De-ning Song

Pre-compensation for continuous-path running trajectory error in high-speed machining of parts with varied curvature features

Parts with varied curvature features play increasingly critical roles in engineering, and are often machined under high-speed continuous-path running mode to ensure the machining efficiency. However, the continuous-path running trajectory error is significant during high-feed-speed machining, which seriously restricts the machining precision for such parts with varied curvature features. In order to reduce the continuous-path running trajectory error without sacrificing the machining efficiency, a pre-compensation method for the trajectory error is proposed. Based on the formation mechanism of the continuous-path running trajectory error analyzed, this error is estimated in advance by approximating the desired toolpath with spline curves. Then, an iterative error pre-compensation method is presented. By machining with the regenerated toolpath after pre-compensation instead of the uncompensated toolpath, the continuous-path running trajectory error can be effectively decreased without the reduction of the feed speed. To demonstrate the feasibility of the proposed pre-compensation method, a heart curve toolpath that possesses varied curvature features is employed. Experimental results indicate that compared with the uncompensated processing trajectory, the maximum and average machining errors for the pre-compensated processing trajectory are reduced by 67.19% and 82.30%, respectively. An easy to implement solution for high efficiency and high precision machining of the parts with varied curvature features is provided.

Influence of spindle speed on tool wear in high-speed milling of Inconel 718 curved surface parts:

High-speed milling, which provides an efficient approach for high-quality machining, is widely adopted for machining difficult-to-machine materials such as Inconel 718. For high-speed milling of Inconel 718 curved surface parts, the spindle speed which determines cutting speed directly is regarded as an important cutting parameter related to tool wear and machining efficiency. Meanwhile, because of the changing geometric features of curved surface, cutting force is changing all the time with the variation of geometric features, which influences not only tool wear but also machining quality significantly. In this study, the influence of spindle speed on coated tool wear in high-speed milling of Inconel 718 curved surface parts is studied through a series of experiments on considering tool life, cutting force, cutting force fluctuation, and machining efficiency. According to the experimental results, the appropriate spindle speed that can balance both the tool life and the machining efficiency is selected as 10,000 r/min for high-speed milling of Inconel 718 curved surface parts. In addition, the coated tool wear mechanism is investigated through scanning electron microscopy–energy dispersive x-ray spectroscopy analysis. The results show that at the beginning wear stage and the stable wear stage, the coated tool wear is mainly caused by mechanical wear. Then, with the increasing cutting temperature due to the blunt tool edge, the tool wear becomes compound wear which contains more than one wear form so as to cause a severe tool wear.

Synergistic real-time compensation of tracking and contouring errors for precise parametric curved contour following systems:

The tracking and contouring errors are inevitable in real computer numerical control contour following because of the reasons such as servo delay and dynamics mismatch. In order to improve the motion accuracy, this paper proposes a synergistic real-time compensation method of tracking and contouring errors for precise parametric curve following of the computer numerical control systems. The tracking error for each individual axis is first compensated, by using the feed-drive models with the consideration of model uncertainties, to enhance the tracking performances of all axes. Further, the contouring error is estimated and compensated to improve the contour accuracy directly, where a high-precision contouring-error estimation algorithm, based on spatial circular approximation of the desired contour neighboring the actual motion position, is presented. Considering that the system structure is coupled after compensation, the stability of the coupled system is analyzed for design of the synergistic compensator. Innovative contributions of this study are that not only the contouring-error can be estimated with a high precision in real time, but also the tracking and contouring performances can be simultaneously improved although there exist modeling errors and disturbances. Simulation and experimental tests demonstrate the effectiveness and advantages of the proposed method.

Machining error reduction by combining of feed-speed optimization and toolpath modification in high-speed machining for parts with rapidly varied geometric features

Parts with rapidly varied geometric features are usually crucial parts in high-end equipment and widely applied in the fields of aerospace, energy and power, which are difficult or inefficient to process because of the more special structure and the higher requirement of machining precision. High-speed machining technology provides an effective method for parts with rapidly varied geometric features to solve the contradiction between high demand and low machining efficiency. However, as the existence of rapidly varied geometric features, the machining toolpath for such parts is always complex free-form curve and the actual moving speed of the workbench of the NC machine tool cannot reach the feed-speed set in the NC program timely due to the drive constraint of NC machine tool. Furthermore, the machine tool would vibrate violently when machining the rapidly varied geometric features. In this way, the big machining error will be formed. A machining error reduction method by combining of feed-speed optimization and toolpath modification in high-speed machining for such parts is proposed. First, considering that the actual feed-speed cannot reach the programmed value when the toolpath curvature is too large, the feed-speed is optimized with the constraints of jerk and acceleration limitations of the feed shafts, and a feed-rate smoothing algorithm is applied. Then, the compensated cutter locations are calculated via machining-error estimation. Finally, the modified NC codes are acquired according to the optimized feed-speed and the compensated toolpath. By combining the feed-speed optimization and toolpath modification, the high precision and high efficiency machining can be realized. The experimental results demonstrate the feasibility of the proposed approach. This study provides an effective approach to reduce the machining error in high-speed machining, and is significant for improving the processing precision and efficiency of parts with rapidly varied geometric features.

Conversion method study from cutter-location points to nonuniform rational B-spline toolpath NC file for high-speed machining

Parts with complex curved surfaces are widely applied and the demands for the machining quality and the machining efficiency of such complex parts become increasingly higher. In order to realize high-speed and high-quality machining, the nonuniform rational B-spline interpolator is widely researched and demonstrated to be superior to the conventional linear or circular interpolators. However, the nonuniform rational B-spline toolpath NC files cannot be directly generated from the computer-aided design (CAD) models by using commercial computer-aided manufacturing (CAM) software. To deal with this problem, a conversion method from the cutter-location points to the nonuniform rational B-spline toolpath numerical control (NC) file is presented. To avoid the bad curve-fitting effect at sharp corners of the toolpath and to meanwhile reduce the computational burden, the cutter-location pre-processing method, consisting data segmentation and data simplification, is provided first. Then, the least-square method is employed to fit the cutter locations to the nonuniform rational B-spline curves, and an iterative fitting approach is proposed for linear/nonuniform rational B-spline hybrid toolpath generation. Finally, a user interface is designed for displaying the fitting results and outputting the NC file with nonuniform rational B-spline toolpaths. By using this method, nonuniform rational B-spline and linear toolpaths hybrid interpolation NC program can be generated for the high-speed machining of complex curved surface parts with the utilization of the nonuniform rational B-spline interpolator. The experimental results demonstrate the feasibility and the advantages of the presented method.

An efficient 5-axis toolpath optimization algorithm for machining parts with abrupt curvature

The kinematical behavior of 5-axis machine tool introduced by toolpath calculation has a close relation with processing efficiency and quality of parts. For the category of sculptured surface parts with abrupt curvature, such as turbine blade and fixed guide vane, the most commonly utilized spiral contouring toolpath modes usually encounter some trouble during actual machining, the reason lies in the difficult tool orientation control near the edge of such parts. However, the global optimization for the spiral toolpath means increased computation burden, and so it is time consuming. In this paper, an efficient 5-axis toolpath optimization algorithm is presented, and the objective is to smooth the rotary axes’ motion caused by drastically changed tool orientation on spiral toolpath for abrupt curvature parts machining. To reduce the computation burden, only path segments on the spiral trajectory-owned weak kinematic performance are selected for further optimizing. To obtain smoother motion for rotary axes, the specific optimization computation for the selected path segments is conducted in the Machine Coordinate System (MCS) instead of the Part Coordinate System (PCS). The optimization model is constructed and a related solution method is presented to ensure the high-performance optimization. The complete optimization algorithm is demonstrated on a spiral 5-axis toolpath for the turbine blade finish machining, and the result shows that only 25.9% optimization computation is needed compared with global optimization algorithm. And then, the actual machining experiments are carried out by using paraffin as cut material, and the machining time with optimized toolpath is decreased by 19.7% compared with initial toolpath. In addition, the surface quality of parts is significantly improved after conducting the optimization. This study proves that the proposed algorithm can significantly improve the processing efficiency and surface quality of parts with abrupt curvature and provides an efficient method to optimize the spiral 5-axis toolpath used for finish machining parts with abrupt curvature.

Contouring-Error Reduction by Combination of Tracking-Error Compensation and Cross- Coupled Control

Abstract: Complex curved surface parts are widely used in important fields such as aerospace, energy and power. In the machining process for the curved features of the parts, the contour-following error of the CNC (computer numerical control) system is increasingly significant due to the existence of servo delay for the feed-drive system and the dynamics mismatch between each axis. Then, the formed contouring error will directly affects the machining quality of the complex curved surface parts. To reduce the contour-following error, this paper presents a cross-coupled controller with tracking-error compensation. First, the tracking error for each feed axis is compensated in real time. On this basis, the numerical efficient contouringerror estimation method is utilized and the proportional cross-coupled controller is integrated with the tracking-error compensator. By using this method, the tracking error and contouring error can be synergistically reduced and the contour accuracy of the CNC system can be dramatically improved. Results of the verification tests demonstrate the feasibility of the presented approach, and the presented approach is significant for high-speed and high-precision machining of complex curved surface parts.