Technology And Equipment Of Helical Milling For Aerospace Components Assembly

During the assembly process of aerospace equipment,a large number of parts or components need to be connected according to the design requirements through rivets or bolts,so it is necessary to make holes on the aerospace components in advance.Making assembly holes on aerospace components faces many difficulties,such as large quantity and complex operating environment.Especially for the new aerospace equipment in China,there are more problems in the process of development or batch production,such as larger amount of difficultto-cut materials,more large diameter and large depth assembly holes and higher quality requirement,which pose new challenges to hole-making technology.Due to some limitations,the existing traditional drilling process combined with boring and reaming has been unable to meet the requirements of high quality and efficient processing for large diameter and large depth assembly holes of difficult-to-cut materials.Helical milling is a new hole-making process.Compared the traditional hole-making process,helical milling has many advantages,such as high quality,high efficiency and low cost,showing a good application prospect in assembly hole machining.Correlative researches on helical milling have been carried out,but there is a lack of in-depth and thorough basic theoretical analysis,few researches on control methods of hole-making and no mature helical milling equipment have been developed.As a result,the practical application of helical milling in aerospace component assembly is restricted.This paper aims to realize the high quality and high efficient processing of assembly holes,focus on the problems existing in the application of helical milling.Firstly,kinematics analysis was carried out and the cutting force model was established,then the causes of machining errors and machining defects of helical milling were studied and the high precision and low damage helical milling processes were developed.Finally,a series of helical milling equipment was developed and the corresponding technology and equipment had been applied in domestic aerospace enterprises.The main research contents and conclusions of this paper are as follows:(1)Considering down milling and up milling,the geometry and kinematics models of helical milling were built and the cutting tool motion trajectory during helical milling was simulated,in order to analyze the cutting process and undeformed chip of bottom cutting edge and peripheral cutting edge.Based on the analysis of undeformed chip,the cutting force model of helical milling considering down milling and up milling was established.Then,the helical milling experiment system was structured and the cutting force coefficients of titanium alloy,composite and aluminum alloy were identified.The verification experiments were carried out and the measured helical milling cutting forces are in agreement with the predicted ones,showing an error of 8.1%.(2)Based on the helical milling cutting force model,the causes and influencing laws of machining errors were analyzed.The results show that the radial cutting force is the main reason of hole diameter deviation during helical milling.The radial force directions in down milling and up milling are different.leading to smaller and larger hole diameter,respectively.The extrusion between the peripheral cutting edge and the hole wall also produces radial force.which causes the hole diameter to decrease with the hole depth.Based on the analysis above.a prediction model of helical milling hole diameter was established.which considered the effect of the milling strategy and hole depth.Then,a method for controlling the machining error of helical milling by means of using a short peripheral edge tool and adjusting eccentricity was presented.The experimental results show that the prediction error of the hole diameter prediction model is less than 0.015 mm and the precision of hole making reaches H7 level after using the proposed machining error control method.(3)The forms and characteristics of machining defects in helical milling were analyzed and it is revealed that the axial cutting force is the main reason for machining defects.such as burr at hole-exit of metal materials and delamination at hole-exit of composite materials.Based on the geometry and kinematics models,the influence of the bottom cutting edge geometrical parameters on the shape of the undeformed chip and the axial cutting force was analyzed.Then.a process method for controlling the machining defects of helical milling by using a large "gap angle" tool was proposed.By increasing the "gap angle",the part near the center of the bottom cutting edge does not participate in cutting.thus reducing the axial cutting force and machining defects.As a result.the delamination factor of CFRP is less than 1.4 and nearly no machining defect was observed at the hole-exit of aluminum alloy material.The process method for controlling the machining defects of helical milling based on reversed feed was further developed.By using the two-way milling cutters and the composite drilling and milling tool,the feed direction is reversed,the stress state of the workpiece material at hole-exit is changed and as a result,the production of machining defects is suppressed and making near defect free holes on composite materials and metal materials is realized.(4)The key technologies in the development of helical milling equipment,such as the accurate measurement of nominal hole diameter and the accurate adjustment of eccentricity.were conquered.A series of portable helical milling equipment and automatic helical milling equipment were developed for different requirements of hole making in aerospace component assembly.The helical milling technology and equipment developed had been applied in domestic aerospace enterprises.Compared with the traditional "drilling-boring-reaming"combined process,the number of holes making processes was reduced by more than 80%,the efficiency was increased by more than 5 times and the cutting tool cost was reduced by more than 80%.