基于极值法与统计公差法的不同尺寸公差分配方法的研究

对装配体的尺寸公差分配是产品公差设计中的一项重要环节。公差的大小决定着产品的制造成本,对装配尺寸公差进行合理分配具有重要意义。然而在两种不同的公差分析方法下其各种公差分配也有所不同。文章介绍了目前装配尺寸公差分配的主要方法,有等公差法、等精度法和最优化分配法等。结合齿轮轴工程实例,分别运用极值法和统计公差法对各种尺寸公差分配方法进行分析研究。其结果表明,在实际运用中利用装配功能公差链统计公差法约束条件并考虑加工方法的经济性公差约束条件对装配尺寸公差进行分配为最优化分析方案。并将其集成到计算机公差设计系统中。     

工艺公差表示方法及其在稳健设计中的应用

公差在机械零件设计、制造中占有重要地位。公差常用表示方法有极值公差、统计公差。本文在分析极值公差、统计公差优缺点的基础上引出工艺公差概念，它可以统一表达极值公差、统计公差，着重对零件加工状态进行公差控制，在输入、输出方差传递变动分析中能够灵活应用，是公差稳健设计的重要工具。     

基于功能表面的公差表示模型研究

为实现概念结构与公差并行设计,提出了基于功能表面的公差表示模型。以功能表面、功能表面最小基准元等概念为基础,结合面向对象的思想,将现有的公差类型进行合理分类,并对类属性进行了深入研究。经实例验证,该模型能够表示功能表面的公差信息。     

考虑几何公差的设计与工序公差并行分配研究

设计与工序公差并行分配大多数集中于尺寸公差的并行设计上,较少考虑几何公差。研究的目标是对并行公差模型进行扩展研究,以便考虑几何公差要求。首先,根据几何公差各自的特点,将其转换为等效尺寸公差或仅作为附加加工约束处理;然后,以期望质量损失和制造成本之和最小为目标函数,以产品功能约束、几何公差约束和经济工序边界约束为约束条件,对并行公差分配模型进行扩展,并采用非线性规划技术对扩展后的模型进行求解以获得优化的设计公差和工序公差值;最后,通过滚轮装配的并行公差优化分配实例验证该方法的有效性。     

Pro/E注射模尺寸公差自动查询及标注系统开发

使用Pro/E进行模具零件设计时,经常需要标注尺寸公差,而Pro/E系统本身没有注射模公差标注的标准规范,尺寸公差的标注需要人工查手册得到各种模具零件的常用公差并查出该公差的上下偏差值最后手工输入。针对这个问题,分析了模具零件公差标注的特殊需求,利用Pro/Toolkit对Pro/E进行二次开发。设计出自动查询和标注注射模零件尺寸公差的软件,实现了模具零件公差的智能查询和标注,同时允许用户根据需要定制自己的公差标准。     

基于凸包域的三维公差分析模型研究

为了更好地解决复杂装配体的公差分析问题,文章提出了基于凸包域的三维公差表达和公差分析方法。以凸包理论为基础,建立了尺寸公差和形位公差域模型;对公差域内的变动特征进行PDF的建模,构建偏差累积方程;根据公差域模型和PDF对蒙特卡罗仿真方法进行修正;运用修正的蒙特卡罗方法和HDMR对偏差累积方程进行敏感度和贡献因子分析。与传统的基于点的建模方法相比,基于凸包的建模方法能真实的体现几何特征的变动与公差标准的契合关系。最后通过实例验证了该方法的可行性和精确性。     

基于修正矩阵的刚柔混合模型公差分析方法

为了在设计阶段进行公差分析、结构优化,文中讨论了刚柔混合模型的装配偏差,提出了一种基于修正矩阵的公差分析方法。将刚柔混合模型的理想位姿、公差域表示为矩阵形式,借助齐次变换进行公差累积。使用有限元模拟装配变形,将仿真结果表示为修正矩阵,对原有的偏差累积路径进行修正,从而实现了对刚柔混合模型关键尺寸的精确预测,为设计优化奠定了基础。以汽车车灯装配为例,验证了该方法的可行性。     

Pro/E环境下尺寸公差自动查询及标注软件的开发

针对在Pro/E环境下进行尺寸公差标注,需人工查手册,且过程繁琐、费时费力的问题.利用Pro/Toolkit程序开发技术,在VC 6.0环境下对Pro/E进行二次开发,并且用Access2000将国标公差做成数据库,设计出了一套公差自动查询与标注软件.应用该软件,用户不必再通过查手册获得偏差数值,只需选择相应的尺寸并选择公差代号和公差等级,即可标注出符合国标的各种尺寸公差,并且可以方便地调整尺寸位置和修改参数,实现了尺寸公差的自动查询和公差的快速自动标注.     

基于公差原则的装配公差统计分析

为了提高机械产品的互换性,减少制造成本,公差原则在零件设计时有着广泛的应用,但是在现有的装配公差统计分析方法中却较少考虑零件的公差原则。为解决此问题,基于雅克比旋量理论建立了考虑公差原则的三维装配公差模型,并结合蒙特卡洛法对装配体进行统计公差分析,期间在用旋量区间表示特征变动时并考虑了旋量间的约束关系,使公差分析结果更加符合实际情况。最后以简易顶尖尾座装配体为实例,对比了零件应用不同公差原则时的公差分析结果,验证了所述方法的有效性。     

机床夹具公差的灰色综合设计理论与方法

根据灰色聚类综合评价的原理和方法，对影响夹具公差选择的多种因素进行了数据处理，建立了夹具公差选择的灰色聚类综合评价模型，为定量确定夹具的公差提供了有效的方法。     

An offset finite element model and its applications in predicting sheet metal assembly variation

Dimensional variation in sheet metal assembly affects product fit and functionality. To predict the variation of assembled products, variation simulation analysis has been gradually adopted in the early design stage. However, variation simulation based on rigid body assumptions usually results in over estimation of the assembly variation. In this paper, we propose an offset beam element model for predicting the assembly variation of deformable sheet metal parts joined by resistance spot welding. The purpose of using the offset beam element is to include the shear effect provided by resistance spot weld nuggets that cannot be captured by the conventional beam element. The offset element is then applied to predict sheet metal assembly variation for one-dimensional (1D) models extracted from industrial practice. The first example evaluates the effects of sheet metal thicknesses on assembly variation. The second example shows how the assembly sequence affects assembly variation. These models provide interesting insights into the mechanisms of variation stackup and will lead to the understanding of more complex sheet metal assemblies.     

A parametric study of joint performance in sheet metal assembly

In sheet metal assembly, joints are designed to facilitate welding the parts and to form various shapes and dimensions of sheet metal products. The most commonly used joints are lap joints, butt joints and butt-lap joints. Different joint configurations have different performance characteristics. One of the performance criteria is the level of dimensional variation in the assembly. This paper presents a parametric study of the dimensional variation characteristics of the three basic joints. An offset finite-element-model-based variation simulation methodology is used in the parametric study in order to take into consideration possible part deformation. This parametric study helps determine the circumstances under which part variation or tooling variation will contribute more variation to assembly variation. These results provide improved understanding of sheet metal assembly processes and provide data for robust product/process design.     

Wavelets-based method for variation analysis of non-rigid assemblies

This paper proposes a new method based on wavelets analysis and finite element method (FEM) for the variation analysis of non-rigid assemblies. It is well known that the part fabrication variation, coupled with the part's deformation during the assembly process, is one of the main factors affecting the assembly quality. But little investigation has been done on how component variations with different scales contribute to the final dimensional variation of non-rigid assemblies. The proposed approach takes the part variation as a signal and applies wavelets transform to decompose it into different scale components. The deformation of non-rigid assemblies that corresponds to these different scale components is calculated by using FEM. Since the part variation is resulted from manufacturing, manufacturing engineers can apply this method to get valuable information to avoid major variation causes in manufacturing process and make a better process plan. The proposed method is illustrated through a case study on an assembly of two flat sheet metal parts.     

Modeling and Control of Compliant Assembly Systems

The assembly of compliant, non-rigid parts is widely used in automotive, aerospace, electronics, and appliance manufacturing. Dimensional variation is one important measure of quality in such assembly. This paper presents models for analyzing the propagation of dimensional variation in multi-stage compliant assembly systems and the use of such models for robust design and adaptive control of assembly quality. The models combine engineering structure analysis with advanced statistical methods in considering the effect part variation, tooling variation, as well as part deformation due to clamping, joining and springback. The new adaptive control algorithm makes use of the fine adjustment capabilities in new programmable tooling in achieving reduction of assembly variation.     

Employing fractals and FEM for detailed variation analysis of non-rigid assemblies

Many studies on non-rigid assemblies, or assemblies of non-rigid components, suggest that the component variation affects the assembly dimensional quality. However, little is known about how the variation of surface micro-geometry of assembly components influences the assembly dimensional quality. In this paper, a new method based on the fractal geometry and finite element method (FEM) is proposed to study such an influence. In the new method, a special fractal function, named the Weierstrass–Mandelbrot (W–M) function, is used to extract and represent the characteristics of surface micro-geometry of assembly components. FEM is applied to analyze the deformation of non-rigid assemblies by integrating the variation of component micro-geometry. The sensitivity matrix between the component variation and assembly variation is obtained by using the existing influence coefficients method. It is found that contributions of the surface micro-geometry of assembly components to the final variation of non-rigid assemblies could be substantial under certain conditions. The proposed method is illustrated through a case study on an assembly of two flat sheet metal components under different fixture-releasing conditions.     

Defect susceptibility of sensile strength to microporosity variation in as-cast magnesium alloys with different grain sizes

The variability in the tensile strength of as-cast AM60 and AZ91 alloys was investigated in terms of the defect susceptibility to the variation in grain size and microporosity. The microporosity was measured from the quantitative fractography analysis through scanning electron microscopy (SEM) observation on fractured surface after tensile test. The ultimate tensile strength (UTS) of both alloys can be characterized as a power law relationship to microporosity variation in terms of the defect susceptibility and maximum strength achievable in the defect-free condition. The defect susceptibility of tensile strength to microporosity variation is decreased remarkably with grain refinement. The defect susceptibility of AZ91 alloy to microporosity variation exhibits more sensitive dependence on the variation in grain size than AM60 alloy. Also, the dependence of UTS on the variation in grain size is described as a power law relationship for various levels of microporosity. The variation on effective void area fraction by the damage evolution of Mg₁₇Al₁₂ phase may introduce a practically significant decrease of load bearing capacity, less than by microporosity variation. The Hall-Petch relation of both alloys in the defect-free condition could be suggested as maximum values of friction stress and locking parameter.     

Stability of milling processes with continuous spindle speed variation: Analysis in the frequency and time domains, and experimental correlation

Up to now, the theory for analysis of continuous spindle speed variation in milling processes was developed for sinusoidal variation only, and for average tooth passing frequency an exact multiple of speed variation frequency. This paper presents the general theory for analysis in the frequency domain and for any speed variation strategy. Results are compared with those obtained by semidiscretization and time integration, as well as with those obtained by experiments. The discrepancies of the results obtained by the different approaches are discussed, and the analysis of the evolution of the stability along the speed variation period is proposed.     

基于轮廓尺寸变动量的三道次机器人滚压包边角度路径设计

采用试验方法,研究机器人滚压包边的滚边角度路径对轮廓尺寸变动量的影响。结果表明,三道次滚压包边过程中,前两道次滚边导致尺寸缩进,终滚边导致尺寸胀出;第一道次滚边和终滚边的尺寸变动较大,第二道次尺寸变动较小;各道次尺寸变动量数值主要由该道次零件初始角度决定,与该道次滚边角关系较小。文章提出了滚压包边角度路径设计方法,为实际生产中滚边工艺规划提供了一定的指导。     

Experimental studies of cutting force variation in face milling

The purpose of this paper is to present a developed cutting force model for multi-toothed cutting processes, including a complete set of parameters influencing the cutting force variation that has been shown to occur in face milling, and to analyse to what extent these parameters influence the total cutting force variation for a selected tool geometry. The scope is to model and analyse the cutting forces for each individual tooth on the tool, to be able to draw conclusions about how the cutting action for an individual tooth is affected by its neighbours.A previously developed cutting force model for multi-toothed cutting processes is supplemented with three new parameters; eccentricity of the spindle, continuous cutting edge deterioration and load inflicted tool deflection influencing the cutting force variation. A previously developed milling force sensor is used to experimentally analyse the cutting force variation, and to give input to the cutting force simulation performed with the developed cutting force model.The experimental results from the case studied in this paper show that there are mainly three factors influencing the cutting force variation for a tool with new inserts. Radial and axial cutting edge position causes approximately 50% of the force variation for the case studied in this paper. Approximately 40% arises from eccentricity and the remaining 10% is the result of spindle deflection during machining. The experimental results presented in this paper show a new type of cutting force diagrams where the force variation for each individual tooth when two cutting edges are engaged in the workpiece at the same time. The wear studies performed shows a redistribution of the individual main cutting forces dependent on the wear propagation for each tooth.     

An examination of the effect of variation in datum targets on part acceptance

The use of datum targets within Geometric Dimensioning and Tolerancing (GD&T) is commonplace in industry today. Datum targets are used primarily for manufacturing and inspection setups where it is not practical to have tooling that contacts the entire surface of the datum. Establishing a simulated datum from datum targets may be done physically or mathematically. In the process of establishing a simulated datum, there are several possible sources of variation. Any variation or error in establishing the datum will manifest itself as variation or error in the measurement of the distance between the feature and the datum. This paper examines the effect of variation in datum targets on part acceptance using both mathematical analysis and simulation. The focus of this examination is diametric positional tolerance of holes that reference the datum established by datum targets. A three-dimensional mathematical model of the hole position error with respect to the datum target variation is developed. Then, simulation is adopted to identify the major sources of hole position error due to errors in datum targets. It is found that the datum target variation has a profound effect on the measured location of holes referencing those datums. Explanation and validation for accepted practice are offered and some additional basic guidelines are developed for the placement of datum targets relative to hole patterns.