It is of great significance to study the dynamic characteristics of twin ball screw (TBS) feed system to improve the precision of gantry-type dual-driven computer numerical control (CNC) machine tools. In this paper, an equivalent dynamic model of the TBS feed system is established utilizing lumped mass method considering the stiffness of joints. Equivalent axial stiffness of screw-nut joints and bearing joints are both calculated by Hertz contact theory. Furthermore, a friction model is proposed because the friction force of the screw nut affects the stiffness of the joints. Then, the friction parameters are obtained by using the nonlinear system identification method. Meanwhile, a finite element model (FEM) is developed to assess the dynamic characteristics of TBS feed system under the stiffness of joints. Finally, validation experiments are conducted, and the results show that the positions of the nut and the velocities of worktable greatly affect the dynamic characteristics of the TBS feed system. Compared with the theoretical calculation, FEM and experiments indicate that the dynamic modeling proposed in this article can reach a higher accuracy.

Gantry-type computer numerical control (CNC) machines are widely used in the manufacturing industry. A novel structure with moveable gantry is proposed to improve the traditional gantry-type machine structure’s disadvantage of taking up too much space. Geometric errors have direct impacts on the actual position of the tool, which significantly reduces the accuracy of machines. Errors of different components are always coupled and have uncertain effects on the total geometric error. Thus, it is essential to find an effective way to identify the dominant errors and do targeted compensation. First, a novel identification method using value leaded global sensitivity analysis (VLGSA) is proposed to find the dominant errors. In VLGSA, weighting factors which show the influence of the error range are used to modify the multi-body system (MBS) error model. Results show that the dominant errors in three directions respectively contribute 80%, 86% and 85% of the total error in their directions. Then, errors identified by VLGSA are modeled by least-square linear fitting and B-spline interpolation methods, respectively, according to the feature of error data. Finally, the models are applied in a new real-time compensation system developed on the Beckhoff TwinCAT servo system. Experimental results from the gantry-type CNC engraving and milling machine show the proposed method can help figure out the most dominant errors and reduce around 90% of the total error.

An investigation on the surface waviness of both the journal and the bearing bush and their impact on the static characteristics of the hydrodynamic journal bearing is presented in this paper. The finite difference method is introduced to solve a Reynolds equation and obtain the unknown pressure field. The static characteristics, including the load carrying capacity, attitude angle, end leakage flow rate and frictional coefficient are studied under different waviness parameters. The numerically simulated results indicate that the waviness of the bearing bush may deteriorate or enhance the bearing system, depending on the phase angle. The waviness of the journal causes periodic changes in bearing behavior, owing to the alteration in the phase angle. The profile of the journal and bearing surfaces near the attitude angle determines the performance of the bearing system.

The position synchronous control of multi-axis gantry-type feed stage is crucial in precision machine tools. Industrial position control which aims to widen the bandwidth and improve disturbance rejection of single axis is not enough to achieve precise synchronization in a dual-driving feed stage. The characteristics diversity, transmission-mechanism deformation, and mechanical coupling effect between dual axes will degrade the control accuracy. Hence, the novel two-degree-of-freedom (2-DOF) dynamic model-based terminal sliding mode control (TSMC) with disturbance and state observer is proposed in this paper for the synchronous control of a 2-DOF dual-driving feed stage. The 2-DOF dynamic model, based on Lagrange equation, is established along with the parameters identification method. The predictive natural frequencies and vibration modes frequencies by the proposed dynamic model are compared by a modal experiment. Then, the 2-DOF dynamic model-based TSMC is provided to satisfy the tracking and synchronization control. In order to reduce the chattering and to increase the robustness against the mechanical coupling, the disturbance and state observer is designed. Moreover, Lyapunov stability criterion is used to analyze the stability of the proposed control scheme. Finally, an industrial application of 2-DOF dual-driving feed stage is utilized to validate the effectiveness of the proposed control scheme. The proposed 2-DOF dynamic model-based TSMC with observer has been effectively demonstrated to improve synchronous performance and tracking accuracy.

Considering the rough surface as a fractal model makes the research of contact parameters more practical. In the fractal model of the machined surface, the parameters describing the surface topography are independent of the measurement resolution. Based on the elastic, elasto-plastic and plastic deformations of a single pair of contact asperities, a normal contact stiffness model using the fractal model for surface topography description is proposed in this paper. The specimens machined by milling and grinding methods are used to verify the proposed contact stiffness model based on the fractal theory. The experimental and theoretical results indicate that the proposed contact stiffness model is appropriate for the machined joint surfaces.