# Impact of Cryogenic Condition and Drill Diameter on Drilling Performance of CFRP
## 1 Introduction  
Carbon fiber-reinforced polymer (CFRP) has such perfect properties as high stiffness, light weight and high strength. Owing to these properties, the material is commonly used for modern aerospace industry applications [1]. Machining of composites is generally carried out to obtain desired geometrical shapes and tolerances [2]. Conventional drilling, the subject of this paper, is a widely utilized machining technique in aircraft production [3]. [PERSON] et al. [4] investigated the influence of drill diameters and cutting conditions on the thrust force and drilling performance of glass fiber-reinforced epoxy (GFRE) composites. They reported that delamination-free holes in the drilling of GFRE composites were not obtained in the range of the tested parameters. [PERSON] et al. [5] presented the effect of the fiber-volume fraction, feed rate, cutting speed, and drill size on the thrust force, torque, and surface roughness in the drilling of fiber-reinforced composite materials. The authors concluded that cutting speed had an unimportant influence on the thrust force and surface roughness of epoxy resin, and drill diameter combined with feed significantly affected surface roughness. The influence of drill diameter, feed rate and spindle speed on delamination in the drilling process of glass fiber-reinforced polypropylene (GFR-PP) thermoplastic composites was studied by [PERSON] et al. [6]. The response surface method was used to make a model and optimize delamination damage during drilling. The result showed that the models utilized to predict delamination in the drilling of GFR-PP composites were effective. Besides this, feed rate was the most influential parameteron delamination. [PERSON] and [PERSON] [7] presented an estimation and evaluation of delamination when using a twist drill, candle stick drill and saw drill. In the experiments, three different drill bits with different diameters, feed rates and spindle speeds were used as cutting variables. The authors demonstrated that the feed rate and the drill diameter were the most influential factors affecting overall performance. [PERSON] et al. [8] made an attempt at studying the effects of cutting parameters on delamination when a bi-directional carbon-fabric-reinforced polymer composite was drilled by using uncoated and TiN-coated solid carbide drills. The authors implied that the diameter of the drill had an important effect on delamination. The experimental study also showed that delamination increased with an increase of drill diameter and feed rate and decreased with an increase of spindle speed. [PERSON] et al. [9] presented the impact of such machining parameters as feed rate, spindle speed, and drill diameter on the thrust force when drilling polypropylene laminates. They revealed that the principal factor influencing the machining process of composites was feed rate, followed by drill diameter. There are many studies in the literature regarding the drilling of metals [10; 11; 12] such as aluminum [13], magnesium [14], steel [15], and white cast iron [16] under cryogenic conditions. However, in contrast to metals, studies in relation to the machining of polymer composites under cryogenic conditions are limited. [PERSON] et al. [17] comprehensively investigated the influence of the cryogenic cooling process on the drilling performance of CFRP. They reported that cryogenic cooling reduced the rounding of the cutting edge of a drill bit and outer corner wear; it also improved the surface quality of drilled holes. However, this cooling process generated larger thrust force and torque, and this caused the occurrence of a larger delamination factor. [PERSON] et al. [18] investigated the influence of minimum-quantity lubrication and liquid nitrogen cooling in the drilling of glass-aluminum-reinforced epoxy fiber-metal laminates.