Determining dimensions for process planning: A backward derivation approach

Computer-aided process planning has shown its great importance in computer integrated manufacturing systems. Dimensioning and tolerancing plays a key role in process planning since the final workpiece must ensure conformance with the dimensions in its blueprint. This paper proposes a simple, efficient approach to derive working dimensions for process planning, based on the tolerance chart technique. The paper first reviews briefly some approaches previously presented, such as the mathematical linear equation approach and the graph tracing approach, it then proposes the backward derivation approach. By setting up the origin of a coordinate system at the left-most surface in a blueprint (B/P), the approach first determines the distances from all the B/P surfaces to the origin by use of the B/P dimensions. Then by updating these dimensions, the final working dimensions are calculated backwards by a simple arithmetical equation.     

Product and process dimensioning and tolerancing techniques. A state-of-the-art review

Dimensioning and tolerancing are important engineering processes in the different phases of a product development cycle. The two main phases in a product cycle where dimensioning and tolerancing techniques are extensively employed are in the areas of product design and process planning. Tolerance and dimension assignment in both product design and process planning has an equally important role in keeping the production cost down and, hence, requires equal attention as far as research into these areas are concerned. Another important motivating factor for research is that manual dimension and tolerance assignment is often tedious, time-consuming and requires a considerable amount of skill and experience on the part of the engineer, resulting in inconsistencies and errors. Extensive research, in the area of dimensioning and tolerancing in both product design and process planning, has been carried out with the advancement in computers since the 1970s. The purpose of this paper is to review the state-of-the-art dimensioning and tolerancing techniques in both product design and process planning and explore the opportunities for future research in these areas.     

A graph theoretic approach linking design dimensioning and process planning

The purpose of this paper and its companion (Part 2) is to present a rigorous graph theoretic model to link designing with process planning. This paper shows how to generate process plans from design dimension trees. It is the foundation for the companion paper, Part 2, which shows how part dimensioning (during designing) can be guided by knowledge of the datum-hierarchy tree structure underlying process plans. Design dimensions are represented as a design dimension tree, which is the basis for defining an ideal (optimal) datum-hierarchy tree of a process plan. The ideal datum-hierarchy tree, in turn, is used to define measures of process planning efficiency. These measures can be utilised to compare actual process plans and improve manufacturing processes. An example is presented to illustrate the concepts and method.     

Automatic datum dimensioning for plastic injection mould design and manufacturing

Datum dimensioning (or ordinate dimensioning) technique is very popular in plastic injection mould drawings where the location dimensions of a large number of hole features must be specified in the drawings of the mould plates. Although commercial CAD/CAM systems provide semi-automatic tools to assist the designer in the dimensioning process, it is still a very tedious process, as the user has to specify the location of each dimension tag. This paper reports a completely automatic method where optimal placements of the dimension tags can be determined. The method employs dynamic programming technique to optimize the dimension process with respect to several criteria that can be selected by the user. The method has been implemented and incorporated into a commercial CAD/CAM system, and examples are given to illustrate the important features of the program.     

Optimum assembly using a component dimensioning method

Many tolerance allocation methods based primarily on cost optimisation have been developed in recent years. However, the dimensions of the assembly parts are usually determined by convenience or design constraints other than the requirements of the assembly itself. The dimensions are often determined to give sufficient clearance during assembly without much consideration given to their tolerances. This is usually followed by a tolerance analysis to check for interference. The dimensions are then adjusted to eliminate any interferences. There is no control over the size of the clearance of an assembly.This paper presents a new approach of dimensioning the components of an assembly which would allow real control over the size of clearance or interference in an assembly. A unique algorithm in conjunction with a relationship matrix is developed to formulate the dimensions in a tolerance chain into a linear equation. This linear equation is then used to develop the tolerance analysis module. The dimensioning module is accomplished using a separate algorithm in conjunction with the linear equation established. The tolerance analysis and the dimensioning modules are subsequently tested on a number of examples.     

工程图形自动标注尺寸的图论方法

有效自动标注工程图形尺寸是CAD领域长期以来未较好解决的一个复杂问题。应用图论方法,从图形中提取由线段和连接点组成的路,研究路中线段和连接点的相互关系,生成关于线段的二分图,既可有效实现尺寸的自动标注,又能直观得出应标尺寸总数。     

Process capability-based tolerance design to minimise manufacturing loss

In this paper we propose a nonlinear programming model, composed of a cumulative standard normal probability function and manufacturing cost, to design process tolerances. The tolerances are standardised in conjunction with the process capability of the machinery to minimise the total manufacturing loss that occurs owing to non-conforming parts. The proposed model has been applied to a workpiece manufactured through ten operations and solved by GINO (General INteractive Optimizer). A comparison between the results obtained by the proposed model and other methods indicates that robust process tolerances can be obtained by the new formulation.     

基于机器视觉的机械零件图像尺寸自动标注方法

机械零件图像二维几何尺寸自动标注是机械零件视觉检测系统的重要组成部分.对机械零件图像二维几何尺寸自动标注进行研究,提出一种在MATLAB环境下基于机器视觉的直线段、圆和圆弧尺寸自动标注方法.通过分析MATLAB中的坐标关系,进行坐标转换,再根据机械零件图像的二维几何特征,推导出对应的尺寸标注公式,并给出该方法的具体实施步骤,最后通过实验验证了该方法的可行性和有效性.     

利用形体特征表达式辅助机械制图中的尺寸标注

为了解决机械制图中尺寸标注容易漏标、重复标注的现象,结合形体分析法和当今CAD/CAM技术的特征造型思想,使用形体分析表达式辅助机械制图中的尺寸标注,使尺寸标注更方便直观表达,便于检查,尺寸标注更准确。实践表明,这种方法具有简单、方便、实用的特点。     

Determining the process tolerances based on the manufacturing process capability

An efficient approach has been developed for determining the process tolerances. This approach simplifies the traditional procedures of tolerance chart balancing and takes into account the capability of the process producing the workpiece. A mathematical model, based on the modified rooted tree chart, has been established. By using the model, the process tolerance can be obtained effectively and accurately without the effort of finding the additional tolerances. The work done previously by Ngoi is examined and several drawbacks are identified. A comparison between the results generated by the proposed method and by Ngoi is also made.Notation d _i additional tolerance to be added - f _j jth functional relationship betweent _i andd _i - m the number of blueprint specifications - n the number of operations - r _i the failure rate - estimated standard deviation ofith operation - T _i initial tolerance - TB _j blueprint tolerance - TP _i upper limit of process tolerance - _i process tolerance for operationi - x _i standardised process tolerance - w _i weighting value - Z the minimum value amongx _i      

Simultaneous process mean and process tolerance determination with asymmetrical loss function

Process design involves process mean and process tolerance determination. Process mean determination is finding the best settings for product quality, without affecting manufacturing cost. However, process tolerance determination is a manufacturing process selection which generally affects manufacturing cost and product quality. In this study, asymmetric quality loss is considered in measuring product quality, and tolerance cost is adopted in representing manufacturing cost. To reflect the combined effect of process mean and process tolerance completely, failure cost is added to the category of manufacturing cost. Then, based on the sum of these three costs, a simultaneous optimization of process mean and process tolerance is determined for process planning in the early stages of development.     

Computer-aided geometrical dimensioning and tolerancing for process-operation planning and quality control

This paper describes an extension of a model which determines an optimum set of dimensions and tolerances for machining processes at minimum manufacturing cost. This optimisation minimizes the cost of scrap, which is a function of manufacturing tolerances, as the objective function. Requirements of design sizes, geometrical tolerances (including both form and position) and machining allowances are expressed mathematically as constraints for the optimisation. A computerised trace method has been extended to determine the relationships between geometrical tolerances and associated relevant manufacturing dimensions and tolerances. In addition to the manufacturing cost, the model takes into account manufacturing sequence, distribution of manufacturing dimensions, process capabilities, tolerances, design sizes, geometrical tolerances, machining allowances and optimum scrap level. The resulting computerized interactive system can be used not only in process planning, but also in quality control.     

Applying linear programming to tolerance chart balancing

This paper explores an important aspect of tolerance charting, i.e. tolerance chart balancing. A mathematical model for tolerance chart balancing, based on the “rooted tree” representation technique, has been established. The mathematical model, which represents a resource allocation problem, is then solved by using the linear programming approach. An example is used to illustrate the method. A comparison between the results generated by the proposed method and the manual method is also included.     

A synthetic control chart for monitoring process dispersion with sample range

A synthetic control chart for monitoring the changes in the standard deviation of a normally distributed process is proposed in this paper. The synthetic chart consists of the sample range (R) chart and the conforming run-length (CRL) chart. The R chart can be viewed as a special case of the synthetic chart. The operation, design and performance of this chart are described. Average run- length comparisons between other procedures and the synthetic chart are presented. It indicates that the synthetic chart is a good alternative for monitoring process dispersion. The variable sampling interval (VSI) schemes, as an enhancement to the synthetic chart, are discussed to further improve the chart performance. An example is presented to illustrate the application of synthetic chart and its VSI scheme.     

计算机辅助求解工艺尺寸链

计算机辅助工艺尺寸链的分析与解算是CAPP中不可或缺的一个环节。针对工艺尺寸链计算中工艺基准与设计基准不重合的情况,开发了一套简单的计算机辅助工艺尺寸链换算系统,并通过实例分析说明计算机辅助建立和计算工艺尺寸链的方法和步骤。该系统增加了实时绘图功能,具有友好的用户界面,进一步提高了尺寸链的解算速度和质量。     

Graphical approach to tolerance charting—A “maze chart” method

This paper presents a simple graphical method to represent the process links between surface planes, and leads to ease in performing the validity of a process plan. The approach used the linear optimisation software, LINDO, to solve the respectively linear working dimension and manufacturing tolerance equations. An example is also included to illustrate the method.     

Concurrent process tolerance design based on minimum product manufacturing cost and quality loss

In a concurrent design environment, a robust optimum method is presented to directly determine the process tolerances from multiple correlated critical tolerances in an assembly. With given distributions of multiple critical assembly dimensions, the Taguchi quadric quality loss function is first derived. The quality loss is then expressed as the function of pertinent process tolerances. A nonlinear optimal model is established to minimize the summation of manufacturing costs and product quality loss. An example illustrates the proposed model and the solution method .     

Optimal Process Tolerance Balancing Based on Process Capabilities

An optimal approach for process tolerance balancing is presented. The new approach is based on process capabilities and is to be used in the stage of process planning. A nonlinear programming model is used to simultaneously optimise process tolerances of required operations. In the optimisation model, the objective function is to minimise the total manufacturing cost with different weighting factors. Using the estimated standard deviations of the dimensions and the manufacturing cost-tolerance functions, the constraint equations for the process tolerance chains and the manufacturing capability indices are established, together with a model for the economical tolerance bounds of machine tools. A practical example was used to verify the usefulness of the proposed approach. The results of the comparative study show that the proposed approach is more advantageous in relaxing tolerance requirements and in reducing scrap rates generally, compared with the existing methods. ID="A1"Correspondance and offprint requests to: Dr Y. Gao, Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: meygao@ust.hk     

A process net model approach for multiple process plans

A process planning system that generates alternative process plans offers multiple process plans for a part, thereby providing the flexibility to cope with changes in shop floor status. In this paper, we introduce the concept of a process net as a model for the generation of alternative process plans. We also show the usefulness of the process net model in generating alternative process plans by implementing the developed system to construct process nets, and devising an algorithm to generate alternative process plans for rotational parts.