Study On Mechanism Of Cutting Edge And Low Defect Hole Making Drill For Drilling Carbon Fiber Reinforced Plastics

Carbon Fiber Reinforced Polymer(CFRP)have excellent mechanical properties and are widely used in high-end equipment,national defense,aerospace,new energy and other fields,and the proportion of applications is increasing year by year.When CFRP components are assembled with other structural parts,drilling is one of the main machining processes.However,the anisotropy,low interlaminar bonding strength,high hardness,low thermal conductivity and dusty chip make the drilling process of CFRP easy to produce delamination,burr,tear and other defects.So,CFRP is a typical difficult-to-machine material.The researches show that one of the effective ways to reduce CFRP drilling defects is to optimize the tool structure.Focusing on the formation and control strategy of drilling defects in CFRP,this paper studies the mechanism of the chisel edge and main cutting edges cutting CFRP and develops the low-defect hole making drill,which provides theoretical guidance for the design and process optimization of low-defect drilling tools.The main research contents are as follows:(1)The fracture behavior of CFRP micro-cutting fibers and the characteristics of macro-cutting surface were studied.The finite element simulation model of 3D micro-cutting is established to analyze the fiber breakage and matrix failure under four typical fiber cutting angles.The experiments of UD-CFRP laminates milling with straight groove milling cutters with different rake and relief angles are carried out,and the effects of rake and relief angles on cutting force,burr and surface quality are studied.The fracture morphology and chip morphology of fibers at different cutting angles are analyzed to study the formation mechanism of cutting damage,to establish the chip formation model at different cutting angles,and obtain the surface defects mechanism of the machined surface as well as the machining entry and exit.(2)The influence of cutting behavior of drill chisel edge on the formation of CFRP hole-making defects and its control strategy are studied.The experiment of drilling UD-CFRP laminates with twist drill is designed to study the influence of processing parameters on the axial force of the chisel edge.The influence of drilling behavior of the chisel edge on the quality of the hole exit is studied by drilling blind holes.In order to reduce the influence of chisel edge on the fomation of hole defects,the influence of step ratio(the ratio of initial bit diameter to secondary bit diameter)on thrust force,exit delamination and sidewall damage is studied,and the optimum step ratio range is obtained according to theoretical analysis and experimental results.(3)The influence of the cutting behavior of the main cutting edge on the formation of CFRP hole-making defects is studied,and the expression equation of the thrust force based on the cutting force coefficient of the cutting edge is established.The effects of four kinds of special drills(dagger drill,double point angle drill,candle stick drill and)on the quality of hole entry and exit are studied to study the geometric characteristics of four drills and the influence of main cutting edge on the quality of hole entry and exit,to analyze the characteristics of time-varying curve of thrust force in drilling process.The method of predicting the key inflection points on the time-varying curve of thrust force is proposed to construct the time-varying curve model for predicting the thrust force,the result shows that the method can better prdict the time-varying curve of the thrust force of straight-edged drills.The existence angle of burr is defined to measure hole burr damage,and the inflence of main edge geometry on existance angle of burr and delamination is studied.The distribution of the thrust force on the main cutting edge of a typical straight-edge drill(double point angle drill)is analyzed.The influence of processing parameters on the cutting force coefficient of the main cutting edge is studied.According to the linear increase of the thrust force when the straight-edge cuts in,the thrust force equation based on the cutting force coefficient of the main cutting edge is established.(4)A new type of compound drill bit is designed to study its drilling performance and tool wear performance.The cutting edge force coefficient is used to characterize the thrust force of the step drill.Combining the structural characteristics of the double point angle drill and the candle stick drill,the cutting edge structure is optimized,and a novel compound drill is developed.The wear morphology and characteristics of double point angle drill,step drill and new compound drill are studied.The variation of tool life evaluation index of thrust force,burr and delamination with tool wear are analyzed.The wear criteria and tool life of the three drills are obtained.The research shows that the double point angle drill fails due to the excessive thrust force.However,the stepped drill and the novel compound drill fail because the exit delamination range reaches the threshold.