基于GA和FEM的夹具布局和变夹紧力优化设计

通过夹具布局和夹紧力大小的优化可以提高薄壁件加工精度．建立了夹具布局和变夹紧力分层优化模型．首先,以工件加工变形最小化和变形最均匀化为目标函数,对夹具布局进行优化设计;其次,基于优化的夹具布局对变夹紧力进行设计．采用有限元法计算工件的加工变形,加工变形求解时综合考虑了接触力、摩擦力、切削力、夹紧力和切屑的影响．采用遗传算法求解优化模型,获得优化的夹具布局和变夹紧力．通过实例分析,验证了分层优化设计方法可以进一步减小工件加工变形,提高加工变形均匀度．     

Analysis and optimization of fixture under dynamic machining condition with chip removal effect

Dynamic interactions in the tool–workpiece and workpiece–fixture systems significantly impinge on the quality of finished workpieces. The presented simulation system integrates the effects of workpiece fixture dynamics with the other factors contributing to the machining process dynamics. It provides more accurate prediction of the process output which helps in the design of the optimum fixture configuration prior to the production stage. Modelling of the frictional contact behaviour between the fixture element and the workpiece helps to improve the prediction accuracy of the simulation system which accelerates the convergence to the optimum fixture configuration design and consequently improves the machined part dimensional accuracy and geometric integrity. The developed simulation is capable of modelling complicated part geometries by interfacing with commercial ANSYS.V10^® packages. This research work minimizes the deformation of workpiece using integrated optimization tool of Genetic algorithm (GA) and ANSYS Parametric Design Language (APDL) of finite element analysis. The same layouts given by the above optimization tool are used in the experimental setup and it is found that the improved geometric tolerance of squarness and flatness of the given workpiece. The chip removal effect and frictional contact between the workpiece and the fixture elements are taken into account based on element death technique and nonlinear finite-element analysis. A Case study of an open slot milling process illustrates the application of the proposed improved geometric tolerance approach.     

On Clamping Planning in Workpiece-Fixture Systems

Deformations of contacts between the workpiece and locators/clamps resulting from large contact forces cause overall workpiece displacement, and affect the localization accuracy of the workpiece. An important characteristic of a workpiece-fixture system is that locators are passive elements and can only react to clamping forces and external loads, whereas clamps are active elements and apply a predetermined normal load to the surface of workpiece to prevent it from losing contact with the locators. Clamping forces play an important role in determining the final workpiece quality. This paper presents a general method for determining the optimal clamping forces including their magnitudes and positions. First, we derive a set of “compatibility” equations that describe the relationship between the displacement of the workpiece and the deformations at contacts. Further, we develop a locally elastic contact model to characterize the nonlinear coupling between the contact force and elastic deformation at the individual contact. We define the minimum norm of the elastic deformations at contacts as the objective function, then formulate the problem of determining the optimal clamping forces as a constrained nonlinear programming problem which guarantees that the fixturing of the workpiece is force closure. Using the exterior penalty function method, we transform the constrained nonlinear programming into an unconstrained nonlinear programming which is, in fact, the nonlinear least square. Consequently, the optimal magnitudes and positions of clamping forces are obtained by using the Levenberg–Marquardt method which is globally convergent. The proposed planning method of optimal clamping forces, which may also have an application to other passive, indeterminate problems such as power grasps in robotics, is illustrated with numerical example.      

Dynamic modeling of the fixture-workpiece system

This paper presents a methodology for dynamic modeling and simulation of a fixture-workpiece system. A simulation approach is required since standards typically do not exist for dynamic situations like machining operations. In addition, an accurate model is developed for the contact interface at each locating and clamping region on the workpiece's surface. An end milling operation is simulated to analyze the effects of various factors on workpiece accuracy and demonstrate the advantage of the simulation approach. The clamping forces required to keep the workpiece in contact with its locators are obtained, and the influences of locator placement, clamp placement, clamping forces, and clamping sequence on linear and angular errors are reported. Elastic effects of the locator-workpiece and clamp-workpiece contacts, yielding nonlinear dynamic equations of motion, are included in the model. Since system dynamics are considered, results are obtained as a function of time. The study compares well with previous experimental work by other investigators, and the method shows promise as a fixture design tool.     

A method to minimize the workpiece deformation using a concept of intelligent fixture

Manufacturing processes are commonly affected by the low stiffness of the components limiting the quality and precision of the final product. Precision is one of the most important issues in the machining process, and the main cause for rejection is the part static deformation and the dynamic vibrations. The static deformation is mainly affected by two factors: deformation due to clamping and process forces, and geometrical distortions due to material removal and residual stress relieving during processing.The deformations caused by the clamping in the fixture are normally associated to existing distortion in the raw workpiece due to previous manufacturing processes and to the clamping forces. These problems lead to uncertainties in the set-up process, hindering the fixture functions, the achievement of a right positioning of the workpiece and the avoidance of deformations due to clamping forces.This paper presents an analysis to identify the causes of the static deformations during clamping and a method to correct the geometrical distortion and deformation of a clamped workpiece by the evaluation of the reaction forces in the selected relevant clamping points. It covers the design and validation of an active clamping unit to minimize the deformation produced by the fixture that could affect the machining process. The developed clamping unit presents an alternative to combine the locator and the clamper in a single component that controls the reaction force and the deformation of the workpiece in the clamping point, and it performs the positioning of that point to minimize the distortion of the workpiece. The clamping unit was verified in laboratory conditions and then tested in an industrial application, evaluating the capabilities related to positioning and reaction force control.     

夹紧顺序、夹具布局和夹紧力对装夹变形影响与同步优化分析研究

为减小工件装夹变形,提高薄壁件加工精度,以薄壁零件装夹变形最小化为目标 函数,通过遗传算法和有限元方法相结合,提出夹紧顺序、装夹布局和夹紧力同步分析方法。用 该方法对一航空薄壁零件装夹进行优化分析,优化结果与经验设计及传统分析结果进行对比,有 效地降低了工件因装夹不当引起的变形,验证了夹紧顺序、夹具布局和夹紧力同步优化方法的 有效性。     

Stability ofWorkpkce Setup in Feature-Based Fixture Planning System

Analyzed the issues of stability of workpiece during clamping and machining. Discussed classification of the stability problem according to the forces acting on the part in the commonly used machining processes; gave the methods of calculating clamping force and measures to guarantee the stability of workpiece in the feature-based fixture planning design system.     

Stability of Workpiece in Feature－Based Fixture Planning System

Analyzed the issues of stability of workpiece during clamping and machining,Discussed classification of the stability problem according to the forces acting on the part in the commonly used machining processes;gave the methods of calculating clamping force and measures to guarantee the stability of workpiece in the feature-based fixture planning design system.     

Non-jamming conditions in multi-contact rigid-body dynamics

This paper addresses an issue related to a multi-rigid-body frictional contact application, the non-jamming condition for the applying force such that the workpiece can move while maintaining existing contacts. The issue arises from the study of fixture loading planning. While rigid-body frictional contacts could restrict workpiece motion in both normal and tangential directions, it is found that the reason for jamming is from the tangential constraints by frictional forces. We first enumerate all the possible contact states for multiple contacts, and the contact constraints are classified into two categories, the configuration constraints and kinematic constraints. We then find an interesting result related with a non-jamming condition. That is, in a general situation, the applying force that can induce all-sliding contacts will never result in jamming. Moreover, a method to find the applied force on the workpiece that results in sliding on all contact points is presented, based on the sufficient condition for non-jamming. Numerical examples are presented and the results of the method are compared with the results of a quasi-static method.     

Model testing of fixture–workpiece interface compliance in dynamic conditions

This paper is focused on the problem of compliance of interface between clamping/locating fixture elements and workpiece, under dynamic loads during machining. In contrast to previous investigations, the authors have developed a special device dedicated to testing of physical models which represent clamping/locating elements and workpiece. This device allows optimization of a large number of input parameters which are critical to interface compliance. It was used in experimental investigations to establish the impact that the radius of the spherical tip of a clamping/locating element has on the interface compliance and load capacity. The results of experimental investigation show that, under certain conditions, the clamping/locating elements with larger-radius spherical tips provide significantly lower interface compliance. Future investigations should be aimed at finding optimum macro- and micro-geometries of contact interface, as well as the selection of materials for clamping/locating elements.     

An integrated fixture planning and analysis system for machining processes

Fixture planning is an important part of computer-aided process planning (CAPP), which is the link between design and manufacturing in a CIM environment. This paper presents a rational approach to computer-assisted fixture planning (CAFP), emphasizing integration of fixture planning with process planning, an issue that has not been adequately addressed until very recently. A systematic approach to fixture selection is outlined for planning of modular fixtures. A prototype CAPP-CAFP system has been developed at UCLA and linked to a commercial CAD system, namely, CADAM. Part design information can be extracted from the CAD model and multiple-view engineering drawings of a part stored in the CADAM system. Modular fixture elements can be selected automatically by the CAPP-CAFP system and the generated fixture layout can be displayed on the screen. Included also in the system is a fixture analysis module for verification and rationalization of a fixturing scheme. The force analysis module has a built-in local optimization routine that can determine the clamping forces of more reasonable magnitudes. The friction forces between the fixture and the workpiece can also be considered for simple cases.     

Multi-Objective Approach to Automated Fixture Synthesis Incorporating Deep Neural Network for Deformation Evaluation

In machining, the synthesis of a fixturing schema significantly impacts the accuracy of the final product. Moreover, A robust and automatic configuration of fixture elements can reduce production costs and eliminate the need for expert labor to perform the task. Given the multi-modal problem of fixture synthesis, this article presents a multi-objective approach to fixture synthesis in the discrete domain. The performance criteria are localization accuracy, detachment of locators, workpiece deformation, severity and dispersion of reaction loads, and the spacing between contact points. Optimization is performed via an improved Declining Neighborhood Simulated Annealing algorithm (DNSA). To achieve consistent performance over different inputs, the number of iterations follows a Shanon entropy index reflecting the recurrence of folds/corners. Except for deformation, all other objectives are derived from the kinematic analysis of the workpiece-fixtures system. In contrast, deformation is estimated via a Constitutive Deep Neural Network (CDNN). Both models incorporate the machining loads as quasi-static intervals. A new strategy is adopted for the trade-off based on the Z-score quantification of objectives through a pre-calibration run of DNSA. Numerical examples demonstrate the implementation flow of our generalized CAD-based tool developed for the purpose. The approach is verified and proved efficient in automating the robust selection of a fixture layout for a prismatic workpiece.     

Multi-objective optimal fixture layout design

This paper addresses a major issue in fixture layout design to determine and evaluate the acceptable fixture designs based on multiple quality criteria and to select an optimal fixture with appropriate trade-offs among multiple performance requirements. The performance objectives considered are related to the fundamental requirements of kinematic localization and total fixturing (form-closure). Three performance objectives are defined as the workpiece localization accuracy and the norm and dispersion of the locator contact forces. The paper focuses on multi-criteria optimal design with a hierarchical approach. An efficient interchange algorithm is extended and used for different practical cases, leading to proper trade-off strategies for performing fixture synthesis. Examples are given to illustrate empirical observations with respect to the proposed approach and its effectiveness.     

Automated modular fixture planning based on linkage mechanism theory

A new approach to automated modular fixture planning is presented. The approach identifies all the location plan candidates of a workpiece using linkage mechanism theory and excludes the infeasible location plan candidates by evaluating their accessibility and fixturability. The algorithms for analyzing accessibility and fixturability and generating feasible clamp positions of a fixture plan are developed based on several new concepts including IRC triangle, locator visible cone, etc. The approach is capable of handling the workpiece whose side clamping faces consist of planar faces or cylindrical faces. To illustrate the effect of the approach, some test results are described.     

Effect of workpiece springback on micromilling forces

The machining forces present in micromilling with tools in the 50–100 m diameter range are dominated by contact pressure and friction between the tool cutting edges and the workpiece. A model of the micromilling process was developed based on the elastic contact between the tool and the workpiece along the side and bottom cutting edges of the tool. Micromilling experiments were conducted on 6061-T6 aluminum to obtain machining forces in the feed and cross-feed directions during slot milling and partial engagement end milling. Comparisons with the experimental data indicate reasonable agreement for full slot milling as well as end milling with radial depths of cut in the range of 2 m to 40 m. It was concluded that this model is adequate for predicting micromilling forces with the precision needed to reduce tool breakage and workpiece clamping forces and for predicting tool deflection that affects wall slope and feature size.This work was supported primarily by the Engineering Research Centers Program of the National Science Foundation under Award Number EEC-9986866. The Engineering Research Center for Wireless Integrated Microsystems is also hereby acknowledged. All machining was performed at the Micromechanical Applications and Processes Laboratory at Michigan Technological University.     

An approach to enhancing the intelligence of a three-fingered automated flexible fixturing system by using adaptive control theory

A new approach to clamping workpieces for the machining process has been developed. The new approach is based on the identification of the workpiece stiffness, during the machining process, which enables the fixturing system to determine whether the clamping force should be changed. Using this approach, a three-fingered intelligent automated flexible fixturing system for planar objects has been successfully designed and implemented. The conventional and traditional clamping methods use a hydraulic vise to clamp workpieces with a fixed amount of clamping forces. This often causes the workpieces to deform under such force, thus contributing to machining error. It is therefore desirable to control the clamping force according to the progress of the machining process. The implementation of this approach to the three-fingered automated flexible fixturing system enhances the system’s ability to achieve intelligent clamping force control and deal with abnormal machining, thus making the system intelligent. A thin ring-shaped workpiece was successfully used as an example to demonstrate the feasibility and validity of this approach. It also demonstrates that the developed system can be used in the highly automated manufacturing system of the future.     

Prediction of workpiece elastic deflections under cutting forces in turning

One of the problems faced in turning processes is the elastic deformation of the workpiece due to the cutting forces resulting in the actual depth of cut being different than the desirable one. In this paper, a cutting mechanism is described suggesting that the above problem results in an over-dimensioned part. Consequently, the problem of determining the workpiece elastic deflection is addressed from two different points of view. The first approach is based on solving the analytical equations of the elastic line, in discretized segments of the workpiece, by considering a stored modal energy formulation due to the cutting forces. Given the mechanical properties of the workpiece material, the geometry of the final part and the cutting force values, this numerical method can predict the elastic deflection. The whole approach is implemented through a Microsoft Excel^© workbook. The second approach involves the use of artificial neural networks (ANNs) in order to develop a model that can predict the dimensional deviation of the final part by correlating the cutting parameters and certain workpiece geometrical characteristics with the deviations of the depth of cut. These deviations are calculated with reference to final diameter values measured with precision micrometers or on a CMM. The verification of the numerical method and the development of the ANN model were based on data gathered from turning experiments conducted on a CNC lathe. The results support the proposed cutting mechanism. The numerical method qualitatively agrees with the experimental data while the ANN model is accurate and consistent in its predictions.     

Constructing minimum deflection fixture arrangements using frame invariant norms

This paper describes a fixture planning method that minimizes object deflection under external loads. The method takes into account the natural compliance of the contacting bodies and applies to two-dimensional and three-dimensional quasirigid bodies. The fixturing method is based on a quality measure that characterizes the deflection of a fixtured object in response to unit magnitude wrenches. The object deflection measure is defined in terms of frame-invariant rigid body velocity and wrench norms and is therefore frame invariant. The object deflection measure is applied to the planning of optimal fixture arrangements of polygonal objects. We describe minimum-deflection fixturing algorithms for these objects, and make qualitative observations on the optimal arrangements generated by the algorithms. Concrete examples illustrate the minimum deflection fixturing method. Note to Practitioners-During fixturing, a workpiece needs to not only be stable against external perturbations, but must also stay within a specified tolerance in response to machining or assembly forces. This paper describes a fixture planning approach that minimizes object deflection under applied work loads. The paper describes how to take local material deformation effects into account, using a generic quasirigid contact model. Practical algorithms that compute the optimal fixturing arrangements of polygonal workpieces are described and examples are then presented.     

A Polyhedral Bound on the Indeterminate Contact Forces in Planar Quasi-Rigid Fixturing and Grasping Arrangements

This paper considers multiple-contact arrangements where several bodies grasp, fixture, or support an object via frictional point contacts. Within a strictly rigid-body modeling paradigm, when an external wrench (i.e., force and torque) acts on the object, the reaction forces at the contacts are typically indeterminate and span an unbounded linear space. This paper analyzes the contact reaction forces within a generalized quasi-rigid-body framework that keeps the desirable geometric properties of rigid-body modeling, while also including more realistic physical effects. We describe two basic principles that govern the contact mechanics of quasi-rigid bodies. The main result is that for any given external wrench acting on a quasi-rigid object, the statically feasible contact reaction forces lie in a bounded polyhedral set that depends on the external wrench, the grasp's geometry, and the preload forces. Moreover, the bound does not depend upon any detailed knowledge of the contact mechanics parameters. When some knowledge of the parameters is available, the bound can be sharpened. The polyhedral bound is useful for “robust” grasp and fixture synthesis. Given a set of external wrenches that may act upon an object, the grasp's geometry and preload forces can be chosen such that all of these external wrenches would be automatically supported by the contacts.     

Rules for an expert fixturing system on a CAD screen using flexible fixtures

The object of this research is to develop rules for an expert system which will aid in the automatic layout of flexible fixture models on a CAD/CAM system for given applications through an interactive process. The rules are developed and implemented in PROLOG adopting the general sets of fixturing coordinates developed based on the 321-clamping concept and the geometric information such as shape and size of the workpiece to be fixtured. Surface milling is used to demonstrate the principle of the rules and applied to regular polyhedral shapes. Input to the program consists of parameters describing the workpiece to be fixtured which will initiate a search through the databases of rules and fixturing coordinates. Appropriate fixturing rules will be matched and fired to create a datafile containing fixturing points in cartesian coordinates and the list of modular fixture components to be used as an input source to the CAD system.