This article proposes a method to measure, model and compensate both geometrically dependent and independent volumetric errors of five-axis, serial CNC machine tools. The forward and inverse kinematics of the machine tool are modeled using the screw theory, and the 41 errors of all 5 axes are represented by error motion twists. The component errors of translational drives have been measured with a laser interferometer, and the errors of two rotary drives have been identified with ballbar measurements. The complete volumetric error model of a five-axis machine has been modeled in the machine's coordinate system and proven experimentally. The volumetric errors are mapped to the part coordinates along the tool path, and compensated using the kinematic model of the machine. The compensation strategy has been demonstrated on a five-axis machine tool controlled by an industrial CNC with a limited freedom, as well as by a Virtual CNC which allows the incorporation of compensating all 41 errors.

Modeling and compensation of volumetric errors for five-axis machine tools

On-machine measurement (OMM) involves the interruption of the machining process and the subsequent measurement of the workpiece without its removal from the machining tool. Chromatic confocal sensing is a well-known measurement technique that is able to evaluate the position of a point on an object surface along the optical axis of the system with high accuracy. The present study integrates a chromatic confocal measurement probe with an ultra-precision diamond turning machine to achieve the non-contact OMM of machined components. The procedure for establishing the position of the rotary axis of the spindle based on dual standard spheres is first described in detail, and the relevant OMM procedure for machined components of different surface topographies is explained. Then, a 50-μm quartz step height standard is employed to investigate the linear measurement accuracy of the chromatic confocal probe. Finally, the measurement accuracy of the proposed OMM method is compared experimentally with that of the stylus method. The results show that the estimated form error value of the OMM method agrees well with the value obtained by the stylus method. The proposed OMM method feasibly achieves non-contact OMM with a nanometer-level accuracy for an ultra-precision turning machine and is capable of reconstructing the 3D surface topography of flat, spherical, and aspheric surfaces. After integrating the OMM method, the ultra-precision turning machine can realize the function of processing-measurement integration.

Non-contact on-machine measurement using a chromatic confocal probe for an ultra-precision turning machine

The geometric errors of rotary axes are the fundamental errors of a five-axis machine tool. They directly affect the machining accuracy, and require periodical measurement, identification and compensation. In this paper, a precise calibration and compensation method for the geometric errors of rotary axes on a five-axis machine tool is proposed. The automated measurement is realized by using an on-the-machine touch-trigger technology and an artifact. A calibration algorithm is proposed to calibrate geometric errors of rotary axes based on the relative displacement of the measured reference point. The geometric errors are individually separated and the coupling effect of the geometric errors of two rotary axes can be avoided. The geometry error of the artifact as well as its setup error has little influence on geometric error calibration results. Then a geometric error compensation algorithm is developed by modifying the numeric control (NC) source file. All the geometric errors of the rotary errors are compensated to improve the machining accuracy. The algorithm can be conveniently integrated into the post process. At last, an experiment on a five-axis machine tool with table A-axis and head B-axis structure validates the feasibility of the proposed method.

Identification and compensation of geometric errors of rotary axes on five-axis machine by on-machine measurement

Waterjet machining has attracted great attention in the conditions of hard-to-machine materials, microstructures, or complicated industrial components, and it has become well-established in all major areas of theoretical researches and already been found across the broad spectrum of technical application areas especially in the specific sectors of scientific frontiers, including the mechanical precision component, advanced functional material, intelligent automotive engineering, aerospace equipment, renewable energy science, leading medical instruments, etc. This paper reviews the historical and latest research developments and integrated applications of waterjet machining in the domains of mechanism and performances, which covers a lot of key aspects such as waterjet machining optimization, dynamic simulation and process monitoring of machining process, and the influence mechanism of waterjet machining as well. Its machining mechanism, performance capability, functional advantages, and inherent disadvantages are characterized and assessed in detail, so that the integrated applications of multifield-assisted waterjet machining can be introduced and focused thereafter. Finally, various future development prospects in all the abovementioned aspects of waterjet machining are discussed systematically and explored subsequently, which contribute to the acquirement of a series of comprehensive conclusions. This review can be used as suitable and effective tools to study and summarize the complicated correlations between waterjet machining mechanism and its actual working performances in different environmental conditions; therefore, this proposed investigation facilitates the precision manufacture or characteristic improvements of industrial product with higher efficiency and better quality in return.

Waterjet machining and research developments: a review

In flank milling of thin-walled parts, the profile tolerance would usually be violated by the excessive static deformations. This paper presents a comprehensive method to compensate deformation errors in five-axis flank milling from the aspect of tool path optimization. Firstly, the machined surface is constructed by imprinting the predicted tool/workpiece deformations on the cutter envelope surface. Secondly, the signed distances from the sample points on the design surface to the machined surface are calculated for the machining error evaluation. Their differential increments that characterize the variation of surface errors with respect to the adjustment of tool path are then derived. On this basis, the mathematical model and algorithm for minimizing the deformation-induced surface errors are developed through slightly optimizing the shape parameters of the tool path surface. Finally, five-axis blade milling experiments are conducted to validate the effectiveness of the proposed method. The results demonstrate that the surface errors mainly caused by machining deformations in flank milling of flexible blades can be largely reduced by using the developed algorithm.

Compensation of deformation errors in five-axis flank milling of thin-walled parts via tool path optimization

Deformation of the part and cutter caused by cutting forces immediately affects the dimensional accuracy of manufactured parts. This paper presents an integrated machining deviation compensation strategy based on on-machine measurement (OMM) inspection system. Previous research attempts on this topic deal with deformation compensation in machining of geometries in 3-axis machine tools only. This paper is the first time that concerned with 5-axis flank milling of flexible thin-walled parts. To capture the machined surface precision dimensions, OMM with a touch-trigger probe installed on machine׳s spindle is utilized. Probe path is planned to obtain the coordinate of the sampling points on machined surface. The machined surface can then be reconstructed. Meanwhile, the cutter׳s envelope surface is calculated based on nominal cutter location source file (CLSF). Subsequently, the machining error caused by part and cutter deflection is calibrated by comparing the deviation between the machined surface and the envelope surface. An iteration toolpath compensation algorithm is designed to decrease machining errors and avoid unwanted interference by modifying the toolpath. Experiment of machining the impeller blade is carried out to validate the methodology developed in this paper. The results demonstrate the effectiveness of the proposed method in machining error compensation.

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Geometric errors of 5-axis machine tools introduce great deviation in real workpiece manufacture and on-machine measurement like touch-trigger probe measurement. Compensation of those errors by toolpath modification is an effective and distinguished method considering the machine calibration costs and productivity. Development of kinematic transformation model is involved in this paper to clarify the negative influences caused by those errors at first. The deviation of the designed toolpath and the real implemented toolpath in workpiece coordinate system is calculated by this model. An iterative compensation algorithm is then developed through NC code modification. The differential relationship between the NC code and the corresponding real toolpath can be expressed by Jacobi matrix. The optimal linear approximation of the compensated NC code is calculated by utilizing the Newton method. Iteratively applying this approximation progress until the deviation between the nominal and real toolpath satisfies the given tolerance. The variations of the geometric errors at different positions are also taken into account. To this end, the nominal toolpath and the geometric errors of the specific 5-axis machine tool are considered as the input. The new compensated NC code is generated as the output. The methodology can be directly utilized as the post-processor. Experimental results demonstrate the sensibility and effectiveness of the compensation method established in this study.

Integrated post-processor for 5-axis machine tools with geometric errors compensation

Curved contour on large thin-walled skin is difficult to be machined since severe deformations usually occur on real skin. It is critical to determine the real geometry of the deformed contour. Under the assumption that only the normal deformation of the large thin-walled skin occurs and the shear deformation is ignored, a novel isometric mapping based adaptive machining method is developed. While straightness distance or local angle is preserved in traditional surface mapping methods, arc-length is preserved in proposed isometric mapping to improve the contour cutting accuracy. The proposed method consists of three steps. (1) The real geometry of the deformed surface is obtained by using a laser scanner based on-machine measurement (OMM) system. The measured point cloud is then transformed into triangular mesh to represent the deformed surface. Some points on the nominal surface are sampled based on uniform sampling strategy, and the corresponding sampling points in the deformed surface are also allocated. (2) Isometric mapping between the two groups of points is constructed. Matching accuracy between nominal surface and real surface is defined based on the deviation of the geodesic distances of the two groups of points. A surface matching optimization model is developed to adjust the mapping point location in the triangular mesh and achieve a minimum surface mismatch error. (3) The toolpath is adaptively adjusted to compensate the deformation error based on the isometric surface mapping results. Both simulation and machining experiments are conducted to demonstrate the feasibility and validity of the proposed method. The experiment results show that accuracy of the machined curved contour on the deformed skin has been significantly improved.

Adaptive machining for curved contour on deformed large skin based on on-machine measurement and isometric mapping

Large thin-walled parts are widely used in aerospace. Due to its low rigidity, force- and thermal-induced cutting deformation immediately affects the dimensional accuracy of machined parts. Multilayer milling strategy is usually utilized due to its low rigidity, which results in reduction of machining efficiency. In this work, a typical large thin-walled part, tank bottom of the rocket, is selected as an application object and an adaptive deformation error compensation method for large thin-walled part is proposed. An integrated on-machine measurement (OMM) system is developed to acquire the part’s geometry. Geometry of outer surface is directly measured and constructed by a touch-trigger probe installed on machine tool’s spindle, while the geometry of inner surface is determined by measuring the thickness at each probe point, using an ultrasonic thickness gage. As such, machining error for each layer cutting is identified by comparing with the designed geometry. A deformation prediction model is established to predict the cutting deformation of the next layer based on the calibrated error in previous layer cutting, so as to compute the compensation value. A machining error compensation algorithm is then developed to eliminate the deformation error by modifying the machining toolpath. At last, machining experiment is conducted to verify the feasibility of the proposed methodology.

Nuodi Huang

Papers

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Identification and compensation of geometric errors of rotary axes on five-axis machine by on-machine measurement

Integrated post-processor for 5-axis machine tools with geometric errors compensation

Adaptive machining for curved contour on deformed large skin based on on-machine measurement and isometric mapping

Error compensation for machining of large thin-walled part with sculptured surface based on on-machine measurement