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.     

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.     

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.     

Optimisation of machining fixture layout under multi-constraints

Fixture layout including locator layout and clamp layout plays a vital role in fixture performance. Current researches have been focused on fixtures for components with regular geometry, the results of which are not always applicable for components with complex geometry. This paper presents a methodology of fixture layout optimisation carried out in consideration of the repeatability, immobility and stability of fixturing. The locator layout is optimised based on the criteria of workpiece repeatability and location accuracy. The clamp layout optimisation offers a method to minimize the magnitude of the clamping force. The procedure of clamp optimisation is (1) to find the feasible clamping position in terms of the immobility requirement; (2) to get the best clamp among the feasible clamping positions under constraint of the stability. The optimal fixture layout is considered in a global range and is especially suitable for the workpiece with complex surfaces. A fixture layout design for the aerofoil of a turbine blade is analysed as an example, and the optimal fixture layout is demonstrated.     

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.     

Error compensation in machine tools — a review: Part I: geometric,cutting-force induced and fixture-dependent errors

Accuracy of machined components is one of the most critical considerations for any manufacturer. Many key factors like cutting tools and machining conditions, resolution of the machine tool, the type of workpiece etc., play an important role. However, once these are decided upon, the consistent performance of the machine tool depends upon its ability to accurately position the tool tip vis-à-vis the required workpiece dimension. This task is greatly constrained by errors either built into the machine or occurring on a periodic basis on account of temperature changes or variation in cutting forces. The three major types of error are geometric, thermal and cutting-force induced errors. Geometric errors make up the major part of the inaccuracy of a machine tool, the error caused by cutting forces depending on the type of tool and workpiece and the cutting conditions adopted. This part of the paper attempts to review the work done in analysing the various sources of geometric errors that are usually encountered on machine tools and the methods of elimination or compensation employed in these machines. A brief study of cutting-force induced errors and other errors is also made towards the end of this paper.     

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.     

Error compensation in machine tools — a review: Part II: thermal errors

Accuracy of machined components is one of the most critical considerations for any manufacturer. Many key factors like cutting tools and machining conditions, resolution of the machine tool, the type of workpiece etc., play an important role. However, once these are decided upon, the consistent performance of the machine tool depends upon its ability to accurately position the tool tip vis-à-vis the required workpiece dimension. This task is greatly constrained by errors either built into the machine or occurring on a periodic basis on account of temperature changes or variation in cutting forces. The three major types of error are geometric, thermal and cutting-force induced errors. Geometric errors make up the major part of the inaccuracy of a machine tool, the error caused by cutting forces depending on the type of tool and workpiece and the cutting conditions adopted. This part of the paper attempts to review the work done in analysing the various sources of geometric errors that are usually encountered on machine tools and the methods of elimination or compensation employed in these machines. A brief study of cutting-force induced errors and other errors is also made towards the end of this paper.     

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。     

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.     

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.     

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。     

压缩机上外罩制作的工艺实施

为了制作合格的上外罩零件,对其结构、成形工艺难点和工序内容进行了分析,确定了落料拉深、冲凸台、车边、冲孔、整形、压印等8道制作工序.结合冲压模具设计的知识和经验,设计了1副复合模和各单工序模,并对每1道制作工序和各模具工作过程进行了详细的阐述.为了保证车边工序的质量,设计了1副车边夹具,并对夹具的工作原理进行了详细说明...     

转向架轴箱专用机床及夹具设计

对转向架轴箱的加工方案进行分析,设计钻孔专用机床。为了实现转架轴箱体一次装夹完成4-12 mm孔的加工,设计一套液压专用自动夹具。实践证明:该夹具结构简单,操作方便,加工过程中实现了对工件的快速定位装夹,提高了生产效率。     

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.     

Model-based form error compensation in the turning of thin-walled cylindrical parts

The turning of thin-walled, hollow, cylindrical parts is often not possible when using the common hydraulically operated 3- or 4-jaw chucks. This is due to the deformation of the part by the clamping force of the chuck. Even low clamping forces cause elastic deformations of the clamped workpiece, which results in surface errors due to machining after the unclamping and elastic recovery of the part. For this reason, several approaches have been adopted in the past in order to quantify and minimise the clamping-induced deformations. In this paper a model-based error compensation for thin-walled, cylindrical parts in turning is presented as a universal solution. In this work, a compensation by different models is designed and realised by an intelligent, adaptive and interchangeable turning tool holder with integrated sensors and actuators.     

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.     

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。     

纵向旋转切削加工对环形零件畸变的影响

通过旋转试验和有限元分析介绍了工件在切削加工过程中产生的畸变情况，分析了工件的装夹方式、切削速度、切削深度和进刀量对100Cr6钢环圆度的影响。通过去应力退火释放冷加工诱发的残余应力后工件的圆度与切削参数有关。另外测试了被试验环的表面残余应力，其表面残余应力与装夹方式有关。将测量的装夹力作为计算参数输入，通过有限元分析方法测试了装夹方式对工件变形的影响。协同测量结果示出了装夹方式影响工件变形的一个主要因素，表面残余应力与工件的径向变形有关，最大的拉伸应力位于夹口位置。旋转切削试验结果表明，提高切削速度圆度会稍有增加；随着切削深度的加大，圆度呈下降趋势，尽管切削力增加了；进给量的增加会导致更高的切削力，因此圆度值也增加；常规的去应力退火可使被加工环的圆度值增加。     

基于层叠式夹持的夹具失效形式分析

目的 为了解决超薄蓝宝石晶片的双平面加工问题,确定层叠式夹具基盘及限位片的材料,并对限位片的失效形式进行分析.方法 通过分析层叠式夹具中工件在双平面加工中的受力状态及传统双平面加工工件受力状态,确定限位片的受力状态.测量蓝宝石与基盘间的摩擦力对基盘材料进行选择,通过受力分析结合摩擦因数计算限位片的剪切强度,对限位片的材料进行初步选择.在平面抛光机上进行加压试验,对限位片的失效形式进行分析.结果 层叠式夹具在双平面加工中受到工件施加的力小于传统双平面加工行星轮受到的力.在3种基盘材料中,不锈钢材料与蓝宝石晶片间的摩擦力较大,铸铁次之,铝合金最小.液滴在2个表面间形成的液膜对不锈钢和铸铁的摩擦因数有一定的增益效果.基盘选择不锈钢材料,限位片选择玻璃纤维板材料的情况下,限位片所承受的加工压力随着夹持厚度的增加而呈现非线性增加.限位片的主要失效形式表现为限位区域被蓝宝石晶片的边缘切割,受基盘及蓝宝石平面度的影响.结论 层叠式夹具对材料强度的要求更低,更加适用于超薄平面零件的双平面加工.限位片失效受基盘高度差的影响,为保证限位片的夹持效果,应尽量降低基盘表面的高度差.