# Impact of Cryogenic Condition and Drill Diameter on Drilling Performance of CFRP
## 2 Materials and Methods  
The CFRP plate used in the experiments was fabricated from plain, woven, carbon-fiber fabric that is of 0/90\({}^{\circ}\) fiber orientation. Epoxy resin was used as the matrix. The plate was 5 mm thick with the dimensions 500 mm \(\times\) 500 mm. The drilling experiments were conducted on a three-axis vertical machining center under dry and cryogenic cutting conditions. The drill bits were solid carbide drills with a diameter of 4 and 6 mm. These drills were chosen due to their special application for the machining of fiber-reinforced materials (Figure 1). A new drill bit was used for machining each hole to avoid any tool wear effect.  
The machining control factors considered for the experiment were feed rate and drill diameter. The cutting speed considered for the experiments was at a constant level of 50 m/min, the feed rates were at four levels--0.075, 0.15, 0.225, and 0.3 mm/rev--and the drill diameters were at two levels, 4 and 6 mm. The cutting parameters and their levels were chosen by taking into account the authors' experiments, previous studies published by other authors, and the recommendations of the cutting tool catalog. Generally, a drill that has the same diameter as the thickness of the plate to be drilled is used. In this study, in order to investigate the effects of drill diameter on the drilling of CFRP, both smaller (4 mm) and larger (6 mm) diameter drills than the thickness of the plate (5 mm) were utilized. A total of 16 experimental runs were conducted; eight were carried out in dry conditions and eight in cryogenic conditions. A photographic view of the experimental set-up is presented in Figure 2. The experimental set-up consisted of a drilling machine, an adjustable fixture, and a piezoelectric dynamometer. The dynamometer was rigidly mounted on the machine table. The CFRP composite was held by an adjustable fixture clamped to the piezoelectric dynamometer. In this study, liquid nitrogen (LN\({}_{2}\)) at \(-\)196 \({}^{\circ}\)C was used as the cryogenic coolant. The CFRP was machined in a thermally insulated fixture that was full of liquid nitrogen. The purpose of using the thermally insulated fixture was to hinder the hazardous effects of LN\({}_{2}\) on the dynamometer and the machining center.  
During the drilling operation, the thrust force was measured and monitored with the help of a Kistler 9257B type dynamometer (Kistler, Winterthur, Switzerland), a data acquisition box, and an amplifier. The signal was transmitted through the amplifier to a PC for data collection. The exit-hole surface delamination was recorded using an Olympus SZX7 stereo trinocular microscope with 12.5\(\times\) magnification. In addition, scanning electron microscope (SEM) (Quanta SEM, FEI Company, Hillsboro, OR, USA) analyses were also utilized to facilitate the delamination of the hole damage and tool wear induced by the drilling. The average surface roughness (Ra), root mean  
Figure 1: Drill bits used in the experiments.  
Figure 2: Experimental set-up.  
square roughness (Rq) and the three-dimensional (3D) view of the drilled hole were measured offline using an atomic force microscope (AFM) (NanoMagnetics Instruments, Oxford, UK).