STIFFNESS ANALYSIS OF THE MAIN MODULE FOR PARALLEL MACHINE TOOLS BY FINITE ELEMENT ANALYSIS

With the aid of commercial finite element analysis software package ANSYS,investigations are made on the contributions of main components to stiffness of the main module for parallel machine tools,and it is found that the frame is the main contributor.Then,influences of constraints,strut length and working ways of the main module have also been investigated.It can be concluded that when one of the main planes of the frame without linear drive unit is constrained,the largest whole stiffness can be acquired.And,the stiffness is much better when the main module is used in a vertical machine tool instead of a horizontal one.Finally,the principle of stiffness variation is summarized when the mobile platform reaches various positions within its working space and when various loads are applied.These achievements have provided critical instructions for the design of the main module for parallel machine tools.

Dynamic behavior of bridge-erecting machine subjected to moving mass suspended by wire ropes

The dynamic behavior of a bridge-erecting machine, carrying a moving mass suspended by a wire rope, is investigated. The bridge-erecting machine is modelled by a simply supported uniform beam, and a massless equivalent ＂spring-damper＂ system with an effective spring constant and an effective damping coefficient is used to model the moving mass suspended by the wire rope. The suddenly applied load is represented by a unitary Dirac Delta function. With the expansion method, a simple closed-form solution for the equation of motion with the replaced spring-damper-mass system is formulated. The characters of the rope are included in the derivation of the differential equation of motion for the system. The numerical examples show that the effects of the damping coefficient and the spring constant of the rope on the deflection have significant variations with the loading frequency. The effects of the damping coefficient and the spring constant under different beam lengths are also examined. The obtained results validate the presented approach, and provide significant references in the design process of bridgeerecting machines.

ERROR COMPENSATION OF COORDINATE MEASURING MACHINES WITH LOW STIFFNESS

A technique for compensating the errors of coordinate measuring machines (CMMs) with low stiffness is proposed. Some additional items related with the force deformation are introduced to the error compensation aquations. The research was carried on a moving colunm horizontal arm CMM. Experimental results show that both the effects of systematic components of error motions and force deformations are greatly reduced, which shows the effectiveness of proposed technique.

Machine learning-based stiffness optimization of digital composite metamaterials with desired positive or negative Poisson's ratio

Mechanical metamaterials such as auxetic materials have attracted great interest due to their unusual properties that are dictated by their architectures.However,these architected materials usually have low stiffness because of the bending or rotation deformation mechanisms in the microstructures.In this work,a convolutional neural network(CNN)based self-learning multi-objective optimization is performed to design digital composite materials.The CNN models have undergone rigorous training using randomly generated two-phase digital composite materials,along with their corresponding Poisson's ratios and stiffness values.Then the CNN models are used for designing composite material structures with the minimum Poisson's ratio at a given volume fraction constraint.Furthermore,we have designed composite materials with optimized stiffness while exhibiting a desired Poisson's ratio(negative,zero,or positive).The optimized designs have been successfully and efficiently obtained,and their validity has been confirmed through finite element analysis results.This self-learning multi-objective optimization model offers a promising approach for achieving comprehensive multi-objective optimization.

A non-uniform allowance allocation method based on interim state stiffness of machining features for NC programming of structural parts

For thin-walled parts,uniform allowance to each machining surface is allocated by the traditional machining method.Considering the sequence of the adjacent machining features,it may cause poor stiffness for some side walls due to a minor wall thickness,which may cause the deformation of the final formed parts to be large,or deduce machining efficiency for some machining features due to too thick remains.In order to address this issue,a non-uniform allowance allocation method based on interim state stiffness of machining features for the finishing of thin-walled structural parts is proposed in this paper.In this method,the interim state model of machining features is constructed according to the machining sequence of the parts,and the stiffness of the side wall is taken as the evaluation index to allocate reasonable allowance value to the corresponding machining surface to ensure the stiffness requirement of the parts in the machining process.According to the finite element simulation results,the non-uniform allowance allocation method proposed in this paper can effectively improve the stiffness of the parts and reduce the deformation of the parts,when compared with the traditional uniform allowance machining method.

Error Prediction in Industrial Robot Machining: Optimization Based on Stiffness and Accuracy Limit

Among the advantages of using industrial robots for machining applications instead of machine tools are flexibility, cost effectiveness, and versatility. Due to the kinematics of the articulated robot, the system behaviour is quite different compared with machine tools. Two major questions arise in implementing robots in machining tasks: one is the robot’s stiffness, and the second is the achievable machined part accuracy, which varies mainly due to the huge variety of robot models. This paper proposes error prediction model in the application of industrial robot for machining tasks, based on stiffness and accuracy limits. The research work includes experimental and theoretical parts. Advanced machining and inspection tools were applied, as well as a theoretical model of the robot structure and stiffness based on the form-shaping function approach. The robot machining performances, from the workpiece accuracy point of view were predicted.

Kinetostatic Modeling and Analysis of an Exechon Parallel Kinematic Machine(PKM) Module

As a newly invented parallel kinematic machine（PKM）, Exechon has found its potential application in machining and assembling industries due to high rigidity and high dynamics. To guarantee the overall performance, the loading conditions and deflections of the key components must be revealed to provide basic mechanic data for component design. For this purpose, a kinetostatic model is proposed with substructure synthesis technique. The Exechon is divided into a platform subsystem, a fixed base subsystem and three limb subsystems according to its structure. By modeling the limb assemblage as a spatial beam constrained by two sets of lumped virtual springs representing the compliances of revolute joint, universal joint and spherical joint, the equilibrium equations of limb subsystems are derived with finite element method（FEM）. The equilibrium equations of the platform are derived with Newton＇s 2nd law. By introducing deformation compatibility conditions between the platform and limb, the governing equilibrium equations of the system are derived to formulate an analytical expression for system＇s deflections. The platform＇s elastic displacements and joint reactions caused by the gravity are investigated to show a strong position-dependency and axis-symmetry due to its kinematic and structure features. The proposed kinetostatic model is a trade-off between the accuracy of FEM and concision of analytical method, thus can predict the kinetostatics throughout the workspace in a quick and succinct manner. The proposed modeling methodology and kinetostatic analysis can be further expanded to other PKMs with necessary modifications, providing useful information for kinematic calibration as well as component strength calculations.

Nonlinear Static and Dynamic Stiffness Characteristics of Support Hydraulic System of TBM

Full-face hard rock tunnel boring machines(TBM)are essential equipment in highway and railway tunnel engineering construction.During the tunneling process,TBM have serious vibrations,which can damage some of its key components.The support system,an important part of TBM,is one path through which vibrational energy from the cutter head is transmitted.To reduce the vibration of support systems of TBM during the excavation process,based on the structural features of the support hydraulic system,a nonlinear dynamical model of support hydraulic systems of TBM is established.The influences of the component structure parameters and operating conditions parameters on the stiffness characteristics of the support hydraulic system are analyzed.The analysis results indicate that the static stiffness of the support hydraulic system consists of an increase stage,stable stage and decrease stage.The static stiffness value increases with an increase in the clearances.The pre-compression length of the spring in the relief valve a ects the range of the stable stage of the static stiffness,and it does not a ect the static stiffness value.The dynamic stiffness of the support hydraulic system consists of a U-shape and reverse U-shape.The bottom value of the U-shape increases with the amplitude and frequency of the external force acting on the cylinder body,however,the top value of the reverse U-shape remains constant.This study instructs how to design the support hydraulic system of TBM.

A Target Grabbing Strategy for Telerobot Based on Improved Stiffness Display Device

Most target grabbing problems have been dealt with by computer vision system, however, computer vision method is not always enough when it comes to the precision contact grabbing problems during the teleoperation process, and need to be combined with the stiffness display to provide more effective information to the operator on the remote side. Therefore, in this paper a more portable stiffness display device with a small volume and extended function is developed based on our previous work. A new static load calibration of the improved stiffness display device is performed to detect its accuracy, and the relationship between the stiffness and the position is given. An effective target grabbing strategy is presented to help operator on the remote side to judge and control and the target is classified by multi-class SVM(supporter vector machine). The teleoperation system is established to test and verify the feasibility. A special experiment is designed and the results demonstrate that the improved stiffness display device could greatly help operator on the remote side control the telerobot to grab target and the target grabbing strategy is effective.

VARIABLE INERTIAL DAMPER AND ITS APPLICATION IN ZERO-DRIVE GEAR HOBBING MACHINE

A variable inertial damper, whose viscous damping coefficient can be adjusted by changing the gap between inertia wheel and motor spindle, is designed in servomechanism of zero-drive hobbing machine and is directly attached on the motor spindle. The mathematical model of servo system with inertial damper is built. By using theoretical analysis and system simulation, it is demonstrated that the variable inertial damper with optimal damping coefficient and moment of inertia should lead to no resonance point in wider frequency range of exciting force. Therefore, its application in zero-drive hobbing machine makes this system not only achieve higher system stiffness to overcome load torque fluctuation, but also gain better stability.

Structural Bionic Design of Machine Tool Structures

A structural bionic design process is systematically presented for lightweight mechanical structures. By mimicking biological excellent structural principles, the stiffening ribs of a machining table and a moving column were redesigned for better load-bearing efficiency. Finite element method（FEM） simulation and model experiments were carried out for performance verification, which showed the increase of structural static and dynamic performance. Structural bionic offers a new solution to change conventional structures for high specific stiffness.

Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

In order to prevent unwanted excited vibrations and to secure better machining precision in large size heavy duty machine tools dynamic stiffness is one of the most desirable and critical properties. In the past decades, many researches on machine tool stiffness test and evaluation methodology have been made. However any methodology for a Pin Turning Device (PTD), which is a special kind of turning lathe for machining big size crankshaft pins, is rarely found among them. This study proposes a test and evaluation process of stiffness of a PTD by measuring frequency response function at the tool center point (TCP). For conformance proving for the proposed methodology, stiffness of a PTD obtained by the proposed method with impact hammer test (IHT) has been compared with that determined by FEM.

STUDY ON ELECTRORHEOLOGICAL FLUID DAMPER FOR APPLICATION IN MACHINING CHATTER CONTROL

The electrorheological fluid(ERF)is a kind of intelligent material withbright prospects for industry applications, which has viscoelastic characteristic under the appliedelectric field. The dynamic model of a milling system with an ERF damper is established, and thechaffer suppression mechanism of the ER effect is discussed theoretically. Both the theoreticalstudy and the experimental investigation show that the additional damping and additional stiffnessproduced by the ERF increase with the rise in the strength of electric field E, but their influenceon the cuffing stability is different. Only when both additional damping and additional stiffnesscooperate, the milling chatter can be suppressed quickly and effectively. In additional, an ERFdamper used on the arbor of horizontal spindle milling machine is developed, and a series of millingchatter control experiments are performed. The experimental results show that the milling chaffercan be suppressed effectively by using the ER damper.

Stiffness estimation of a parallel kinematic machine

This paper presents a simple yet comprehensive approach to quickly estimating the stiff-ness of a tripod-based parallel kinematic machine. This approach can be implemented in two steps. Inthe first step, the machine structure is decomposed into two substructures associated with the machineframe and parallel mechanism. The stiffness models of these two substructures are formulated bymeans of virtual work principle. This is followed by the second step that enables the stiffness model ofthe machine structure as a whole to be achieved by linear superposition. The 3D representations of themachine stiffness within the usable workspace are depicted and the contributions of different componentrigidities to the machine stiffness are discussed. The result is compared with that obtained through finiteelement analysis.

A stiffness-matching based evaluation approach for compliance of mechanical systems in shield tunneling machines

As the most important performance,compliance of shield tunneling machines(STM) is defined as the capability to accommodate the sudden change of the load induced by the variable geological conditions during excavation.Owing to the different requirements of the compliant tasks,the existing methods in the robotic field cannot be utilized in the analysis and design of the mechanical system of shield tunneling machines.In this paper,based on the stiffness of the mechanical system and the equivalent contact stiffness of the tunnel face,the tunneling interface-matching index(IMI) is proposed to evaluate the compliance of the machine.The IMI is defined as a metric to describe the coincidence of the stiffness curves of the mechanical system and the tunnel face.Moreover,a tunneling case is investigated in the paper as an example to expound the validation of IMI and the analytical process.In conclusion,the IMI presented here can be served as an appraisement of the capability in conforming to the load fluctuation,and give instructions for the design of the thrust system of shield tunneling machines.

Design and development of a five-axis machine tool with high accuracy,stiffness and efficiency for aero-engine casing manufacturing

In order to satisfy the machining requirements of aero-engine casing in modern aviation industry, this paper investigates three main issues during the design and development process of a five-axis machine tool with high accuracy, stiffness and efficiency, including whole structure design,key components design, and supporting stiffness design. First, an appropriate structure of five-axis machine tool is determined considering the processing characteristics of aero-engine casing. Then, a dual drive swing head and a compact motorized spindle are designed with enough drive capability and stiffness, and related structure, assembly method, cooling technology, and performance simulation are given in detail. Next, a design method of supporting stiffness of guide is proposed through the deformation prediction of the spindle end. Based on above work, a prototype of machine tool is developed, and some experiments are carried out, including performance tests of swing head and motorized spindle, and machining of a simulated workpiece of aero-engine casing. All experimental results show that the machine tool has satisfactory accuracy, stiffness and efficiency, which meets the machining requirements of aero-engine casing. The main work can be used as references for engineers and technicians, which are meaningful in practice.

A semi-analytical approach for stiffness modeling of PKM by considering compliance of machine frame with complex geometry

Stiffness modeling is one of the most significant issues in the design of parallel kinematic machine (PKM). This paper presents a semi-analytical approach that enables the stiffness of PKM with complex machine frame geometry to be estimated effectively. This approach can be implemented by three steps: (i) decomposition of the entire system into two sub-systems associated with the parallel mechanism and the machine frame respectively; (ii) stiffness modeling of each sub-system using the analytical approach and the finite element analysis; and (iii) generation of the stiffness model of the entire system by means of linear superposition. In the modeling process of each sub-system, the virtual work princi- ple and overall deflection Jacobian are employed with special attention to the bending rigidity of the constrained passive limb and the interface stiffness of the machine frame. The stiffness distribution of a 5-DOF hybrid robot named TriVariant-B is investigated as an example to illustrate the effectiveness of this approach. The contributions of component rigidities to that of the system are evaluated using global indices. It shows that the results achieved by this approach have a good match to those obtained through finite element analysis and experiments.

Structure Improvement and Optimization of Gantry Milling System for Complex Boring and Milling Machining Center

To enhance the efficiency and machining precision of the TX1600G complex boring and milling machining center,a study was conducted on the structure of its gantry milling system.This study aimed to mitigate the influence of factors such as structural quality,natural frequency,and stiffness.The approach employed for this investigation involved mechanism topology optimization.To initiate this process,a finite element model of the gantry milling system structure was established.Subsequently,an objective function,comprising strain energy and modal eigenvalues,was synthesized.This objective function was optimized through multi-objective topology optimization,taking into account certain mass fraction constraints and considering various factors,including processing technology.The ultimate goal of this optimization was to create a gantry milling structure that exhibited high levels of dynamic and static stiffness,a superior natural frequency,and reduced mass.To validate the effectiveness of these topology optimization results,a comparison was made between the new and previous structures.The findings of this study serve as a valuable reference for optimizing the structure of other components within the machining center.

基于双信号融合的主轴/刀柄结合面刚度退化程度预测

为了预测主轴/刀柄结合面刚度退化程度,提出了一种基于激励和响应信号融合的主轴/刀柄结合面刚度退化程度预测方法。首先进行钛合金矩形工件侧铣实验,采集瞬时铣削力信号和主轴/刀柄结合面附近的响应振动信号,构建反映主轴/刀柄结合面刚度退化的数据库。然后根据数据库中瞬时铣削力和振动信号各方向的时域、频域和时频域特征,基于相关性分析优选出瞬时铣削力信号和振动信号的时域均值、频域中心频率、时频域一阶小波包能量3个特征,分别使用低频滤波卷积核和高频滤波卷积核对优选后的特征矩阵进行双通道卷积池化处理,获取深度融合的主轴/刀柄结合面刚度退化程度特征向量。最后以支持向量机模型(SVM)的概率模式转化为朴素贝叶斯分类器(NBC)的条件概率,构建混合分类器模型(NBC-SVM),提高了分类器的分类性能。在主轴/刀柄结合面刚度退化数据库的基础上,基于双通道卷积池化的特征融合方法(CP-FF)和NBC-SVM模型实现了主轴/刀柄结合面刚度退化程度的预测,预测精度达96%。

小盒商标纸在线压痕装置的设计与应用

目的解决中支双铝包小盒商标纸因模切压痕不稳定出现的烟包小盒侧边弹开翻折、烟包变形等问题。方法设计小盒商标纸在线压痕装置,该装置包括机架、传动轴、辗送轮、压痕件、导纸轮、对压轮、压痕避让部和托纸轮,同时开展正交试验,获取压痕件半径、刃缘弧长和包装机作业速度最佳参数组合。结果应用在线压痕装置后,小盒商标纸压痕挺度的平均绝对误差由6.0减小至1.9,次品烟包频率由37.6包/班次降低至8.4包/班次,改进后比改进前降低了77.7%。结论设计的小盒商标纸在线压痕装置可使商标纸挺度处于适宜水平,降低了该类缺陷烟包频率,提高了烟包包装质量。该技术可为提高双铝包包装机的产品质量、作业效率及稳定性提供支持。