Chapter 1 Introduction

Transcription

1 Chapter 1 Introduction This chapter presents an introductory background to the research work undertaken, the problem statement, research issues involved and objectives of the research work. The contribution of this research work to the body of knowledge is highlighted briefly. The organization of the thesis is also described at the end of the chapter. 1.1 Background Increased product variety, better product quality, reduced production volumes, and pressure from customer to reduce unit costs and shorten lead times are some of the challenges faced by industry in recent times. Adapting to flexible manufacturing systems is one of the responses given by industry to this situation. In any kind of manufacturing system, fixturing technology is a key element. Fixturing methodology adapted in product design cycle has a great bearing on the product quality and development costs. Flexible fixturing is a necessary component of flexible manufacturing system. Flexible fixture hardware and fixture design and analysis software are necessary elements of reconfigurable (flexible) fixturing technology. Reconfigurable fixturing technique converts existing fixture designs to new ones in order to meet changed product requirements in short time. This technique involves minimum changes in hardware and eliminates extensive testing requirements. (Rong, 2004) The cost of designing and fabricating the fixtures in a manufacturing system is estimated to amount to 10-20% of the total product cost. This includes the cost of fixture hardware, fixture designing and fixtures assembly. There are two approaches to reduce the cost of fixturing as follows: (Bi and Zhang, 2001) a. Developing flexible fixture systems (FFS), which deals with the hardware component of fixturing system, and b. Simplifying the designing process (design automation), which deals with application of computers for design and verification of fixturing systems. 1

2 Different flexible fixturing systems such as modular fixtures, adjustable fixtures for part family, fixture with phase change materials, and programmable clamps are studied by researchers for their suitability in manufacturing processes. (Rong, 1999) The details are discussed in Chapter 2. In spite of a number of flexible fixturing systems developed so far, modular fixtures and modularized adjustable fixtures are two main types of flexible fixturing systems commonly used in production. Modular fixtures consist of a set of modular components or elements. These elements perform the functions of locators, clamps, supports and fixture body. These components can be assembled to build fixtures capable of handling wide variety of part shapes and sizes. (Hrgrove 1994) Adjustable fixtures are single fixtures which consist of certain functional components that can be adjusted in position or quickly changed to meet different shape and size requirements. Although modular types of fixtures are used commonly in industry, the potential application areas are limited. This is because of high initial cost, skilled manpower requirements for design and assembly of modular fixtures, and lack of specific fixturing performances such as fixturing stiffness, compact spacing, long life of fixturing components, easiness of fixturing operations and the insignificant benefits from the reuse of fixtures. Because of these limitations, modular fixtures are not particularly suitable for small and medium scale industrial units. On the contrary, adjustable fixturing technology is more suitable for small and medium scale industrial units. By adapting to adjustable fixturing technology, a typical small/ medium scale unit can achieve a number of benefits in product development cycle, such as rapid product development and reduced inventory costs. Similarly, development of adjustable fixtures into modularized and standard designs is expected to increase advantages of flexibility, as well as specified fixturing performances. (Rong, 1999; 2004). However, relatively small numbers of adjustable fixture systems are commonly used by industry. Moreover, benefits obtained by Indian industry are not reported. Second important aspect of flexible fixturing system is the deployment of Computer Aided Fixturing Design (CAFD) for design and verification of fixturing systems. CAFD is defined as application of computers in different aspects of fixture design such as setup planning, fixture planning, fixture element design, and fixture evaluation. 2

3 Three types of CAFD methodologies are investigated commonly by the researchers. Firstly, knowledge-based expert systems are examined for the selection of locating methods, fixture elements, and fixture configuration design and optimization. Second approach examines automatic fixture design based on kinematic analysis and a series of design rules. Third approach utilizes successful knowledge available in existing fixture designs to generate new designs.(rong, 1999) Computer Aided Fixture Design (CAFD) methodologies are applied separately and in fragmented form to the above mentioned aspects of fixture design. There is no integrated and comprehensive fixture design methodology reported so far. (Chen 2007) The CAFD methodologies that are based on the kinematic analysis and rule based optimization techniques can only deal with simple parts with regular shapes. However most of the industrial workpieces are complex in shape and size. Rule-based expert systems make use of existing rules for fixture design, and function relatively well for an initial model. This methodology provides appropriate conceptual designs which can be further used. However, these systems are inflexible and can not improve themselves over a period of time. Detailed fixture design can not be generated through rule-based expert systems. In such situation Case-Based Reasoning approach may prove to be particularly useful. (Sun 2007) Fixture design process is highly experience-dependent. There is a potential for application of Case Based Reasoning (CBR) in fixture design process. Case based reasoning is a methodology that solves new problems by using knowledge and experience from previous cases (Maher 1997). A small number of researchers have investigated fixture design methodology using CBR approach. However, most of the reported work is limited to the deployment of case representation, case indexing and case retrieval in fixture design. Investigations in deployment of case modification and case verification in fixture design are reported scarcely. (Chen, 2006) The research reported so far is only in the area of modular fixturing. There are no reported applications of case-based reasoning in the area of adjustable fixture design. The above-mentioned points form a basis for the motivation of this research candidate into the present inquiry. The prevailing status of flexible fixturing technology and 3

4 trends in CAFD methodology augment a need for a systematic and scientific inquiry into this body of knowledge. It is this background on which the agenda of this particular research inquiry is set. 1.2 Problem Definition On the background discussed in the previous section, an extensive literature review is taken (findings of this review are presented in Chapter 2). In the light of the status of the body of knowledge in fixturing technology, the problem statement of the research is defined as follows: In order to face challenges of global competition in manufacturing, it is essential to cut down the costs and lead times associated with product development. With advancements in CAD/CAM technology and CNC machine tools, one of the promising sources to improve product development performance is to deploy flexible fixturing methodology. There is a need to adopt flexible fixturing system (for example, adjustable fixturing system for part families), with appropriate CAFD methodology. It is required to quantify and investigate this need and critically examine issues related to development and performance of flexible fixturing systems. The need of flexible fixturing system for part family is justified with following points: 1. Unsuitability of dedicated fixtures for contemporary manufacturing systems, 2. Unaffordable cost of modular fixturing systems, 3. Increased setup time required with prevailing fixturing systems, 4. Blocked inventory in fixtures and their accessories, 5. Delayed product development cycle and sluggish response to changing market requirements. As would be shown in the subsequent chapters, this need is felt severely by the small and medium scale manufacturing ancillary industry in India and such community would be benefited by the findings of the present research inquiry. 4

5 1.3 Research Issues A thorough review of the literature published in the field of the proposed research is conducted (i.e. development of flexible fixturing systems and Computer Aided Fixture Design). From the review, it is found that there exist some research gaps in the above mentioned field of knowledge. These gaps are summarized in the form of research issues as follows: 1. In spite of different flexible fixturing systems reported in the published literature, modular fixtures remain the most commonly investigated fixturing systems. However, studies addressing issues related to adjustable fixturing systems are very small in number. 2. Modular fixtures have certain limitations i.e. high initial costs, skilled manpower requirement, and lack of specific fixturing performance. However, contemporary research does not investigate alternative solutions to these problems. 3. Adjustable fixtures for part family combine advantage of both dedicated and modular fixtures, and are especially beneficial in batch production and mass customization. There is a scope in developing adjustable fixture design systems considering part families as units. 4. The part classification and coding system for part family formation for adjustable fixture system is not reported. Also the fixture similarity analysis of adjustable fixturing system is lacking. 5. The standardized and modularized design data for adjustable fixtures for different part families are not summarized and generalized by designer community. Such summarization and generalization may enhance the ability of adjustable fixture design methodology to take advantage of flexibility of manufacturing systems. 6. Computer Aided Design methodology for designing adjustable fixtures for part families are not investigated and reported. This establishes a need of development of a software application for rapid design and verification of fixture design with variations. These issues are discussed at length in Chapter 2 (Literature Review). 5

6 1.4 Research Objectives In the light of the research issues highlighted in section 1.3, the overall objective of this research is set to examine suitability of CBR methodology for adjustable fixture design process and to assess feasibility of development of an integrated fixture design methodology. The specific objectives set are: 1. To propose a suitable flexible fixture design system for manufacturing of parts with large variety and low volumes. 2. To develop a generic classification and coding system for workpiece with fixturing features and develop the similarity coefficient algorithm for the analysis of part family and adjustable fixture family. 3. To assess and select a methodology suitable for Computer Aided Adjustable Fixture Design System (CAAFD). 4. To define task elements of CAAFD those are required for a part family. 5. To develop generic design heuristics for adjustable fixture design for selected part families. 6. To develop a prototype Computer Aided Adjustable Fixture Design (CAAFD) System for a given part family based on Case-Based Reasoning methodology. 6

7 1.5 Proposed Work and Methodology The proposed work and the research methodology is explained in following sections, Proposed Work 1. To establish the need of application of flexible fixturing system and Computer aided Adjustable Fixture Design System (CAAFD) through literature review and a survey of small and medium scaled industries. 2. To study and collect different variety of workpieces to represent the part families and their corresponding adjustable fixtures information. This will be further utilized for building the library of database of workpieces, fixture elements and fixture assemblies. The same can be used to generate and develop the knowledge base of standardized and modularized adjustable fixture design data base suitable for part family. 3. To establish the criterion for part family formation for adjustable fixtures and form the basis of similarity analysis for workpieces and fixtures. 4. To develop the Expert System for Adjustable Fixture Design based on Case-Based Reasoning including the following steps: a. Case representation and case indexing b. Case retrieval by adjustability index (Proposed by research) c. Case modification d. Case verification e. Case storage. 5. To develop the CAAFD software and implement the same with case study. 7

8 1.5.2 Methodology In order to carry out the work mentioned above a generic methodology is formed which is given in the flow diagram as shown in Figure 1.1 Decide the broad area of the research Literature Review in the Area of Research Identify the Research Issues and Set the Research Objectives Define the Problem Statement Identify the need through the Literature review and Survey Study of Variety of Workpiece Part Families and Corresponding Fixtures to Build the Database Review Different Expert Systems applied to Fixture Design and Select the Appropriate one for the area of research. Development of the CAAFD Architecture Development of the prototype CAAFD Software Implementation with Case Study Validation of the Methodology Figure 1.1 : Methodology 8

9 1.6 Scope of Work As discussed in the research issues, an effort is made to generate a knowledge base for standardized and modularized adjustable fixture design data base for part family. The workpieces to be processed on VMC are considered and their corresponding fixture design options are summarized. This study focuses on building an overall framework of Computer Aided Adjustable Fixture Design System (CAAFD) for part families through subtasks such as: (i) Case Representation and case memory organization in fixture design domain (ii) case indexing and case retrieval mechanism, (iii) case modification by adjustment and case verification. The case verification includes interference analysis and fixture stability analysis. Other tasks of fixture design such as tolerance analysis and assembly automation are not examined in this study. As adjustable fixture design methodology is suitable for product-based production and mass customization hence the focus of proposed CAAFD is on these two production systems. Therefore design considerations for job and mass production are not included in this study. The scope of this work is limited to adjustable fixture design for the components to be machined on HMC and VMC machines. Components processed on conventional machines are not considered in this study. 1.7 Contribution of the thesis The contribution of this research is categorized into three broad areas: (i.) Contribution to the body of knowledge of the selected field of research (ii.) Methodological contribution (iii.) Implications to the industry and the design professionals. 9

10 1.7.a System Level This study as a whole creates a framework for CAAFD expert system with Case Based Reasoning. The database of workpiece, fixturing elements and adjustable fixture units is created. A survey is conducted to study the potential of application of Adjustable Fixtures for Part Family and Computer Aided Fixture Design System in small and medium scale industries. Based on the study, part family classification with fixturing features and workopiece adjustability criterion for adjustable fixture selection is suggested which is further expandable. 1.7.b Theoretical Level The classification and coding system for workpiece with fixturing features is developed. The concept of adjustability of a workpiece with fixture is developed. Adjustability Index is defined which is further used for the retrieval of most suitable fixture case for modification. Parametric case modification is used for assembly modification of adjustable fixtures. The fixture elements such as locators, clamps and supports are modeled as point model with specific ID number, which is the least possible representation of any entity. These models are further utilized for the interference analysis and stability analysis. A coordinate based rapid interference checking algorithm is developed. Criterion for stability analysis is also discussed and implemented. 1.7.c Implementation Level Algorithms for case indexing and case retrieval based on absolute similarity, relative similarity and for case modification are developed. An algorithm for case verification for interference analysis is also developed and stability analysis is carried out. An interactive prototype expert system (CAAFD) based on case based reasoning (CBR) for adjustable fixture design is developed, through VB-6 having interface with 3D modeling software CATIA V5-R16 of DASSAULT SYSTEMS. 10

11 1.8 Organization of thesis. The thesis is organized into five sections as shown below: Part I : (Chapter 1 and 2) Introduction and Literature Review - Chapter 1 introduces the background, problem definition and objectives of the research, the research issues of the CAAFD system, and the key technologies applied in this research, the proposed work and methodology as well as the scope of the research. - Chapter 2 gives review of the flexible fixturing systems and CAFD technologies, the existing state-of-the-art has been compared and summarized according to their applied technologies. The critical appraisal is presented at the end. The details of the survey of industries are discussed and the findings of the same are presented. Part II : (3 to 5) Research Methodology and Theoretical Development - Chapter 3 explains the research objectives and proposed methodology. - Chapter 4 describes the theoretical developments of the research. It explains the part family formation for workpieces, the development of classification and coding for workpiece and adjustable fixtures, the criterion for similarity coefficient and adjustability index. The issues related to the application of CBR in fixture design are discussed. The developments in the different phases of CBR such as Case Representation, Case Indexing, Case Retrieval and Case Modification as applicable to adjustable fixture design are presented. The proposed point model of fixturing element is presented. Based on this model, a coordinate based rapid interference checking method is developed and the same is discussed. - Chapter 5 Worked Examples - describes the practical applications of the theoretical development carried out in the chapter 4. 11

12 Part III : (Chapter 6) System Implementation and Validation - Chapter 6 describes the detailed software design, information flow and stages of implementation in the prototype CAAFD. The validation strategy and the results are presented at the end. Part IV : (Chapter 7) Summary and future work - Chapter 7 gives summary of the research, contributions, limitations and future scope. Part V : References and Appendices 12