**5. Chip formation**

Chips are formed during drilling operation as the aluminum alloy is removed by the cutting edges. Understanding the chip formation and morphology is important because it can influence the tool wear and hole quality. A drilling operation on Al 7075-T6 was conducted by the authors using 6.5 mm carbide drills at cutting speeds of 120 and 160 m/min as well as feed rates of 0.01 and 0.1 mm/rev. It was found that the chips produced are in the form of continuous, spiral, and discontinuous, as shown in **Figure 2a**.

The continuous chip in the form of a spiral or ribbon, as shown in **Figure 2b**, formed during drilling with increasing cutting speed. This is supported by previous research [15–17], which explained that the continuous chips formed due to material straining caused by thermal softening as a result of high cutting temperature. The material became more ductile, which led to the formation of continuous chips. Whereas, thinner continuous chips, as shown in **Figure 2c** are formed when the feed rate is decreasing. Continuous chips are typically undesirable because they could cause difficulty in chip evacuation as the chips entangle around the drill and jam in the flutes, which consequently could cause high surface roughness and poor drill hole quality [18]. In this case, a chip breaker is needed to break the chips into smaller pieces during drilling to facilitate chip evacuation from the drilled hole.

Based on [18], the chip becomes shorter as the drilling operation progresses as the resistance between the material and the drilling bit decreases. In addition, the chip produced unfolds due to the opposite of the torque of the friction to the chip rotation. The break-up of the chip into pieces is when the chip reaches its fracture point, whereas the continuous cone-shaped spiral chips break into smaller pieces when the chip exceeds the breaking torque between the tool and hole wall [18]. In addition, the use of chilled air during drilling Al 7075 with carbide drill could be favorable as it was found to result in shorter chips than dry drilling by 20 and 32.5% at cutting speeds of 120 and 160 m/min, respectively, as shown in **Figure 3**. The chip started to change into a segmented chip from a continuous chip due to the lower cutting temperature which lowered the thermal softening and material straining.

#### **Figure 2.**

*Chip morphology of Al 7075-T6 during drilling operations (a) continuous, discontinuous and smaller pieces chips (b) cone-shaped spiral chips (c) thinner continuous chips.*

#### **Figure 3.**

*Comparison of chip length when drilling Al 7075-T6 at cutting speeds of 120 and 160 m/min and a constant feed rate of 0.05 mm/rev.*
