# Study of Drilled Holes after a Cryogenic Machining in Glass Fiber-Reinforced Composites
###### Abstract  
Glass fiber-reinforced composites are widely used in industry, with machining operations frequently performed, drilling, in particular, for later assembly. Although there is a smaller increase in temperature during drilling in composites than in metals, further cooling of the tool can produce improvements in some variables, such as thrust force, diameter, or surface roughness. This has been seen in studies where lower temperatures were achieved by cooling compressed air, reaching around \(-20~{}^{\circ}\)C in plates of polyether-ketone and polyamide, reinforced with glass fiber at 30% (PEEK-GF30 and PA-GF30, respectively). This paper analyzes the results of cryogenic drilling in plates of PEEK-GF30 and PA-GF30, specifically assessing thrust forces, diameter, and average surface roughness. The experimental methodology was carried out by monitoring thrust forces during cryogenic drilling using a piezoelectric dynamometer, measuring diameters with a coordinate measurement machine, and assessing surface quality with a roughness profilometer. During the cutting, the temperature of the cutting tool achieved a temperature near \(-120~{}^{\circ}\)C from cooling with liquid nitrogen. Conducting an analytical and statistical study allowed us to determine the relationships between the measured variables and cutting conditions. Our results showed that cooling the tool during the drilling processes improved results of the cutting process. c 2022  
at temperatures near \(-20\)\({}^{\circ}\)C [7]. In drilling operations of PEEK-GF30, thrust forces can also decrease at temperatures close to \(-20\)\({}^{\circ}\)C and oversized holes can be avoided [8]. In a review on the cooling process using compressed cold air by vortex tube in machining, it was concluded that improvements in surface quality and tool life were also achieved [9]. On the other hand, cryogenic machining is another method of cooling that is considered, first, because this avoid using refrigerants in the cutting process of alloys; thus, this process is considered sustainable and safe, without health risks [10]. [PERSON] et al. [11], in their literature review, found that the cryogenic LN\({}_{2}\)-assisted machining process was economically sustainable, but proximity between LN\({}_{2}\) production and the machining workshop would be convenient to reduce the carbon footprint generated by its transport. In general, cryogenic cooling improves machinability, reducing the cutting temperature [12] to temperatures that can reach below \(-100\)\({}^{\circ}\)C. Benefits have also been found in terms of improved machinability under high performance conditions, albeit in metals, particularly in 42 CrMo4 steel [13]. Besides the improvements in machinability of composites, cutting forces and tool life have been shown to improve in Kevlar fiber composites [14]. In addition, the cutting forces [15] or surface roughness [16] can be reduced compared with dry milling in carbon fiber composites; in general, the mechanical proprieties in these composites are lower after cryogenic drilling than after a conventional drilling, although this is not due to damage [17]; variables such as surface quality or tool wear are better in cryogenic drilling than in dry drilling, particularly with 4 mm diameter drill bits [18]. In carbon fiber-reinforced polymers, factors such as the resin used can affect the results, with epoxy showing good results for the hole diameter [19]. With these fibers, a lower delamination factor and higher thrust forces have been found in cryogenic drilling, compared with dry drilling [20]. However, lower thrust forces have also been reported, but with better surface quality and lower specific cutting energy consumption [21]. In fiberglass-reinforced composites, such as an S2 glass fiber with epoxy matrix, cryogenic drilling, with 6 mm diameter bits, achieves diameter deviations greater than the tolerances between \(\pm 20\) and \(\pm 40\)\(\upmu\)m that exceeds recommendations in aeronautical structures for hole size [22].