Computer Numerical Control Machine Tools Research Articles

Research on processing technology of grinding aspheric workpiece in the five-axis machine tool

Aspheric optical components have been widely used recently because of their unique advantages. Thus, it is necessary to investigate processing technology of such aspheric parts. In this paper, an aspherical mirror is ground with cup wheel in five-axis computer numerical control (CNC) machine tool and corresponding processes were investigated. To improve machining efficiency and reduce tool wear, the workpiece is firstly ground to be a spherical mirror, and then it is ground to be the aspheric contour. The best fitting spherical mathematical equation is derived by the axisymmetric quadratic aspheric generatrix equation. A ∅90 mm aspheric mirror was ground with cup wheel with five-axis computer numerical control (CNC) machine tool. The aspheric curve is processed into a paraboloid shape. The experimental results show that the surface profile of the aspherical lens is consistent with the designed profile, and the peak value (PV) value of the error is 4.77μm. Such results can be accepted in consideration of working allowance for polishing. According to the experimental results, the cause of the error was analysed and methods for improving precision were proposed.

Thermal Error Modeling for Machine Tools: Mechanistic Analysis and Solution for the Pseudocorrelation of Temperature-Sensitive Points

The temperature-sensitive point is the input variable of the thermal error compensation model of computer numerical control (CNC) machine tools. At present, the most commonly used selection method is to measure the multipoint temperature and thermal error of the machine tool synchronously and select several temperature measurement points with the highest thermal error correlation as temperature-sensitive points according to the measurement data. This study reveals that this method sometimes has obvious mis-selection and causes the model to fail. The reason is that the weak correlation temperature measurement point away from the machine tool heat source amplifies the volatility of the correlation evaluation result, owing to the small overall change. If the calculated result exceeds the true strong correlation temperature measurement point, it will be incorrectly selected as the temperature-sensitive point. This scenario has been termed herein as the pseudocorrelation problem. With the gradual popularization of thermal error compensation technology for CNC machine tools, pseudocorrelation will seriously affect the mass production pass rate. Therefore, the study analyzes and rigorously proves the mathematical mechanism of this problem, and the temperature measurement point preselection algorithm based on the correlation coefficient volatility determination factor (CCVDF) is proposed to eliminate potential pseudocorrelation temperature points before selecting temperature-sensitive points. After 1000 random simulation experiments, the failure rate of Z-direction thermal error modeling after preselection decreased from 5.1% to 0.5%, and the Y-direction error decreased from 33.7% to 1.5%. This algorithm can greatly improve the qualification rate of large-scale equipment of thermal error compensation technology.

Development Trend of NC Machining Accuracy Control Technology for Aeronautical Structural Parts

High-performance five-axis computer numerical control machine tools are widely used in the processing of Aeronautical Structural parts. With the increase of service life, the precision of CNC machine tools equipped by aeronautical manufacturing enterprises is declining day by day, while the new generation of aircraft structural parts are developing towards integration, large-scale, complexity, thin-walled and lightweight. It is very easy to produce dimension overshoot and surface quality defects due to unstable processing technology. The machining accuracy of aircraft structural parts is also affected by complex factors such as cutting load, cutting stability, tool error, workpiece deformation, fixture deformation, etc. Because of the complexity of structure and characteristics of Aeronautical Structural parts, the consistency and stability of cutting process are poor. It is easy to cause machining accuracy problems due to tool wear, breakage and cutting chatter. Relevant scholars have carried out a lot of basic research on NC machining accuracy control and achieved fruitful results, but the research on NC machining accuracy control of Aeronautical structural parts is still less. This paper elaborates from three aspects: error modeling method of NC machine tools, error compensation method, prediction and control of machining accuracy, and combines the characteristics of Aeronautical Structural parts, the development trend and demand of NC machining accuracy control technology are put forward.

A new solution of measurement using 3D artefact to identify all location errors related to rotary axis of five-axis machine tool

In the manufacturing industry, computer numerical control (CNC) machine tool has been applied widely due to their high adaptability and precision in the machining of diversity shape, especially five-axis CNC machine tools. However, more error terms related to rotary axes, in which location errors are always considered as one of the most fundamental error sources, affect directly to machine tool performance. Thus, ensuring that a five-axis machine tool is machining within tolerance is a crucial demand in the market. This study investigates an efficient strategy using a specific 3D artifact to identify and characterize the location errors in rotary axes of a five-axis machine tool. The intended 3D artifact consists of a base plate, 2 standards ball with high roundness embedded on the top surface of the base. A Touch Trigger Probe will be utilized as the calibration instrument to capture the coordinate of 6 arbitrary contact points of the balls, then an optimization algorithm is applied to calculate the coordinate of the centers of the balls. By comparisons the relative position of the imaginary circles made of those centers with the ideal ones, all location errors will be measured individually.

Research on the application of multiplication dimension reduction method in global sensitivity analysis of CNC machine tools

Three-axis computer numerical control (CNC) milling machines have a wide range of application in practical processing. The error transfer principle of machine tools is an important factor to improve the precision of machine tools. Based on multibody system theory and homogeneous transformation matrix theory, a geometrical error model of three-axis CNC milling machines is proposed in this paper. In addition, the global sensitivity analysis of machine tool errors was carried out to identify important errors. The analysis model has two layers of higher dimensional integration; hence, in the aim of solving this issue, a multiplication dimension reduction method was used in this paper. By this means, a one-dimensional integration model was obtained, and sensitivity analysis of machine tool geometric errors was realized. The present work provides a good theoretical basis for the optimization design and error compensation of machine tools.

An optimal process of machining complex surfaces of anti-backlash roller enveloping hourglass worms

Anti-backlash roller enveloping hourglass worm gear drives, as a new precision transmission system, has been extensively applied in robot joint design, and CNC (computer numerical control) machine tools. However, tooth profiles of the roller enveloping hourglass worms are spatial complex surfaces and very difficult to machine. The meshing and transmission properties such type of worm gear drive are especially sensitive to the errors in machining the tooth profiles of such worms. Thus, in order to improve the accuracy in machining their tooth profile surfaces and reduce the surface roughness, barriers to achieve precision machining of these complex surfaces have to be overcome. In the present study, an equation of grinding contact line for the roller enveloping hourglass worm gear drives with seven critical error parameters was for the first time established based on modern theory of gear meshing. Influence of each error parameter on the grinding contact line was decided through numerical analysis. Also, in order to find the best relationship between the grinding speed and the grinding feed rage, real contact patterns between the grinding rod and the machined worm tooth surface at different grinding speeds were observed using a metallurgical microscope and compared with the theoretical contact pattern calculated from the developed line of contact equation. Based on the obtained results, an innovative grinding method that can offset the loss of radius of the grinding rod was developed and a grinding process under the optimal rotational speed was presented. The proposed grinding process was then applied to generate the complex helical surface of a high precision roller enveloping hourglass worm. Outcomes of this study formed a profound theoretical and practical background for manufacturing and machining of the roller enveloping hourglass worm gear drives.

Instantaneous Drill Bit Wear Level Detection in CNC Machine using Wavelet Transform

The usage of machine tools is widely increased to industrial automation, manufacturing, production technology and etc. The machine tool wear condition monitoring is playing a key role to increase accuracy of the dimension in the final product. By monitoring the wearing level, the life time of the tool is accurately detected and tools can be replaced at the correct time and it can be used to minimize the process time of the task. But it is difficult to monitor and detect the machine tool weariness level from the direct methods. From the indirect methods, the weariness levels of Computer Numerical Control (CNC) machine tool for Acoustic Emission(AE) property is approached in this paper. The AE signals are recorded and preprocessed to extract the features of different wearing conditions using Wavelet Transform(WT). The WT is used to extract the discriminating features that are indirectly reflecting the wearing levels of machine tools. The CNC machines tool weariness at various stage is evaluated from statistical indexes and analyzed based on the relation between the energy distribution of machined surface and wear state of the bit. This approach effectively detects real-time wearing levels of drilling tools by AE using Wavelet technique.

Experimental study on the repeatability of positioning of linear axes of machine tools

This study presents experimental investigations on the mechanisms of the repeatability of positioning of linear axes of computer numerical control machine tools. First, the factors affecting the repeatability of positioning of linear axes are obtained based on orthogonal experiments. Second, the mechanism of each factor affecting the repeatability of positioning is studied based on single-factor experiments. Finally, an assembly method to improve the repeatability of positioning of linear axes is proposed. The method can provide designers and workers with informative guidelines for improving the repeatability of positioning in design and assembly processes.

Development of a cooperative system for wire and arc additive manufacturing and machining

Wire and arc additive manufacturing (WAAM), which is an additive manufacturing (AM) process that uses metal materials, has a higher fabricated volume per unit time but a lower fabricated shape accuracy compared with other methods. With this process, the surface roughness of fabricated objects is several hundred micrometers or more, and a finishing process is necessary. However, the fabricated objects after finishing can have uncut areas or can be overcut during the finishing process owing to the large difference between the target and actual fabricated shapes. Therefore, the objective of this study is to develop a cooperative system for WAAM and machining that includes a process that measures the shape of the fabricated object. First, the three-dimensional (3-D) shape of the fabricated object was measured by structure from motion (SfM) and compared with the 3-D computer-aided design (CAD) data. Second, the original design was modified, and the amount of material removed during finish cutting was optimized with the developed software. Finally, the fabricated hollow object was finished by milling to obtain a uniform wall thickness without any defects. A 3-D fabricated object was measured by SfM, and it was observed that the measurement accuracy was sufficiently high for the requirements of the system. In addition, a fabricated hollow quadrangular pyramid with a closed shape was machined with a computer numerical control (CNC) machine tool with the modification of the work origin. As a result, the amount of material removed during finish cutting was optimized, and the inclined wall thickness was uniform compared with that without modification. In addition, a hollow turbine blade with a freeform shape was successfully finished without any defects.

Quality characteristic decoupling planning of different meta-action units for computer numerical control machine tool

Since computer numerical control machine tool is composed of multiple meta-action units to achieve one specific function, including the meta-action units' own quality, it still needs to control the coupling relationships among different meta-action units' quality characteristics to guarantee the whole machine's quality. In this article, a method of quality characteristic decoupling planning based on meta-action unit for computer numerical control machine tool was proposed. Firstly, the coupling constraint models based on meta-action unit were established. Secondly, the comprehensive coupling strengths of meta-action units were calculated and introduced into the design structure matrices. Thirdly, multidisciplinary design optimization method was adopted to obtain the optimized control sequence of different meta-action units' quality characteristics. What is more, automatic pallet changer rotary motion of computer numerical control machine tool was taken as an example to illustrate the rightness and effectiveness of this method.

Experimental study of stability prediction for high-speed robotic milling of aluminum

It has been fully demonstrated that the regenerative chatter theory is applicable for predicting chatter-free milling parameters for computer numerical control machine tools, but researchers are still arguing whether it is effective for robotic milling processes. The main reason is that the robot’s modes greatly shift, depending on its varying dynamic parameters and joint configurations. More experimental investigations are required to study and better understand the mechanism of vibration in robotic machining. The present paper is focusing on finding experimental support for chatter-free prediction in robot high-speed milling by the regenerative chatter theory. Modal tests are first conducted on a milling robot and used to predict stability lobes by zeroth order approximation. A number of high-speed slotting tests are then carried out to verify the prediction results. Thus, the regenerative chatter theory is proved to be also applicable to robotic high-speed milling. Furthermore, low-frequency modes of the robot structure are investigated by more modal experiments involving a laser tracker and a displacement sensor. The low-frequency modes are identified as the main part of the prediction error of the zeroth order approximation method, which could also be dominant in low-speed robotic milling processes. In addition, robots are different from computer numerical control machines in terms of stiffness, trajectory following error, forced vibration, and motion coupling. These long-period trend terms have to be carefully taken into account in the regenerative chatter theory for robotic high-speed milling.

A Framework of Next Generation Adaptive CNC Controller

Modern manufacturing industries have increasingly demanded to bring comprehensive input data described using high-level languages such as STEP-NC, rather than outdated G&M codes into computer numerical control machine tool levels. In current dynamic shop floor environments, predefined numerical control (NC) command generated in early stages is regularly found unusable or unsuitable for the dedicated resources, causing useless efforts used up in the initial process planning and NC code generation. This research aims to propose a new structure of an adaptive CNC controller by taking the advantages of well-known IEC61499 and STEP-NC standards. For realising adaptive CNC controller capability, integration of the native process planning decision-making function into CNC controller will be established. The activities such as cutting tool selection, machining parameter determination and toolpath generation will be issued automatically by the controller itself that subject to available online machine resources. The generic STEP-NC file is employed as data input and arranged accordingly in the IEC 61499 function block software editor. The system is developed in the JAVA environment by using proposed language.

Research on remote state monitoring and intelligent maintenance system of CNC machine tools

In order to improve the operation reliability of high-grade computer numerical control (CNC) machine tools and reduce the maintenance time of failures, the remote state monitoring and intelligent maintenance system for CNC machine tools was proposed. Through collecting the basic information of equipment, maintenance information, and the information of the vibration, temperature, main current and power of the machine tool based on soft-bus technology, the system analysed the real-time state data, got the reliability parameters, extracted the fault features and established the maintenance knowledge base. Combining the dynamic monitoring data and static data, the system evaluated the key parts of machine tools. By the fault knowledge base, the system analysed the reason for failure and recommended the maintenance method. With the function modules, the system can improve the intelligent maintenance level of CNC machine tools and improve the using efficiency of machine tools.

A Low-Cost Vision-Based Monitoring of Computer Numerical Control (CNC) Machine Tools for Small and Medium-Sized Enterprises (SMEs).

In the new era of manufacturing with the Fourth Industrial Revolution, the smart factory is getting much attention as a solution for the factory of the future. Despite challenges in small and medium-sized enterprises (SMEs), such as short-term strategies and labor-intensive with limited resources, they have to improve productivity and stay competitive by adopting smart factory technologies. This study presents a novel monitoring approach for SMEs, KEM (keep an eye on your machine), and using a low-cost vision, such as a webcam and open-source technologies. Mainly, this idea focuses on collecting and processing operational data using cheaper and easy-to-use components. A prototype was tested with the typical 3-axis computer numerical control (CNC) milling machine. From the evaluation, availability of using a low-cost webcam and open-source technologies for monitoring of machine tools was confirmed. The results revealed that the proposed system is easy to integrate and can be conveniently applied to legacy machine tools on the shop floor without a significant change of equipment and cost barrier, which is less than $500 USD. These benefits could lead to a change of monitoring operations to reduce time in operation, energy consumption, and environmental impact for the sustainable production of SMEs.

Lz-Transform Method and Markov Reward Approach for Flexible Manufacturing System Performance Evaluation

This article presents a performance evaluation of a flexible manufacturing system (FMS), in which the system and its components can have different performance levels ranging from perfect functioning to total failure. An original multistate transition process for the FMS’s main component, Computerized Numerical Control (CNC) machine tools, proposed in the current article and treated as an aging process because of its increasing failure rate is investigated. The Markov reward approach is utilized to analyze the availability of CNC machine tools and sensitivity analysis of each non-operational state to the machine’s downtime. To overcome the “state explosion problem” in the subsequent FMS performance evaluation process, an application of the Lz-transform method is proposed to determine the performance evaluation and availability of such aging multistate FMS. An illustrated case is presented to demonstrate the performance evaluation process. The results show that the suggested methods can be implemented in engineering decision making and construction of various aging multistate FMS related to demands, availability, and output performance.

Binocular vision-based 3D method for detecting high dynamic and wide-range contouring errors of CNC machine tools

Periodic checking for contouring error in unloading conditions can effectively evaluate the dynamic performance of machining centers. Existing measurement devices have limitations in the high accuracy three-dimensional (3D) measurement of arbitrary contouring errors (e.g. ballbar and cross-grid encoder). Thus, in this paper, a cost-effective binocular vision-based 3D method for detecting high dynamic and wide-range contouring error of computer numerical control machine tools is proposed. With this method, the strobe lighting method is first presented to suppress the blurring effect of the newly designed 1024 coded targets that were embedded in the measurement fixture. Thereafter, an encoding and decoding method based on finding the optimal initial non-encoding region is proposed to automatically identify, match, and reconstruct the time-varying targets. Then, to enhance the performance of the vision system, a calculated method based on three visible non-collinear coded targets is presented to deduce the dynamic and large-scale contouring error. Finally, contouring error detection experiments are carried out in a self-built five-axis machine tool to validate the advantages of the method. Both the proposed vision method and cross-grid encoder are used to measure contouring errors of three types of trajectories under different feed rates. By comparing the difference between the two trajectories measured by the two devices, the experimental results illustrate that the mean vision detection error at 7 m min−1 is about 4.3 µm, which verifies the accuracy and effectiveness of the proposed vision measurement method.

Intensity-Based Registration With Voxel-Based Computer-Aided Manufacturing for Adaptive Machining

Abstract This paper presents a novel application of intensity-based volume registration to manufacturing using voxel-based computer-aided manufacturing (CAM) models. The introduced techniques are presented in the context of machining irregularly shaped materials by integrating volumetric imaging feedback to computer numerical control (CNC) machine tools. This requires a comparison and alignment to be performed in the software to geometrically “fit” the source design model inside a rendered starting material. The requirements differ from many typical registration applications in that the workpiece will necessarily be larger (i.e., greater in volume) than the desired final computer-aided design (CAD) file. Therefore, models need to be aligned for toolpath generation to workpiece counterparts that have been either volumetrically offset or contain additional material/volume. Intensity-based registrations are unique in that they consider only the voxel values over the entire volume. Although advancements in medical imaging have produced efficient, robust voxel registration algorithms, these techniques have not yet been applied to manufacturing. This research introduces the use of maximization of mutual information (MMI) for voxel-based CAM to drive an alignment registration for systems integrating imaging technology. A simple but novel method, which the authors have named minimization of distance variance (MDV), is also introduced. This minimizes the variance between voxel intensities to demonstrate the design of a similarity metric for a simple case in machining rough castings.

Research on meta‐action decomposition and meta‐action unit modeling technology for electromechanical product

AbstractThe current decomposition methods are not suitable for electromechanical product; the smallest decomposition units obtained by these methods are static parts or components, which cannot reflect the characteristics of electromechanical product that ‘action determines motion, and motion determines function'. Meta‐action is the smallest action to realize the function of electromechanical product, and it is reasonable to regard meta‐action as the smallest decomposition unit of electromechanical product. Meta‐action unit is the smallest structural unit to ensure the normal operation of meta‐action, and it is also the smallest carrier of electromechanical product quality characteristics. Meta‐action unit modeling technology is the basis of other subsequent research, so it is necessary to study it. In this paper, detailed criteria for meta‐action decomposition and meta‐action unit separation are formulated, and the standardized structural model, symbolized conceptual model, and assembly model of meta‐action unit are also studied, which provide a basis for mechanical, kinematic, and failure mechanism research of electromechanical product. A CNC (computer numerical control) machine tool made in China is taken as an example for meta‐motion decomposition and meta‐action unit modeling, and the results verify the applicability and correctness of the method proposed in this paper. The proposed method is also applicable to other types of electromechanical product.

Hybrid Multi-Domain Analytical and Data-Driven Modeling for Feed Systems in Machine Tools

Position error-compensation control in the servo system of computerized numerical control (CNC) machine tools relies on accurate prediction of dynamic tracking errors of the machine tool feed system. In this paper, in order to accurately predict dynamic tracking errors, a hybrid modeling method is proposed and a dynamic model of the ball screw feed system is developed. Firstly, according to the law of conservation of energy, a complete multi-domain system analytical model of a ball screw feed system was established based on energy flow. In order to overcome the uncertainties of the analytical model, then the data-driven model based on the back propagation (BP) neural network was established and trained using experimental data. Finally, the data-driven model was coupled with the multi-domain analytical model and the hybrid model was developed. The model was verified by experiment at different velocities and the results show that the prediction accuracy of the hybrid model reaches high levels. The hybrid modeling method combines the advantages of analytical modeling and data-driven modeling methods, and can significantly improve the feed system’s modeling accuracy. The research results of this paper are of great significance to improve the compensation control accuracy of CNC machine tools.

Research on the Error Averaging Effect in A Rolling Guide Pair

By studying the effects of geometric precision on kinematic accuracy, an error mapping model has been established, based on the hypothesis that a motion pair and its installation surface are rigid. However, when using this assumption, there is a significant error induced in high-precision computer numerical control (CNC) machine tools as compared with reality. One of the most important reasons for this error is failing to consider the error averaging effect of motion pair elements. Therefore, this work examines a high-precision horizontal machining center as its research object, and analyzes the error averaging mechanism of a rolling guide pair under a deformation of the rolling elements. The carriage bearing forces caused by guideway straightness errors are obtained by constructing a geometric error model of a single carriage. The relationship between guideway straightness errors and carriage bearing forces is described by a transfer function in the spatial frequency domain, and its characteristics are analyzed. It quantifies the so-called error averaging effect of the rolling guide system and, on this basis, a static model for four carriages is established to reflect the error averaging effect of the rolling guide pair on the position and orientation errors of the motion pair. In addition, it is found that the wavelengths and phase differences of guideway errors affect this error averaging mechanism, but the amplitude and preload have little influence thereon. The experiment result shows that the kinematic straightness errors in the x- and y-directions were approximately 1/3 to 1/2 of the guideway straightness errors in the corresponding directions. The results can be used to guide the precision design and assembly of machine tools.