Vibration control of cutting process in milling using dynamic absorber

Machine tool chatter, a violent vibration between the cutting tool and the workpiece material, is developed under some machining conditions, and often limits productivity. The objective of this dissertation is to propose a design concept for a dynamic absorber in the milling process to suppress this undesirable vibration. An investigation and analysis into the characteristics of, and interaction between, forced vibration and machining chatter in milling is carried out by the use of a two-degree-of-freedom structural model simulation. An absorber mass is connected to the main system through passive elements (spring and damper) in the passive dynamic absorber, and active force generating element (piezoelectric translator) in the active dynamic absorber. The use of the inertia force is considered by controlling the motion of the absorber to attenuate the vibration of the milling system. The optimal stiffness and damping are obtained for the passive control, and the optimal feedback gain is derived for the active control. A theoretical approach for the prediction of machine tool chatter in milling including cutting dynamics permits calculation of borderlines of stability for the milling structure model. Transient responses of the milling process are obtained from the numerical integration of the system's dynamic equations based on the fourth-order Runge-Kutta method. The borderline of stability obtained in terms of critical axial depth of cut versus spindle speed is verified by the transient responses of the system in the time domain under varying cutting conditions. The corresponding chatter frequency is also identified by the frequency spectrum of the system's responses. The harmonic response of the milling is obtained also. Comparison of the results with and without control are made to show the effectiveness of the control techniques. A proof-of-the-concept experiment is designed and conducted to verify the theoretical prediction. A milling extension has a low static stiffness and is prone to machine tool chatter, therefore it is used as the main system in the experiment. A finite element model for the milling extension is considered. A fabricated absorber mass, an annular ring, is connected to the milling extension through a commercial available piezoelectric translator in our experiment. The annular ring and the actuator are functioning as an active dynamic absorber in the theory to suppress the vibration of the milling process. The real-time feedback control is performed and achieved by the use of analog circuits. Oscillation of the milling extension equipped with the active dynamic absorber is attenuated appreciably. Transfer functions are also obtained experimentally with and without the feedback control to show the superiority of the active control technique. Comparisons of the simulation and experimental results are made and presented in this dissertation.