Influence Of Double Point Angle Drill Geometric Parameters On Thrust Force And Delamination In Drilling CFRP

Drilling of carbon fiber reinforced polymer (CFRP) is an necessary procedure in the assembly of composites, however, CFRP consists of soft viscous resin matrix and high-strength high-hardness fiber buildup cross, making this material unique anisotropy and lamination property, which would easily cause many kinds of damage during drilling process, such as delaminantion, spalling and burrs.Drill point is main region which takes part in cutting, and the geometric characteristics of drill point directly determine cutting process and the contact between tool and workpiece, so drill point geometry is an important factor affecting defects. This paper takes T800 CFRP laminates as research object, in order to explore the suitable tool geometry which achieves low defect, firstly, several comparative experiments were carried out adopting special drill bits, it is determined that double point angle drill point is a suitable drill shape for CFRP drilling. Then the influneces of double point angle drill point geometric parameters on thrust force and outlet delamination were studied combining the methods of theoretical analysis and experiments, the results would provide some evidences for directing tool design. The main research contents and conclusions are as follows:(1) Series of drilling experiments were implemented on CFRP laminates by twist drill, step drill, double point angle drill and arc drill. Based on the measured thurst force and cutting temperature during the drilling process, the reason for outlet delamination was analyzed. Results show that double point angle drill drills out of the buttom of workpiece by stages, which would cause thrust force returning to zero slowly in multi stages, and the drilling temperature is relatively low, so the final hole has a small outlet delamination facror and good hole quality.(2) Parameterization was conducted for the dimension of double point angle drill, then a theoretical chisel thrust force model was established based on Hertz contact, and a cutting edges thrust force model was established based on orthogonal cutting. Undetermined coefficients were calibrated by drilling experiments of thirteen different geometric parameters double angle point angle drill bits, and the model was valided by comparing predicted and experimental thrust force of two different double point angle drill. The average error of predicted chisel edge thrust force is about 11%, the average error of predicted main cutting edge is about 2%. Finally, by the analysis of thrust force model, the rules that thrust force changing with geometric parameters was obtained.(3) Double point angle drill bits are classified into convex and concave double point angle drill by different configuration of point angles. By analyzing the thrust force distribution on cutting edges, the thrust force changing process of last fiber layer and outlet delamination. It is found that tool peripheral thrust force of convex double point angle drill is smaller, whih could shorten the crack length, generating better hole quality. It is found that total thrust force is not proportional to outlet delamination factor, but thrust force distribution on cutting edges has a more direct effect on delamation factor. This paper shows that tool design should consider the distribution rules of thrust force at first, then the total reduction of thrust force. Drill bits should be designed by controlling a small thrust force in the periphery of drill bits to get a smaller delamination factor in the final hole exit.