**4. Cutting parameters**

Cutting parameters are important in drilling operations because they have a significant influence in determining the rates of material removal, tool wear, and tool life which could affect the drilled hole quality. The important cutting parameters which need to be appropriately considered and selected in drilling operations are cutting speed and feed rate.

## **4.1 Cutting speed**

A cutting speed is measured in terms of the rate at which the outside or periphery of the tool moves to the work being drilled. The range of cutting speeds that are normally used in drilling Al 7075 is within 50 to 250 m/min for 6 to 8 mm diameter cutting tools. It was reported that increasing cutting speed caused a reduction in the machined surface roughness due to the improved material shearing [9]. Based on [9], which conducted drilling experiments using 180, 200, 220, and 240 m/min of cutting speeds, it was found that the surface roughness of Al 7075 decreases by 5.49% when the cutting speed is increased from 180 to 240 m/min. However, an increase in cutting speeds could increase the tool chatter which affects the surface roughness of the machined surface. This view is supported by previous research [10], which also reported that increasing cutting speed could increase the tool vibration caused by the spindle rotation, which would lead to poor machined surface finish. To avoid the tool chatter and vibration due to high cutting speed during drilling, proper fixture and clamping of the work material, as well as a secured spindle head must be ensured before starting the drilling operation.

Furthermore, high cutting speed can also cause an increase in cutting temperatures between the tool and workpiece due to high heat generated during drilling operations which may lead to a higher tool wear rate. When the cutting speed is increased, the cutting temperature also increases which would result in the workpiece material sticking at cutting edges. This is supported by a previous study [11], which found that increasing cutting speed from 60 to 100 m/min caused the cutting temperature to increase from 195 to 240°C during drilling of Al 7075. Another previous study [9] shows that the flank wear increased from 0.08 to 0.19 mm due to an increase in cutting speeds from 180 to 240 m/min with 0.1 mm/rev feed rate which also resulted in decreased surface roughness from 4.015 to 3.619 μm. Although, higher cutting speed causes higher tool wear, using too low cutting speed is not recommended as it may cause the formation of Built-Up Edge (BUE) on the cutting edge which leads to high machined surface roughness and also cause low productivity. Based on [12], BUE formation was observed when drilling using a low cutting speed of 40 m/min with a cutting temperature of 160°C which resulted in surface roughness, Ra of 1.16 μm. Hence, using moderate cutting speeds within the range of 100 to 220 m/min is generally recommended when drilling Al 7075 to maintain good productivity.

#### **4.2 Feed rate**

Feed rate is a major factor that influences chip formation, cutting forces and hole quality. Feed rate is the distance that the drill moves into the workpiece for each complete turn of the cutting tool. The range of feed rates that are typically used in drilling operations of Al 7075 is within 0.01 to 0.10 mm/rev. A previous study [13] found that an increase in feed rate from 0.05 to 0.2 mm/rev at a constant cutting speed of 50 m/min caused the thrust force to increase from 825 to 1020 N and produce continuous chips which could entangle in the drill flutes hence causing poor machined surface finish. Another previous study [14] examined the effect of

feed rate on drilled holes and found that at high feed rates, there is a deterioration in the shape of the holes. This is due to the high thrust forces caused by the thick chip formation. Generally, a low feed rate is recommended when drilling Al 7075 as it causes low thrust force, produces a good machined surface finish and prolongs the tool life.
