# Study on Burr Formation and Tool Wear in Drilling CFRP and Its Hybrid Composites
## 2 Materials and Methods
### Machine Setup and Drilling Experiment Details  
A series of experiments were carried out on a bed-type computer numerical control (CNC) machine (HISUPER-4, Hwacheon, Co., Ltd., Seoul, Korea). The CNC machine is a 3-axis model consisting of linear motors and FANUC 0-Mate controller with a maximum spindle speed and power of 10,000 rpm and 7.5 kW, respectively. The main CNC machine specifications are displayed in Table 1. The schematic of the drilling experimental setup is shown in Figure 2. A Kistler 9271A piezo-electric type force-torque dynamometer was employed to measure thrust force and torque during the drilling experiment. A complementary Kistler 5017A 8-channel charge amplifier was connected to the force dynamometer to amplify the output charge signals. All data were acquired and recorded by an A/D card converter and was further analyzed by software on a PC computer. The tested laminate was clamped in a jig attached to a dynamometer, which was mounted on a CNC machining table. In order to better observe the burrs formation and tool wear, the jig was drilled with 15 mm diameter holes. Before commencing the series of experiments, the whole setup was thoroughly calibrated.  
The cutting tool used for all drilling tests was an 8 mm diameter carbide drill with a point angle of 90\({}^{\circ}\), a relief angle of 10\({}^{\circ}\), a helix angle of 30\({}^{\circ}\) and a total length of 63 mm. The cutting tool and its specifications are displayed in Figure 3. The size of the cutting tool was chosen since it is one of the most common sizes used to produce holes for rivets in aerospace structures. Moreover, the smaller point angle is helpful to reduce thrust force and is more conducive to drilling operation. [PERSON] et al. [35] found that a 90\({}^{\circ}\) drill point angle gives better results, as compared to 104\({}^{\circ}\) and 118\({}^{\circ}\), in drilling-induced damage while drilling UD-GFRP composite laminates. On the other hand, the spindle speed and feed rate are two of the most important parameters in analyzing drilling quality and are always selected as research objects. Many researchers have pointed out that the burr formation and the hole circularity depend on different cutting parameters [36]. In this study, 300 holes were, respectively, drilled on CFRP and C-AFRP laminates with back support at a feed rate of 200 mm/min and a spindle speed of 2000 rpm. Oil cooling cannot be used in drilling  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Item** & **Unit** & **Specifications** \\ \hline Table size & mm & 1350 \(\times\) 450 \\ X, Y, Z axis & mm & 950 \(\times\) 430 \(\times\) 500 \\ Rapid traverse (X, Y, Z axis) & m/min & 12 \(\times\) 12 \(\times\) 8 \\ Spindle motor & kW & 7.5 \\ Max spindle speed & rpm & 10,000 \\ Machine weight & kg & 5300 \\ Electric power & kVA & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Computer numerical control (CNC) machine specifications.  
Figure 1: Images of (**a**) CFRP and (**b**) C-AFRP.  
operation because it will lead to the contamination of the FRP materials. Thus, the cooling system was developed by means of a Vortex Tube Magnet Base 2 (Seyang Mechatronics Co., Ltd., Gyeonggi-do, Korea) with a hot component and a cool component. The advantage of this cooling system is that the cold air flow and temperature can easily be controlled to obtain more homogeneous distribution of cooling by adjusting the slotted valve in the hot air outlet.