rate [15]. Based on [15], which conducted experiments on drilling aluminum alloy using a 6 mm diameter drill, the hole roundness error was found to be lower (0.030–0.038 mm) when using a lower feed rate of 0.04 mm/rev compared to the higher feed rate of 0.08 mm/rev which causes higher roundness error (0.05–0.06 mm). However, no significant difference in the roundness error was observed when cutting speeds changed from 25 to 50 m/min [15]. Therefore, using a low feed rate is recommended to ensure the hole is produced with minimum dimensional error. However, as cutting speed is likely to have less effect on the hole diameter, a higher cutting speed is recommended to improve productivity. Nevertheless, the tool wear needs to be observed as the material adhesion could also affect the hole dimensional accuracy.
