| Ultrasonic vibration assisted machining has been widely used in turning,drilling,grinding and other machining methods because of its excellent properties such as reducing cutting force,reducing cutting temperature and improving surface quality.However,the development of ultrasonic milling is much slowly.As the study in ultrasonic milling is restricted to ultrasonic vibration worktable and pure longitudinal vibration machining,the superiorities of ultraoinc cutting have been seriously limited.Therefore,it is necessary to study the multi-dimensional ultrasonic tool vibration milling method.This paper is aimed at the research of the ultrasonic milling system which has 3D vibration trajectory when excitated by single longitudinal vibration.Based on the system,the characteristic cutting process of longitudinal torsional ultrasonic milling is analyzed and the performance of machining weak stiffness structure is studied.In order to apply the ultrasonic vibration milling tool on the common milling machine without affecting the normal operation of the spindle,an ultrasonic transducer with shared longitudinal and torsional vibration nodes is introduced in this paper at first.Based on the sound wave propagation and reflection theory,the mathematical model of force and torque produced by longitudinal torsional conversion structure under longitudinal vibration excitation is established and the longitudinal torsional conversion structure is designed based on the model.Based on equivalent circuit method and finite element method,the structure dimension of longitudinal-torsional vibration transducer is established.The frequency and node characteristics of thetransducer are verified by impedance test and finite element analysis.The longitudinal and torsional vibration amplitudes of the transducer are measured by a laser displacement measuring equipment.These create the foundation for further analysis of longitudinal torsional ultrasonic milling process.In order to provide electrical energy to the high speed rotating ultrasonic transducer,a contactless energy transmission unit is designed based on loosely coupled transformer.The contactless energy transmission has a primary coil and a secondary coil which can rotates to each other.Through theoretical calculation and circuit simulation,the effects of different circuit compensation methods on the power transfer efficiency,the amplitude and the waveform of the input and output voltage are studied when using the contactless energy transmission unit.The principle of selecting circuit compensation method for the contactless power supply device under different power supply types and load is formulated,and the optimal circuit compensation method is chosen for the contactless power supply when the longitudinal and torsional ultrasonic transducer is the output load.The cutting process characters are studied through the cutting edge trajectory and uncutting time.A mathematical model of the cutting tool trajectory of longitudinal torsional ultrasonic vibration milling is established based on the measured frequency and vibration amplitude of the ultrasonic transducer.The difference among longitudinal torsional ultrasonic milling,pure longitudinal milling and conventional milling is analyzed through the unfolded cutting tool vibration trajectory.A mathematical model of uncutting time in a single ultrasonic vibration period is established based on the mathematical model of the cutting tool motion trajectory and the geometric feature of the unfolded trajectory in the two-dimensional plane.The mathematical model can be used to predict the uncutting coefficients in ultrasonic milling process under different milling parameters.Based on the mathematical model of ultrasonic milling tool trajectory,the effect of longitudinal torsional ultrasonic vibration on the decreasing of cutting force in milling process is studied through cutting simulation.Then the relationship between uncutting coefficient and cutting force reduction in ultrasonic machining is analyzed.In order to investigate the machining performance of longitudinal and torsional ultrasonic vibration assisted milling for weak stiffness structures,the stiffness strengthening effect of weak stiffness structure in milling process is studied based on the mechanical vibration theory and the longitudinal torsional ultrasonic milling uncutting time mathematical model.The effect of ultrasonic milling on improving the equivalent stiffness of weak stiffness structure is verified by cutting simulations and milling experiments.A dynamic model of longitudinal torsional ultrasonic vibration milling is established based on the mathematical model of uncutting time and the stability region of the ultrasonic milling is obtained by the revised semi-discrete method.The stability of milling weak stiffness structure under different milling parameters and ultrasonic vibration parameters can then be predicted.Taking the thin plate structure milling process as an example,the effect of longitudinal torsional ultrasonic vibration on milling weak stiffness is verified by testing the acceleration,cutting force and vibration displacement of the thin plate. |
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