No abstract

For a slender ball-screw feed system, the position of the screw–nut joints will change greatly when the moving worktable moves along its whole stroke. Therefore, the compression/tension and torsion stiffness of the screw-shaft on both sides of the screw–nut joints will change. Thus, the system transmission stiffness will also vary and affect its dynamics. In this article, considering the changes of the worktable position and using hybrid element method, a dynamic model was established for the slender ball-screw feed system, and a calculation method of the system stiffness and mass matrix was provided. The experiments on the slender ball-screw feed system with different positions were also performed to verify the proposed dynamic model. Finally, the influences of length, screw-tension force, pitch, nominal diameter of screw and rated dynamic load of screw-nut joints on the slender ball-screw feed system’s natural frequency along its whole stroke were studied. The results showed that the slender ball-screw feed system possesses obvious position-dependent variable dynamics along its whole stroke; the ratio of length to diameter of the screw ( Lf/ d0) would be better designed smaller than 50 in order to obtain the relative uniformity of the system dynamics when manufacturing large machine tools.

The linear motor feed system can realize high-precision motion with high-speed and high-acceleration, which has a broad application in high-speed machine tools. In its unique zero transmission structure, the motor is connected directly with the mechanical system. The thrust harmonics and other disturbances act on the motor mover directly, which makes the tracking fluctuation more prominent. The traditional accuracy and repeatability of positioning cannot reflect the precision of the linear motor feed system in the process of high-speed movement. In this paper, a novel precision evaluation method which takes the dynamic precision as the index is proposed for the linear motor feed system in high-speed machine tools. The performance and precision of linear motor feed system are evaluated by the transient error, steady-state follow error and tracking fluctuation, respectively. Then the servo control model is established oriented to dynamic precision. The influences of NC instructions, servo system and mechanical system on the dynamic precision are discussed. It can be obtained that this new evaluation method on the precision can evaluate systematically the performance of the linear motor feed system in high-speed machine tools. Meanwhile it can reveal efficiently main factors which lead to the decline of dynamic precision and propose effective improvement methods.

With the development of the high-speed and high-precision CNC machine tools, the interaction between mechanical system and servo drive torque in the feed system becomes more and more serious which affects the surface quality of the workpiece. In this paper, taking a small-sized vertical milling center as the research object, the characteristics of servo torque and mechanical system are analyzed, respectively. Then the influence of electromechanical matching on the tracking fluctuation of the feed system is discussed. Furthermore, aiming at the surface roughness of the workpiece, the influence of the interaction between servo torque and mechanical system on the surface quality of the workpiece is analyzed under different milling ways. Finally, a large number of experiments are carried out to verify the analysis above. At last, the optimization methods of machining precision based on electromechanical matching are put forward. It can be found that in the high-speed machining, the servo torque has lots of harmonics which act on the mechanical system with kinds of modals, leading to the vibration. The surface quality of the workpiece will be deteriorated rapidly when the mode of vibration is consistent with the sensitive direction of machining error. The surface quality of the workpiece can be significantly improved through optimizing the feed velocity and the processing method and realizing the electromechanical matching.

In a ball screw feed system of high-speed/high-acceleration machine tools, large frictional and inertial forces may change the real contact state of the kinematic joints, resulting in changes in the contact and transmission stiffnesses and, hence, changes in the dynamic characteristics of the system. In this study, a variable–coefficient dynamic modeling method for a ball screw feed system is proposed, considering the influence of changes in the no-extra-load running states, such as position, speed, and acceleration. Based on Timoshenko beam elements with two nodes and four DOFs, an equivalent dynamic model of a ball screw feed system is established using the hybrid element method. The expression for the equivalent axial stiffness of individual kinematic joints is derived, considering the influence of the feed speed/acceleration under the no-extra-load running state of the system. In addition, the stiffness and mass of the screw shafts on both sides of the screw nut are calculated, considering the influence of the system’s feed position. Hence, we obtain the total stiffness and mass of the system in the no-extra-load running state and analyze the natural frequency. Finally, we conduct validation experiments on a ball screw feed system of a large gantry-type machine tool with different no-extra-load running states.