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.     

An Integrated Model of a Fixture-Workpiece System for Surface Quality Prediction

Surface quality is a major factor affecting the performance of a component. The machined surface quality is strongly influenced by the external loads during the fixturing and machining processes. In machining process development, it is highly desirable to predict the quality of a machined surface. For this purpose, an integrated finite element analysis (FEA) model of the entire fixture–workpiece system is developed to investigate the influence of clamping preload and machining force on the surface quality of the machined workpiece. The effects of fixture and machine table compliance (from experimental data), and the workpiece and its locators/clamps contact interaction, and forced vibration, on the machined surface quality are taken into account. This simulation model provides a better understanding of the causes of surface error and a more realistic prediction of the machined surface quality. The deck face of a V-type engine block subjected to fixture clamping and a face milling operation is given as an example. A comparison between the simulation result and experimental data shows a reasonable agreement.     

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.     

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.     

Computer-Aided Fixture Design Verification. Part 3. Stability Analysis

In fixture design, a workpiece is required to remain stable throughout the fixturing and machining processes in order to achieve safety and machining accuracy. This requirement is verified by a function of the computer-aided fixture design verification (CAFDV) system. This paper presents the methodologies of fixturing stability analysis in CAFDV. A kinetic fixture model is created to formulate the stability problem, and a fixture stiffness matrix (FSM) is derived to solve the problem. This approach not only verifies fixturing stability, but also finds the minimum clamping forces, fixture deformation, and fixture reaction forces. The clamping sequence can also be verified with this approach.     

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.     

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.     

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.     

Machining fixture layout design using ant colony algorithm based continuous optimization method

In any machining fixture, the workpiece elastic deformation caused during machining influences the dimensional and form errors of the workpiece. Placing each locator and clamp in an optimal place can minimize the elastic deformation of the workpiece, which in turn minimizes the dimensional and form errors of the workpiece. Design of fixture configuration (layout) is a procedure to establish the workpiece–fixture contact through optimal positioning of clamping and locating elements. In this paper, an ant colony algorithm (ACA) based discrete and continuous optimization methods are applied for optimizing the machining fixture layout so that the workpiece elastic deformation is minimized. The finite element method (FEM) is used for determining the dynamic response of the workpiece caused due to machining and clamping forces. The dynamic response of the workpiece is simulated for all ACA runs. This paper proves that the ACA-based continuous fixture layout optimization method exhibits the better results than that of ACA-based discrete fixture layout optimization method.     

基于多重夹紧的工件—夹具间接触力的预测方法

在夹具设计过程中,工件-夹具之间的接触力是工件稳定性分析和加工精度估算的关键因素.为此,根据多重夹紧力对工件的作用过程,建立了接触力与多重夹紧力的大小、作用点以及夹紧顺序之间的接触力模型.基于总余能原理,提出了接触力模型的求解算法.最后通过典型实例,详细说明了接触力的分析预测过程.     

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。     

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.     

Optimization of the number and positions of fixture locators in the peripheral milling of a low-rigidity workpiece

In this paper, a method is proposed to optimize the fixture layout in the peripheral milling of a low-rigidity workpiece. Because the locators on the secondary locating surface directly influence the deformation of the workpiece in peripheral milling, this paper deals with the optimization of the number and positions of the locators on the secondary locating surface. The method proposed in this paper includes two stages. In the first stage, the initial number and positions of the locators are determined by adding the locators at the position with the maximum deformation. In the second stage, the number and positions of the locators are optimized. Using the method proposed in this paper, the number of locators is reduced, while the machining accuracy of the workpiece is retained.     

A combined contact elasticity and finite element-based model for contact load and pressure distribution calculation in a frictional workpiece-fixture system

Contact forces between workpiece and fixture define fixture stability during clamping and influence workpiece accuracy during machining. In particular, forces acting in the contact region are important for understanding deformation of the workpiece at the contact region. This paper presents a model that combines contact elasticity with finite element methods to predict the contact load and pressure distribution at the contact region in a workpiece-fixture system. The objective is to determine how much clamp forces can be applied to generate adequate contact forces to keep the workpiece in position during machining. The model is able to predict the normal and tangential contact forces as well as the pressure distribution at each workpiece-fixture contact in the fixturing system. Model prediction is shown to be in good agreement with known industry practice on clamp force determination. The presented method has no limits on the types of materials that can be analyzed.     

Elastic deformation of a fixture and turbine blades system based on finite element analysis

Finite element analysis (FEA) has been used to recapitulate the interactions between fixtures and components over the last decade. Most of the researches were focussed on the 3-2-1 fixture for components with regular geometry using point-to-point contact elements, where the fixture element is represented by a point-contacting component. Due to predicable behaviour of the fixture–component pair, such a point-to-point contact representation may be sufficient. However, when components with complex geometry, e.g. B-spline surfaces, which are widely used in the automotive and aero-engine industries, are of interest, the point-to-point method can no longer be satisfactory. This paper proposes a method of FEA on a system of a fixture and turbine blades by considering the complex contact geometry and complicated contact status of fixture–component pairs using surface-to-surface contact elements. A complete procedure of FEA modelling including geometry simplification, contact modelling, stiffness of locators, mesh generation, boundary condition and loading sequence is explained in detail. Having verified the FEA prediction of the elastic deformation with the displacement of the workpiece measured by coordinate measurement machines (CMMs), the influential factors of deformation, such as friction and machining directions, are analysed.     

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。     

Workpiece locating error prediction and compensation in fixtures

In machining process, fixture is used to keep the position and orientation of a workpiece with respect to machine tool frame. However, the workpiece always cannot be at its ideal position because of the setup error and geometric inaccuracy of the locators, clamping force, cutting force, and so on. It is necessary to predict and control the workpiece locating error which will result in machining error of parts. This paper presents a prediction model of a workpiece locating error caused by the setup error and geometric inaccuracy of locaters for the fixtures with one locating surface and two locating pins. Error parameters along 6 degrees of freedom can be calculated by the proposed model and then compensated by either using the “frame transformation” function of a numerical control (NC) system or modifying NC codes in post-processing. In addition, machining error caused by the workpiece locating error can be predicted based on a multi-body system and homogeneous transfer matrix. This is meaningful to fixture design and machining process planning. Finally, a cutting test has shown that the proposed method is practicable and effective.     

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。     

薄壁件的装夹变形机理分析与控制技术

系统地提出一个分析与优选夹紧力大小、作用点以及夹紧顺序的通用方法.基于由摩擦力引起的接触力历史依赖性,定量地分析多重夹紧元件及其作用顺序对薄壁件变形的影响,并建立装夹方案的数学模型.同时提出基于最小总余能原理的有限元求解方法.另一方面,基于装夹方案的优化模型,提出装夹变形的控制技术以便获得最高的工件加工精度.以典型铝合金航空材料构件为例,模拟与分析夹紧力及夹紧顺序对其变形的影响过程.