Analysis of the effects of fixture clamping sequence on part location errors

Several fixture-related error sources are known to contribute to part location error, which can lead to poor part quality. In addition to typical error sources, such as fixture geometric error and elastic deformation of the fixture and part due to clamping forces, the clamping sequence used can also influence part position and orientation. In this paper, the effect of clamping sequence on workpiece location error is modeled analytically for a fixture–workpiece system where all major compliance sources and fixture geometric error are considered. Part location error is quantified by the displacement of a response point on the part surface. An algorithmic procedure designed to understand how forces and deformations change as clamps are applied sequentially is presented. The effect of clamping sequence on part location error and locator reaction force is examined through model simulations and experiments via an example involving a 3-2-1 machining fixture.     

Machining fixture layout optimization using the genetic algorithm

Dimensional and form accuracy of a workpiece are influenced by the fixture layout selected for the machining operation. Hence, optimization of fixture layout is a critical aspect of machining fixture design. This paper presents a fixture layout optimization technique that uses the genetic algorithm (GA) to find the fixture layout that minimizes the deformation of the machined surface due to clamping and machining forces over the entire tool path. The advantages of the GA-based method over previously reported non-linear programming methods for fixture layout optimization are discussed. Two GA-based fixture layout optimization approaches are implemented and compared by applying them to several two-dimensional example problems.     

发动机进排气导管座圈锪铰专用机床设计

针对企业摩托车发动机缸体现有工艺存在加工精度、合格率低及生产效率跟不上市场需求的问题,在分析现有加工手段优劣的基础上,利用工序集中原则,将缸体进排气座圈孔、导管孔及平面的精加工集中于一台设备上进行加工,提出了两次装夹、多工件、多工位、多刀具复合加工的全新工艺方案。根据工件加工姿态确定了双工位、双夹具,每个夹具装夹两个工件的整体布局方案,工位一、二相互协作完成缸体工序加工内容;最后,根据整体方案对专用夹具、专用刀具及专用主轴箱等关键零部件进行了详细设计及阐述。     

FANUC机器人机床上下料系统设计与仿真

工业机器人已经广泛应用到现代制造工厂,机器人选型和仿真分析是机器人应用的关键。根据产品的加工工艺和上下料的要求,设计了分度自动夹具和专用夹爪。基于FANUC ROBOGUIDE仿真软件,将机床、机器人、夹具、工件、料框等三维模型导入仿真软件中进行组合,通过示教做上下料仿真。仿真分析的结果验证了机器人选型的合理性。通过对机器人运动轨迹的分析,调整了整个系统的布局,使布局更加合理。     

An experimental evaluation of coefficients of static friction of common workpiece–fixture element pairs

Most machining fixtures utilize clamping forces and friction at fixture–workpiece joints to help prevent the workpiece from slipping out of the fixture during machining. The magnitudes of the clamping forces required are a direct function of the coefficients of static friction at the joints. Recently, analytical methods have been developed to predict minimum clamping forces. However, these methods require accurate estimates of the friction coefficients.One source of friction data are handbooks. However, these data are typically listed relative to the materials of the contacting elements and are otherwise completely generalized. This paper will illustrate that the coefficient of static friction for typical fixture–workpiece joints is not a simple function of the workpiece materials. Instead it is also a function of factors such as fixture element geometry, workpiece surface topography, clamping forces, the presence or absence of cutting fluids, and normal joint rigidity.     

Prediction of workpiece deformation in a fixture system using the finite element method

Knowledge of workpiece deformations induced by loading in a fixture–workpiece system is important to ensure quality part production. Suitable methods for accurately predicting such deformations are essential to the design and operation of fixtures. In this regard, finite element modeling has been widely applied by researchers and practitioners. However, these studies generally neglect the role of compliance of the fixture body on workpiece deformation. Also lacking is knowledge of the effects of different finite element model parameters on workpiece deformation. This study uses finite element analysis (FEA) to model a fixture–workpiece system and to explore the influence of compliance of the fixture body on workpiece deformation. In addition, the effects of certain finite element model parameters on the prediction accuracy are also examined. Experimental verification of the workpiece deformations and locator reaction forces predicted by the FEA model shows agreement within 5% of the experimental data. For the fixture–workpiece system analyzed in this study, it was found that 98% of all system compliance is captured by modeling just the workpiece and fixture contact tips. The remainder of deformation occurred in the other fixture components. The accuracy and computational time tradeoffs are given for various fixture models.     

Determination of minimum clamping forces for dynamically stable fixturing

This paper presents a model-based framework for determining the minimum required clamping forces that ensure the dynamic stability of a fixtured workpiece during machining. The framework consists of a dynamic model for simulating the vibratory behavior of the fixtured workpiece subjected to time- and space-varying machining loads, a geometric model for capturing the continuously changing geometry and inertia of the fixture–workpiece system during machining, a static model for determining the localized fixture–workpiece contact deformations due to clamping, a model for checking the dynamic stability of the fixtured workpiece, and a model for determining the optimal set of clamping forces that satisfies the stability criteria for a given machining operation. The clamping force optimization problem is formulated as a bilevel nonlinear programming problem and solved using the Particle Swarm Optimization (PSO) technique featuring computational intelligence. A simulation example solved using the developed approach reveals that the minimum required clamping forces for dynamically stable fixturing are significantly affected by the fixture–workpiece system dynamics and its continuous change during machining due to the material removal effect.     

A model for synthesis of the fixturing configuration in pin-array type flexible machining fixtures

This paper presents a model for the synthesis of the fixturing configuration in pin-array type flexible machining fixtures. These fixtures possess an array of pins that hold parts by conforming to their shape. The proposed algorithm aims to achieve a specified level of fixture–workpiece conformability and stable equilibrium while keeping the workpiece rigid body motion due to fixture elastic deformation at or below a user-specified value. Specifically, the model finds the minimum clamping loads and the optimal number, position and dimensions of the pins necessary to achieve the conformability and stiffness goals for a workpiece having an arbitrary geometry and subjected to quasi-static machining/assembly forces. Experimental data for the X-clamp^® flexible pin-array vise is used to validate the proposed synthesis model.     

A novel approach to fixture layout optimization on maximizing dynamic machinability

In the aerospace industry, the reasonable layout of fixture can efficiently suppresses machining vibration of thin-walled aerospace structure during machining. Based on the analysis of typical structural components encountered in the aerospace industry, a general frame-structure workpiece with fixture constraints can be equivalent as Mindlin plates with simultaneous elastic edges and internal supports. On basis of the equivalent models, the powerful pb-2 Ritz method defined by the product of a two-dimensional polynomial and basic functions can be introduced to be taken as trial functions. Substituting displacement functions into energy functional and minimizing total energy by differentiation leads to eigenfrequency equations of the workpiece–fixture system. Consequently, a novel nonlinear programming problem based on the frequency sensitivity can be built to optimize the layout of fixture supports to maximize the fundamental nature frequency of the workpiece–fixture system. The feasibility of the proposed approach is validated by a machining case.     

A model to predict minimum required clamp pre-loads in light of fixture–workpiece compliance

The determination of minimum required clamp pre-loads is an important process in the design of machining fixtures. This paper presents a linear, clamp pre-load (LCPL) model that can be applied to fixture–workpiece systems whose compliance is load invariant. The model considers the static deformation of the fixture–workpiece system in response to the clamping process and the machining process. Sources of compliance throughout a fixture–workpiece system are considered. The model computes the minimum required pre-loads necessary to prevent workpiece slip at the fixture–workpiece joints throughout the machining process.This paper also describes an experimental study that was used to characterize the accuracy of the LCPL model with regard to the application of a ramping external load to a fixture–workpiece system. Over the contact conditions tested, the LCPL model was observed to overestimate the minimum required clamp pre-loads by an average of 7%. This experimental study also revealed the sensitivity of the computed pre-loads to the relative compliance of the fixture elements as well as the coefficient of friction.     

变速箱拨叉轴圆弧槽铣夹具的设计及制造

根据变速箱拨叉轴上圆弧槽相对位置准确、系列拨叉轴可以在一套夹具上装夹、多个零件同时加工的要求,设计了一种分体式气动铣床夹具。该夹具采用定位块限位拨叉轴端面、燕尾面支承拨叉轴外圆的定位方式,对工件外圆多点气动夹紧;通过两级增力机构,增大夹紧力;柔性传动力矩,消除了交变铣削力作用产生的振动。依据计算的切削力选择了气缸规格。最终实现了工件在夹具中成组布置,提高了生产效率,保证了加工质量。     

Experimental determination of the friction coefficient on the workpiece-fixture contact surface in workholding applications

Surface flatness, geometric integrity and micro-surface finish characteristics are crucial for automotive industry to properly seal joints, reduce leakage and consequently increasing engines efficiency and reducing emissions. Optimum fixture layout is a key element in achieving this goal. Machining of flexible parts impose further challenges to the selection of a proper fixture scenario.Workpiece motion arising from localized elastic deformation at the workpiece/fixture contacts due to machining and clamping forces significantly affect the workpiece location accuracy and hence the machined part quality. The tangential friction force plays an important role in fixture configuration design as it can be utilized to reduce the number of fixture components, thereby the workpiece features accessibility to machining operations and providing a damping mechanism to dissipate input energy from machining forces out of the workpiece/fixture system.Although the literature is full of research on friction and its application, it lacks research that relates to the contact found in workpiece/fixture systems. This paper presents the results of an experimental investigation of the workpiece/fixture contact characteristics.     

快速加工中心孔的专用夹具设计

简述了专用夹具的作用,设计了在车床上快速加工中心孔的专用夹具。通过夹紧力的验算,该夹具完全可以实现对工件的夹紧。介绍了此夹具的结构及使用方法,分析了其加工优势。此夹具结构简单、适用范围广、夹紧可靠,可降低劳动强度、提高生产率,具有较强的人性化效果和达到降低生产成本的目的。     

基于KRG代理模型的超声悬浮夹具设计

在超精密制造领域,如晶圆、硅片的制造与运输,需要在非接触、超净化条件下进行。为提高非接触式夹具的夹持稳定性,分析了无侧向约束力的超声悬浮夹具存在的不足,设计了一种有侧向约束力的新型超声悬浮夹具。基于KRG代理模型技术,建立了新型超声悬浮夹具的设计模型,并用遗传算法优化了新型超声悬浮夹具的结构参数。为验证新型超声悬浮夹具非接触夹持效果,运用工业视觉测量技术,搭建了试验平台。试验结果表明,新型超声悬浮夹具能够稳定夹持工件。     

A unified framework of error evaluation and adjustment in machining

Errors of machine tool, fixture, and datum on workpiece to be machined influence the machining accuracy of the workpiece. The objective of this paper is to provide a framework for abstracting an error model that integrates three types of errors, i.e., machine tool, fixture, and datum errors, into a unified one. Differential motion theory is used to build the evaluation model of three types of errors. The resultant deviation model of the tool with respect to the workpiece is derived by using the model. For the purpose of eliminating the deviation, the resultant geometric variation is mapped into the locator errors on the fixture. Then the position and orientation errors of the tool with respect to the workpiece may be reduced by adjusting the length of locators. Finally, the effectiveness of the resultant deviation model is verified by examples.     

Flexible multibody dynamics based fixture-workpiece analysis model for fixturing stability

The machining force and torque exerted on a workpiece vary as the cutter moves along the tool path, therefore a dynamic approach is essential for fixturing stability analysis. This paper presents a technique to dynamically model and analyze the fixture-workpiece system subjected to time-varying machining loads. Combining the advantages of FEA (Finite Element Analysis) and nonlinear rigid body dynamics, a flexible multibody dynamic model is formulated to incorporate the overall interaction (clamping forces, machining loads, and contact friction) between flexible workpiece and compliant fixture elements. Three major parameters affecting the fixturing stability, namely the magnitude, application sequence, and placement of fixturing clamps, are analyzed. Additionally, the time dependent deformation of a flexible workpiece under clamping and machining loads is estimated. A scaled engine block with the 3–2–1 fixturing scheme subjected to face milling operation is given as an example. Comparison between the simulation result and experimental data shows a reasonable agreement.     

Improved workpiece location accuracy through fixture layout optimization

Inaccuracies in workpiece location lead to errors in position and orientation of a machined feature on the workpiece. The ability to accurately locate a workpiece in a machining fixture is strongly influenced by rigid body displacements of the workpiece caused by elastic deformation of loaded fixture–workpiece contacts. This paper presents a model for improving workpiece location accuracy in fixturing. A discrete elastic contact model is used to represent each fixture–workpiece contact. Reduction in workpiece locating error due to rigid body displacements is achieved through optimal placement of locators and clamps around the workpiece. The layout optimization model is also shown to improve the overall workpiece deflection and reaction force characteristics.     

面向自由曲面零件装夹的自适应夹具设计

针对叶片零件顶端修复过程中由于几何形体差异导致的难以通过固定夹具夹紧叶片的问题,开展自适应于叶片型面的夹具设计与加工过程变形分析。根据叶片顶端特征的修复需求,采用球端针式螺钉阵列对叶片型面进行夹紧。通过有限元软件分析叶片模型夹紧力与加工变形,可知:合理确定螺钉阵列的位置分布与夹紧力大小,使叶片受力均匀,可减少叶片在顶端加工过程中的变形量。     

多腔缸体零件端面铣削加工的高精密夹具设计及其优化

针对发动机多腔体缸体的结构复杂和刚度小的问题,对该缸体铣削端面加工工序确定高精密夹具的定位和夹紧方案.在初步分析夹具方案之后,运用有限元法对夹具的主要零件进行了变形分析和计算,并在此基础上对夹具结构进行了优化,最终确定了高精密夹具设计新方案.结果表明优化得到的设计方案优于经验设计方案,提高了夹具的定位和夹紧精度,从而保证了加工质量,在实际生产中效果良好.     

Contact condition modelling for machining fixture setup processes

Manufacturing automation often requires high precision and highly accurate equipment to achieve satisfactory performance. In many cases, precision and accuracy depend highly on the surface contact conditions between machine components. Described in this paper is the modelling of the response frequency of a modular fixturing system with multiple components in contact, and it has demonstrated that it is possible to monitor the contact conditions based on a fixture system's dynamic response frequencies. The virtual spring element based on the Hertz theory is used to recapitulate the fixture contact conditions in the finite element modelling. Possible contributing factors to contact conditions, such as material properties, surface finish, hardness and the contact area between the workpiece and fixture device, are investigated in the estimation of the virtual spring constant. Experimental analysis is also conducted to verify the analytical model. The application of this model can be used for control, monitoring, and design of general clamping devices in many manufacturing processes.