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1、Application and development Of case based reasoning in fixture designAbstract: Based on the case based designing (CBD) methodology, the fixture similarity is in two respects: the function and the structure information. Then, the computer aided fixture design system is created on case based reasoning
2、 (CBR),in which the attributes of the main features of work piece and structure of fixture as case index code are designed for the retrieve of the similar cases, and the structure and hierarchical relation of case library are set up for store. Meanwhile, the algorithm based on the knowledge guided i
3、n the retrieve of the similar cases, the strategy of case adapt at ion and case storage in which the case indent if cat ion number is used to distinguish from similar cases are presented. The application of the system in some projects improves the design efficiency and gets a good result .Keywords:
4、case based reasoning ;fixture design; computer aided design(CAD) Fixtures are devices that serve as the purpose of holding the work piece securely and accurately, and maintaining a consistent relationship with respect to the tools while machining. Because the fixture structure depends on the feature
5、 of the product and the status of the process planning in the enterprise, its design is the bottleneck during manufacturing, which restrains to improve the efficiency and lead-time. And fixture design is a complicated process, based on experience that needs comprehensive qualitative knowledge about
6、a number of design issues including work piece configuration, manufacturing processes involved, and machining environment. This is also a very time consuming work when using traditional CAD tools (such as Epigraphic, CATIA or Pro/E), which are good at performing detailed design tasks, but provide fe
7、w benefits for taking advantage of the previous design experience and resources, which are precisely the key factors in improving the efficiency. The methodology of case based reasoning (CBR) adapts the solution of a previously solved case to build a solution for a new problem with the following fou
8、r steps: retrieve, reuse, revise, and retain 1. This is a more useful method than the use of an expert system to simulate human thought because proposing a similar case and applying a few modifications seems to be self explanatory and more intuitive to humans .So various case based design support to
9、ols have been developed for numerous areas2-4, such as in injection molding and design, architectural design, die casting die design, process planning, and also in fixture design. Sun used six digitals to compose the index code that included work piece shape, machine portion, bushing, the 1st locati
10、ng device, the 2nd locating device and clamping device5. But the system cannot be used for other fixture types except for drill fixtures, and cannot solve the problem of storage of the same index code that needs to be retained, which is very important in CBR6.1 Construction of a Case Index and Case
11、Library1.1 Case index The case index should be composed of all features of the work piece, which are distinguished from different fixtures. Using all of them would make the operation in convenient. Because the forms of the parts are diverse, and the technology requirements of manufacture in the ente
12、rprise also develop continuously, lots of features used as the case index will make the search rate slow, and the main feature unimportant, for the reason that the relative weight which is allotted to every feature must diminish. And on the other hand, it is hard to include all the features in the c
13、ase index.Therefore, considering the practicality and the demand of rapid design, the case index includes both the major feature of the work piece and the structure of fixture. The case index code is made up of 16 digits: 13 digits for case features and 3 digits for case identification number.The fi
14、rst 13 digits represent 13 features. Each digit is corresponding to an attribute of the feature, which may be one of“*”, “？”, “1”, “2”,“A”,“B”, “Z”, etc. In which, “*” means anyone, “?” uncertain, “0” nothing.The system rules: fixture type, work piece shape, locating model cannot be “*”or“?”. When t
15、he system is designed, the attribute information of the three items does not have these options, which means the certain attribute must be selected. The last three digits are the case identification number, which means the 13 digits of the case feature are the same, and the number of these three dig
16、its is used for distinguishing them.The system also rules: “000” is a prototype case, which is used for retrieval, and other cases are “001”,“002”, which are used for reference cases to be searched by designers. If occasionally one of them needs to be changed as the prototype case, first it must be required to apply to change the one to “000”, and the former is changed to referential case automatically.The construction of the case index code is
