| Carbon fiber reinforced polymer(CFRP)has been widely adopted in manufacturing key load-bearing parts in large airplanes due to its light weight,high specific strength,etc.A huge number of holes need to be drilled on the CFRP parts in the whole assembling process.CFRP is prone to defects such as delaminations in drilling,which is detrimental to the overall service performance of final product after assembling.In order to improve drilled hole quality,the one-shot drill bit is currently employed in drilling CFRP.Due to the special staging drilling structure,the one-shot bit can effectively reduce thrust force and then minimize delaminations.However,for aeronautical-grade CFRP,the fiber reinforcement has high strength and high hardness,thus it has strong abrasiveness,which leads to rapid and severe tool wear as well as subsequent drilling defects.For drilling tools with complex structures,various types of wear can occur on the different cutting edges involving in drilling process,which induces different defects as well.Therefore,it is of great necessity to study the different wear mechanism and law along various cutting edges of one-shot drill bit and then focus on the specific wear impacts on drilled hole quality.Finally it is proper to explore how to reduce corresponding tool wear of cutting edges and subsequently improve drilling quality.The research in this paper utilizes the tungsten carbide one-shot drill bit,and mainly focuses on wear study of the chisel edge influencing hole-exit delamination and the secondary cutting edge influencing final quality under the recommended drilling parameters.By shorting wear measurement interval,cutting edge rounding(CER)and flank wear(VB)are utilized to represent the wear progression of chisel edge and secondary cutting edge.For one-shot drill bit,it is the secondary cutting edge that determines the drilled hole quality.On the basis of tool wear research of secondary cutting edge above,the tool wear impacts on the cutting capabilities of secondary cutting edge are systematically analyzed,and then the wear-induced defects and defect initiation mechanisms are revealed.In order to improve final quality of drilled holes,tool wear reduction should be carried out on specific positon of secondary cutting edge especially its end part.Different coolant outlet holes are designed on secondary cutting edges of separate drill bits,which helps to deliver MQL mists out.And the effects of cooling and lubricating position on wear reduction of end part of secondary cutting edge are compared and analyzed.The major results are listed below:(1)Based on micro observation of cutting edge and variation of cutting edge rounding,chipping is found to be dominant at the chisel edge,and secondary cutting edge is reground and re-sharpened after flank wear.(2)On the basis of wear analysis of secondary cutting edge and chip formation mechanism,the cutting edge rounding mainly determines the cutting capability of secondary cutting edge under acute fiber cutting angles.For prepreg laminates cut under 120°-170° fiber cutting angles,denting defects can be generated due to severe tool wear,while in adjacent laminates,uncut fibers easily occur.(3)Ejecting MQL mists at the end of secondary cutting edge can help to reduce specific tool wear at the end part of the edge,and thus improves drilled hole quality by reducing drilled hole surface defects.The paper explores the tool wear along different cutting edges of one shot drill bit,and then analyzes specific tool wear impacts on drilled hole surface quality.Finally the tool wear reduction of specific position on the cutting edge is done to effectively improve drilling quality.The results in the paper help to guide researches on drill bit design with long tool life and superb drilling capability.Also the results are valuable for the development of drilling process for further improvement of drilled hole quality. |
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