Study of an assembly tolerance allocation model based on Monte Carlo simulation

The traditional design methods of assembly tolerance allocation are usually based on engineers' experience, or the worst on worst tolerance analysis (WOW) method, or the root sum square tolerance analysis (RSS) method. However, the above-mentioned methods, whilst used frequently in the analysis of a single-dimensional chain, are not suitable for the analysis of geometrical tolerance and multi-dimensional chains. Also, the relationship between tolerance and manufacturing cost is not considered and a suitable tolerance allocation based on minimum manufacturing cost can not be obtained. Some research works have applied linear or non-linear programming methods to optimize the tolerance allocation of each part in an assembly. However, the convergence of the solution is not ensured. The purpose of this study is to provide an integrated approach, including tolerance design, manufacturing cost analysis and multiple chains consideration, using the Monte Carlo method to optimize the tolerance allocation with minimum cost. The Monte Carlo method, a statistical simulation method, was used to simulate the dimension variance of each part and each dimensional chain. The contribution percentage of each part on each dimensional chain was calculated. Tolerance cost was chosen as an object function and the tolerance allocation model as a constraint condition, the optimum tolerances of each part being obtained by the iteration method. Results of computer simulation for several examples were compared with published data for demonstrating the feasibility of the proposed method. It can be concluded that the tolerance-allocation model combined with a tolerance-cost relationship can provide a very practical and useful approach for design engineers.     

A rule-based system for angular tolerance charting

Different manufacturing processes and their sequences have different effects on tolerance build-ups. An oversight on the effect of tolerance build-up often leads to rework and excessive scrap. This paper describes a rule-based system for developing angular tolerance charts. Mathematical equations for computing the mean working dimensions and their tolerances for typical manufacturing processes are presented. The procedures involved in calculating the mean working dimensions, mean stock removal and their tolerances are shown. The general rule-sets adopted to compute the above are also presented.     

The application of tool deflection knowledge in process planning to meet geometric tolerances

Machine tool deflections due to cutting forces can result in dimensional errors on workpieces. The problem is most severe when flexible tools such as end mills are used. When dimensioned features are specified with tolerances, process planning should examine the compromise between achieving high productivity rates and meeting dimensions within the specified tolerances. The use of geometric dimensioning and tolerancing permits interaction between size and position and makes bonus tolerances available. The errors occurring in end milling are first examined and modelled using regression methods. A procedure is proposed for selecting optimal feed rates that ensure that tolerances can be met. The process is demonstrated in machining a slot using the down milling mode. The use of a tolerance analysis chart clarifies the results of the test in relation to the tolerance standards. The need to consider the transient errors at the exit of the cut is demonstrated.     

基于矩阵的工艺尺寸链分析与解算

文章基于尺寸链原理,借助一个机械加工实例,详细分析了工艺尺寸式方程的建立、查找及解算过程;并在此基础上引入基本尺寸矩阵方程和公差矩阵方程,采用Matlab语言编写计算机程序.系统完整地解决了机械加工过程中涉及到的所有尺寸及公差的计算机解算问题,明显地提高了尺寸链的解算速度和质量.     

挖掘机行走马达正反向输出扭矩一致性技术研究

针对某型号挖掘机液压行走马达正反向输出扭矩不一致问题展开研究,分析得出马达正反向输出扭矩不一致的主要原因为马达正反向进出口压差偏差大,而导致压差偏差大的主要因素为马达正反向回油平衡阀阀口尺寸公差的不一致性。建立马达平衡阀阀口尺寸链模型,分析平衡阀阀芯、弹簧座、平衡阀阀头及端盖割槽尺寸公差对平衡阀阀口尺寸公差的影响。通过CFD仿真及阀口各组成环尺寸链优化设计,结合零部件实际加工难度进行公差分配,使马达回油平衡阀阀口尺寸保持在0.5~0.7 mm,从而缩小马达背压变化范围。试验结果表明：优化后的马达平衡阀口尺寸公差使马达进出口压差保持一致,从而保证了马达正反向输出扭矩的一致性。     

电力机车电机风罩冲压工艺与模具设计

分析了电力机车某电机风罩零件的结构特点,计算了该风罩的展开尺寸,确定了该风罩的冲压成形工艺及各工序尺寸,并对全套模具（共4副）的总体结构设计进行了比较详细的论述。     

Dimensional Relevance Shifts in Category Learning

A category learning experiment involving human participants compared the difficulties of four types of shift learning. Initial learning was of an exclusive-or (XOR) structure on two of three stimulus dimensions. One shift type was a reversal, a second shift was to a single previously relevant dimension, a third shift was to a single previously irrelevant dimension, and a fourth shift was to an XOR on one previously relevant dimension and one previously irrelevant dimension. Results showed that reversal shift was easiest, followed, in order, by shift to a single previously relevant dimension, shift to a single previously irrelevant dimension, and a shift to a new XOR. An extended version of the ALCOVE model, called AMBRY, qualitatively fits the data. The model incorporates two essential principles. First, internal category representations that can be quickly remapped to overt responses are important for accounting for the ease of reversal shift. Second, perseverating dimensional attention is important for accounting for the ease of shifting to a previously relevant dimension as opposed to a previously irrelevant dimension. It is suggested that any model of these effects will need to implement both of these principles.     

铸钢件下芯和合箱过程尺寸控制的研究

本文主要以下芯前砂芯和砂型尺寸测量以及下芯、合箱尺寸调整为例,结合多年的铸钢件生产经验介绍了铸钢件生产中下芯、合箱过程的尺寸控制要求以及对工艺、模型芯头在造型/制芯过程的操作要点。阐述了通过造型、制芯工序生产出符合工艺要求、尺寸合格的砂型、砂芯,在合箱工的装配下,组合成合格的浇注铸型的过程。对提高大型铸钢件产品的尺寸精度有一定的借鉴作用。     

注射模成型零件工作尺寸计算方法分析

通过对2种注射模成型零件工作尺寸计算方法的理论分析 ,指出了采用平均收缩率法存在的理论缺陷和实际模具设计中存在的问题 ,对极限公差法计算模具成型尺寸和公差作了分析和实例举证 ,说明了该计算方法能够较好地保证在注射模设计中成型尺寸和公差计算的正确性。     

Relative influence of injection molding processing conditions on HDPE acetabular cups dimensional stability

The effects of processing conditions on the characteristics of injection-molded high density polyethylene (HDPE) acetabular cups for the total hip replacement implants have been studied. The study has been focused on the effects of filling rate (injection speed), packing pressure, packing time and hot nozzle temperature of the mold on the dimensional stability, weight variation and visual appearance of the acetabular cups. A considerable influence of these processing conditions on the weight and dimension variation was found. Information regarding cycle time duration was obtained through numerical simulation of the entire molding process using a commercial dedicated code Moldflow Plastics Insight 6.1 (Moldflow Corporation, 2007). Within the influencing process parameters studied, the packing pressure was found to be the most significant one affecting the part dimensions and weight. An increase in the injection speed (decrease in the injection time) had a negative effect on the weight and on the most of the part’s analyzed dimensions. The influence of the hot nozzle temperature and packing time was considered irrelevant, for the range of process parameters under analysis. For all the set-point variation tests, two singularities on the linear dimension variations were identified: anisotropic shrinkage in flow and normal to flow direction and a slight plastic expansion at the location around the hall furthest away from the injection point.     

刹车盘尺寸和形位公差自动检测研究

针对典型汽车刹车盘尺寸和形位公差的设计制造要求,提出一种采用自动化仪器检测刹车盘尺寸和形位公差的方法。分析了激光位移传感器的工作原理及其刹车盘厚度的检测方法,提出自动化检测机的传感器布置形式。设计了自动化检测机的机械结构、电气系统、操作流程,推导了直径、厚度、平面度、平行度、跳动等主要检测项目的计算公式,为刹车盘尺寸和形位公差的检测提供自动化解决方案。     

后弹性支座成形模设计

分析了后弹性支座零件的成形工艺及毛坯展开尺寸计算 ,并介绍了成形模具的结构特点及工作过程     

工艺因素对桥梁支座消失模铸件尺寸的影响

采用消失模铸造工艺生产铁路桥梁支座,可以减小加工余量,降低综合成本。指出了铁路桥梁支座生产过程中,白模模样材料的不同、模样干燥时间、干燥温度以及浇注时负压的变化等因素都会对铸件的尺寸产生影响。保持这些工艺参数的一致性,才能将铸件的尺寸控制在要求的公差范围以内。     

薄壁管管端成形的有限元分析

针对管端成形过程中金属塑性流动引起的轴向伸缩及卸载回弹导致的成形精度问题,研究金属的塑性流动和回弹对管端成形的影响。以不锈钢卡压式直通管件为研究对象,运用LS-DYNA的有限元显式算法和隐式算法对薄壁管管端成形进行数值模拟,得到管材的金属流动分布和卸载后的回弹结果,并进行了实验验证。研究表明,管端扩孔和缩孔中,在变形急剧处,金属出现一定程度的累积,壁厚有较明显地增加。金属的流动对管端的径向尺寸影响可以忽略,但是对管端的轴向尺寸影响较大,需对坯料进行补偿;管端卸载后的回弹尺寸影响成形的精度,需通过模具进行补偿。对金属塑性流动和回弹进行有限元分析,能有效地指导管端成形的模具设计及优化加工工艺,极大缩短产品开发和模具设计周期。     

三坐标测量机在超硬材料制品检测领域的应用

三坐标测量机是一种高精度的自动化测量设备,检测范围广泛。不仅适用于常规产品的尺寸公差、形位公差检验,而且也适用于复杂工件以及复杂尺寸的测量,同时也可应用于产品的逆向开发,为我们在日常的产品设计和制造过程中提供了重要的技术资料。我们通过列举在实际工作中的几个测量事例,说明三坐标测量机在超硬材料行业中已经发挥着重要作用,为我们的科研生产和产品开发提供技术支持。      

A theoretical and experimental analysis of variances in weld bead morphologies

Predictable and reproducible weld bead morphologies and dimensions are a major concern in welding. In bead-on-plate welding, the heat flow is controlled by the heat source parameters (power, speed, and radius) and the physical properties and dimensions of the workpiece, especially its thickness. Complex models that account for weld pool circulation have been developed to quantify welds. However, to some extent, fluctuations in weld dimensions can be explained with conduction models of moving Gaussian heat sources. In early investigations with point and line heat sources, relationships between process parameters and plate thickness were derived to differentiate between two- and three-dimensional heat flow. To date, the heat source radius (R) has not been taken into account. The dimensionless ratio (D*) of the plate thickness (D) to the heat source radius (R) is actually a variable to consider. With the introduction of additional dimensionless parameters (*)—speed (v*), power(q*)— relationships among governing variables, heat flow dimension, and weld bead dimensional fluctuations can be derived. Weld bead fluctuations are found to depend on dimensionless variables (v*, q*, D* ) and occur when the heat flow dimension is intermediate between two- and three-dimensional. Occasionally, experimental data exhibit trends that differ from predictions. This article presents a dimensionless version of a heat flow model and discusses the discrepancies between experimental and theoretical results. Formerly with ECN.     

Analysis–synthesis of dimensional deviation of the machining feature for discrete-part manufacturing processes

Dimensional deviation analysis has been an active and important research topic in mechanical design, manufacturing processes, and manufacturing systems. This paper proposes a comprehensive dimensional deviation evaluation framework for discrete-part manufacturing processes (DMP). A generic, explicit, and transmission model is developed to describe the dimensional deviation accumulation of machining processes by means of kinematic analysis of relationships between fixture error, datum error, machine tool geometric error, fixturing force inducing error and the dimensional quality of the product. The developed modeling technology can deal with general fixture configurations. In addition, the local contact deformations of the workpiece–fixture system are determined by solving a nonlinear programming problem of minimizing the total complementary energy of the frictional workpiece–fixture subsystem in machining system. Moreover, the deviation of an arbitrary point on machining feature can be also evaluated based on a point deviation model with prediction dimension deviation from the transmission model. The dimensional error transmission within the machining process is quantitatively described in this model. A systematic procedure to construct the model is presented and validated. This model can be also applied to process design evaluation for complicated machining processes.     

多层压配组合冷挤压凹模的优化设计

讨论多层压配组合冷挤压凹模的优化设计 ,推导了多层压配组合冷挤压凹模各层的应力和预紧力公式 ,并论证了屈服点分别在各层的内表面上 ;分别建立了在给定 1～ 3层组合凹模的外半径的条件下 ,以提高组合冷挤压凹模的承载能力为目标的最优化设计数学模型 ,以及在满足凹模的承载能力的条件下 ,以减少凹模的结构尺寸为目标的最优化设计数学模型 ;通过优化计算 ,得到了 1～ 3层组合凹模承载能力与最小凹模外半径等关系曲线。     

凸、凹模工作部分尺寸的自动计算及其图形的绘制和标注

介绍了利用ObjectARX2000和VC 6．0对MDT6．0进行二次开发，可以自动判断凸、凹模工作部分磨损后尺寸的变化情况，从而可以选择相应的计算公式自动计算其尺寸和公差，并可以根据计算出来的尺寸自动完成凸、凹模的绘图工作及尺寸和公差的标注。大大减少了计算和绘图的工作量，从而缩短了模具设计周期，提高了模具设计效率。     

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 热轧无缝钢管经调质处理后,与轧态相比各尺寸参数均发生变化,导致下道加工工序无法正常进行或无法满足交货技术条件。通过对不同规格的样管在现场进行跟踪测量各项尺寸参数,分析数据后表明,对不同规格的钢管,热处理工序对尺寸变化的影响程度不同。轧制尺寸公差的合理控制能抵消因热处理而引起的附加尺寸变形,在轧制小规格无缝钢管时,壁厚按负公差控制,外径按公差中线控制;在轧制大口径厚壁无缝钢管时,外径应按负公差控制,壁厚按正公差控制,能使成品钢管各项尺寸参数达到最终标准要求。