**6. Tool wear**

Tool wear is an important element that can affect tool life, the machined surface finish of the drilled hole and hence hole quality. Tool wear is the change in the geometry and dimension, particularly at the cutting edges, which indicates the failure rate of the cutting tool due to drilling a significant number of holes. The tool wear mechanisms that typically occur in drilling Al 7075 are abrasive and adhesive wear, as shown in **Figure 4**, which was observed during experiments conducted by the authors**.** The abrasive wear, as shown in **Figure 4a**, occurs as the tool material (e.g., carbide and cobalt), especially at the cutting edges, were abraded due to the removing process of the harder workpiece material (e.g., Al7075) in the drilling process.

#### **Figure 4.**

*Tool wear mechanism when drilling Al 7075-T6 (a) abrasive wear, (b, c) adhesion of material, and (d) chipping at the cutting edge of carbide drills.*

#### **Figure 5.**

*Measurement of tool wear is taken as the change in flank length indicated by A, B, and C after drilling each 10th hole compared to the initial flank length.*

Based on [19], scratches at the cutting edges parallel to the cutting direction will show the proof and region of abrasive wear. It was reported [20] that the abrasion mechanism occurs at a low cutting speed due to a low chip removal rate compared to drilling at a higher cutting speed. To minimize the abrasive wear, a cutting tool with a harder material than the workpiece material should be used in the drilling operation. Adhesive wear, as shown in **Figure 4b** and **c**, occurs when the chip or machined material adhere to the cutting edges due to high cutting temperature and pressure, which can cause edge chipping, as can be seen in **Figure 4d** when the adhered material breaks off [21, 22]. Previous studies [20, 23] reported that increasing cutting speed from 76 to 198 m/min in dry drilling resulted in material adhesion due to an increase in cutting temperature, thus will lead to higher tool wear.

The tool wear is typically affected by the cutting parameters and process conditions used. The authors experimented drilling Al 7075-T6 heat-treated using 6.5 mm carbide drills to investigate the effect of cutting speed and feed rate on tool wear. The tool wear was determined by measuring the change in the flank, as shown in **Figure 5**. This measurement follows ISO 3685:1993, which recommends that the end of tool life is reached when the flank wear is equivalent to 0.3 mm.

From the experiment conducted by the authors, it can be seen in **Figure 6** that increasing cutting speed from 80 to 120 and to 160 m/min at a constant feed rate of 0.05 mm/rev resulted in increasing tool wear. This is likely due to increasing heat generation, which caused weakening of the tool material as well as material adhesion on the cutting edges. Feed rate also has a significant effect on tool wear. As shown in **Figure 7**, increasing feed rate from 0.01 to 0.05 and to 0.1 mm/rev

#### **Figure 6.**

*Tool wear in drilling Al 7075-T6 (heat-treated) using 6.5 mm carbide drills at cutting speeds of 80, 120 and 160 m/min and a constant feed rate of 0.05 mm/rev.*

**Figure 7.** *Tool wear in drilling Al 7075-T6 (heat-treated) using 6.5 mm carbide drills at feed rates of 0.01, 0.05 and 0.1 mm/rev and a constant cutting speed of 120 m/min.*

at a constant cutting speed of 120 m/min was found to increase tool wear. This is likely due to the increasing volume of material removed per tool rotation that could cause higher thrust force and weaken the cutting edges leading to higher tool wear. This indicates that a low cutting speed and low feed rate can produce low tool wear therefore a longer tool life.

Even though using a low cutting speed and a low feed rate can cause a lower tool wear rate in drilling Al 7075-T6, it is usually not preferable in the industry as this can lead to longer production time hence low productivity. Therefore, moderate cutting speed and feed rate could be used when productivity is a concern, although frequent tool change may be needed to ensure the tool wear does not affect the hole quality. Drilling in a dry condition typically results in high tool wear due to heat generated between workpiece material and cutting tool. Therefore, the presence of cutting fluid and chilled air during drilling Al 7075 could improve the tool wear. Referring to [24], chilled air can act as a coolant to reduce the temperature at the cutting zone, thus will result in low tool wear, hence longer tool life.
