Study On Chatter Performance Of Aircraft Intersection Holes Fine Boring And Its Numerical Simulation Analysis

Robotic boring is an effective way to achieve the final machining of intersection holes in aircraft manufacturing. However,compared with a standard machine tool,the chatter as a kind of self-excited instable vibration is more likely to happen than in the robotic boring system. Although the capacity to resist vibration can be greatly improved by the pressure foot, the robotic boring system is still easily subjected to chatter during the process due to the series structure of the robot,which seriously affects the boring efficiency and the surface quality of the workpiece. Therefore, in this dissertation, the chatter mechanism of the robotic boring with its identification,forecast and suppression are proposed to guratantee the surface quality and the boring efficiency. The main work and innovation points are as follows:The importance and the development status of the intersection holes, the system structure and the cutting process of robotic boring are first introduced. The calibration methods of the robot coordinate system machining coordinate system with established the robot coordinate system is then given. Next, the system characteristic that is more prone to chatter and the research status of machining chatter mechanism are elaborated.Finally, the kinematics model of the used robot ABB6600-175/2.55 is established according to the D-H method. Based on this, the stiffness identification experiment and the modal test are conducted to obtain the structural property and the dynamic property of the system.Based on the obtained structural property and the dynamic property of the system,a dynamic model is established to predict the vibration performance of the system. The robot stiffness coupling, the chip deformation and the plowing interference affecting the cutting force are considered using the principles of cutting mechanics and the Oxley orthogonal cutting model. The results show that the average cutting forces are predicted within an error margin of 13% for stable boring and 21% for vibrated boring,which verifies that the proposed model is acceptable and valid, and has application value in terms of suppressing vibration during robotic boring.The effect of the pressure foot on the stability and the structure of the system are analyzed in detail, and a forced model of the system with a pressure foot based on the dynamic cutting force obtained from the established dynamic model is presented. The chatter mechanism and the stability of the system with a pressure foot are analyzed.The chatter and stability models of the system with pressure foot are then established.Next,the stability lobe diagram is plotted. Finally, the orthogonal experiments were performed twice on high-strength steel to verify the chatter mechanism and stability.The result shows that the stiffness can significantly restrain the growth of chatter amplitude and the two most significant factors that affect the stability of the system are the feed rate and the depth of cut, which means that an appropriate method should be found to optimize the robot posture to enhance the robot stiffness.A new approach to identify and forecast the chatter of the system based on the measured force signal of the dynamometer and the amplitude measured of the grating scale is presented. The proposed approach consists of three steps. First, the measured signal is decomposed a series of intrinsic mode functions (IMF) and a residue by empirical mode decomposition (EMD). Secondly, Hilbert transform is invoked for each IMF to obtain the instantaneous frequencies and the instantaneous magnitudes,which comprise the Hilbert-Huang spectrum of the original signal. Finally, the chatter feature is extracted by analyzing the Hilbert spectrum of each IMF and a statistical method is used to detect the chatter symptom. In addition, the influence of the robot stiffness, the machining parameters and the tool geometry parameters on the chatter is analyzed according to the established chatter model, which helps to put forward the suppression strategy of the chatter from robotic boring system.Using the obtained structural property and dynamic property of the system, based on an equivalent stiffness,a new approach is proposed to gain a fundamental understa-nding for the vibration performance. In the approach, the whole robotic boring system is equivalent to a mass-spring-damping group in 3D space,and then an equivalent 3D-finite element model based on this approach is established to simulate the robotic boring process. The results show that the simulated cutting force components and the amplitude in the feed direction are in good agreement with the experiment under different cutting conditions, and this proposed approach is feasible.Finally, the whole work in this dissertation is summarized, and the future work is discussed.