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Table of Content

    10 July 2020, Volume 31 Issue 13
    Gain Design Method of Machine Tool Contour Controllers Based on Stability Margin
    ZHAN Chengpeng, YANG Lei, YANG Xiao, ZHAO Wanhua
    2020, 31(13):  1513-1517. 
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    The contour controllers were usually designed as a P controller, and the gain value was the key parameter to determine the control effectiveness. In order to design the gain values of contour controllers quantitatively, a gain design method of contour controllers was proposed based on stability margin herein. Firstly, the discrete transfer functions of each axis of the machine tool were identified by applying the least squares method. Secondly, the stability domain of the gain was calculated based on the Jury criterion and transfer functions. Finally, the objective function was constructed, and the gain values satisfying the stability margin requirements were obtained in the stability domain. A contour controller was designed on a three-axis machine tool, and a trajectory tracking experiment was carried out to verify the effectiveness of the proposed method. The experimental results show that the contour controller designed based on stability margin may reduce the maximum trajectory error of the spiral trajectory by 39.06% and the maximum trajectory error of the fan-shaped trajectory by 34.33%, under the conditions of ensuring the motion stability of the machine tools.
    Error Sensitivity Analysis of Motion Axis for Five-axis CNC Machine Tools with Geometric Error Contribution
    FU Guoqiang, RAO Yongjian, XIE Yunpeng, GAO Hongli, DENG Xiaolei
    2020, 31(13):  1518-1528. 
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    Aiming at the problems that the sensitivity analysis of the existing error elements was not closely related to the subsequent error compensation,the geometric error contribution modeling of the motion axes and error sensitivity analysis of the motion axes were built to obtain the key motion axes whose geometric errors had great influence on the accuracy of the five-axis CNC machine tool. The geometric error contributions of all motion axes were established based on the error sensitivity matrix by using POE(product of exponential) theory and the transforming differential changes among coordinate frames. The integrated geometric error model of the CNC machine tool was the summation of the error contribution of all motion axes. The weight components and the comprehensive weight of error contribution for all motion axes were calculated  to realize the error sensitivity analysis of the axes. The motion axis with the maximum mean comprehensive weight was selected as the key motion axis of the CNC machine tool, and the error compensation method of the key motion axis was analyzed and discussed. Finally, simulations and real cutting experiments were carried out on five-axis CNC machine tool of JINGDIAO. The results show that geometric error contribution modeling and error sensitivity evaluations of the motion axis are effective, and only compensating the geometric error contribution of the key motion axis may effectively improve the accuracy of the five-axis CNC machine tool.
    A Method for Thermal Error Modeling of FAMT
    HUANG Zhi, LIU Yongchao, DENG Tao, ZHOU Tao, ZHU Yun
    2020, 31(13):  1529-1538. 
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    Aiming at the complicated thermal error measurement and control problems caused by the superposition of multiple heat sources in FAMTs, a FAMT thermal error modeling method was proposed, the important parameters of the thermal error models were evaluated by using LSO optimized least squares support vector machine (LSO-LSSVM), and then the efficiency and accuracy of the thermal error prediction models were effectively improved. The partial correlation analysis was used to screen a large number of temperature sensor positions, and the temperature variables with large correlation were selected. According to the selected measured temperature data, the multiple linear regression, particle optimization optimized LSSVM, and LSO-LSSVM modeling methods were respectively used to conduct thermal error modeling, and the prediction capabilities of the respective thermal error models were compared and analyzed. The results show that the accuracy and robustness of the thermal error prediction models established by LSO-LSSVM were greatly improved. The thermal error compensation tests were also carried out on the main parts of the FAMTs. The test results show that the LSO-LSSVM modeling method reduces the errors of the specimens in the three directions of X, Y and Z by 35.3%, 32.2% and 43.9% respectively.
    A Methodology for Identifying Crucial Geometric Error Elements of Horizontal Machining Centers
    HU Teng, GUO Xipeng, MI Liang, YIN Guofu
    2020, 31(13):  1539-1547. 
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    Taking a horizontal machining center as the research object, a complete volumetric error model was established by defining the initial position and position error characteristic matrix for local coordinates of each components of machine tools, so as to solve the  problems that some geometric error elements(GEE) were lost when concerning about the conventional volumetric error modeling methodology. The proposed complete volumetric model was experimentally validated by measuring the volumetric diagonal positioning errors. The concept, as well as the calculation approaches, of GEEs actual contribution factors(ACF) were presented on the basis of the complete volumetric error model. A novel methodology for identifying the crucial GEEs was furtherly formulated with the help of complete volumetric error model and ACF. Based on the calculated ACFs or sensitivities, the crucial GEEs of the given machining center were respectively identified. The comparative analyses show that the proposed identification methodology, fundamental of which is the concept of ACF, is more accurate and effective than that based on traditional sensitivities analyses. The identified results indicate that there are 7 crucial GEEs for the studied machining center, which are all position-dependent. In particular, 4 elements of the crucial GEEs are related with feeding motions in X-axis, which suggests that the machine tool movable components in X-axis may have massive flaws affected the manufacturing accuracy.
    A Method for Error Identification of Rotation Axes of Dual-five-axis CNC Milling Machines
    RUAN Dawen, MAO Jian, LIU Gang, MA Li
    2020, 31(13):  1548-1554. 
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    In order to quickly and systematically identify the geometric errors of rotation axes of dual-five-axis CNC milling machines, an error measurement and identification method was proposed based on R-test. According to the error model of R-test, the relationship among the error measurement values and various error parameters was studied, and the installation errors and motion errors of rotation axes were obtained by identifying the geometric error items of rotation axes. Two displacement errors and two perpendicularity errors were identified respectively by the method of plane circle fitting and line fitting based on the principle of least square method. Based on the theory of multi-body system and the method of homogeneous coordinate transformation, the homogeneous coordinate transformation model of tool coordinate system and workpiece coordinate system was established to identify three movement errors and three rotation errors. Finally, the X-direction and Y-direction displacement errors were identified according to the identified values compensation. The experimental results show that the X-direction and Y-direction displacement errors are significantly reduced after compensation, and the error compensation results verify the accuracy and effectiveness of measurement and identification.
    Design of an Automatic Drilling-riveting End-effector
    JIN Jie, TIAN Wei, LI Bo
    2020, 31(13):  1555-1561. 
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    According to the development requirements of automatic assembly equipment for aerospace products, a product design and evaluation method was proposed by combining functional requirements and optimization objectives, and the method was applied to the structural design of multi-functional automatic drilling-riveting end-effectors with weight reduction requirements. Various schemes were proposed and evaluated for each functional module at the end, and the optimal scheme combination was selected to improve the overall structure. Finally, a kind of automatic drilling-riveting end-effector considering the weight reduction requirements was proposed, which might realize the detection, make holes, rivet and other functions. The effectiveness of this method was verified by the comparative analysis with other design methods and the motion simulation of the end structures.
    Key Assembly Quality Control Point Judgment Method in Motion Mechanisms Based on PLSR
    JU Pinghua, CHEN Zi, RAN Yan, XIAO Bing
    2020, 31(13):  1562-1569. 
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    To enhance the assembly quality of complex motion mechanisms, a key assembly quality control point judgment method in motion mechanisms was proposed based on PLSR. Firstly, the definitions of meta-action and meta-action chain were introduced based on “function—motion— action” functional structural decomposition. Secondly, the kinematics parameters of the terminal meta-action in meta-action chain were taken as the assembly quality analysis object,the power-transmitting model for meta-action chain was developed based on the state space model and the hierarchical iteration method, which reflected the relationship between the assembly quality analysis object and the influential factors in meta-action. Thirdly, the PLSR was used to solve the multicollinearity problems and few sample data problems of the influential factors, and the variable importance projection index was utilized to measure the importance degree of each influencial factor. Finally, the meta-action chain under the X-axis feed motion of grinder carriage was taken as an example, the results prove that the method may effectively figure out the most influential factors related to assembly quality.
    Evaluation and Analysis on Bolt Pre-tightening Forces of Nine-stage Disc Assembly
    ZHAO Bing, ZHANG Shouyang, WANG Hui, WU Dongbo, QIAO Tingqiang, HUANG Xin
    2020, 31(13):  1570-1576. 
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    In order to find out the ranges of effective pre-tightening forces for bolts reused in the assembly of nine-stage discs in aviation, the assembly environment was simulated and the experiments were carried on the thread friction testing machine to research the repeated tightening forces of aerial fasteners and the variation law. The experiment of tightening and pulling relationship was repeated 15 times. The experiment set up a comparison group of sufficient lubrication, local lubrication and without lubrication, and analyzed the effect of repeated tightening on the changes of pre-tightening forces under different lubrication conditions. The results show that lubrication has positive significance for reducing the friction coefficient values of the contact surfaces and improving the stability of the friction coefficient, which may further improve the efficiency and stability of torque conversion into pre-tightening forces. Repeated tightening will change the conversion efficiency and stability of the pre-tightening forces, and the laws shown under different lubrication conditions are different, which provides a support for the evaluation of the pre-tightening forces of the nine-stage disc bolts reused.
    Process Optimization Decision Evaluation Method Based on Meta-activity
    CHU Wangwei, ZHAO Meijia
    2020, 31(13):  1576-1584. 
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    Aiming at the problems that the process optimization objectives of aircraft complex structural parts were changeable and the optimization elements were difficult to be determined, the value engineering theory was introduced and the process optimization value evaluation method was proposed based on process meta-activity. Orienting to the optimization requirements for dynamic changes in the enterprises, the value expected weight allocation algorithm was put forward, and the process value evaluation model of aircraft structural parts were established from meta-activity evaluation, cost evaluation and value evaluation. Finally, the optimization values of different process links were analyzed and the effectiveness of the method was verified through an optimization example.
    Scale Thorn Generation Regularity and Its Inhibitory Measures in Turning Processes of Typical Titanium Alloy TC17
    HE Genghuang, YAN Guohong, LI Lingxiang, CHENG Cheng
    2020, 31(13):  1585-1592. 
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    The scale thorn generation regularity and its inhibitory measures were researched during the turning processes of titanium alloy. The main factors of scale thorn formation were analyzed. The break law of scale thorns was analyzed by moment method. Then, a mathematical model was established among cutting parameters, tool geometric parameters with moments of tool nose. By solving through MATLAB, it is found that the cutting speed has the least influences on the moments of tool nose, and the sizes of the moments of tool nose are determined by cutting depth , feed rate, corner radius and tool cutting edge angle. In order to verify the trend of the model, the intermittent experiments of turning typical titanium alloy TC17 were carried out. The sample data of scale thorns were collected with constant cutting speed, different cutting depths, different feed rates, different corner radius and different cutting edge angles. Furthermore, the curve of scale thorn break regularity was gained. The experimental results show that, with the cutting parameters less than the critical cutting depth and larger than the critical feed rate, the experimental results are consistent with the overall trend of mathematical model, which proves the rationality of the model. This study may provide data support for high quality machining of titanium alloys in terms of technology and tool selection.
    An Adaptive Machining Surface Reconstruction Method for Leading and Trailing Edges of Rolling Blades Based on Geometric Adjustment
    CUI Kang, WANG Wenhu, JIANG Ruisong
    2020, 31(13):  1593-1600,1605. 
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    To solve the CNC milling problems of the material forced outwards at leading and trailing edges of rolling compressor blades of aeroengine, an adaptive machining surface reconstruction method was proposed based on geometric adjustment of the design surfaces. Firstly, according to the free-form deformation theory and the surface energy control principle, a geometric control model of free-form surfaces with the use of deformation constraints was established. Then, based on this model, an adaptive surface reconstruction strategy was established based on genetic algorithm for leading and trailing edges of the rolling blades. By slightly adjusting the geometric shapes of the blade design surfaces, the deformed surfaces might move close to the measurement points of the blade bodies without losing their initial geometry seriously. Thus, the machining surfaces for the leading and trailing edges of the rolling blades might be reconstructed. The results show that the leading and trailing surfaces reconstructed with the above method may meet the shape accuracy and realize smooth transition with the actual blade bodies at the same time.
    Influences of TaC(NbC) on Cemented Carbide Tool Wear Resistance in High Speed Milling
    LI Yousheng
    2020, 31(13):  1601-1605. 
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    Aiming at the problems of fast wear of cemented carbide tools in the machining processes of aerospace titanium alloys, two WC-Co cemented carbide materials with the same main elements and different contents of TaC(NbC) were prepared herein. High temperature hardness and high temperature toughness of two carbide materials were measured by high temperature Vickers hardness tester. Then, the milling experiments of titanium alloy TC4 were performed with end-mills which were made of these two materials. The experimental results show that the high temperature hardness and high temperature toughness may be improved by adding TaC(NbC) to the cemented carbide materials. Under the same cutting conditions, the wear resistance of cemented carbide tools with TaC(NbC) is better than that without TaC(NbC). The tools with TaC(NbC) have less fracture cracks, longer tool life, and are more suitable for high speed milling of aerospace titanium alloy TC4.
    Multi-axis Loading Device for Reliability Tests of CNC Machine Tools
    LIU Hongyi, FAN Rui, GUO Jiangzhen, ZHANG Wei, ZHAO Qinzhi, CHEN Wuyi
    2020, 31(13):  1606-1612. 
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    A multi-axis loading device with load-while-track capacity was proposed to generate multi-axis feeding resistance for the reliability test of CNC machine tools. This provided the research foundation to simulate complex cutting forces in real machining processes. The loading device was developed by a 6-PUS parallel mechanism. Based on a fuzzy PID controller, an explicit force control system was established, which could adaptively adjust the proportional and integral gains according to loading errors. Multi-axis loading experiments were conducted on a real five-axis CNC machine tool. Results show that the loading device may track the uniaxial, three-axis and five-axis feeding motion of the spindle and simultaneously apply three-axis forces to the spindle; the maximum loading error is less than 3.2%. This illustrates that the device may effectively generate multi-axis feeding resistance to the spindle that is performing various feeding trajectories. Research fruits may supply the theoretical support and equipment to the future work on simulating cutting forces by dynamic loading. The loading device provides a new loading method for the reliability test of machine tools, and assists large-scale development and standardization of the test. Besides the reliability test, the loading device may also be utilized in other performance evaluations of machine tools, such as accuracy retainability, overload, and run-in test.
    Reliability Modeling for Hydraulic Components of Heavy Duty Machine Tools in Distribution of Degradation Amount for Oil Contamination Profile
    JIN Tongtong, YANG Zhaojun, WANG Dachuan, ZHAO Xinyue, TIAN Hailong, CHEN Chuanhai,
    2020, 31(13):  1613-1620,1628. 
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    The failures of heavy-duty machine tool was mainly on the hydraulic systems and caused by oil contamination. Aiming at the problems, the trend change tests of oil contamination were carried out. After the time domain analysis on test data of oil samples, the dimensional and non-dimensional parameters of particle numbers was obtained. Through the analysis by Q-Q plots and K-S tests on dimensional parameters, it might be seen that the degradation data of contaminated particles obeyed the normal distribution. The correlation test of oil contamination and the environment have been carried out. Analyzed by using correlation coefficient method, the change values of oil contamination were hardly dependent of temperature, flow rate and pressure. Hydraulic components were summarized into three categories, pipelines, valves and filters. The blockages of pipelines and valves were studied by the particle number with diameter about 5 μm. The wears of pipelines and valves were studied by the particle number with diameter larger than 15 μm. The blockages of filters were studied by particles with similar diameter with filtering accuracy. The threshold of the particle number was set according to the 20/17 level of ISO4406 standard. Reliability models of various components for a single failure mode were established based on the distribution of degradation amount. The reliability model under multiple failure modes was fused by the competitive failure model.
    Accuracy Analysis of Multi-axis CNC Machine Tools Based on Meta-action Module
    YANG Bin, RAN Yan, ZHANG Genbao
    2020, 31(13):  1621-1628. 
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    In order to give full play to the value of big data of multi-axis CNC machine tools and reduce the difficulty of the motion accuracy prediction, an accurace research method was proposed based on meta-action module. A multi-body model was used to describe the structures and motion relationships of machine tools' motion systems. The coordinate error model for motion accuracy evaluation was deduced by screw theory and differential method. The structure framework of accuracy analysis was planned for multi-axis CNC machine tools driven by big data, and the method of constructing distributed meta-action database was emphatically discussed based on meta-action module. The value of historical big data and real-time dynamic data were brought into full play to ensure the stability and accuracy of simulation and accurace prediction of motion systems. The experiments were performed on the tool motion systems of CNC 5-axis machining center which verified the simplicity and applicability of the accuracy analysis method.
    Research on Status and Prospects of Optimization Design Method of High-speed CNC Turntables
    LIU Shihao, LIN Mao
    2020, 31(13):  1629-1637. 
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    During the high-speed precision machining of the machine tools, the CNC turntable was subjected to variable loads and a variety of heat sources to produce a thermal-mechanical coupling effect. The current CNC turntable optimization design method often ignored the influences of the thermal-mechanical coupling effect on the dynamic characteristics of the turntables. The paper proposed to explore the thermal-mechanical coupling effect and the mechanism based on an in-depth analysis of the research results of CNC turntables, and then the thermal-mechanical coupling model of the CNC turntables was established to obtain the thermal-mechanical coupling dynamic characteristics of high-speed CNC turntables. In view of the good heat dissipation and mechanics properties of the honeycomb structure, the thermal-mechanical coupling bionic design criteria of high-speed CNC turntables was proposed based on the bionic principle. In order to improve the thermal-mechanical coupling dynamic characteristics of high-speed CNC turntables, the structural multi-objective optimization was carried out through bionic design criteria, and then a novel thermal-mechanical coupling bionic optimization design method of high-speed CNC turntables was established.