Errors Of CNC Machine Tools Research Articles

A Method for Simultaneously Measuring 6DOF Geometric Motion Errors of Linear and Rotary Axes Using Lasers.

A novel method for simultaneously directly measuring six-degrees-of-freedom (6DOF) geometric motion errors of CNC machine tools was proposed, and a corresponding measurement system was developed. This method can not only be applied for measuring a linear axis, but also for a rotary axis. A single-mode fiber was used to separate the measuring unit from the laser source in order to ensure system thermal stability and measurement accuracy. The method has the advantages of high efficiency and good accuracy, and requires no complicated decoupling calculation. The positioning error of the linear axis and radial motion error of the rotary axis are measured by laser interferometry and other 5DOF geometric motion errors by laser collimation. A series of experiments were performed to verify the feasibility and effectiveness of the developed measurement system.

A Knowledge-Driven Approach for 3D High Temporal-Spatial Measurement of an Arbitrary Contouring Error of CNC Machine Tools Using Monocular Vision.

Periodic health checks of contouring errors under unloaded conditions are critical for machine performance evaluation and value-added manufacturing. Aiming at breaking the dimension, range and speed measurement limitations of the existing devices, a cost-effective knowledge-driven approach for detecting error motions of arbitrary paths using a single camera is proposed. In combination with the PNP algorithm, the three-dimensional (3D) evaluation of large-scale contouring error in relatively high feed rate conditions can be deduced from a priori geometrical knowledge. The innovations of this paper focus on improving the accuracy, efficiency and ability of the vision measurement. Firstly, a camera calibration method considering distortion partition of the depth-of-field (DOF) is presented to give an accurate description of the distortion behavior in the entire photography domain. Then, to maximize the utilization of the decimal involved in the feature encoding, new high-efficient encoding markers are designed on a cooperative target to characterize motion information of the machine. Accordingly, in the image processing, markers are automatically identified and located by the proposed decoding method based on finding the optimal start bit. Finally, with the selected imaging parameters and the precalibrated position of each marker, the 3D measurement of large-scale contouring error under relatively high dynamic conditions can be realized by comparing the curve that is measured by PNP algorithm with the nominal one. Both detection and verification experiments are conducted for two types of paths (i.e., planar and spatial trajectory), and experimental results validate the measurement accuracy and advantages of the proposed method.

A two-stage friction model and its application in tracking error pre-compensation of CNC machine tools

Friction modelling and compensation is an important artifice to improve the tracking accuracy of CNC machine tools used for welding, 3D printing and milling, etc. This paper proposes a two-stage friction model to describe the characteristics of friction. First, a novel method is proposed to distinguish the boundary between the presliding and sliding regimes of non-linear friction by estimating the breakaway velocity of static friction based on its elastic characteristics. Then, a two-stage feedforward friction compensation model which only has relationship with the velocity is developed. The proposed friction model not only can describe the Stribeck phenomena of friction, but also represent the characteristics of the static friction. A tracking error pre-compensation model is developed thereafter based on the good performances of feedforward friction compensation, in order to further reduce the tracking error of CNC machine tools. Extensive experiments and analyses verify that the proposed friction model and tracking error pre-compensation method can significantly improve the tracking accuracy of CNC machine tools. Specifically, comparison with existing tracking-error based friction model shows that the proposed model can reduce maximum and average tracking errors more than 70% and 20%, respectively. Furthermore, contouring errors of multi-axis motion are also reduced to some extent.

The design and development of a real-time compensation controller of thermally-induced errors for CNC machine tools

Thermal deformation of the machine tool structure is one of critical factors affecting the machining accuracy level. These thermally-induced errors can account for as much as 70% of the dimensional errors on a machined work piece. Although thermally-induced errors can be reduced by applying the new materials with low-expansion coefficient to machine tools, designing the well symmetrical structure of machine tools, as well as controlling the temperature of the machining process, the costs will rise greatly. Furthermore, thermally-induced errors cannot be eliminated completely at the design stage. Hence if thermally-induced errors can be predicted, the use of indirect compensation systems will emerge as the most economical and inevitable course of action. In this paper, a real-time compensation controller of thermally-induced errors for CNC machine tools is designed and developed with the embedded technology. Test results demonstrate this compensation controller predict and compensate thermally-induced errors of CNC machine tools very well.

THE DESIGN AND DEVELOPMENT OF A REAL-TIME COMPENSATION CONTROLLER OF THERMALLY-INDUCED ERRORS FOR CNC MACHINE TOOLS

Thermal deformation of the machine tool structure is one of critical factors affecting the machining accuracy level. These thermally-induced errors can account for as much as 70% of the dimensional errors on a machined work piece. Although thermally-induced errors can be reduced by applying the new materials with low-expansion coefficient to machine tools, designing the well symmetrical structure of machine tools, as well as controlling the temperature of the machining process, the costs will rise greatly. Furthermore, thermally-induced errors cannot be eliminated completely at the design stage. Hence if thermally-induced errors can be predicted, the use of indirect compensation systems will emerge as the most economical and inevitable course of action. In this paper, a real-time compensation controller of thermally-induced errors for CNC machine tools is designed and developed with the embedded technology. Test results demonstrate this compensation controller predict and compensate thermally-induced errors of CNC machine tools very well.

Robust modeling method for thermal error of CNC machine tools based on ridge regression algorithm

For thermal error compensation technology of CNC machine tools, the collinearity between temperature sensitive points is the main factor for determining the predicted robustness of thermal error model. The temperature sensitive points are input variables of the thermal error model. This paper studies the thermal error of Leaderway V-450 type CNC machine tools during different seasons. It is found that although the commonly used temperature sensitive point selection methods can significantly reduce the collinearity between temperature sensitive points, the correlation between some of the selected temperature-sensitive points and thermal error is weak. This causes the temperature-sensitive points to be variable and the predicted accuracy and robustness of thermal error to be reduced. Therefore, in this paper, the temperature sensitive points are selected directly by their correlation with thermal error to eliminate variability. However, the experimental results also show that the collinearity between temperature sensitive points is very large. Hence, the ridge regression algorithm is used to establish a thermal error model to inhibit the bad influence of collinearity on the thermal error predicted robustness. Thus, the “robustness ridge regression machine tool thermal error modeling method” is proposed, the “RRR method” for short. In addition, in the “RRR method”, the correlation coefficient is used to measure the correlation between temperature sensitive points and thermal error instead of the commonly used gray correlation; because this paper finds that the gray correlation algorithm is essentially inapplicable for measuring a negative correlation. Based on the thermal error experiment data for the whole year, the “RRR method” is compared with two currently used methods, and the results show that the “RRR method” can significantly enhance the long-term predicted accuracy and robustness of thermal error. Finally, the application effect of practical compensation shows that the “RRR method” is usable and effective.

Global Quantitative Sensitivity Analysis and Compensation of Geometric Errors of CNC Machine Tool

A quantitative analysis to identify the key geometric error elements and their coupling is the prerequisite and foundation for improving the precision of machine tools. The purpose of this paper is to identify key geometric error elements and compensate for geometric errors accordingly. The geometric error model of three-axis machine tool is built on the basis of multibody system theory; and the quantitative global sensitivity analysis (GSA) model of geometric error elements is constructed by using extended Fourier amplitude sensitivity test method. The crucial geometric errors are identified; and stochastic characteristics of geometric errors are taken into consideration in the formulation of building up the compensation strategy. The validity of geometric error compensation based on sensitivity analysis is verified on a high-precision three-axis machine tool with open CNC system. The experimental results show that the average compensation rates along theX,Y, andZdirections are 59.8%, 65.5%, and 73.5%, respectively. The methods of sensitivity analysis and geometric errors compensation presented in this paper are suitable for identifying the key geometric errors and improving the precision of CNC machine tools effectively.

Compensation for CNC Machine Tools Comprehensive Error Based on Interpolation Algorithm

In order to reduce the in∞uence of comprehensive error on CNC machine’s precision, the modeling method of comprehensive error was presented in this work based on linear interpolation and Newton interpolation. By using the CNC external machine zero point ofiset function, real-time error compensation was realized online. The experimental results on a vertical milling machine show that the model is convenient in calculation and performs excellent in accuracy prediction. Therefore it is suitable for the compensation for comprehensive error of CNC machine tools.

A generalized on-line estimation and control of five-axis contouring errors of CNC machine tools

Nonlinear and configuration-dependent five-axis kinematics make contouring errors difficult to estimate and control in real time. This paper proposes a generalized method for the on-line estimation and control of five-axis contouring errors. First, a generalized Jacobian function is derived based on screw theory in order to synchronize the motions of linear and rotary drives. The contouring error components contributed by all active drives are estimated through interpolated position commands and the generalized Jacobian function. The estimated axis components of contouring errors are fed back to the position commands of each closed loop servo drive with a proportional gain. The proposed contouring error estimation and control methods are general, and applicable to arbitrary five-axis tool paths and any kinematically admissible five-axis machine tools. The proposed algorithms are verified experimentally on a five-axis machine controlled by a modular research CNC system built in-house. The contouring errors are shown to be reduced by half with the proposed method, which is simple to implement in existing CNC systems.

Dimensional and geometrical errors of three-axis CNC milling machines in a virtual machining system

Virtual machining systems are applying computers and different types of software in manufacturing and production in order to simulate and model errors of real environment in virtual reality systems. Many errors of CNC machine tools have an effect on the accuracy and repeatability of part manufacturing. Some of these errors can be reduced by controlling the machining process and environmental parameters. However geometrical errors which have a big portion of total error need more attention. In this paper a virtual machining system which simulates the dimensional and geometrical errors of real three-axis milling machining operations is described. The system can read the machining codes of parts and enforce 21 errors associated with linear and rotational motion axes in order to generate new codes to represent the actual machining operation. In order to validate the system free form profiles and surfaces of virtual and real machined parts are compared in order to present the reliability and accuracy of the software.

Study in Interface Appearance Design of Postal Equipment Based on Man-Machine Engineering

In order to systematically study the CNC machine tools panel appearance of various factors on the operating efficiency, on the basis of theoretical research in cognitive psychology, ergonomics and other design principles put forward the manmachine interface for CNC machine tools equipment operating efficiency and apply it to the improved design of the CNC machine tools panel formed for the panel color key position, key symbols single factor and combination of factors and improved design to reduce the error of CNC machine tools in operation, reduce operator fatigue and psychological discomfort, allowing more perfect, more practical CNC machine tools.

Volumetric Errors Prediction of CNC Machine Tool Based on the Laser Interferometer

The volumetric errors modal is built with the coordinate errors of each axis of CNC machine tool through the laser interferometer. The volumetric errors in the workspace are calculated with the errors prediction modal. With the calculated results of volumetric errors, the errors tendency of CNC machine tool can be predicted. Besides, the specific accuracy testing projects can be conducted by the errors areas taken care more. The main errors of CNC machine tool will be tested rapidly and efficiently.

Tool path accuracy enhancement through geometrical error compensation

Kinematic and geometric errors of CNC machine tools, introduce large deviations in the real path traveled by the cutting tool. Tool path deviation reduces geometrical and dimensional accuracy of the machined features of the component. Tool path modification is an effective strategy to increase accuracy of the machined features. An improved error estimation model based on kinematic transformation concepts has been developed and used to calculate the volumetric overall error. These calculations are applicable for each arbitrary target positions of the machine's work space. Also a NC Program editor software has been developed in order to manage the calculations, modifications and to generate the new compensated NC program. The compensation procedure includes: fragmentation of nominal tool path to small linear elements, translating nominal position of elements to real positions using the Kinematics error model, finding compensated positions using the error compensation algorithm, converting newly generated elements to new tool paths using the packing algorithms and finally editing old NC program using NC code generator algorithm. Experimental tests showed 4–8 times accuracy improvement for linear, and S-pline tool paths deviations.

Measurement and compensation for volumetric positioning errors of CNC machine tools considering thermal effect

The measurement and compensation of volumetric positioning errors can be used to significantly improve the accuracy of machine tools. In this paper, a sequential step diagonal measurement is introduced to measure nine volumetric positioning errors in a short time. Measurements under various thermal conditions are preformed to understand the relationship between the volumetric positioning errors and the machine temperature field and variations. A radial basis function neural network is used to predict the volumetric positioning errors at all positions based on the temperature distribution of the machine. Compensation experiment is carried out to validate the performance of the measurement and the prediction method. The experimental results show that the volumetric accuracy of the machine tool is significantly improved by the error compensation.

Thermal Error Robust Modeling and Real Time Compensation Based on the GA-BPN Theory on a Vertical Machine Centre

Thermal errors generally account for as much as 70% of the total errors of CNC machine tools, are the most contributor to the workpiece dimensional precision in precision machining process. Thermal error compensation is an effective way to decreasing thermal errors. Precision mode is a key to thermal error compensation. In this paper thermal error modeling method based on the artificial neural networks (ANN) algorithm is applied for a vertical machining center. Four key temperature points of a vertical machining center were obtained based on the temperature field analysis. A novel genetic algorism-Back propagation neural network (GA-BPN) thermal error model was proposed on the basis of four temperature points. The emulations and experiments prove that there was about a 60% increase in machine tool precision.

Volumetric Error Modeling and Analysis for CNC Machine Tool Based on Multi-Body System

With the development of the CNC precision machining and the ultra-precision machining, machine tools error issue has became the most active research topics and concerned by more and more experts. In this paper, a rapid simulation software platform based on multi-body system theory and Matlab software for measurement and analysis geometric errors of CNC machine tool is presented, which includes the generation of simulated measurement data, data processing, error separation and error compensation etc. To verify the feasibility of the developed software, the sequential step diagonal vector measurement method has been analyzed. The experimental results show that after error compensation the machine performance is improved by 39%. It demonstrates that the sequential step diagonal vector measurement method could significantly improve the machine accuracy and the developed software platform is reliable. This platform is not only effective for step diagonal vector measurement method, but also can be applied to other measurements. Therefore, the software platform provides a fast, reliable and objective tool for machine error measurement and analysis.

A Synchronization Approach for the Minimization of Contouring Errors of CNC Machine Tools

This paper presents a synchronization control approach for the minimization of contouring errors of multi-axis CNC machine tools. The contouring errors are presented by the position synchronization errors that are defined as differential position errors between each axis and its adjacent ones. Using cross-coupling concept, a decentralized tracking controller is developed with feedback of both position and synchronization errors, formed with a combination of feedforward, feedback and a saturation control. It is proven that this controller can asymptotically stabilize both position and synchronization errors to zero. The proposed controller does not require significant use of the system dynamic models. Experiments performed on a multi-axis machine tool demonstrate improved performance especially in the contouring error minimization.

Study on Checking and Measurement of Double-Ball Bar for Thermal Error of CNC Machine Tools

Thermal errors of CNC machine tools was checked and measured by double-ball bar in this paper. Mathematical model of measuring thermal error of machine tools by DBB on the principle of its basic measurement was derived. Combining it with kinematics model of machine tool, the identification law of error component parameters was established when thermal error was checked and measured by DBB. Thus, all of the thermal error component would be separated from the synthetical error. Finally, the entire analysis was provided to adjust machine tool and actualize error compensation of machine tool through grasping main factors influencing the machining precision of machine tool.

NURBS-based fast geometric error compensation for CNC machine tools

In this paper, a novel method for the compensation of geometric errors of CNC machine tools is presented. The key idea is to use the basis functions of the setting NURBS path to approximate its error compensation function and to generate a new compensated NURBS path. In this way, both the setting and the compensated NURBS path have the same NURBS form. More importantly, the control points of the error compensation function can be obtained by simply calculating the positioning deviations of the control points of the setting NURBS path using the error model. A high compensation accuracy can be achieved through the systematic insertion of new knots, which creates new control points and raises the flexibility of NURBS in representing the error compensation function. The real-time interpolation of the compensated NURBS path completes the error compensation automatically. Simulations and experiments have shown that the new method delivers the same positioning accuracy as a model-based real-time geometric error compensation method does, but without additional real-time CPU loading. The proposed method can also be implemented in the post-processor of a CAM system for off-line compensation.

Verification and evaluation method for volumetric and positional errors of CNC machine tools

This paper provides a testing device and an automatic sampling technique for fast checking of volumetric errors of CNC machine tools. With the proposed automatic sampling technique, the verification work is carried out automatically and reliably. Furthermore, the analyzed result obtained via the traditional least-squares method is not the minimum zone solution, and may overestimate the volumetric error. Thus, genetic algorithms (GAs) are employed to obtain the minimum zone solution in this paper. Experimental test results show that the proposed method is superior to the least-squares method.