铣床半轴夹具设计【铣φ38端面】【说明书+CAD】
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Journal of Materials Processing TechnologyJournal of Materials Processing Technology 128（2002）7-18：15101515Elsevier Science B.V.Knowledge model as an integral way to reuse the knowledge for fixture design processCharles W. BeardsleyAbstract:The fixture design is considered a complex process that demands the knowledge of different areas, such as geometry, tolerances, dimensions, processes and manufacturing resources. Nowadays, the fixture design process is o-riented to automated systems based on knowledge models. These models describe the characteristics and relationships of the physical elements together withthe inference processes that allow carrying out the activity of fixture design. With the employment of the knowledge models, besides the automation, it ispossible to systematize and structure the knowledge of the fixture design process.With the use of specific methodologies, as the knowledge template, it ispossible to reuse the knowledge represented in a model, for its use in a different design process. The knowledge template represents a pattern that defines the common entities and inference processes to use in the design process .In this work, with the use of knowledge template we propose the reuse of the knowledge described in the design process of fixtures for machining to other types of fixtures uses like inspection, assembly or welding. 2005 Elsevier B.V. All rights reserved.Keywords: Knowledge model; Knowledge template; Fixture designPage 91. IntroductionThe continuous challenge that involves the knowledge representation hasoriented to many different research groups to develop methodologies that describe stages for capture and representation of the knowledge in design and manufacturing systems 13. This has allowed to define knowledge models as a tool that helps us to clarify the structure of intensive knowledge and information-processing tasks. In this sense, a knowledge model provides a specification of the data and inference processes required by the system of study 4. A first approach in the development of knowledge models applied to machining fixtures design process has been proposed by Hunter 5.During the last decade, the use of modelling techniques has allowed usto represent the fixture design process employed in some manufacturing operations, such as machining, assembly and inspection, etc. 6. Due to the complexity and the wide scope of the fixture design process, different researchgroups have been focused in the analysis of specific activities of this process,such as fixture configuration, tolerance analyses, stability and accessibility.A great number of investigations has taken in consideration the way inwhich represent the knowledge used in the fixture design process. These researches are focused in the documentation of the design parameters, the structuring of the information of the fixture and the description of the fixture elements used in fixture design 2,7. On the other hand, the implementation of the knowledge used in the fixture design can be classified regarding the artificial intelligence technique(AI) used 8,9 and on the automation level of t-he design system2.However, whatever it is the artificial intelligence technique used and the automation level of this type of systems, the process of knowledge modelling in the fixture design is important for several reasons: the need to specify the concepts used in the fixture design; to establish the relationships amongdifferent knowledge groups; to develop knowledge based systems (KBS), and finally, to provide a conceptual base for reusing the knowledge. In this sense, the entities and structures defined in a knowledge model for design process of machining fixtures can be partially reused to develop new models for fixture design process, as the inspection or assembly fixture. The entities and structures reused has been defined using the method of knowledge template4.The work presented is a detailed proposition of the knowledge model for machining fixture design and the definition of the knowledge groups that can be reused in the inspection fixture design process, using the knowledge template method. Fig. 1 presents a general view of the contents of this ex Fig. 1. The structure of the work. planation.2. Present state of fixture design process knowledge modellingThe fixture concept arises from the need to establish a physical connection between part, and tool, and part and machine-tool. This connection should fulfil some requirements for support the machining operation to carry out. The mainly functionality of the fixture is to support, locate and clamp the part to the machine tool. However, in order to interpreting correctly the needed knowledge for develop the fixture design process, it is necessary to define the basic information related with this process according to the classification exposed in Table 1.All this information has been represented in models that describe the entities, attributes and relationships between each knowledge group in the fixture design process. The definition of these models can be carried out using methodologies that describe the activities to capture, represent and reuse the knowledge of a design system, for example MOKA and CommonKADS.The MOKA methodology is based on the definition of two models.These models allow to capture and to structure the knowledge of a system. The first model, uses a group of forms (ICARE: Illustrations, Constraints, Activities, Rules,Entities) that allow to capture and to represent the knowledge in a semistructured way; the second model allows to represent knowledge ina structured way, using the extension of UML 10.The CommonKADS methodology proposes the use of tools and techniques to carry out the capture and representation of the knowledge. In the case of reusing knowledge, CommonKADS proposes the use of the knowledgetemplate: a knowledge template is a piece of a knowledge model, in which the data and reasoning processes can be employed in the development of other applications 3.3. Methodology for development a knowledge model: structural modelThe methodology proposed for development of a knowledge model includes the realization of two stages. The first stage represents the knowledgeof the objects like part geometry, machining process, functional and detailed fixture design,and fixture resources (see Table 1). The second stage describesthe inference process (design and interpretation rules) needed to obtain a firstsolution for the machining fixture. These two stages allow to describe the structural model and behaviour model of the objects that compose the knowledge model for machining fixtures. This work is focused mainly in the descriptionof the structural model for machining fixture design process. Table 1Knowledge group for machining fixtureKnowledge group CharacteristicsPart geometry Geometry: holes, slots, etc.DimensionsTolerancesMachining process Type of machining process Machining phase and sub phaseMachining operationFunctional and detailed Methodology of designfixture design process Design rulesInterpretation rulesDesign constrainsFixture resources Type of fixture (modular, general, or dedicated)(functional elements Type of fixture elements (support, locate, clamp)and commercial elements) Type of machine tool (vertical milling machine, horizontal milling machine, etc.)The proposed structural model contains a general structure of the knowledge groups related with the fixture design process. The description of theaspects related with the knowledge groups are presented in Table 1. Fig. 2 shows the general structure of the knowledge groups for machining fixture design process. Due to the complexity of the fixture design process, the fixture design cannot be considered as an independent process with respect to the manufacturing process. In this sense, the information of the manufacturing process isdirectly represented in the fixture design process. In a similarway, the resources involved in the manufacturing process have a narrow relationship with the fixture resources, in terms of machine tools and commercial elements of fixture. The definition of each knowledge groups (see Table 1) has taken intoconsideration the attributes and necessary operations to represent the knowledge relative to these knowledge groups. The applications of these models are presented in the following sections.Fig. 2. Structural model for machining fixture.4. Knowledge template modelIn this section, we describe those pieces of the knowledge model thatcan be reused in other applications using the method of knowledge template.From a conceptual point of view, a knowledge template describes a piece of the knowledge model in which the inference and the knowledge tasks are defined with the objective of reuse this knowledge in other similar applications. In this sense, it is necessary to distinguish among the analytic and the synthetic tasks. The analytic tasks define the classification of the objects involvFig. 3. Knowledge tasks based on the structural model.ed in the fixture design process. The synthetic tasks have relationship with the reasoning way procedure from which a fixture solution is obtained.Using these two types of tasks, a first approach has been established to define the knowledge groups that can be classified under the analytic and synthetic tasks. Fig. 3 shows the objects of the structural model that describe the analytic and synthetic tasks of the machining fixture design process.The division of these two tasks allows to set in a first level the knowledge groups, that it objects and attributes that can be employed in the development of new applications. Also, this separation allows us to identify those knowledge groups that describe inference procedures in the design process,as the functional fixture design and the detailed fixture design. This section presentsthe definition of the tasks of the knowledge model classified under theconcept of analytic and synthetic tasks that can be reused in other applications.4.1. Analytic task definitionThe entities (or classes) defined under this category can be classified regarding the level of dependence level that present the objects involved in the machining fixture design. The first level defines those knowledge groups that are not consequence of the fixture design process, as geometry, dimensions and tolerances of the part. In this level, the entities that compose these knowledge groups can be totally reused in their structure, relationships and attributes. Fig. 4 shows an example of the knowledge group of geometry that can be reused in other applications.Fig. 4. Knowledge template for part entity (analytic task).The second level describes those entities that present a similar structure and relationships in the fixture design process(fixture functions and commercial elements for machining fixtures). In this level, can be reused only a portion of the structure and relationships that are not conditioned by the fixture design process.The third level describes those entities that present a complete dependence to the fixture design process. In this level, cannot be reused the knowledge previously defined (structures, relationships and attributes), due to dependence of the process developed.4.2. Synthetic task definitionThe definition of the synthetic tasks involves the identification of those objects linked with the inference procedure carried out in the fixture design process. In this type of tasks, it cannot have a total reutilization of the knowledge, because the inference process carried out using a group of production rules that depend of the type of process executed.Under this classification, the knowledge group of functional design establish the functional solution of the fixture definition: the supporting surfaces,locating and clamping of the part. The definition of these surfaces is depending to the manufacturing process developed. This last characteristic makes thatthe functional design possesses depend of the machining processes developedduring the manufacturing of the part. In this sense, the sharing knowledge ofthis group is limited to the definition of the surfaces and supporting points,locating, clamping for machining fixture and to selection of the functional elements. However, the knowledge group of functional elements can be reusedFig. 5. Knowledge template for functional elements entity.in other applications, due to the functional elements can be employed in multiple domains in the fixture design process. Fig. 5 shows an example of the knowledge template for functional elements used in the fixture design process.In the detailed design occur similar situations to those of the functional design. In this case, the detailed design depends on the fixture functional design through a correspondence between functional and commercial elements.The knowledge group for fixture elements can be partially reused to define anew group of fixture elements. For it, we must use the structure, relationships and entities defined for the following categories, base, support, locate, clampand auxiliary elements.Table 2Initial information for fixture machiningInformation CharacteristicsInitial geometr Final geometry Machining operations Face milling Side millingDrillingManufacturing resources Vertical milling machineFixture resources Modular fixture elements5. Application of the knowledge modelIn the next two sections, we present the application of the knowledge model for machining and inspection fixture design. These models taking into consideration two different parts, because we wish express the potentiality of the use of knowledge template.The implementation of the structuralmodel,discussed in Section 3, isbased on the instantiation of each attribute defined defined in the knowledge groups that compose this model. The instantiation is defined as the assignment of a concrete value for a specific attribute. For it, the initial conditions are exposed for the application of the knowledge model, which include the description of the initial geometry, final geometry, lists of machiningoperations,machine-tool and fixture resources. Table 2 shows the initial information for the application of the knowledge model for machining fixture.Mechanical Engineeringupon the principles of mechanics, such as those of static for reaction forces and for the optimum utiliza- tion of friction; of dynamics for inertia, acceleration, and energy; of elasticity and strength of materials for stress and deflection; of physical behavior of materials; and of fluid mechanics for lubrication and hydrodynamic drives. The analyses may be made by the same engineer who conceived the arrange- ment of mechanisms, or, in a large company, they may be made by a separate analysis division or research group. As a result of the analyses, new arrangements and new dimensions may be required. Design is a reiterative and cooperative process, whether done formally or informally, and the analyst can contribute to phases other than his own.Finally, a design based upon function and reli- ability will be completed, and a prototype may be built. If its tests are satisfactory, and if the device is to be produced in quantity, the initial design will undergo certain modifications that enable it to be manufactured in quantity at a lower cost. During subsequent years of manufacture and service, the design is likely to undergo changes as new ideas are conceived or as further analyses based upon tests and experience indicate alterations. Sales appeal, customer satisfaction, and manufacturing cost are all related to design, and ability in design is intimately involved in the success of an engineering venture.Some Rules for DesignIn this section it is suggested that, applied with a creative attitude, analyses can lead to important improvements and to the conception and perfec- tion of alternate, perhaps more functional, eco- nomical, and durable products. The creative phase need not be an initial and separate one. Although he may not be responsible for the whole design, an analyst can contribute more than the numerically correct answer to a problem that he is asked to solvemore than the values of stress, dimensions, or limitations of operation. He can take the broader view that the specifications or the arrangements may be improved. Since he will become familiarwith the device and its conditions of operation be- fore or during his analysis, he is in a good position to conceive of alternatives. It is better that he suggest a change in shape that will eliminate a moment or a stress concentration than to allow construction of a mechanism with heavy sections and excessive dynamic loads. It is better that he scrap his fine analysis, rather than that he later see the mechanism scrapped.To stimulate creative thought, the following rules are suggested for the designer and analyst. The first six rules are particularly applicable for the analyst, although he may become involved with all ten rules.1. Apply ingenuity to utilize desired physical properties and to control undesired ones.2. Recognize functional loads and their significance.3.Anticipateunintentional loads.4. Devise more favorable load- ing conditions.5. Provide for favorable stress distribution and stiffness with minimum weight.6. Use basic equations to proportion and opti- mize dimensions.7. Choose materials for a combina- tion of properties.8. Select carefully, between stock:and integral components.9. Modify a functional design to fit the manu- facturing process and reduce cost.10. Provide for accurate location and nonin- terference of parts in assembly.Machine DesignThe complete design of a machine is a complex process. The designer must have a good back- ground in such fields as statics, kinematics, dy- namics, and strength of materials, and in addition, be familiar with the fabricating materials and proc- esses. The designer must be able to assemble all the relevant facts, and make calculations, sketches, and drawings to convey manufacturing informationto the shop.One of the first steps in the design of any product is to select the material from which each part is to be made. Numerous materials are avail- able to todays designers. The function of the prod- uct, its appearance, the cost of the material, and the cost of fabrication are important in making a selec- tion. A careful evaluation of the properties of a. ma- terial must be made prior to any calculations.Careful calculations are necessary to ensure the validity of a design. Calculations never appear on drawings, but are filed away for several reasons. In case of any part failures, it is desirable to know what was done in originally designing the defective components. Also, an experience file can result from having calculations from past projects. When a similar design is needed, past records are of great help.The checking of calculations (and drawing di- mensions) is of utmost importance. The misplace- ment of one decimal point can ruin an otherwise acceptable project. For example, if one were to de- sign a bracket to support 100 lb when it should have been figured for 1,000 lb, failure would surely be forthcoming. All aspects of design work should be checked and rechecked.The computer is a tool helpful to mechanical designers to lighten tedious calculations, and pro- vide extended analysis of available data. Interactive systems, based on computer capabilities, have made possible the concepts of computer aided de- sign (CAD) and computer-aided manufacturing (CAM). Through such systems, it is possible for one to transmit conceptual ideas to punched tapes for numerical machine control without having formal working drawings.Laboratory tests, models, and prototypes help considerably in machine design. Laboratories fur- nish much of the information needed to establish basic concepts; however, they can also be used to gain some idea of how a product will perform in the field.Finally, a successful designer does all he can to keep up to date. New materials and productionmethods appear daily. Drafting and design person- nel may lose their usefulness by not being versed in modern methods and materials. A good designer reads technical periodicals constantly to keep abreast of new developments.Engineering TolerancingIntroductionA solid is defined by its surface boundaries. Designers typically specify a components nominal dimensions such that it fulfils its requirements. In reality, components cannot be made repeatedly to nominal dimensions, due to surface irregularities and the intrinsic surface roughness. Some variability in dimensions must be allowed to ensure manufac- ture is possible. However, the variability permitted must not be so great that the performance of the assembled parts is impaired. The allowed variability on the individual component dimensions is called the tolerance.The term tolerance applies not only to the ac- ceptable range of component dimensions produced by manufacturing techniques, but also to the output of machines or processes, For example, the power produced by a given type of internal combustion engine varies from one engine to another. In prac- tice, the variability is usually found to be modeled by a frequency distribution curve, for example the normal distribution (also called the Gaussian distri- bution). One of the tasks of the designer is to spec- ify a dimension on a component and the allowable variability on this value that will give acceptable performance.Component TolerancesControl of dimensions is necessary in order to ensure assembly and interchangeability of com- ponents. Tolerances are specified on critical di- mensions that affect clearances and interference fits. One method of specifying tolerances are to state the nominal dimension followed by the per- missible variation, 50 a dimension could be stated as 40.000 0. 003mm. This means that the dimen- sion should be machined so that it is between 39-0 000997 and 40. 003mm. Where the variation can vary either side of the nominal dimension, the tolerance is called a bilateral tolerance. For a unilateral tol- erance, one tolerance is zero, e.g. 40. 000 +0.006 .Most organizations have general tolerances that apply to dimensions when an explicit dimension is not specified on a drawing. For machined dimen- sions a general tolerance may be 0. 5mm. So a dimension specified as 15.0mm may range between14.5mm and 15.5mm. Other general tolerances can be applied to features such as angles, drilled and punched holes, castings, forgings, weld beads and fillets.When specifying a tolerance for a component, reference can be made to previous drawings or general engineering practice. Tolerances are typi- cally specified in bands as defined in British or ISO standards. For a given tolerance, e. g. H7 /s6, a set of numerical values is available from a correspond- ing chart for the size of component under consid- eration. The section following gives specific exam- ples of this for a shaft or cylindrical spigot fitting into a hole.Standard Fits for Holes and ShaftsA standard engineering task is to determine tolerances for a cylindrical component, e. g. a shaft, fitting or rotating inside a corresponding cylindrical component or hole. The tightness of fit will depend on the application. For example, a gear located on to a shaft would require a tight interference fit, where the diameter of the shaft is actually slightly greater than the inside diameter of the gear hub in order to be able to transmit the desired torque, alternatively, the diameter of a journal bearing must be greater than the diameter of the shaft to allow rotation. Given that it is not economically possible to manu- facture components to exact dimensions, some variability in sizes of both the shaft and hole dimen- sion must be specified. However, the range of vari- ability should not be so large that the operation of the assembly is impaired. Rather than having an infinite variety of tolerance dimensions that could be speeded, national and international standards havebeen produced defining bands of tolerances, ex- amples of which are listed e.g. Hll/cll. To turn this information into actual dimensions corresponding tables exist, defining the tolerance levels for the size of dimension under consideration. In order to use this information the following list and give definitions used in conventional tolerancing. Usually the de- based systern is used, as this results in a reduction in the variety of drill, reamer, brooch and gauge tooling required within a company.Size: a number expressing in a particular unit the numerical value of a dimension.Actual size: the size of a part as obtained by measurement.Limits of size: the maximum and minimum sizes permitted for a feature.Maximum limit of size: the greater of the two limits of size.Minimum limit of size: the smaller of the two limits of size.Basic size: the size by reference to which the limits of size are fixed.Deviation: the algebraic difference between a size and the corresponding basic size.Actual deviation: the algebraic difference be- tween the actual size and the corresponding basic size.Upper deviation: the algebraic difference the maximum limit of size and the corresponding basic size.Lower duration: the algebraic difference be- tween the minimum limit of size and the corre- sponding basic size.Tolerance: the difference between the maxi- mum limit of size and the minimum limit of size.Shaft: the term used by convention to designate all external features of a part. Hole: the term used by convention to designate all internal features of a part.Conceptual design is the generation of solu- tions to meet the specified requirements. Concep- tual design can represent the sum of all subsystems and component parts which go on to make up the whole system. Ion and Smith describe conceptualdesign as an iterative process comprising a series of generative and evaluative stages which converge to the preferred solution. At each stage of iteration the concepts are defined in greater detail, allowing more thorough evaluation.It is important to generate as many concepts and ideas as possible or economically expedient. There is a temptation to accept the first promising concept and proceed towards detailed design and the final product. This should be resisted as such results can invariably be bettered. It is worth noting that sooner or later your design will have to com- pete against those from other manufacturers, so the generation of developed concepts is prudent.According to McGrath, concepts are often most effectively generated by working individually and then co
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