Five-axis Machining Center Research Articles

Effect of the dynamic stiffness of a five-axis machine tool on the quality of sculptured surface

To achieve high quality of sculptured surface, a five-axis machine center must have excellent dynamic behavior. This article gives a new way to characterize and present the performance of machine center on milling sculptured surface which is constrained by the dynamic stiffness. First, the formation mechanism of surface quality is explained in intermittent milling process. It is mainly affected by the vibration frequency of cutting behavior. Second, the concept of dynamic stiffness of machine center is proposed and calculated based on the movement of multi-axis. To elaborate such effect, the map of dynamic stiffness on a sculptured surface, S part is obtained. The change of stiffness causes the different vibration frequency and then leads to different surface quality. Finally, experiments are conducted on S part by microscope and the waviness measurement. The results demonstrate the proposed method is very close to the true one. Presenting, measuring, and controlling the surface quality are easy for the user to understand the challenging performance of the machine center on milling sculptured surface.

Tool posture dependent chatter suppression in five-axis milling of thin-walled workpiece with ball-end cutter

In aerospace industry, five-axis ball-end milling is widely used for flexible thin-walled aerospace parts by considering the process mechanics. A new analytical method is proposed to investigate the effects of the change of tool posture, especially lead and tilt angle, on chatter stability in milling. In this method, the dynamic cutter-workpiece engagement regions, which are essential to predict dynamic cutting forces and investigate chatter stability of the thin-walled workpiece, are determined considering the dynamic machining conditions. Then, according to dynamic cutter-workpiece engagement region data, the dynamic cutting force model considering the effects of lead and tilt angle is developed in machining the thin-walled aerospace parts with ball-end cutter. Subsequently, the milling chatter stability model is established based on the dynamic cutting force model, and the milling stability limits are determined in different lead and tilt angles. Finally, the feasibility and effectiveness of the proposed method are verified by experiments with ball-end cutter in five-axis machining center. The results are shown that the predicted values well match with the experimental results.

An error compensation scheme for multi-axis machine tool using machining method template

In this paper, a software-based geometric error compensation method using the machining method template is proposed to modify the long and complicated G-code file with multiple machining features. Firstly, an error model is established to evaluate the error distribution. Then, a new segmentation method is proposed to split the long multi-axis line into several parts with the same length to eliminate the nonlinear effect. Furthermore, a compensation scheme according to the machining method is presented to determine which segments and axes should be compensated when facing with a long and complicated G-code file. In addition, a framework is developed to process the G-code file with high efficiency. The G-code file is firstly separated into the motion and non-motion command files, which are then merged together to form a new G-code file after the motion commands are modified by an error compensation module. Finally, a machining experiment with a test workpiece containing multiple features is conducted on a five-axis machining center to validate the effectiveness and correctness of the proposed method.

Geometric error compensation method based on Floyd algorithm and product of exponential screw theory

Error compensation technique is a recognized and cost-effective method to improve machining accuracy of machine tools. In this article, a new compensation method for geometric error is proposed based on Floyd algorithm and product of exponential screw theory. Based on topological structure and measured data, volumetric geometric error modeling is established by product of exponential screw theory. Then, the improved Floyd minimum-distance method was used to establish an error compensation model by adjusting weight unceasingly. In order to verify the effectiveness and generality of the method proposed in this article, two experiments were designed. A total of 5 five-axis machining centers of the same type with different use time were selected to carry out the simulation experiments. Results show that the Floyd method can provide higher compensation precision, that is, Floyd algorithm compensation method can keep positioning errors within the range [−8 µm, 9 µm]. In addition, roundness error, coaxial error, and surface roughness were reduced in the actual machining experiments of two machined conical tables. Therefore, it can be seen that the proposed compensation method is effective to improve machining accuracy of machine tools.

The tool following function-based identification approach for all geometric errors of rotary axes using ballbar

This paper proposes a tool following function-based identification approach (TFFIA) for geometric errors of two rotary axes for one five-axis machine tool. It is comprehensive to identify all geometric errors of rotary errors. Firstly, synthetic error formulas of ballbar originate from the geometric error model of machine tools in order to consider the influences of 21 errors of three translational axes. It makes the approach more reasonable and precise. Secondly, the structures of three measurement patterns of TFFIA are described. Thirdly, in each pattern, errors of rotary axes affecting the accuracy of the sensitive direction are identified. As the result, the identification equations of all 20 errors coincide with the geometric properties of errors. Moreover, the impacts of setup errors of ballbar are eliminated with least square method to improve the precise of TFFIA. According to the structures of three patterns, only three installation of workpiece ball of ballbar are needed in the whole identification of two rotary axes to obtain the required ballbar readings. It greatly shortens the measurement time. Twenty geometric errors of two rotary axes are calculated with identification equations and ballbar readings. Finally, TFFIA is applied to a SmartCNC500 five-axis vertical machining center. The corresponding comparisons are proposed to verify the effectiveness and accuracy of TFFIA.

A parametric modeling method for the pose-dependent dynamics of bi-rotary milling head

Bi-rotary milling head is one of the core components of five-axis machining center, and its dynamic characteristics directly affect the machining stability and accuracy. During the sculptured surface machining, the bi-rotary milling head exhibits varying dynamics in various machining postures. To facilitate rapid evaluation of the dynamic behavior of the bi-rotary milling head within the whole workspace, this article presents a method for parametrically establishing dynamic equation at different postures. The rotating and swing shafts are treated as rigid bodies. The varying stiffness of the flexible joints (such as bearings and hirth coupling) affected by gravity and cutting force at different swing angles is analyzed and then a multi-rigid-body dynamic model of the bi-rotary milling head considering the pose-varying joint stiffness is established. The Lagrangian method is employed to deduce the parametric dynamic equation with posture parameters. The static stiffness, natural frequencies and frequency response functions at different postures are simulated using the parametric equation and verified by the impact testing experiments. The theoretical and experimental results show that the dynamics of the bi-rotary milling head vary with the machining postures, and the proposed method can be used for efficient and accurate evaluation of the pose-dependent dynamics at the design stage.

An approach of comprehensive error modeling and accuracy allocation for the improvement of reliability and optimization of cost of a multi-axis NC machine tool

Machining accuracy is critical for the quality and performance of a mechanical product, and the reliability of a multi-axis NC machine tool reflects the ability to reach and maintain the required machining accuracy. The objective of this study is to propose a general methodology that will simultaneously consider geometric errors and thermal-induced errors to allocate the geometric accuracy of components, for improving machining accuracy reliability under certain design requirements. The multi-body system (MBS) theory was applied to develop a comprehensive volumetric error model, showing the coupling relationship between the individual errors of the components of this machine tool and their volumetric accuracy. Additionally, a thermal error model was established based on the neural fuzzy control theory and was compared to the common thermal error modeling method called BP neural network. Based on the traditional cost model and the reliability analysis model, a geometric error-cost model and a geometric error-reliability model were established, taking the weighted function principle into consideration. Then, an allocation approach of the geometric errors, for optimizing total cost (manufacture and QLF) and reliability, subject to the geometrical and operational constraints of the machine tool, was proposed and formulated into a mathematical model, in order to perform the optimization process of accuracy allocation by using the advanced NSGA-II algorithm. A case study was also performed in a five-axis machine tool, and the traditional NSGA algorithm was used for comparison. The optimization results for the five-axis machining center showed that the proposed approach is effective and able to perform the optimization of geometric accuracy and improve the machining accuracy and the reliability of the machine tool.

Smooth Geodesic Interpolation for Five-Axis Machine Tools

The smoothness of tool trajectories would be as important as that of axis trajectories for five-axis machining. The present study proposes a geodesic interpolation for five-axis machine tools. The tool positions and orientations are fitted to geodesics, respectively, to achieve smooth accurate motions of the tool center point (TCP), smooth axis motions, and the constant velocities of the TCP position and orientation. Not only the linear and circular geodesic interpolation are proposed, but also the acceleration/deceleration and block connection schemes are considered. The method is illustrated by simulations and experiments on a five-axis machining center, and compared to the interpolator with the TCP control. The real-time implementation of geodesic interpolation is well within the scope of modern CNC systems.

Laser assisted milling device: A review

Laser-Assisted milling is a type of thermally-assisted machining process in which a workpiece is locally softened by a laser heat source before machining. This method is effective solution for machining materials such as Inconel series alloys, titanium alloy, and ceramics, which are more difficult to machine compared with conventional materials. It is also a green machining process, because it saves energy by reducing the cutting force. Laser-Assisted milling has only been used in limited fields including single-direction machining of flat surfaces. When the laser-assisted milling has a complex tool-path, it is difficult to control the heat source and cutting tool simultaneously. To apply the process in industrial field studies of workpieces having various shapes are needed. This paper provides a review of current laser-assisted milling devices, and then develops a three-dimensional laser-assisted milling device for complex tool-path machining. A high-power diode laser with additional axes was retrofit to the spindle of a five-axis machining center. The device could be used in industrial fields whose machined products have complicated shapes.

Study on a high thrust force bi-double-sided permanent magnet linear synchronous motor

A high thrust force bi-double-sided permanent magnet linear synchronous motor used in gantry-type five-axis machining center is designed and its performance was tested in this article. This motor is the subproject of Chinese National Science and Technology Major Project named as “development of domestic large thrust linear motor used in high-speed gantry-type five-axis machining center project” jointly participated by enterprises and universities. According to the requirement of the application environment and motor performance parameters, the linear motor’s basic dimensions, form of windings, and magnet arrangement are preliminarily specified through theoretical analysis and calculation. To verify the correctness of the result of the calculation, the finite element model of the motor is established. The static and dynamic characteristics of the motor are studied and analyzed through the finite element method, and the initial scheme is revised. The prototype of the motor is manufactured based on the final revised structure parameters, and the performance of the motor is fully tested using the evaluation platform for direct-drive motor component. Experimental test results meet the design requirements and show the effectiveness of design method and process.

Evaluation of dynamic behavior of rotary axis in five-axis machining center (Behavior around motion direction changes)

Evaluation of dynamic behavior of rotary axis in five-axis machining center (Behavior around motion direction changes)

5 軸マシニングセンタの円錐台加工における旋回運動に関する幾何学的解析

NAS 979 is a standard for evaluate motion accuracy of five-axis machining centers. Five-axis machining centers evaluate motion accuracy by to cut cone frustum in NAS 979. The geometric movement of five-axis machining centers is determined by using form-shaping function. This paper mathematically verified that effect of the inclination angle α and the half apex angle ϕ of cone frustum on command value of rotational axes. The movement of rotary axes can be divided into three types by relationship between magnitudes of half apex angle and tilt angle. The amplitude and turning range of the rotary axes are divided into three types by relationship between magnitudes of half apex angle and tilt angle.

Online Parameter Estimation for Model-Based Force Control in Milling Processes

In order to adapt to a milling machine’s changing behavior while operating due to changing tool-workpiece contact conditions adaptive controllers are an advantageous approach to control the machine’s feed rate. Otherwise inaccuracies can occur leading to poor quality. In this paper Model Predictive Control (MPC) is used to control the feed rate of the machine. By applying a parameterized model to the current state of the milling process its future trend can be predicted. An n-th order system with delay time is used to model the behavior of the milling machine’s velocity control loop. By continually re-estimating the model parameters at run time, the controller adapts to the current machine behavior. We developed solutions of the maximum likelihood estimators of the model’s independent parameters for a state-space representation that can be numerically efficiently calculated. These estimators can be used to estimate the parameters of the n-th order system and the delay time iteratively based on the collected process data. Two validation studies, one emulating an online estimation based on real data from the five-axis machining center Mazak Variaxis 630II-T, and one simulation study with a simulated machine, were carried out to validate the iterative parameter estimation algorithm. In these studies, the time variant model, whose parameters are re-estimated on the basis of the developed estimators, is compared to a time invariant model, for which the controller does not adapt to changing machine behavior.

Pitch deviation measurement and analysis of curve-face gear pair

The curve-face gear pair is proposed as a new type of gear pair which was put forward in recent years. It can achieve variable ratio transmission of motion and power between the intersection axes. In the matter of the inspection of the pitch deviations of this gear pair, there is no practical possibility for a reliable metrological traceability to national or international measurement standards can be provided. Hence, a new method was proposed in this article, which was based on the CNC gear measuring center, aimed at the gear artefacts processed by the five-axis machining center. With the coordinate data which were obtained by the gear measuring center, the pitch deviations of the gear pair were obtained. In addition, based on the values of the pitch deviations, the angular deviations of curve-face gear pair can be worked out, and the results turn out that the measurement method is feasible.

Three degrees of freedom stability analysis in the milling with bull-nosed end mills

When bull-nosed end mills are applied to machine the eccentric brackets of special-shaped parts, the chip regeneration mechanism is influenced by relative vibrations between the cutter and the workpiece in three directions. Three degrees of freedom stability is modeled analytically in this paper, in which dynamics of the cutter and the workpiece are both taken into consideration. In order to obtain the dynamic equation, a universal vector method is proposed to calculate dynamic chip thickness of the infinitesimal cutting flute in three directions based on the characteristics of relative vibrations between the cutter and the workpiece. A checking value point-by-point method is introduced to determine whether the infinitesimal cutting flute is in cutting or not. The full-discretization method is adopted to solve the dynamic equation. Since the dynamics of the thin walls is varied with the position, there exists a significant difference in the stability lobes at different positions. The stability lobes show that the stability limit at the end of the thin walls is lower than that in the middle of the thin walls. The verification experiments of three degrees of freedom stability were conducted in milling thin walls with the bull-nosed end mills on the five-axis machining center GMC 1600 H/2. The predicted three degrees of freedom stability lobes agree well with the experimental results. For comparison, the viability of the two degrees of freedom stability is proven in milling the plate with the same cutters.

Building 3D Geometric and Kinematic Models of Five-Axis Machine-Tools for Manufacturing Prosthetic Devices

The paper presents the process of building and testing a geometric and kinematic model of a five-axis machining center, which can be used for accurate cutting processes simulation of complex parts. After building the model, it was be tested by simulating the machining process of a hip joint prosthetic device. The prosthetic device was chosen because of its complex shape and because it was obtained by a 3D scanning process, which means that the part was reconstructed using meshes, instead of surfaces, which makes the toolpath control more difficult.

A uniform expression model for volumetric errors of machine tools

This paper proposes a new method, a Self-adaptive Mathematical Expression Model (SMEM), to describe volumetric errors of machine tools based on Non-Uniform Rational B-Spline (NURBS). The NURBS parameters of expression model were optimized by an improved Genetic Algorithm (GA). Simulation method was adopted to verify the effectiveness of the parameter optimization method of the SMEM, and the measurement experiment and machining experiment with error compensation based on the SMEM were conducted on a five-axis machining center with a titling rotary table. It was found that the SMEM can be used to uniformly express the position-dependant error parameters. Compared with the traditional methods, which adopt largely discrete database tables or polynomial method, the presented method is more concise, accurate and robust. In addition, volumetric errors of any position among the workspace of the machine tools can be quickly obtained by searching the SMEM of error parameters. And volumetric error of tool paths and the actual surface of the machining parts also can be expressed by the SMEM. The accuracy of the linear measured paths can have a great improvement of 70.63% with error compensation based on the SMEM. The accuracy of the part's predicted machining precision using the SMEM was 76.34%, and the surface profile error of the part can be improved significantly, up to 61.29%, when the SMEM was used for error compensation. Therefore, the expression model established in this study is feasible and robust, and could be used to express error parameters and volumetric errors. Moreover, it could be used to predict machining precision of part before machining and provide the basis for error compensation.

A new test part to identify performance of five-axis machine tool-Part II validation of S part

A new test part, the S part, has been further discussed on its validation. Researches have shown dynamic errors, especially for follow-up errors that mainly contribute to dimensional errors of the S part. The models of mechanical system, servo system, are set up to find out the key dynamic machine factors. Coupled with the motions of five axes, the follow-up errors, the actual positions of the tool center, and the contouring error are calculated, which are used to analyze the effect of factors. Results show that different machine factors have different error curves on the S part surface. So the algorithm to track the cause of the processed error has been developed. Then, the S part cutting experiments are carried out to testify the analysis results. Both analysis and experiment results have a good agreement. Finally, the characteristics of the S part are compared with the commonly used test part NAS979. Obviously, the S part presents more machine abilities than NAS979, which test well the performance of a five-axis machine center.

0218 Kinematics analysis on machining model of posihtion-attitude for machining test of cone frustum by five-axis machining centers

0218 Kinematics analysis on machining model of posihtion-attitude for machining test of cone frustum by five-axis machining centers

Five axis machine tool volumetric error prediction through an indirect estimation of intra- and inter-axis error parameters by probing facets on a scale enriched uncalibrated indigenous artefact

The volumetric accuracy of five-axis machine tools is affected by intra-axis geometric errors (error motions) and inter-axis geometric errors (axes relative position and orientation errors). Self-probing of uncalibrated facets on the existing machine tool table is proposed to provide the necessary data for the self-calibration of the machine error parameters and of the artefact geometry using an indirect approach. A set of 86 non-confounded coefficients are selected from the ordinary cubic polynomials used to model both the intra- and inter-axis errors. A scale bar is added to provide the isotropic scale factor. The estimated model is then used to predict the actual tool to workpiece position. Experimental trials are conducted on a five-axis horizontal machining centre using its original unmodified machine table as an artefact. For validation purposes only, the estimated artefact geometry is compared to accurate coordinate measuring machine (CMM) measurements. A study of the volumetric error predictive capability of the model for selected subsets of estimated error coefficients is also conducted.