Study On Geometrical Error In Helical Milling Of Titanium Alloy

Helical milling is a new hole-making technology in aircraft assembly.Compared with conventional drilling technology,helical milling technology is widely used in difficult-tomachine materials because of its advantages such as high processing efficiency,low cost and wide applicability.Because the technological characteristics of helical milling,such as the axis of the tool and the axis of the hole are not coincident,which leads to the radial force generated in the process of machining cannot be offset,resulting in the deformation of the tool and the hole geometrical error.There is a direct impact on the quality of the assembly and connection of aerospace structural parts,resulting in reduced safety and reliability of aircraft.Hole geometrical error includes hole diameter error and hole shape error in helical milling,Aiming at two kinds of hole errors,titanium alloy material is selected as the research object to carry out the hole error test of helical milling The influence of down and up milling and other different machining parameters on the hole size error and the hole shape error was analyzed,and the mechanism of the hole error was theoretically analyzed.The main research contents are as follows:Based on the helical milling kinematics principle,the undeformed chip shape and material removal process under down and up milling were studied.According to the classic cutting force model,the radial cutting force models of helical milling under different conditions of down and up milling were established respectively.The coefficients of the radial cutting force model are calibrated through the calibration test of the cutting force coefficient.Test is carried out,and the accuracy of model is verified.Finally,the radial cutting force in helical milling is analyzed.The results show that under different conditions of down and up milling,the direction of the radial cutting force is opposite when the tool cuts the material.In view of the hole diameter error in the process of helical milling,the hole diameter error test is carried out for titanium alloy plate(7 mm).The influencing factors,changing rules and causes of hole diameter error are analyzed The empirical formula between hole diameter error and radial force is obtained by linear fitting.The influence of machining parameters on the radial cutting force is calculated by the multiple linear analysis,and then the influence degree of parameters on hole diameter error is analyzed.The research results show that tool is affected by radial cutting force toward machining hole center,which results in deflection deformation toward hole center,resulting in the actual hole diameter smaller than the nominal hole diameter under down milling.It is opposite under up milling.Hole diameter error increases with the increase of feed speed and pitch,and decreases with the increase of cutting speed.In view of the hole shape error in the process of helical milling,the hole diameter error test and radial cutting force test is carried out for titanium alloy plate(25 mm).The influence law of cis-inverse milling and machining parameters on hole shape error is analyzed.The forces are analyzed separately when its cutting materials under down and up milling,and the causes of hole shape errors are studied.Finally,a special tool is used to verify the results.The results show that the shape of the hole is a tapered hole with a large entrance and a small exit regardless of down-milling or up-milling.As the depth of the hole is increased,the radial cutting force toward the center of the machining hole gradually increases during down milling,and the radial cutting force away from the center of the machining hole gradually decreases during up milling,which leads to the decrease of the hole diameter from the entrance to the exit.