# Impact of Cryogenic Condition and Drill Diameter on Drilling Performance of CFRP
## 3 Results and Discussion
### Evaluation of Thrust Force and Delamination Factor  
During a drilling operation, the determination of the thrust force is important in order to define (determine) the delamination of the CFRP material. Figure 3a illustrates a sample chart of overall thrust force-time cycles for CFRP drilling. The feed rate and cutting speed were held constant at 0.075 mm/rev and 50 m/min. Figure 3b shows a sample chart of the overall thrust force for the four feed rates examined (0.075, 0.15, 0.225 and 0.3 mm/rev.) at 50 m/min cutting speed. At the beginning of the cycles when the center and chisel edges of the cutting tool come into contact with the workpiece, the thrust force rises quickly. This is due to the high thrust force at the chisel edge region of the drill. The thrust force remains stable for a short time and reaches the maximum rapidly. All cutting edges of the cutting tool commence machining at this point. Then, the thrust force decreases as the drill emerges from the laminate and goes to zero when the chisel edges and the cutting lips exit the material. The thrust forces shown in Figure 4 were obtained from the experiments by using a 50 m/min constant cutting speed, 4 and 6 mm diameter drill bits, and 0.075, 0.150, 0.225 and 0.300 mm/rev feed rates as parameters under dry and cryogenic conditions. As shown in the figure, larger thrust forces occurred under the cryogenic conditions when compared with dry conditions, and thrust force also increased depending on the increase of the feed rate and the drill diameter. An increase in the feed rate and the drill diameter increased the cross-sectional area of the unremoved chip. This led to an increased load on the tool; thus the thrust forces generated by using larger diameter drills and feed rates are greater [5; 20; 21; 22]. In dry conditions, it is expected that heat formation at the cutting zone is higher during drilling since cutting fluid is not used. This heat occurrence softens the matrix (epoxy) and the thrust force decreases. In cryogenic conditions, the material is exposed to extreme cold LN\({}_{2}\) and the Young modulus and tensile strength of the CFRP increases as the temperature of the material decreases. Therefore, the thrust force generated in the drilling of CFRP rises in cryogenic conditions [17]. As can be seen in Figure 4, much more force was generated in drilling with a 6 mm  
Figure 3: (**a**) Thrust force–time cycles for a hole; (**b**) Relationship between feed rate and thrust force. diameter drill bit under the cryogenic conditions. The reason for the higher thrust force when drilling in cryogenic conditions with a 6 mm diameter drill is in accordance with the explanations above. In the drilling of composites, delamination is a critical damage characterized by the separation of adjacent piles caused by an external action [23]. In a machining operation, minimizing the delamination factor is an important criterion [24]. The delamination factor was calculated by the ratio of the maximum diameter (D\({}_{\text{max}}\)) of the damage zone to the nominal hole diameter (D). Figure 5 demonstrates the delamination factor variation with different feed rates. As shown in the figure, larger delamination factors occurred under cryogenic conditions when compared with dry conditions. The delamination factor increased with the increasing of the feed rate. Also, the 6 mm diameter drill generated larger delamination than the 4 mm drill. As can be seen, the difference between the delamination factors which occur under dry and cryogenic conditions increase depending on the feed rate. Also, it can be seen that there is a compliance between Figures 4 and 5. This shows that there is a strong correlation between thrust force and the delamination factor. In order to investigate the effects of the number of holes on the delamination factor and thrust force, 342 holes were drilled on a CFRP plate using 50 m/min cutting speed, 0.15 mm/rev feed rate  
Figure 4: The effect of drill diameter, feed rate, and dry and cryogenic conditions on thrust force.