**7.3 Burr formation**

Burrs are the common defect that occurred due to the machining process. A burr is the protrusion of workpiece material typically at the hole edges (entry and exit of the hole), as shown in **Figure 13a** and **b**. The burr height is measured as indicated in **Figure 13b**. The burr is hard and sharp, which can cause difficulty in assembly and may cause injury to the operators. Therefore, the burr is unwanted and needs to be deburred to ensure the good assembly of the parts.

The machining process could result in primary and secondary burrs. The primary burr occurs during the drilling operations after the material has been removed by the cutting edges. The secondary burr is the remaining material at the edge of the drilled hole after breakage of the primary burr due to deburring process. The formation of burr depends on the cutting parameters (cutting speed and feed rate), ductility of workpiece material, and tool geometry especially point angle. The authors experimented on dry drilling of Al 7075-T6 using carbide drills and it was found that using a higher cutting speed of 120 m/min generally resulted in a higher burr compared to 80 m/min, as shown in **Figure 14**. This is likely due to an increase

#### **Figure 13.**

*(a) Burr at the edge of the drilled hole (b) Burr height which is measured after the hole was sectioned into a half.*

#### **Figure 14.**

*The burr height when drilling Al 7075-T6 (heat-treated) using cutting speeds of 80 and 120 m/min at a constant feed rate of 0.05 mm/rev.*

#### **Figure 15.**

*The burr height when drilling Al 7075-T6 (heat-treated) using feed rates of 0.01 and 0.05 mm/rev at a constant cutting speed of 120 m/min.*

in ductility of the alloy as more heat is generated with a higher cutting speed, causing the material to project at the edge [28]. In addition, the burr formation can also be influenced by feed rate. As shown in **Figure 15**, the usage of a higher feed rate of 0.05 mm/rev resulted in higher burr compared to the lower feed rate of 0.01 mm/ rev. This is likely due to the higher volume of material removed.

A previous study [7] on the burr formation of Al 5083 caused by cutting parameters has also been conducted, in which the finding could be related to the case of Al 7075. Feed rates within the range of 0.04 to 0.14 mm/rev and cutting speed range within 19 to 57 mm/min were used [7]. It was found that the feed rate highly influences the growth of burrs during drilling compared to cutting speed. Drilling at feed rate 0.14 mm/rev with 57 m/min cutting speed produced high burr formation, which is 7 μm, while drilling at 0.04 mm/rev feed rate produced the lowest burr formation which is 3.8 μm. The burrs resulting from drilling Al 5083 were observed to be more visible at the exit holes compared to entry holes. Therefore, using a low cutting speed and feed rate in drilling Al 7075 could be beneficial to minimize the burr formation, however, this may cause low productivity in the industry.

## **8. Future development in drilling technology**

Drilling operations have always been necessary for the manufacturing and assembly of mechanical components, which are made of aluminum alloys. This motivates the industry and researchers to further investigate the drilling technology that can produce good hole quality with longer tool life, hence optimizing productivity. Ultrasonic Assisted Drilling (UAD), which employs a cutting tool that is vibrated during the material removal process, has been receiving attention in the industry. The application of UAD on high-performance materials, including aluminum alloys, Carbon Fiber Reinforced Polymer (CFRP), and titanium alloys, has shown potential to reduce tool wear, reduce thrust forces and increase material removal rate, which is reported to be due to intermittent cutting and reduced chip resistance [21, 29]. In addition, the use of cryogenic coolant (i.e., carbon dioxide and liquid nitrogen) could be useful to facilitate heat removal during the drilling process, especially for deep hole production, which could result in less material adhesion and improved hole surface finish. Therefore, further research involving

these drilling technologies, particularly on aluminum alloys, is necessary for industrial applications.
