# Preassembly Cryogenic Drilling in Carbon Fiber Sandwich Sheets with Inner Foam Core
## 2 Materials and Methods
### Experimental Method  
The materials used are carbon fiber sandwich sheets with inner foam core, and they were manufactured by prepreg, pressing, and oven-curing. This material is light-weight and exhibits good mechanical properties. The orientation of the carbon fibers was bidirectional 90\({}^{\circ}\)/0\({}^{\circ}\), with 3000 filaments per yarn strand and 200 g/m\({}^{2}\) and 2 \(\times\) 2 twill fabric type; its moisture content was of 0.5%. The core consisted of foamed polyvinyl chloride (PVC), closed cell, and a density of 60 kg/m\({}^{3}\). Epoxy resin was used as the binder between the layers. Two dimensions were used: 12 mm and 6 mm of thickness, and a surface of \(175\times 110\) mm\({}^{2}\) in both specimens. The thickness of each carbon fiber sheet was 1 mm. Thus, two types of plates, with core thicknesses of 4 and 10 mm, were used.  
These plates were drilled in a Manga Tongtai TMV-510 (Tong-tai Machine and Tool Co., Ltd., Kaohsiung Hsien, Taiwan) machining center, using a 6 mm diameter polycrystalline diamond (PCD) drill bit, with a 120\({}^{\circ}\) point angle and a 30\({}^{\circ}\) helix angle and designed for drilling composites. The twist drill provided better results for sandwich materials with carbon fiber than other drill bits [10]. The cutting conditions were drill rotation speeds (N) between 2000 rpm and 6000 rpm and feed rates (F) between 200 mm/min and 600 mm/min. These conditions allow high performance due to high feed and maintain a balanced rotation speed and feed rate relationship; previous tests have allowed these conditions to be identified as acceptable. During the machining, the temperature reached slightly lower than \(-120\)\({}^{\circ}\)C due to the liquid nitrogen (LN\({}_{2}\)) used as a refrigerant and supplied by Air Liquide (AL Air Liquide Espana, Madrid, Spain); LN\({}_{2}\) impacts the drill through a 2 mm diameter nozzle. This temperature was measured using an infrared pyrometer (Optris Infrared Sensing, Portsmouth, NH, USA). The drill bit after cryogenic machining is shown in Figure 1a. During drilling, the thrust forces (Fz) were monitored by a piezoelectric dynamometer type Kistler 9257B (Kistler Group, Winterthur, Switzerland); this equipment was connected to a multichannel signal amplifier, Kistler 5070A, to transform electrical signals into units of force; finally, the data collected were processed by DasyLab 9.0 software (Measurement Computer, Norton, MA, USA).  
The inlet (Di) and outlet (Do) diameters of the holes were measured by a co-ordinate measuring machine, Mitutoyo BX 303 (Mitutoyo Corp., Kawasaki-shi, Japan) (see Figure 1b), using the least squares method to determine the diameter value through the measured points in the hole. The points were measured at 0.5 mm from the outer surface. The difference between the nominal diameter of the drill bit and the inlet diameter (\(\Delta\)Di) and outlet diameter (\(\Delta\)Do) of the drilled hole were taken as the study variables. Moreover, a three-dimensional measurement device with a TESA VISIO optical sensor (TESA SA, Renens, Switzerland) was used to identify possible loose fibers after drilling.  
The experiments were repeated four times; as a result, 72 tests were performed under the defined cutting conditions.