# Preassembly Cryogenic Drilling in Carbon Fiber Sandwich Sheets with Inner Foam Core
## 1 Introduction  
Drilling, as a preassembly operation in processes such as riveting, requires that the machined holes achieve good surface quality to avoid subsequent finishing operations. The importance of this operation is demonstrated in the standards set in the industry for holes, and numerous studies have been carried out on all types of materials, such as reinforced polymers [1, 2], to find adequate cutting conditions. More recently, sandwich materials have been increasingly studied because they can expand industrial use due to their combination of material properties. These types of materials can be found in aerospace, marine, construction, and automotive applications. An essential reason is that these composites can allow lighter structures, especially when combined with skin materials such as carbon fiber [3]. Recent efforts have been undertaken to determine mainly mechanical properties, for example, in panels of cork and basalt-fiber-reinforced material,used as core and skin materials, respectively, to determine the relationship between stress and strain therein [4]. For this reason, the drilling operation is particularly complex. In general, machining sandwich materials presents greater difficulties than using more typical materials, even leading to different results depending on the arrangement of the same materials [5]. During drilling, the thrust forces are heterogeneous and could hinder the quality of the outlet holes. Differences in the quality of the inlet and outlet holes are observed in sandwich materials, such as the composite of two external aluminum plates bound to a polyethylene core [6] or a carbon/glass fiber sandwich structure [7]. In this type of material, such as balsa wood composite sandwich panels and corrugated foam and PVC foam sandwich panels, the feed rate has been shown as the most significant factor among the delamination and uncut fiber factors during drilling [8]. In other composites such as polyurethane foam sandwiched between glass-fiber-reinforced plastic laminates, the thrust force increases as the thickness of the material increases and as the feed rate increases. However, even if the thickness increases, if the rotational speed increases, the force decreases [9]. In drilling with aluminum honeycomb as the core material, with carbon-fiber-reinforced polymer (CFRP) as the top and bottom surface, the thrust force has been increased with increased feed rate and aluminum thickness and with worse surface results at the exit of the drill compared to the entry [10]. [PERSON] et al. [11] have found that, in drilling CARALL (material composed of aluminum stack plus CFRP layers), lower diameter values were obtained at the hole exit compared to the hole entry in drilling. Another material is CFRP with foam-core polyvinyl chloride, which has important marine applications [12], including providing a protection structure for polar engineering equipment [13] and responding to hailstone impact [14] and impact damage in artic conditions [15], and yet its behavior in manufacturing operations has not been analyzed. Thus, low temperatures can influence the behavior of fibers in composites [16]. However, recently, in a literature review regarding drilling temperature, similar findings can be highlighted, such as that the different cooling methods and workpiece and tool materials used in the experiments may result in different outcomes, as can be seen in different machining processes [17; 18]. Moreover, the influence of these factors on drilling temperature does not display a purely linear relationship [19], and the temperature has little influence on the thermal expansion coefficient, though more so in the fibers, at least in the polyether-ether-ketone and the glass fibers [20]. As can be seen [21], cooling is achieved through a Ranque-Hilsch vortex tube, with compressed air impinging on the tool during machining; this method has allowed a decrease in the cutting force, increasing the tool life and improving the surface finish, among other advantages.