Precision CNC Machine Tools Research Articles

Analysis of thermal characteristics with multi-physics coupling for the feed system of a precision CNC machine tool Junjian

Analysis of thermal characteristics with multi-physics coupling for the feed system of a precision CNC machine tool Junjian

Modeling and compensation of the axial thermal error of electric spindles based on HHO-GRU method

As the core component of precision CNC machine tools, a lot of heat is generated from the internal heat source of electric spindles during operation, resulting in thermal deformation and thermal errors that affect machining accuracy. Thermal error compensation is an economical method for reducing thermal errors, through which the impact of thermal errors on machining accuracy can effectively decrease. Taking a high-speed electric spindle as the research object, the temperature measurement points are selected as its front and rear bearing seat, as well as some positions far from the heat source. The temperature changes at the front and rear bearing as well as in the environment are monitored, then the thermal errors are measured using a Lion spindle rotation accuracy instrument. The optimal training parameters of the gated recurrent unit (GRU) network are optimized utilizing the global optimization ability of a Harris Hawks optimizer (HHO). Finally, the thermal error prediction model of the GRU electric spindle optimized using the Harris Hawks optimizer (HHO-GRU) is established, based on which axial thermal error compensation experiments are conducted. The results show that using the HHO-GRU prediction model for compensation, the axial thermal errors of the electric spindle can be reduced by more than 80%, which can be controlled within 5 μm.

Influence of nonlinearity of servo system electrical characteristics on motion smoothness of precision CNC machine tools

The ball screw feed system of CNC machine tools is a typical mechatronic motion mechanism. During the operation of the machine tool, not only the dynamic characteristics of the mechanical structure will affect its performance, but also the electrical characteristics of the servo system will affect the movement of the worktable, which will lead to displacement fluctuation and bring adverse effects on the precision of the machine tool and even the quality of the processed parts. Therefore, the electrical system and the mechanical system are strongly coupled to each other due to their high integration. In this paper, the fluctuation of current, torque, and displacement during the motion of the worktable is studied. Firstly, the mechatronic model of the ball screw feed system, which not only consists of all components of the feed system but also takes different nonideal characteristics of each component into consideration, is established to realize the physical simulation of the whole system. Then, with the consideration of the impacts of inverter dead zone, motor cogging torque, rotor mixed-eccentricity, winding asymmetry, and collecting error of feedback current, the frequency components of fluctuations caused by the nonlinear electrical characteristics of inverter and servo motor in the servo drive system are obtained. Finally, the influences of load, feedrate, servo parameters, and dynamic characteristics of mechanical structure on fluctuation are analyzed in the time domain and frequency domain. The phenomenon of displacement fluctuation of the worktable caused by the electrical characteristics of the servo system indicates that it is necessary and meaningful to build the integrated model to study the ball screw feed system from the view of the whole system.

Fault monitoring and diagnosis of motorized spindle in five-axis Machining Center based on CNN-SVM-PSO

A spindle fault diagnosis method based on CNN-SVM optimized by particle swarm algorithm (PSO) is proposed to address the problems of high failure rate of electric spindles of high precision CNC machine tools, while manual fault diagnosis is a tedious task and low efficiency. The model uses a convolutional neural network (CNN) model as a deep feature miner and a support vector machine (SVM) as a fault state classifier. Taking the electric spindle of a five-axis machining centre as the experimental research object, the model classifies and predicts four labelled states: normal state of the electric spindle, loose state of the rotating shaft and coupling, eccentric state of the motor air gap and damaged state of the bearing and rolling body, while introducing a particle swarm algorithm ( PSO) is introduced to optimize the hyperparameters in the model to improve the prediction effect. The results show that the proposed hybrid PSO-CNN-SVM model is able to monitor and diagnose the electric spindle failure of a 5-axis machining centre with an accuracy of 99.33%. In comparison with the BP model, SVM model, CNN model and CNN-SVM model, the accuracy of the model increased by 10%, 6%, 4% and 2% respectively, which shows that the fault diagnosis model proposed in the paper can monitor the operation status of the electric spindle more effectively and diagnose the type of electric spindle fault, so as to improve the maintenance strategy.

Thermal error prediction of machine tool spindle using segment fusion LSSVM

The key temperature points are the input variables of the thermal error model for prediction and compensation of thermal errors for precision CNC machine tools. However, the revealed time-varying characteristics of the key temperature points may jeopardize the robust prediction. To this end, the segment fusion least squares support vector machine (SF-LSSVM) thermal error modeling method is proposed. Firstly, the temperature data and thermal error data are divided into different segments according to time. Then, using the LSSVM with excellent nonlinear mapping capabilities as the basic model, the sub LSSVM thermal error model building and the corresponding key temperature points selection in each segment are fulfilled with genetic algorithm (GA) in a wrapper manner to preserve the corresponding local prediction characteristics. Finally, pick some of or all the sub LSSVM thermal error models to fuse together as the final thermal error model which may incorporate both the local and global prediction characteristics. The modeling and prediction experiment results on the spindle thermal error of a horizontal machining center demonstrate that the mean root-mean-square error (RMSE) on 5 spindle speeds after compensation is only 3.1 μm. Comparing with two traditional thermal error models, the prediction performance of the present model is improved by up to 51%. This research casts new light on both the mechanism of key temperature points and the prediction method of thermal errors.

Adaptive fuzzy PID cross coupled control for multi-axis motion system based on sliding mode disturbance observation.

Multi-axis motion system is widely applied in commercial industrial machines such as precision CNC machine tools, Robot manipulator and laser cutting machines, etc. Contour accuracy plays a major role for the multi-axis servo motion system. The contour machining accuracy is related to the synthesis of single-axis position accuracy and multi-axis linkage accuracy. Only improving the single-axis tracking performance cannot effectively guarantee the machining accuracy of multi-axis system. The primary objective of this study was to design a contour control method to improve single-axis tracking accuracy and multi-axis contour accuracy. A control strategy that combines a sliding mode tracking controller, a disturbance observer and an adaptive fuzzy PID cross coupled controller is proposed. Sliding mode control is simple and has strong robustness to parameter changes and disturbance, which is especially suitable for control of such as non-linear multi-axis motion system. Besides, disturbance is inevitable in practical application, which degrades the motion accuracy. In order to eliminate the influence of external disturbance and uncertainty, disturbance observer is adopted to accurately estimate external disturbance and reduce the chattering phenomenon of sliding mode control, then improve the single-axis tracking accuracy. In order to further consider the coordination between different motion axes and improve the contour accuracy, the PID cross coupled control is used. Owing to conventional PID control cannot satisfy the multi-axis servo motion system with nonlinearity and uncertainty, an adaptive fuzzy method with on-line real-time PID parameters adjustment is proposed. The three-axis motion platform driven by PMLSM is used as the control object, to analysis the influence of disturbance observer on sliding mode control signal and analysis adaptive fuzzy PID cross coupled control performance respectively. The disturbance observer is used to observe the disturbance signal and estimate the disturbance well. The chattering of the sliding mode control signal is obviously improved. Next, compared with the conventional PID-CCC control, adaptive fuzzy PID- CCC control can significantly reduce the tracking error, the contour accuracy is also obviously improved. The disturbance observer can effectively eliminate the influence of external disturbance, reduce the chattering of sliding mode control, and ensure the single-axis accurate tracking. The self-adaptive fuzzy PID cross coupled controller can eliminate the influence of the dynamic characteristics mismatching and parameter difference of each axis, and improve contour accuracy. The simulation results clearly demonstrate the effectiveness of the proposed control method.

Review of geometric error measurement and compensation techniques of ultra-precision machine tools

Inevitable machine motion errors change the cutting tool trajectory and degrade the machined surface quality. Compared to the mature error measurement technologies developed for traditional precision CNC machine tools, the increasing use of ultra-precision machine tools (UPMTs) has shown some distinctive characteristics in error modelling, measurement, and compensation. This paper attempts to summarise state-of-the-art research in the calibration of geometric errors of UPMTs. A general routine for a UPMT error calibration is proposed in this literature review. Various error modelling methods, instruments, and measurement methods applicable to the geometric error measurement of both the linear and rotary axes are discussed using typical case studies. With respect to these achievements, there is a real concern regarding the reproducibility of measurement sensors used for the calibration of UPMTs and it remains challenging to decompose the volumetric motion error of UPMTs. Owing to the high flexibility in practice, trial cutting and a sensitivity analysis-based error measurement and compensation provide a promising solution to achieve a fast UPMT calibration.

Time-varying reliability modelling and quasi-static accuracy optimization of precision CNC machine tools

Aiming at the influence of time-varying characteristics of geometric error on precision CNC machine tool quasi accuracy retention, a time-varying reliability accuracy maintenance method based on multi-objective reliability and maintenance cost tradeoff is proposed. Firstly, on the basis of geometric error transfer model and Achard adhesive wear theory, a quasi-static accuracy reliability limit state function to meet time-varying characteristics is established and the time-varying accuracy reliability is solved based on the First order second moment method. Secondly, according to the time-varying characteristics of the quasi-static accuracy of machine tools, the maintenance cost model and reliability multi-objective optimization model are established, and the trade-off between cost and reliability function is realized. Finally, the optimization results of three-axis machining center show that the method can realize the optimization of geometric accuracy and determine the quasi-static error compensation value which meets the requirements of reliability accuracy.

Recent Patents on the Structure of High-Speed Motorized Spindle

Background: As one of the core components of high-speed CNC machine tool, high-speed motorized spindle is the core functional component of high precision CNC machine tool, which has become the key research and development object of the world. Objective: By comparing and discussing the patents of high-speed motorized spindle, some valuable conclusions have been drawn to predict the future research and development of high-speed motorized spindle. Methods: By analyzing the characteristics of high-speed motorized spindle structure, the influence of high-speed motorized spindle on high-speed machining technology was explicated. Combining with the key technology of high-speed motorized spindle, the patents related to high-speed motorized spindle structure were used for investigation. Results: With the rapid development of high-speed cutting and numerical control technology and the need of practical application, the requirement for high-speed spindle performance has increased. Motorized spindle technology has the characteristics of high speed, high strength, high power, high torque and low speed, high precision, high reliability and long life, offering diversified bearing and lubrication cooling methods and serving as an intelligent system. Conclusion: The different levels of improvement and renovation of the structure with high-speed motorized spindle, by adding lubrication and cooling device to the spindle have improve the performance of spindle, addressing the loopholes in the technology and making it more practical.

Active and intelligent control onto thermal behaviors of a motorized spindle unit

Motorized spindle unit is the core component of a precision CNC machine tool. Its thermal errors perform generally serious disturbance onto the accuracy and accuracy stability of precision machining. Traditionally, the effectiveness of the compensation method for spindle thermal errors is restricted by machine freedom degrees. For this problem, this paper presents an active, differentiated, and intelligent control method onto spindle thermal behaviors, to realize comprehensive and accurate suppressions onto spindle thermal errors. Firstly, the mechanism of spindle heat generation/dissipation-structural temperature-thermal deformation error is analyzed. This modeling conveys that the constantly least spindle thermal errors can be realized by differentiated and active controls onto its structural thermal behaviors. Based on this principle, besides, the active control method is developed by a combination of extreme learning machine (ELM) and genetic algorithm (GA). The aim is to realize the general applicability of this active and intelligent control algorithm, for the spindle time-varying thermal behaviors. Consequently, the contrasting experiments clarify that the proposed active and intelligent control method can suppress accurately and synchronously all kinds of spindle thermal errors. It is significantly beneficial for the improvements of the accuracy and accuracy stability of motorized spindle units.

Form error compensation in soft wheel polishing by contact force optimization

An efficient sub-aperture soft wheel polishing was realized on a four-axis precision CNC machine tool. The stable and controllable contact force was supplied by a voice coil motor during the polishing process. Considering the machining efficiency and CNC tool technical restrictions, a time-variant tool influence function (tv-TIF) regime was introduced to improve form accuracy for rotational symmetric workpiece polishing with a spiral tool path in which it is easy to produce a center dent caused by velocity truncation. A constrained polishing force optimization was employed in the tv-TIF regime, and it was expressed as a piecewise function. As the variant polishing force, the size and the material removal rate of TIF changed simultaneously. Applying the tv-TIF regime, a simulation was conducted to obtain the optimized polishing force and the crossfeed velocity for restraining the center dent during the polishing process. In addition, the pitch-variant tool path combined with the variant contact patch was taken into consideration in generating the removal track. Using the tv-TIF regime and the main spindle speed compensation, the polishing experiments were conducted and the form error compensation was realized by alleviating the center dent successfully.

The Impact of Different Direct-Driving Motor Design on Swivel Spindle Head

Swivel spindle head is a key component used in gantry type five-axis machine tool of high performance and is of great importance in its application and design. Nowadays, more and more components are manufactured by high precision CNC machine tools, such as components of spaceflight, renewable energy and automobile, etc. Therefore, high precision machine tools of multiple axes are more and more urgently demanded, while dual axis rotary head is one of the most important components for a multi-axis machine tool. Hence, it will be a key to develop dual axis spindle head that meets high precision needs. The study explores the highly responsive direct-driving motor able to drive the spindle head to rotate with multi-driving rotary technology. The dual-driving motor rotates via multi-driving units, generates torsion that magnifies and eliminates its clearance, and then drives the spindle head to rotate. Results of the test show that the completed machine tool can meet the standards of dual axis rotary head with high precision in, no matter, speed, distance, positional accuracy, repeated accuracy or maximum torque, etc.

Application of SCM and PLC Technology in CNC Machine Tool

With the rapid development of the mechanical and electrical industry, enterprises are increasingly concerned about the degree of precision CNC machine tools. The traditional CNC machine can only perform simple parts processing operations, gradually exposing the shortcomings of high precision parts. To solve this problem, in this paper, we propose that combined SPCE061A SCM and MASTER-K10S PLC, to improve the function and precision of CNC machine tool control systems, and promote the efficiency and reliability of industrial production.

Thermal Structure Analysis and Optimization of the Feeding System of the Permanent Magnet Linear Synchronous Motor

Because of its high dynamic response speed and acceleration, high rigidity and positioning accuracy, the permanent magnet linear synchronous motor (PMLSM) is able to meet the requirements of high-speed precision CNC machine tools. However, its thermal problems have seriously hampered its development. This paper establishes a PMLSM temperature field model, designs a cooling structure, determines the boundary conditions of the finite element model and conducts computer simulation of the model. The result shows that the designed cooling structure can effectively alleviate the motors thermal problems.

Cutting Force Analysis in Micro Milling of Steel

An analysis of cutting force performance in the micro milling on steel has been carried out based on an experimental work using micro flat end mills on a precision CNC machine tool. It has been found that cutting forces occurred at low feed per tooth are relatively high by assessing the averaged peak forces from the experiments. When feed per tooth is relatively close to tool edge radius, the forces were not growing in linearity with the increasing feedrate. This finding indicates the significance of ploughing phenomenon as an effect of tool edge radius in micro milling.

Thermal Characteristic Analysis on the Headstock of a High Precision CNC Machine Tool

The performances of the precision machine tool will be influenced by its thermal characteristics seriously, and accurately predict the thermal characteristic of the key component of the machine tool is helpful to improve the design level. The headstock of a high precision CNC lathes has been regarded as the research objects, and the thermal properties and its influence on the performance of the machine tool are studied. Finite element analysis model of the headstock has been constructed, and the simulation calculations of the steady temperature field distribution and thermal equilibrium time of the headstock are calculated, and then the analysis to identify the thermal deformation trends of the spindle assembly and the heat distortion of the headstock are also been done. Some of the key factors that have significant influence on the thermal characteristic of the high precision machine tools are also studied. The analysis reveals that the performances of the machine tool will be influenced by the hot asymmetric, the study lays a foundation for the optimization design and thermal error compensation of the spindle assembly.

Study of the friction error for a high-speed high precision table

Friction is one of the important factors resulting in the contour error of feed servo systems for high-speed, high precision CNC machine tools. A mathematical model of a high speed high precision feed servo table with PID controlling has been established with consideration of friction effects. The friction can be calculated according to the sliding speed and the oil film between the two contact surfaces by using a two-component mixed friction model. The protrusion error occurring at the quadrant boundary in the circular motion is profoundly studied by means of numerical simulation and experiments under different work conditions, and the conclusion that the quadrant protrusion error lags with the rising of the feed rate is obtained. Comparisons between the simulation results and the experimental results demonstrate that with the mathematical model, the dynamics of high speed high precision servo feed systems under PID controlling can be accurately simulated, and the friction error and its characteristics in the course of feeding can be precisely predicted.

414 超精密 6 軸加工機による自由曲面ホログラム光学素子のマイクロ切削加工

414 超精密 6 軸加工機による自由曲面ホログラム光学素子のマイクロ切削加工

The development of high precision CNC machine tools

Description de quelques exemples de developpement de machines-outils a CNC repondant aux demandes croissantes de forte productivite, de fiabilite et de generation de formes complexes tres precises