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.     

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.     

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.     

Dynamics model of active fixturing systems for thin-walled parts under moving loads

This work presents a modelling approach that captures the dynamic response of active fixture elements and thin-walled workpieces subjected to dynamic moving loads. The impedance coupling method is deployed to achieve the integration of analytical expressions for the active elements and a discretised finite element-based model of a workpiece. The proposed model is validated experimentally and analytically via a test case, involving a single active fixture element in contact with a thin aluminium plate subjected to an evenly distributed dynamic moving load. A very good agreement is observed between the time response of the modelled and the experimental system to step and harmonic inputs. The moving load modelling approach presents a maximum error of ε?=?5.82% in terms of predicting the dynamic elastic deformation of the workpiece in comparison to an analytical model.     

The Application of Chip Removal and Frictional Contact Analysis for Workpiece–Fixture Layout Verification

In this paper, a workpiece–fixture layout verification approach with the application of frictional contact and chip removal effects using a finite-element technique, is presented. The objective of the proposed system is to overcome the deficiencies of existing fixture design approaches. Workpiece–fixture layout verification analysis is carried out for time varying machining forces to ensure that the workpiece will be held against the cutting and clamping forces. The chip removal effect and frictional contact between the workpiece and the fixture elements are taken into account using a material removal approach based on element death technique and nonlinear finite-element analysis. A case study illustrates the application of the proposed approach. ID="A1"Correspondance and offprint requests to: Professor F. ?ztürk, Department of Mechanical Engineering, Mühendislik-Mimarlik Fakültesi, G?r¨kle Kampusu, Bursa 16059, Turkey. E-mail: ferruh@uladag.edu.tr     

The study of fixture stiffness part I: a finite element analysis for stiffness of fixture units

This paper presents a systematic finite element model to predict the fixture unit stiffness by introducing nonlinear contact elements on the contact surface between fixture components. The contact element includes three independent springs: two in tangential directions and one in the normal direction of the contact surface. Strong nonlinearity is caused by possible separation and sliding between two fixture components. The problem is formulated by the penalty function method and is solved by the modified Newton--Raphson procedure. The model was validated by two cases of analysis of a linear cantilever beam and a simple fixture unit with two components. Results are in agreement with the corresponding analytical solution of beams and the previous experiment results for fixture in the literature.     

A Hybrid Intelligent Systems Approach for Die Design in Sheet Metal Forming

Die design is heavily experience based and the die design process is an iterative procedure of trial and error in order to obtain a final die design for the successful manufacture of stampings. Most automotive industries use internal guidelines and past experience for die design. Even though powerful computer-aided design systems are being used in automotive industry, the lack of adequate analysis tools at the initial die geometry design stage hinders the die manufacturing process, and also necessitates lead times of the order of 5–30 weeks [1]. At the concept design stage, and during the initial die development process, the variations in geometry and process conditions are so large that it is prohibitively expensive to use 3D finite element analysis. The complexity of die design heuristic knowledge hinders the development and application of knowledge-based systems. Hybrid intelligent systems are computer programs in which at least one of the constituent models simulates intelligent behaviour [2]. These models could be knowledge-based systems, artificial neural networks, fuzzy logic systems, etc. In this approach both artificial neural networks, knowledge-based systems and finite-element analysis (FEA) for modelling the design process are used. A simulation-based design approach [3] for the die design process is followed. Artificial neural networks (ANNs) are preliminary design tools which indicate the formability of the component geometry, for the selected process and material conditions. The ANN module is trained from FEA results for a generic set of component geometries, process conditions, and material properties. The final die design validation is carried out by FEA. The intelligent frame-work incorporates rules for material selection, process parameter selection and their modification. Component geometry is a critical parameter which affects the manufacturability of the given part. Hence, an intelligent geometry handling module, which automatically modifies and optimises the geometry of the designed die, is implemented in the present system. Knowledge-based blackboard architecture is used for the integration of various analysis models such as CAD, FEA, and ANN, as an intelligent framework for die design [4]. The hybrid intelligent system provides an integrated decision support environment for simulation and analysis of the forming process, both during the initial die design phase and during the die tryout phase. The hybrid intelligent systems approach supports the capability for automatic evaluation of prospective die design for manufacturability, and performs automatic modification of design inputs. Applications of the hybrid intelligent system for die design are described together with a comparison with shop floor data.     

FLEXIBLE ASSEMBLY FIXTURING LAYOUT MODELING AND OPTIMIZATION BASED ON GENETIC ALGORITHM

There are many welding fixture layout design problems of flexible parts in body-in-white assembly process, which directly cause body assemble variation. The fixture layout design quality is mainly influenced by the position and quantity of fixture locators and clamps. A general analysis model of flexible assembles deformation caused by fixture is set up based on "N-2-1" locating principle, in which the locator and clamper are treated as the same fixture layout elements. An analysis model for the flexible part deformation in fixturing is set up in order to obtain the optimization object function and constraints accordingly. The final fixture element layout could be obtained through global optimal research by using improved genetic algorithm, which effectively decreases fixture elements layout influence on flexible assembles deformation.     

Surface error decomposition for fixture development

As an integrated element of the manufacturing system of components, the machining fixture is a major contributing factor of the profile and orientation errors of component features. An effective way to control the accuracy of components is to decompose error sources and evaluate individual influential factors. This paper proposed a systematic method of error identification and calculation, in which locating error and machining error were studied. The locating error, which is the surface error generated before machining, is obtained from the calculation of the surface error based on tolerances of the locating positions and the decomposition of clamping deformation using finite element analysis (FEA). The machining error, the surface error generated from machining operations, is gained mainly from the coordinate measurement machine’s (CMMs) measurements. The surface error of multi-machining operations is investigated and the resultant surface error is evaluated against tolerance. The analysis of a sample feature of a turbine blade is provided as an example.     

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.     

The application of the simulated annealing method and heuristic algorithms in the selection of fixture elements

This paper deals with the selection of suitable fixture element combinations and fixture allocation for a workpiece to allow for the different positions of the fixturing points, which are already given based on related theories. The advanced modelling extension (AME) of AutoCAD R12 was used as the 3D graphic interface. The function suitable for the AME environment was also controlled to obtain detailed data of the workpiece.First of all, the concept of group technology (GT) was used to categorise and encode the modular fixture elements with similar functions for cutting purposes. Next, the AME was combined with API (application programming interface) and Autolisp to establish the graphic illustration system for fixture elements. Then different heuristic algorithms for fixture element selection were developed to correspond to different requirements in fixture functions. The required fixture elements were then selected with the help of the simulated annealing calculation method to derive a suitable and economical number of fixture element combinations. The height of the fixture element combination derived may not match the height of the fixture point originally located. The workpiece shape also affects the allocated position of the fixture elements. Therefore, a number of heuristic algorithms were developed in this study to process all the factors involved and to obtain a desirable selection for fixture elements.     

Boundary Element Method for Elasto-hydrodynamic Lubricated Multi-body Contact

In this paper, a formulation for isothermal Elasto-Hydrodynamic Lubricated (EHL) contact problems, using the Boundary Element Method (BEM) is proposed. The BEM is used for modelling both bodies in contact. The proposed formulation evaluates directly the film thickness considering the elastic deformation of the bodies after treating the problem as a coupled system (solid–fluid–solid). The fluid is modelled using the Finite Difference Method (FDM). The forward iterative method is used as the solution algorithm of the elasto-hydrodynamic problem. The analysis demonstrate that the BEM is efficient for modelling both solids in contact and that the computational time remains low.     

Contact stresses in the glenoid component in total shoulder arthroplasty

Several studies of retrieved glenoid components from total shoulder arthroplasty show an erosion of the rim, surface irregularities, component fracture and wear resulting from polyethylene deformation in vivo. Particles resulting from polyethylene wear might be one of the reasons for the very high rate of glenoid component loosening found clinically. Because wear can be the result of high contact stresses, the aim of this study is to find out whether or not contact stresses are high enough to cause wear of the glenoid component and what influence the component type and geometry have on polyethylene contact stresses for different humerus abduction angles. Elasticity theory is used in a parametric study of contact stresses in several glenoid component designs. A finite element method is used to confirm the accuracy of the analytical solution. The analysis shows that the peak stress generated in glenoid components under conditions of normal living can be as high as 25 MPa; since this exceeds the polyethylene yield strength, wear and also cold flow of the components can be expected. It is predicted that more conforming components have lower contact stresses, which might result in lower wear rate and less cold flow. It is also found that a metal-backed component promotes higher contact stresses than an all-polyethylene component with the same total thickness, therefore it can be expected that metal-backed components have inferior wear properties.     

Numerical simulation of sliding wear for a polymer–polymer sliding contact in an automotive application

Injection moulded polymer-based components are important for cost effective and fast production/assembly of auxiliary mechanisms in automotive industry. Wear is one of the critical factors, which influences the reliability and useful life in such mechanical components. Experimental determination of life parameters in terms of wear has both a cost and time impact. Therefore, the ability to predict wear at the development stage enables the designers to come up with a better design, longer useful life and more reliable products. This paper presents a numerical simulation of wear for a polymer–polymer sliding surface contact in dry conditions. Finite element analysis (FEA) is used as a tool to calculate nodal pressures at the contact area for small sliding steps. These pressures are then inputted to a customized wear calculating routine. The routine uses averaged wear coefficients (wear rates) obtained from custom designed experiments. The FE contact geometry is modified after each sliding step to account for the contact height decay thus determining wear volume loss over usage time and predicting the worn geometry.     

Segment-to-segment contact elements for modelling joint interfaces in finite element analysis

This paper presents an efficient approach to model contact interfaces of joints in finite element analysis (FEA) with segment-to-segment contact elements like thin layer or zero thickness elements. These elements originate from geomechanics and have been applied recently in modal analysis as an efficient way to define the contact stiffness of fixed joints for model updating. A big advantage of these elements is that no global contact search algorithm is employed as used in master–slave contacts. Contact search algorithms are not necessary for modelling contact interfaces of fixed joints since the interfaces are always in contact and restricted to small relative movements, which saves much computing time. We first give an introduction into the theory of segment-to-segment contact elements leading to zero thickness and thin layer elements. As a new application of zero thickness elements, we demonstrate the implementation of a structural contact damping model, derived from a Masing model, as non-linear constitutive laws for the contact element. This damping model takes into account the non-linear influence of frictional microslip in the contact interface of fixed joints. With this model we simulate the non-linear response of a bolted structure. This approach constitutes a new way to simulate multi-degree-of-freedom systems with structural joints and predict modal damping properties.     

Fixture layout optimization for multi point respot welding of sheet metals

The high quality of welding in the automotive industry is achieved by proper positioning of the fixture elements. A new method, N-3-2-1 (N ≥ 1), is proposed for fixture layout optimization of sheet metals. The flexible nature of the sheet metals requires N+3 fixture elements to constrain it normal to the surface (primary plane), but 2-1 fixture elements for other two directions (secondary and tertiary). The objective function is to achieve high stiffness of the workpiece and is calculated in terms of strain energy. Finite element analysis (FEA) was combined with genetic algorithm. A method was also proposed to find the optimum fixturing position of the workpiece in multipoint respot welding operation. Two different kinds of case studies were solved and the performance of the proposed method was also tested in the industrial scenario by fixturing the workpiece and completing the respot welding operation with satisfactory results.

     

A Generalised Asperity-Based Friction Model

Despite considerable research effort, the use of physics-based modelling to predict frictional behaviour is still a debatable question in modern tribological research. This article presents a dry-friction model, based on physical phenomena such as adhesion, elastic–plastic contact and deformation. This contribution offers a means to simulate all kinds of frictional behaviour that is observed in experimental research. The contact of two bodies through their surfaces is transformed into the contact of a body that is provided with asperities and containing material and geometrical information of both of the mating surfaces, and a counter profile, holding solely geometrical information. The local adhesion between the asperity tips and the counter profile, together with the elastic–plastic behaviour of the asperities themselves, form the basis for this model. The simulation results show qualitatively good agreement with experimental study. Friction and contact phenomena such as normal creep, increasing static coefficient of friction with increasing dwell time, pre-sliding hysteresis with nonlocal memory, Stribeck and viscous effect, frictional lag, stick–slip and dynamical oscillations are revealed by this model. Furthermore, future improvement and refinement of the model is possible (and ongoing) so as to incorporate lubrication and asperity wear.