They reported that the committee on Shape Tolerance Resource Model within the Standard for the Exchange of Product Model Data STEP part 47 is attempt-ing to define an unambiguous represe
Trang 1and so on Guilford and Turner [1992] identified some of the deficiencies in the tolerancing models proposed by researchers prior to the year 1990 They reported that the committee on Shape Tolerance Resource Model within the Standard for the Exchange of Product Model Data (STEP part 47) is attempt-ing to define an unambiguous representation of tolerances compatible with the ANSI Y14.5 and the ISO
1101 family of standards They identified a problem that exists in identifying locations and directions while defining tolerances and data In STEP, these are represented by Cartesian vectors, but the problem
of locating the part in the co-ordinate system exists Guilford and Turner [1992] modified the approach employed by STEP in order to overcome the problems For example, STEP describes the direction along which the straightness tolerance is measured as a vector in the global co-ordinate system, while the authors described it by using some virtual geometry entities attached to the actual geometry of the part The authors have discussed the representation which covers almost all of the ANSI Y14.5 tolerances except for items such as knurls, gears, and screws; equalizing data, free state variations, conical position tolerance zones, and position tolerances on elongated holes
Feature Data Exchange Mechanisms
Standards for Exchange of Product Data (STEP) is an international standard (designated as ISO 10303) that deals with the computer interpretable representation and exchange of product model data The intent is to provide a neutral interface which is capable of describing all the life-cycle properties of a given artifact independent of the CAD platform used for product modeling This will also serve as a basis for implementing and sharing product databases and archives The various parts of ISO 10303 are divided into the following categories: description methods, application protocols, abstract test suites, implemen-tation methods, and conformance testing
Application protocols (AP) provide a basis for developing implementations of STEP (ISO 10303) and abstract test suites for conformance testing of Application Protocol (AP) implementations AP 224 (devel-oped by TC184/SC4/WG3) is a part of the application protocol category that defines the context, scope, and information requirements of producing mechanical product definition for process planning appli-cation, and it directs the integrated resources necessary to satisfy these requirements These requirements specify items such as part identification, shape, and material data, necessary for product definition The basic premise of AP 224 is that the process planning function will be greatly assisted by identifying machining features present on the part model Knowledge about the machining features will help in proper identification of machining equipment, tooling, and processes to manufacture a part AP 224 provides a schema for representation and exchange of part feature information
ISO 10303-AP 224 employs two ways to represent the shape of part features: implicit shape represen-tation and explicit shape represenrepresen-tation The explicit shape represenrepresen-tation is specified by using a B-Rep (boundary representation) scheme The implicit shape representation is specified by defining parameters (attributes) associated with each type of feature Currently, three basic types of features are employed in
AP 224, namely, machining features (such as hole, groove, boss, thread, etc.), replicate features (such as circular pattern, rectangular pattern, etc.), and transition features (such as chamfer, fillet, and rounded edge) Compound features (user defined features) can be created by the union of one or more machining features The technical content of AP 224 provides good coverage on part features and associated attributes However, it can be extended in scope (though the actual approval process needs input from representatives of several countries) to include some of the following issues
• Multiple part mechanical parts as opposed to single piece mechanical parts
• Inclusion of features produced by manufacturing processes other than turning and milling
• Interacting features and feature relations deemed critical from the process planning standpoint
• Multiple “viewpoints” of features
• Support other CAD representation schemes than just the B-Rep scheme
• Support “redesign” product development by providing part retrieval mechanisms
• Provide definition of commonly used catalog parts such as nuts, bolts, gear, etc
© 2001 by CRC Press LLC
Trang 23 Flexible Factory Layouts: Issues in Design, Modeling,
and Analysis
3.1 Introduction
3.2 Literature Review
3.3 Flexible Layout Configurations
3.4 Measuring Layout Flexibility
Geometry-Based Measures • Flow-Based Measures
3.5 A Procedure for Flexible Layout Design
Solution Procedure • A Heuristic Approach • Flexible Layout Selection • Software Implementation and Analysis
3.6 Conclusion
In this chapter, we address several issues related to design, modeling, and analysis of flexible factory layouts We present a framework for defining and identifying sources of layout flexibility and for mapping different dimensions of flexibility to specific layout configurations We use this framework to develop several potential measures of layout flexibility and compare the usefulness and limitations of each We also examine the relationship between factory layout and material handling and show that the realization
of layout flexibility largely depends on the configuration of the material handling system Finally, we present an integrated procedure for flexible layout design in stochastic environments We use the pro-cedure to highlight the desirability of disaggregating functional departments into smaller subdepartments and distribute them throughout the factory floor We show that increased disaggregation and distribution can be, indeed, effective in enhancing a layout’s ability to cope with variability
3.1 Introduction
In today’s volatile and competitive environment, manufacturing facilities must be designed with enough flexibility to withstand significant changes in their operating requirements The shortening of product life cycles and the increased variety in product offerings require that facilities remain useful over many product generations and support the manufacturing of a large number of products Because the prolif-eration of products makes it exceedingly difficult to produce accurate forecasts of demand volumes and demand distribution, facilities must be able to rapidly reallocate capacity among different products without major retooling, resource reconfiguration, or replacement of equipment Furthermore, increased emphasis on customer satisfaction places simultaneous requirements on shorter manufacturing lead
Saifallah Benjaafar
University of Minnesota
© 2001 by CRC Press LLC