Design and optimization of machining fixture layout using ANN and DOE

In machining fixtures, minimizing workpiece deformation due to clamping and cutting forces is essential to maintain the machining accuracy. This can be achieved by selecting the optimal location of fixturing elements such as locators and clamps. Many researches in the past decades described more efficient algorithms for fixture layout optimization. In this paper, artificial neural networks (ANN)-based algorithm with design of experiments (DOE) is proposed to design an optimum fixture layout in order to reduce the maximum elastic deformation of the workpiece caused by the clamping and machining forces acting on the workpiece while machining. Finite element method (FEM) is used to find out the maximum deformation of the workpiece for various fixture layouts. ANN is used as an optimization tool to find the optimal location of the locators and clamps. To train the ANN, sufficient sets of input and output are fed to the ANN system. The input includes the position of the locators and clamps. The output includes the maximum deformation of the workpiece for the corresponding fixture layout under the machining condition. In the testing phase, the ANN results are compared with the FEM results. After the testing process, the trained ANN is used to predict the maximum deformation for the possible fixture layouts. DOE is introduced as another optimization tool to find the solution region for all design variables to minimum deformation of the work piece. The maximum deformations of all possible fixture layouts within the solution region are predicted by ANN. Finally, the layout which shows the minimum deformation is selected as optimal fixture layout.     

Optimal Fixture Design Accounting for the Effect of Workpiece Dynamics

This paper presents a fixture layout and clamping force optimal synthesis approach that accounts for workpiece dynamics during machining. The dynamic model is based on the Newton– Euler equations of motion, with each fixture–workpiece contact modelled as an elastic half-space subjected to distributed nor-mal and tangential loads. The fixture design objective in this paper is to minimise the maximum positional error at the machining point during machining. An iterative fixture layout and clamping force optimisation algorithm that yields the "best" improvement in the objective function value is presented. Simulation results show that the proposed optimis-ation approach produces significant improvement in the work-piece location accuracy. Additionally, the method is found to be insensitive to the initial fixture layout and clamping forces.     

薄壁弧形件装夹布局有限元优化

关于航空结构件加工变形控制的研究是高效数控加工研究的一部分。薄壁弧形零件加工中的弹性变形对加工质量影响很大,而装夹布局影响切削变形的大小和分布。以减少加工中工件最大弹性变形为目标,建立了弧形件铣削加工装夹布局的优化模型,采用商业有限元软件的设计优化模块进行计算。在对计算结果进一步分析的基础上,提出了最终的装夹布局方案,采用该方案可以得到整个加工过程中更低的变形量,变形分布更均匀,为采取相应数控补偿措施提供条件。优化方案和实际加工方案结果基本一致。所提出方法可推广至其他类型工件夹具布局优化设计。     

弱刚度工件夹紧顺序与夹具布局的同步优化

针对弱刚度工件在定位、夹紧过程中易变形的问题,建立了夹紧顺序与接触力及节点位移增量之间关系的数学模型,给出了各夹紧步骤中工件夹具系统的静力平衡方程;在此基础上,根据最小余能原理及库仑摩擦定律,构建了装夹方案优化模型,提出了基于遗传算法的夹具布局与夹紧顺序同步优化方法。算例结果表明,该方法有效降低了由于装夹所引起的工件变形。提高了加工精度。     

Fixture Clamping Force Optimisation and its Impact on Workpiece Location Accuracy

Workpiece motion arising from localised elastic deformation at fixture-workpiece contacts owing to clamping and machining forces is known to affect significantly the workpiece location accuracy and, hence, the final part quality. This effect can be minimised through fixture design optimisation. The clamping force is a critical design variable that can be optimised to reduce the workpiece motion. This paper presents a new method for determining the optimun clamping forces for a multiple clamp fixture subjected to quasu-static machining forces. The method uses elastic contact mechanics models to represent the fixture-workpiece contact and involves the formulation and solution of a multi-objective constrained oprimisation model. The impact of clamping force optimisation on workpiece location accuracy is analysed through examples involving a 3-2-1 type milling fixture.     

Recent developments on machining fixture layout design, analysis, and optimization using finite element method and evolutionary techniques

A review of the recent development of the machining fixture configuration/layout is presented in this paper. The literature review is mainly focused on the recently developed optimal fixture configuration under the dynamic conditions of the workpiece. In this review paper, the fixture design, fixture analysis, fixture synthesis, fixture layout design, optimization of fixture layout design, various optimization algorithms, and case studies of two- and three-dimensional workpiece geometries under dynamic conditions have been emphasized specially. The further scopes of the research are finally summarized.     

多目标的装夹方案优化及变夹紧力优化

夹具布局和夹紧力大小影响切削变形的大小和分布.基于遗传算法和有限元方法,提出一种夹具布局和夹紧力优化设计方法.该方法将同步优化夹具布局和夹紧力大小以及施加变夹紧力相结合,首先以加工变形最小化和变形分布最均匀为目标同步优化夹具布局和夹紧力大小,然后在优化后的夹具布局的基础上求解使得加工变形最小的变夹紧力大小.使用该方法进行底座薄壁零件的夹具优化设计,结果表明优化得到的设计优于经验设计和多目标优化方法,该方法有效地降低了加工过程中工件的变形,提高变形均匀度.     

薄壁件加工变形控制快速仿真平台开发

为控制薄壁件装夹变形和加工变形,建立了集装夹优化、加工变形预测、切削参数优化及误差补偿功能为一体的快速仿真平台.在平台实现中,装夹方案的优化采用基于形位公差控制的方法,通过多种装夹方案的比较,确定优化方案.加工变形预测时考虑了前-层变形对后-层切削深度的影响,并使切削力和加工变形达到动态平衡.为获得优化切削参数,建立了以变形控制为目标的优化模型.采用有限元法计算加工变形,采用遗传算法求解优化模型.为解得优化补偿量,仿真时考虑了变形与力的耦合效应.完成了基于ABAQUS的快速仿真平台开发.以镜座零件为例进行仿真,求得了优化的装夹方案和切削参数,验证了平台的可行性.     

Multi-objective optimization design of a fixture layout considering locator displacement and force–deformation

The fixture determines the workpiece position in a machining process; therefore, an increasing amount of attention has been given to fixture layout design. While machining, the workpiece position is affected by two major sources: (a) the locator displacement and (b) the force–deformation of the workpiece–fixture system. In the beginning of this paper, a geometric model considering the shape of a locator is developed to analyze the location performance, followed by the presentation of a simplified solving method and a location layout performance index. Second, to complete the force–deformation analysis, a finite element method-based force–deformation model is built and accelerated by a new method with a lower computer memory cost. Based on these two models, multiple objects of fixture layout optimization problems are proposed, and a multi-objective genetic algorithm-based optimization method is constructed. Finally, testing examples are approved to examine the validity of the method represented in this paper. These methods can provide a more accurate prediction of the locating performance in more widely used cases, and they have faster calculating speeds with lower computer memory costs.     

A feature-based approach for optimal workpiece localization and determination of feasible clamping regions

In this paper, we present an approach for optimally selecting the locating positions of workpieces and identifying feasible clamping regions that meet the requirements of the form-closure principle for fixture layout. This approach firstly finds an optimal configuration of locating points on a set of faces of the workpiece which minimizes the error transfer from locating points to machining features. In the formulation of the optimization, the error transfer is modeled using an error transfer matrix. The eigenvalues of the matrix are used as objective functions of the optimization. Secondly, this approach computes the clamping wrench cone and its member wrench can form a form-closure fixture layout, together with the formerly selected locating points. Finally, it generates the feasible clamping regions by projecting the clamping wrench cone onto faces of the clamping feature faces. An example is included to illustrate the effectiveness of the proposed approach. Compared with traditional methods in which only error transfer from the locating points to the workpiece’s mass-center is considered, the error transfer control in this approach is more effective and of greater efficiency. In addition, the clamping wrench cone projection method for the generation of feasible clamping regions is more suitable for handling cases with concave clamping faces than traditional convex-combination-based methods.     

数控车削轮毂的夹爪改进与工艺优化

阐述在轮毂的数控车削加工过程中,由于受到夹具的夹紧力和切削力的作用,致使工件在加工时产生变形,尺寸精度大大超差,严重时甚至工件会脱落于夹具,导致工件报废。经过本人认真研究,通过改进夹爪、调整工件夹紧位置,适当改变夹头夹紧力及优化加工工艺等,从而大大提高了加工精度和生产效率,降低了劳动强度,节约生产成本。希望以上的方法能对从事相关工作的人员有一定的借鉴作用。     

Design and optimization of machining fixture layout for end-milling operation

High product quality and productivity are the important objectives of manufacturing industries. They are greatly affected by the fixture layout design, and it requires modeling and analysis of fixture-workpiece interactions. In particular, the position of fixture elements has explicit influence on the deformation of the workpiece which needs to be minimized during machining. In order to ensure effective fixture layout design, the relationship between the position of fixture elements and workpiece deformation has to be modeled and optimized. In this research paper, workpiece deformation is modeled using response surface methodology. The developed model is tested for model adequacy, and the results obtained are matched with the simulated data. Then, it is used to minimize the workpiece deformation by determining the appropriate positions of locators and clamps using sequential approximation optimization and LINGO solver. It is found that integration of response surface methodology, with sequential approximation optimization, produces better results than LINGO solver.     

基于ABAQUS的弱刚度结构件装夹布局优化技术

装夹布局是弱刚度结构件加工变形的重要影响因素。以减小工件装夹的最大弹性变形量为目标,利用ABAQUS建立了弱刚度结构件的铣削加工装夹布局模型,并对各装夹方案进行了模拟分析,得出了最终的优化布局方案,采用该方案在加工过程中可以得到更低的变形量,变形分布更均匀。所提方法可应用于类似工件的夹具设计。     

Error Compensation of Thin Plate-shape Part with Prebending Method in Face Milling

Low weight and good toughness thin plate parts are widely used in modern industry, but its flexibility seriously impacts the machinability. Plenty of studies focus on the influence of machine tool and cutting tool on the machining errors. However, few researches focus on compensating machining errors through the fixture. In order to improve the machining accuracy of thin plate-shape part in face milling, this paper presents a novel method for compensating the surface errors by prebending the workpiece during the milling process. First, a machining error prediction model using finite element method is formulated, which simplifies the contacts between the workpiece and fixture with spring constraints. Milling forces calculated by the micro-unit cutting force model are loaded on the error prediction model to predict the machining error. The error prediction results are substituted into the given formulas to obtain the prebending clamping forces and clamping positions. Consequently, the workpiece is prebent in terms of the calculated clamping forces and positions during the face milling operation to reduce the machining error. Finally, simulation and experimental tests are carried out to validate the correctness and efficiency of the proposed error compensation method. The experimental measured flatness results show that the flatness improves by approximately 30 percent through this error compensation method. The proposed method not only predicts the machining errors in face milling thin plate-shape parts but also reduces the machining errors by taking full advantage of the workpiece prebending caused by fixture, meanwhile, it provides a novel idea and theoretical basis for reducing milling errors and improving the milling accuracy.     

接触变形对工件加工误差的影响

基于工件的准静态受力分析 ,运用经典Hertz接触理论 ,计算夹具 工件接触区的变形。根据多刚体运动学 ,建立表面加工误差和接触变形的关系 ,对加工误差进行预报。此方法也可用来计算定位基准误差对加工误差的影响。     

基于遗传算法的夹具布局和夹紧力同步优化

夹具设计是机械加工中一个重要步骤。夹具优化旨在得到最合理的夹具布局和夹紧力。为了弥补分步优化夹具布局和夹紧力以及应用传统优化算法而存在的不足,本文提出了应用遗传算法同步优化夹具布局和夹紧力的方法。使用该方法进行夹具优化的算例结果表明优化得到的设计优于经验设计,该方法是一种有效的夹具优化方法。     

Development of a finite element analysis tool for fixture design integrity verification and optimisation

Machining fixtures are used to locate and constrain a workpiece during a machining operation. To ensure that the workpiece is manufactured according to specified dimensions and tolerances, it must be appropriately located and clamped. Minimising workpiece and fixture tooling deflections due to clamping and cutting forces in machining is critical to machining accuracy. An ideal fixture design maximises locating accuracy and workpiece stability, while minimising displacements.The purpose of this research is to develop a method for modelling workpiece boundary conditions and applied loads during a machining process, analyse modular fixture tool contact area deformation and optimise support locations, using finite element analysis (FEA). The workpiece boundary conditions are defined by locators and clamps. The locators are placed in a 3-2-1 fixture configuration, constraining all degrees of freedom of the workpiece and are modelled using linear spring-gap elements. The clamps are modelled as point loads. The workpiece is loaded to model cutting forces during drilling and milling machining operations. Fixture design integrity is verified. ANSYS parametric design language code is used to develop an algorithm to automatically optimise fixture support and clamp locations, and clamping forces, to minimise workpiece deformation, subsequently increasing machining accuracy. By implementing FEA in a computer-aided-fixture-design environment, unnecessary and uneconomical “trial and error” experimentation on the shop floor is eliminated.     

Analysis of Reactions and Minimum Clamping Force for Machining Fixtures with Large Contact Areas

This paper presents a model for analysing the reaction forces and moments for machining fixtures with large contact areas, e.g. a mechanical vice. Such fixtures transmit torsional loads in addition to normal and tangential loads and thus differ from fixtures using point or line contacts. The model is developed using a contact mechanics approach where the workpiece is assumed to be elastic in the contact region and the fixture element is treated as rigid. Closed-form contact compliance solutions for normal, tangential, and torsional loads are used to derive the elastic deformation model for each contact. A minimum energy principle is used to solve the multiple contact problem yielding unique predictions of the fixture–workpiece contact forces and moments due to clamping and machining forces. This model is then used to determine the minimum clamping force necessary to keep the workpiece in static equilibrium during machining. An example is given to demonstrate its effectiveness in analysing the clamping performance of a mechanical vice during machining.     

套筒形工件钻通孔的定位误差分析及改进方案

机械加工中合理选择夹具定位方案对保证工件的加工精度非常重要。为保证工件的精度要求,往往对夹具中定位误差进行计算。通过对常用套筒形工件钻通孔定位方案分析与计算,提出新的工件定位方法,以消除误差,达到更高的精度要求。     

夹紧方案的数学建模及夹紧力的优化设计

夹紧变形有两大产生原因:由夹紧副变形导致的工件位置误差以及由夹紧力导致的工件变形。本文主要建立了夹紧副变形与工件位置误差的关系模型;并基于该模型,以最小工件位置误差为目标,实现了夹紧力的优化设计。