A comparative evaluation of sequential set point adjustment procedures for tolerance control

The machining of complex parts typically involves a logical and chronological sequence of n operations on m machine tools. Because manufacturing datums cannot always match design datums, some of the design specifications imposed on the part are usually satisfied by distinct subsets of the n operations prescribed in the process plan. Conventional tolerance control specifies a fixed set point for each operation and permissible variation about this set point to insure compliance with the specifications. Sequential tolerance control (STC) uses real-time measurement information at the completion of one stage to exploit the available space inside a dynamic feasible zone and reposition the set point for subsequent operations. This paper introduces an extension of STC that utilizes the variability of the operations to scale the problem data and further enhance the ability of STC to optimize the production of an acceptable part.     

An adaptive sphere-fitting method for sequential tolerance control

The machining of complex parts typically involves a logical and chronological sequence of n operations on m machine tools. Because manufacturing datums cannot always match design constraints, some of the design specifications imposed on the part are usually satisfied by distinct subsets of the n operations prescribed in the process plan. Conventional tolerance control specifies a fixed set point for each operation and a permissible variation about this set point to insure compliance with the specifications, whereas sequential tolerance control (STC) uses real-time measurement information at the completion of one stage to reposition the set point for subsequent operations. However, it has been shown that earlier sphere-fitting methods for STC can lead to inferior solutions when the process distributions are skewed. This paper introduces an extension of STC that uses an adaptive sphere-fitting method that significantly improves the yield in the presence of skewed distributions as well as significantly reducing the computational effort required by earlier probabilistic search methods.     

Process parameters determination for precision manufacturing

Conventional process planning in manufacturing operations presents fixed process means and process tolerances for all operations and allows actual outputs to be distributed around these fixed values, as long as the final outputs fall within acceptable specifications. Some approaches attempt to maximize the process tolerances of all manufacturing operations for part production. Other approaches intend to minimize the tolerance cost or quality loss based on known functions. Most of them consider process mean and process tolerance as independent decision variables in process planning, with the condition that the resultant working dimensions are equal to the design target values of blueprint dimensions. These approaches assume that there is no process drift or deterioration. However, these conventional approaches are inappropriate for small‐volume, high‐value and precision processing, particularly of a complex part. Hence this study introduces an alternative approach to the tolerance‐balancing problem that does not provide specific objective functions, which determines process means and process tolerances simultaneously and adjusts them sequentially. Copyright © 2000 John Wiley & Sons, Ltd.     

Variation transmission analysis and diagnosis of multi-operational machining processes

In a Multi-operational Machining Process (MMP), final product variation is an accumulation, or stack-up of variations generated at all the manufacturing operations. In this paper, variation transmission in the MMP is analyzed by relating part variations to operational errors from machine tools, fixtures and datums. At each operation, total part variation is separated into several components corresponding to different variation sources. The result can be applied in both process design and diagnosis. A methodology is developed to identify faulty operations. Process diagnosability is also discussed. A case study is provided to illustrate the developed diagnostic methodology.     

Process plan selection

Process planning is the task of generating a plan for transforming raw material to its finished form according to design specifications. The existence of alternative feasible process plans for a part and the manufacture of several parts in a single facility sharing constrained resources, makes careful selection of process plans essential. This paper formalizes the selection of process plans with the objective of minimizing the total processing time and the total number of processing steps. The set of plans are then consolidated to minimize the resources used. This is accompanied by intangible benefits such as simplified planning and scheduling, fewer constraints on designing the layout, and an overall reduction in the manufacturing cost. As a word of caution, it may be added that process plans have wide ranging impact on factory operations and it is unrealistic to expect a single model to be comprehensive in addressing the problem. The model outlined in the paper highlights the key issues and provides an analytical basis for selecting process plans.     

Design of delayed reconfigurable manufacturing system based on part family grouping and machine selection

To improve the convertibility of reconfigurable manufacturing system (RMS), the concept of delayed reconfigurable manufacturing system (D-RMS) was proposed. RMS and D-RMS are both constructed around part family. However, D-RMS may suffer from ultra-long system problem with unacceptable idle machines using generic RMS part families. Besides, considering the complex basic system structure of D-RMS, machine selection of D-RMS should be addressed, including dedicated machine, flexible machine, and reconfigurable machine. Therefore, a system design method for D-RMS based on part family grouping and machine selection is proposed. Firstly, a part family grouping method is proposed for D-RMS that groups the parts with more former common operations into the same part family. The concept of longest relative position common operation subsequence (LPCS) is proposed. The similarity coefficient among the parts is calculated based on LPCS. The reciprocal value of the operation position of LPCS is adopted as the characteristic value. The average linkage clustering (ALC) algorithm is used to cluster the parts. Secondly, a machine selection method is proposed to complete the system design of D-RMS, including machine selection rules and the dividing point decision model. Finally, a case study is given to implement and verify the proposed system design method for D-RMS. The results show that the proposed system design method is effective, which can group parts with more former common operations into the same part family and select appropriate machine types.     

Optimization of machining datum selection and machining tolerance allocation with genetic algorithms

In machining process planning, selection of machining datum and allocation of machining tolerances are crucial as they directly affect the part quality and machining efficiency. This study explores the feasibility to build a mathematical model for computer aided process planning (CAPP) to find the optimal machining datum set and machining tolerances simultaneously for rotational parts. Tolerance chart and an efficient dimension chain tracing method are utilized to establish the relationship between machining datums and tolerances. A mixed-discrete nonlinear optimization model is formulated with the manufacturing cost as the objective function and blueprint tolerances and machine tool capabilities as constraints. A directed random search method, genetic algorithm (GA), is used to find optimum solutions. The computational results indicate that the proposed methodology is capable and robust in finding the optimal machining datum set and tolerances. The proposed model and solution procedure can be used as a building block for computer automated process planning.     

Operational dimensioning and tolerancing in process planning: setup planning

Operational dimensioning and tolerancing play an important role in process planning. It ensures that resultant part dimensions and tolerances do not exceed specified design values. Tolerance chart analysis is an effective technique for process planners to calculate mean values and tolerances of operational dimensions. However, a tolerance chart can be built only after all the intial engineering decisions have been made concerning the process plan. Specifically, setups and setup datums should be identified. While many researchers focused their attention on tolerance chart analysis, the selection of setups and setup datums (setup planning) was overlooked. No systematic approaches for setup planning can be found in the literature. This paper discusses the importance of setup planning to tolerance control in process planning. A graphical approach is then proposed to generate optimal setup plans based on design tolerance specifications.     

Sculptured surface tolerance verification with design datums

Sculptured or free-form surfaces are widely used in many fields with extensive applications. Once such surfaces are manufactured, surface inspection compares the manufactured surfaces with the surface design specifications to verify conformance. Although significant research and development efforts have been devoted to the design and manufacturing of products consisting of partial or sole free-form surfaces, the inspection of these surfaces is still a difficult task. For many engineering applications, a free-form surface is assigned a profile tolerance with reference to design datums for assembly, functionality and other manufacturing requirements. The paper discusses developments of surface inspection techniques for profile tolerance of free-form surfaces. The concept of datum direction frame is proposed to find the transformation information that localizes measurement data to design model. The technique consists of two major steps: localization of measurement data to the design system, based on the datum reference information; and further localization based on the information from free-form surfaces. Testing examples were carried out to validate the developed techniques. The new method does not need corresponding points from the datums of the design model and measured surfaces. Therefore, it is simpler, yet more robust. It can also be used conveniently in manufacturing processes.     

Operation allocation and materials-handling system selection in a flexible manufacturing system: A sequential modelling approach

Materials-handling systems are an integrating component of manufacturing operations and as such must be considered within a common framework in manufacturing systems design. This work proposes a first approach to the simultaneous consideration of the operation allocation and the materials-handling system selection problems in a flexible manufacturing system. The objective of the operation allocation model is to select a group of machines where the operations of the part types will be performed and then to assign those operations to the selected machines. The operation allocation model interfaces with the materials-handling system selection model by providing input data in the form of the manufacturing operations to be performed at each machining centre. The selection of the materials-handling system is centred on the matching of the parts visiting a machining centre to perform a manufacturing operation and the abilities of the handling equipment to perform the required materials handling functions of those part types. The objective is to select an optimal group of materials-handling equipment to be assigned to a cell.     

Cell formation: the need for an integrated solution of the subproblems

Cellular manufacturing is a viable option in many manufacturing systems. There are various subproblems in the design of a cellular manufacturing system. These are machine group and part family formation, machine duplication, intracell layout and intercell layout. The only comprehensive design strategy that attempts to address all of these is production flow analysis. However, this technique is a sequential strategy where the subproblems mentioned above are assumed nested within each other and are solved in a forward pass with no feedback. This is a satisfactory approach only in cases where the part families are relatively disjoint and machine groups are formed without constraints on machine duplication to eliminate intercell flow. The presence of bottleneck machines and parts renders the problem considerably more complex, as the subproblems influence each other substantially. This paper presents an integrated framework for solving these subproblems by generating a limited set of feasible alternative solutions.     

Effect of Prestrain on Formability and Forming Limit Strains During Tube Hydroforming

The tube hydroforming process is a relatively complex manufacturing process; the performance of this process depends on various factors and requires proper combination of part design, material selection and boundary conditions. In manufacturing of automotive parts, such as engine cradles, frames rails, sub-frames, cross members, and other parts from circular tubes, pre-bending and per-forming operations are often required prior to the subsequent tubular hydroforming process to fit the tubular blank in the complex die shape. Due to these pre-- hydroforming operations, some of the strains are already developed before going to the actual hydroforming process. Such developed strains before hydroforming process in the part is called as prestrain. In this paper the study of effect of prestrain on formability and forming limit strains during tube hydroforming is done by simulation by taking the material prestrain value. The forming limit strains of pre-strained tube during hydroforming are predicted. A series of tube bulge tests for tube hydroforming are simulated by a commercial finite element solver to predict the limit strains. Numerical simulation of forming limit strains in tube hydroforming with different internal pressure and different simulation set up with or without axial feeding are considered to develop wide range of strain paths in the present work. The effects of process conditions on the forming limit strains are detailed. In this paper the forming limit strains during tube hydroforming are simulated for prestrain and compared with zero prestrain. Prediction of limits strains is based on a novel thickness based necking criterion.     

The impact of ignored attributes on a CMS design

The formation of part families and machine cells is an important design step in implementing a Cellular Manufacturing System (CMS). The effectiveness of this design step, for a practical CMS, depends heavily on the proper consideration of the relevant set of attributes. Generally, the relevant attribute set consists of both geometrical and manufacturing attributes of parts. Most published CMS design procedures use a very small number of manufacturing attributes for two reasons. First, precise information about relevant attributes may not be available, and second, considering an adequate number of attributes causes computational intractability. This paper identifies the sources of imprecision and offers an efficient sensitivity analysis procedure for examining the impact of ignored attributes on a CMS design. The sensitivity analysis is based on the theory of Rough Sets and assumes that an initial design has been obtained by using a computationally efficient CMS design procedure. In addition, this procedure can be used to revalidate the configuration of an existing CMS when product mix or parts design has changed. Background on the theory of Rough Sets is provided and appropriate Rough Sets measures are discussed. A numerical example illustrates the proposed use of Rough Sets theory in CMS design.     

Sequencing of robot activities and parts in two-machine robotic cells

This paper deals with the development of analytical methods for determining an optimal sequence of parts and robot activities to minimize cycle time in robotic cells with two machines. A robot is used to feed parts to the two machines or in the absence of machines two robots perform processing operations on parts. Three different types of robotic cellular layouts, namely the robot-centered cell, the mobile robot cell, and the in-line robot cell have been considered in an automated flow line manufacturing system with no storage buffers. Optimal sequences have been established for both single and multiple part types when part production is performed in a repetitive fashion. In order to keep abreast with the current trend toward just-in-time manufacturing, production of a quantity known as the minimum part set (MPS) is considered for multiple part types. The analysis for a single MPS has further been extended to include the cyclic production of multiple MPSs. The problem of determining the optimal sequence of multiple part types has been shown equivalent to a two machines no-wait flow shop problem, and has been solved by Gilmore and Gomory's (1964) algorithm.     

System for early cost estimation of die-cast parts

Early cost estimate of a part is important information and forms a basis for preparing quotations, which are competitive from a market point of view. It is seen that a commonly adopted approach of variant cost estimation based only on geometric information of the component is not always accurate. This is also true in the case of die-cast parts. The geometric complexity of the part, tooling complexity, part and tool material, processing cost, and manufacturing resources for producing the part all need to be considered for accurate cost estimation. This paper deals with a comprehensive system developed to estimate and analyze the manufacturing cost of die-cast parts. A computer-aided cost-estimation system has been developed that applies manufacturing process as well as manufacturing resource considerations in addition to part feature complexity. Use of the proposed system is demonstrated in selecting the optimum number of cavities and the appropriate manufacturing resources under machine-related constraints. Further, the cost-estimation system developed herein is used for carrying out feature sensitivity analysis to identify design features that add significant cost to the part. The use of this system for optimal machine loading in multiple parts situation is also demonstrated.     

Expert system for autonomous handling of elementary assembly operations†

This paper addresses the new conception of a real-time expert system for elementary two-arm assembly operations under the inclusion of operation-based vision. Every operation is represented by a local blackboard which contains a recognition part, a world model and a measurement part. The total set of these behaviour-oriented modules is supervised and scheduled by an operation monitor, which is again a blackboard. The local-knowledge sources of each individual blackboard receive sensor signals as input and can directly produce a reaction without going to a higher-level global-inference engine. This local, short connection between sensor signal and reflex is decided on the basis of compiled rules which have the form of production rules. The main types of these rules are regulation tasks depending on the operation and analysis rules for situation assessments. Global plans are produced as assembly graphs and transmitted in parts to the operation monitor as task specifications. A 3-D CAD modeller is used to support the object representation and recognition     

Manufacturing cell design: an integer programming model employing genetic algorithms

The design of a cellular manufacturing system requires that a part population, at least minimally described by its use of process technology (part/machine incidence matrix), be partitioned into part families and that the associated plant equipment be partitioned into machine cells. At the highest level, the objective is to form a set of completely autonomous units such that inter-cell movement of parts is minimized. We present an integer program that is solved using a genetic algorithm (GA) to assist in the design of cellular manufacturing systems. The formulation uses a unique representation scheme for individuals (part/machine partitions) that reduces the size of the cell formation problem and increases the scale of problems that can be solved. This approach offers improved design flexibility by allowing a variety of evaluation functions to be employed and by incorporating design constraints during cell formation. The effectiveness of the GA approach is demonstrated on several problems from the literature.     

Technical and industrial approaches for super plastic forming and diffusion bonding (SPF/DB) titanium alloy leading edge manufacturing

The A330/340 HTP leading edge has shifted the prior composite into a new metallic design based on a SPF/DB manufacturing process for a two sheet D‐nose. Superplastic forming and diffusion bonding were the technological means to obtain an integrated detailed part. Moreover, this component was part of the set that made introduction and development SPF/DB feasible within EADS CASA at its facilities located in Cadiz. The main driving factor to launch this program was the weight saving intended for the A330/340 horizontal stabilizer. The design concept was based on two raw material sheets (with initial nominal thicknesses of 3 mm and 0.8 mm) forming skins and spars. The skin was shaped into an aerodynamic nose and the flat spar was also stiffened by swages. It was diffusion bonded to the above mentioned skin by means of two flanges. The final shape is denominated by the term D‐nose, due to the resulting geometrical configuration. The initial manufacturing process was based on a two step press forming process, with part integration as the main goal. This process was robust enough to achieve structural parts with industrial tolerance ranges. Since tighter tolerances ranges and higher than expected ramp up arose, creative improvements were set up to meet the program expectations. A single press forming phase was implemented, resulting in a lower press labour time and a higher quality product. Further increased mass production rates challenged the industrial means to reach higher efficiency. A compromise between part integration, costs and industrial means best practices has shown how detail parts assembly constitutes also a cost effective solution under specific industrial conditions. This article describes the advantages and drawbacks of three alternative SPF manufacturing processes to fabricate superplastic titanium alloys leading edges, which have been validated for mass production purposes.     

Additive Design and Manufacturing of Jet Engine Parts

The additive design (AD) and additive manufacturing (AM) of jet engine parts will revolutionize the traditional aerospace industry. The unique characteristics of AM, such as gradient materials and micro-structures, have opened up a new direction in jet engine design and manufacturing. Engineers have been liberated from many constraints associated with traditional methodologies and technologies. One of the most significant features of the AM process is that it can ensure the consistency of parts because it starts from point(s), continues to line(s) and layer(s), and ends with the competed part. Collaboration between design and manufacturing is the key to success in fields including aerodynamics, thermodynamics, structural integration, heat transfer, material development, and machining. Engineers must change the way they design a part, as they shift from the traditional method of “subtracting material” to the new method of “adding material” in order to manufacture a part. AD is not the same as designing for AM. A new method and new tools are required to assist with this new way of designing and manufacturing. This paper discusses in detail what is required in AD and AM, and how current problems can be solved.     

An optimal cart moving policy for a flexible manufacturing system

A flexible manufacturing system is composed of many stations such as a load/unload station, a set of workstations, and a common buffer, that are linked together with a material handling system. Each workstation consists of a limited input buffer, a single machine and a limited output buffer. The material handling system consists of a single cart moving parts in the system according to the process paths required by the parts. A part is blocked when it is moved to a workstation but cannot enter the workstation. The function of the common buffer is to temporarily store blocked parts. A blocked part is treated in accordance with a flexible manufacturing system blocking mechanism. We model the flexible manufacturing system by a closed queueing network with the flexible manufacturing system blocking mechanism and a block-dependent static Markov part routing. An optimal cart moving policy that maximizes the expected system throughput is formulated as an undiscounted semi-Markov decision process. Several properties of the optimization problem are characterized. A loop approach is developed for finding an optimal policy. An example is given to illustrate the methodology, and investigate its convergence.