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American Society of Mechanical Engineers (ASME)

This paper presents a new S trajectory kinematic measurement method to evaluate the dynamic accuracy of five-axis machine tools based on R-test measurements. The S trajectory is obtained by scaling the machining path of the S-shaped test piece to the measuring stroke of the R-test. A geometric and dynamic accuracy simulation model is established to analyze the influence of various error factors on the S trajectory test and compare with the conical kinematic test described in ISO10791.6 quantitatively. The simulation results shown that the S trajectory test is more sensitive to most geometric errors and dynamic errors than the conical test. Particularly, to reflect the dynamic error factors such as poor rigidity of machine tools, servo mismatch, reverse error and nonlinear error, the S trajectory test has obvious advantages. Meanwhile, compared with the actual S-shaped piece machining, which is under development for inclusion in ISO 10791.7, the proposed non-cutting kinematic test method can simply reflect the dynamic accuracy of five axis machine tools itself without the interference of other factors and does not require additional workpiece and testing equipment. The proposed method is verified through experiments on a five-axis machine tool.

Dynamic accuracy evaluation for five-axis machine tools using S trajectory deviation based on R-test measurement

The dynamic error of CNC machine tools, which often exceeds the quasi-static error at high-speed machining, becomes the main reason affecting the machining error of the sculptured surface parts. Although much research efforts have been dedicated to dynamic error, there is a lack of systematical summaries. In this review, firstly, the dynamic error is defined as the deviation of actual displacement of effector end of axis relative to reference displacement during feed motion. Secondly, according to the mechanical and control structure of the servo feed system, the dynamic error is divided into two components: dynamic error inside the servo loop (component 1) and dynamic error outside the servo loop (component 2). Based on the two components, the causes resulting in the dynamic error are analyzed from the points of view of the servo feed system itself and its input (setpoints). Thirdly, the basic strategies for reducing the dynamic error of individual axis, as well as for reducing the trajectory dynamic error by coordinating the dynamic error of individual axis, are summarized. Finally, the problems and future research directions on dynamic error are analyzed. It is concluded that resolving the contradiction between the setpoints and the servo feed system is still a great challenge for dynamic error in high-speed machining. To achieve high dynamic accuracy at high-speed machining, the control strategies on the dynamic error outside the servo loop should be further developed and integrated into dynamic error inside the servo loop-oriented control strategies. Meanwhile, the servo feed system itself and its input need to be investigated as a whole, so that the servo feed system of each axis can adapt to the differences and changes of the setpoints, and the differences in the servo dynamics of each axis can be considered in the setpoints.

Dynamic error of CNC machine tools: a state-of-the-art review

Tool path planning is critical to fully realise the producing capabilities of multi-axis machine tools and improve the precision and efficiency in multi-axis machining. Up to now, more than 400 art...

A review on tool orientation planning in multi-axis machining

Contour error is a main factor that affects the quality of products in numerical control (NC) machining. This article presents a contour control strategy based on digital curves for high-precision control of computer numerical control (CNC) machines. A contour error estimation algorithm is presented for digital curves based on a geometrical method. The dynamic model of the motion control system is transformed from time domain to space domain because the contour error is dependent on space instead of time. Spatial iterative learning control (sILC) is developed to reduce the contour error, by modifying the reference trajectory in the form of G code. This allows system improvement without interference of low-level controllers so it is applicable to many commercial controllers where interpolators and feed-drive controllers cannot be altered. The effectiveness of this method is verified by experiments on a NC machine, which have shown good performance not only for smooth trajectories but also for large curvature trajectories.

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Reference Trajectory Modification Based on Spatial Iterative Learning for Contour Control of Two-Axis NC Systems

In this paper, a convenient way to detect machining precision of five-axis CNC machine tool is suggested in theory. In a general way, NAS979 is cut to test machine tool; however, it fails to evaluate the combination motions of rotary axes sufficiently. Therefore, a novel S-shape test part, called ‘S’ test piece, has been presented to demonstrate the machine tool’s capabilities. As a new test specimen, ‘S’ test piece has some advantage to exhibit the machining precision of five-axis machine tool. There are some visible marks related to performance of machine tool experimentally, however, the reasons for these abnormal marks are uncertain theoretically, the performance of the servo system may be one of the causes. In order to figure out the definite cause of the abnormal morphology, a simulated platform with the servo system is set up to amplify and the normal errors that come from the tracking of axes are presented. The simulation results of the abnormal morphology of ‘S’ test piece is provided. And the surface quality is evaluated by the peak-to-peak value (Vpp). There are obvious marks in four special regions of ‘S’ test piece that simulated with poor performance servo system, and these marks are invisible in the surface of ‘S’ test piece that simulated with good performance servo system. Vpp in these four regions changes greater than the other regions of ‘S’ test piece. The Vpp that simulated by the poor performance servo system is about 15 times larger than the error simulated by the optimized performance servo system in these four special regions. While, Vpp of other regions is essentially invariant. Then, the machining experiments of ‘S’ test piece are conducted with the standard suggested process. The abnormal morphology of machined ‘S’ test piece is so obvious that it can be observed by the naked eyes, without any test equipment. And the result of the experiment is consistent with the simulation result, which means that tracking errors of servo system have direct influence on surface morphology abnormality, and the surface quality of ‘S’ test piece could display the dynamic performance of the servo system intuitively in theory. As a standard comparison object, the surface quality of NAS979 test piece is analyzed through the same platform with the poor performance servo system, the largest Vpp is about 0.00022 mm, one eightieth smaller than ‘S’ test piece at least. And machining experiment is also carried out with the poor performance machine tool, the surface is so smooth that unexpected texture cannot be observed by the naked eyes, it should be test by the special measurements. The simulation results and experiment results both show that the surface quality of ‘S’ test piece is hugely worse than NAS979. Besides, there are several special regions of ‘S’ test piece to exhibit the surface texture waving with certain parameters. In a word, ‘S’ test piece is high effectively to exhibit the dynamic performance of the servo system of five-axis machine tool.

Modeling and simulation of surface morphology abnormality of ‘S’ test piece machined by five-axis CNC machine tool

The invention discloses a five-axis machine tool cutter posture and cutter point position error synchronous detection mechanism. The five-axis machine tool cutter posture and cutter point position error synchronous detection mechanism comprises a tilt angle measurement device, a spatial displacement measurement device, a rotating angle measurement device, an installation device and a follow-up platform, wherein the tilt angle measurement device, the spatial displacement measurement device, the rotating angle measurement device and the follow-up platform are all installed on the installation device, the spatial displacement device is used for measuring the three-axial-direction micrometric displacement of a cutter point when the rotating center of a cutter of the machine tool to be detected moves along a set track, and the tilt angle measurement device and the rotating angle measurement device are used for measuring the tilt angle and the rotating angle of the cutter of the machine tool to be detected respectively. By the adoption of the five-axis machine tool cutter posture and cutter point position error synchronous detection mechanism, real-time synchronous measurement of the tilt angle and the rotating angle of the cutter of the five-axis machine tool and the three-axial-direction micrometric displacement of the cutter point is achieved. The five-axis machine tool cutter posture and cutter point position error synchronous detection mechanism has positive significance in detecting high-precision machine tools.

Five-axis machine tool cutter posture and cutter tip position error synchronous detection mechanism

The role of five-axis CNC machine tools (FAMT) in the manufacturing industry is becoming more and more important, but due to the large number of heat sources of FAMT, the thermal error caused by them will be more complicated. To simplify the complicated thermal error model, this paper presents a new modelling method for compensation of the thermal errors on a cradle-type FAMT. This method uses artificial neural network (ANN) and shark smell optimization (SSO) algorithm to evaluate the performance of FAMT, and developing the thermal error compensation system, the compensation model is verified by machining experiments. Generally, the thermal sensitive point screening is performed by a method in which a large number of temperature sensors are arranged randomly, it increases the workload and may cause omission of the heat sensitive point. In this paper, the thermal imager is used to screen out the temperature sensitive points of the machine tool (MT), then the temperature sensor is placed at the position of the heat sensitive point of the FAMT, and the collected thermal characteristic data is used for thermal error modeling. The C-axis heating test, spindle heating test, and the combined movement test are applied in this work, and the results show that the shark smell optimization artificial neural network (SSO-ANN) model was compared to the other two models and verified better performance than back propagation artificial neural network (BP-ANN) model and particle swarm optimization neural network (PSO) model with the same training samples. Finally, a compensation experiment is carried out. The compensation values, which was calculated by the SSO-ANN model are sent to the real-time error compensation controller. The compensation effect of the model is then tested by machining the ‘S’-shaped test piece. Test results show that the 32 % reduction in machining error is achieved after compensation, which means this method improves the accuracy and robustness of the thermal error compensation system.

Thermal error analysis, modeling and compensation of five-axis machine tools

Since their ability to machine complex curved surface, five-axis CNC machine tools have been widely used in the machining field. To guarantee the accuracy of the complex surface machining, multi-axis linkage performance detection of five-axis machine tools is necessary. RTCP (rotation tool center point) is one of the basic essential functions for five-axis machine tools, which could effectively reduce the nonlinear errors caused by rotation axes when five axes move synchronously. On the basis of RTCP function, a way to detect multi-axes linkage performance of five-axis machine tools is introduced in this paper. Some simulations that exhibit the relationship between the tool center error trajectories and the factors influencing the linkage performance of the machine tool are established based on the linkage error model. Analyzing the rule of tool center error trajectories, a linkage error tracing method is proposed based on feature extraction and trajectory similarity. The experiments based on RTCP are carried out, and the tool center point error variation is shown to be consistent with the simulation result. Furthermore, the error of the error tracing method is less than 5%, which means the error tracing method could find the mismatch parameters effectively. This attempt provides a theoretical support for linkage performance detection and error tracing of five-axis CNC machine tools.

Research on error tracing method of five-axis CNC machine tool linkage error

The evaluation of contouring error is important for multi-axis CNC machines because the tolerance specifications of manufactured parts are directly affected by contouring error. One of the fundamental quality inspections to verify that a manufactured part meets the expected tolerance is via form error evaluations. However, the existing estimation methods of contouring error are based on the position tolerance requirements. To meet the form tolerance requirements for the parts, this paper focuses on developing a high-accuracy estimation method of contouring error that is not related to a datum (ND-contouring error). In the proposed estimation method, at first, the minimum zone tolerance (MZT) method is used to transform the ideal tool tip path to match the actual one. Subsequently, by comparing the position and orientation between the actual point and the nearest point on the transformed ideal tool path, the ND-contouring error and orientation contouring error of the tool can be estimated, respectively. In addition, the difference between the proposed estimation method and previous evaluation methods is comparatively analyzed. Finally, simulations and experiments are conducted by applying the S-shaped and B-shaped machining trajectories, respectively, and the results all verify the estimation accuracy of the ND-contouring error estimation method. By adoption of compensation based on the ND-contouring error estimation, the contouring error could be significantly reduced, which will improve the quality of parts.

Li Du

Papers

Modeling and simulation of surface morphology abnormality of ‘S’ test piece machined by five-axis CNC machine tool

Five-axis machine tool cutter posture and cutter tip position error synchronous detection mechanism

Thermal error analysis, modeling and compensation of five-axis machine tools

Research on error tracing method of five-axis CNC machine tool linkage error

A new contouring error estimation for the high form accuracy of a multi-axis CNC machine tool