With the development of CAD/CAPP/CAM technology, traditional fixture design methods can no longer meet the flexible and rapid requirements of modern manufacturing systems. Computer-aided fixture design (fixture CAD) was created to meet this requirement. Fixture CAD on the one hand overcomes the shortcomings of the traditional fixture design, reduces the labor intensity of the designer and improves the quality of the fixture design. On the other hand, the fixture information system based on the computer-aided fixture design also constitutes an information inheritance in CIMS. Important link.
MDT (AutoCAD Mechanical Desktop) is a general-purpose three-dimensional mechanical CAD platform developed on the basis of AutoCAD, which mainly includes mechanical CAD parametric feature modeling, surface modeling and assembly modeling. MDT has good secondary development tools and programming interfaces. This article describes the author's use of solid modeling technology developed on the basis of the MDT three-dimensional software platform fixture CAD system function module and fixture component library design method.
Fig. 1 Computer aided fixture design system
Fig. 2 Function module of fixture CAD systemFig. 1 Structure design of fixture CAD system
The general fixture CAD system is mainly composed of two parts: one is the application program for the fixture design service, which is stored in the computer in the form of a program library and a graphic library; the other part is the design workflow of the fixture. As shown in Figure 1. 1 Application program: The program library is mainly used to store various programs needed for fixture design, including design calculation programs, read database programs, and various interface programs. The database is used to store various parameters used in fixture design calculations, tables, and data files generated during the design process. The main function of the database is to provide various data for the design process, various manuals to be searched, and standards. 2 Workflow: Workflow of interactive 3D CAD system: Start → Enter part information → Select positioning method and positioning part → Select guide method and guide → Select clamping method and clamp → Determine other components and clamps → Enter MDT assembly of three-dimensional system assembly diagram → return to the main system → end.
2 function module of fixture CAD systemThe functional modules of the universal fixture CAD system include parts information input, process information input, interactive fixture design and other modules. as shown in picture 2.
Part information input module. In this module, a certain number of typical parts are stored in the computer for the user to choose. If there are parts needed, the user can directly enter the CAD environment, draw a sketch of the part, and save it as a block. Then, the corresponding size information is marked in the figure, and the corresponding part is finally classified. Figure group. Process information input module. The typical parts in the part graphics library have already been stored in the corresponding process. Designers can obtain their process information by reading the process menu and enter it into the system to obtain the process information of the process. Including process dimensions, tolerances, technical requirements, positioning surfaces and clamping points. Fixture structure design module. The basic function of the interactive fixture structure design module is to use an interactive way to design the fixture structure and finally generate a three-dimensional fixture assembly diagram. It includes five parts: positioning device design, clamping device design, guiding device design, clip specific design and assembly drawing design. Each part is independent in its form, but it is interconnected among internal parameters, and thus constitutes a unified body that is independent and interconnected. The link is the part size parameters. 3 The establishment of 3D fixture component graphics libraryThe data model is the data representation of the entity model, which indirectly reflects the relationship and interdependence between the objective entities. There are three data models such as hierarchy, network, and relationship. The fixture component graphics library application relation model represents its data model.
Figure 3 Overall framework of the fixture component library
According to the above requirements for the fixture component graphics library, the fixture component library framework structure is established. As shown in FIG. 3, this framework has five parts: the user interface, the database, the database interface, the fixture component generation and the library building tool. Of course, it needs a 3D software platform to run it. The function of each part is described below.
User Interface It is the window where the user interacts with the fixture component graphics library. Database database is stored in the fixture components of the structural parameters, materials, processing methods and other data, various types of fixture components of the data and related index tables are stored in the database, the user can expand the database, of course, can also create new A database to store user-defined fixture components. Database Interface The database interface is mainly used to call, query, add, delete, and modify the data in the database. The generation of the fixture component It calls the template data from the database to generate a three-dimensional entity part document, and drives the part document with the structural parameters called from the database. The template data is equivalent to a three-dimensional solid part document. In this data, in addition to defining the shape of the generated part, but also defining the part's variables, or even the relationship between variables, it determines whether the part's shape can be properly driven by the part's variables. The database creation tool is used to create new databases, including library creation, table creation, data entry, and so on.
Figure 4 Pipe joint processing position 4 design example
In this section, the drilling fixture is taken as an example to discuss the three-dimensional design process of the drilling fixture for pipe fittings in the MDT environment.
Pipe joint parts information and processing technology informationThe machined part of the fixture is a pipe joint. The machining process is 4-Ø9 on the flange of the pipe joint. In order to facilitate the processing, it is necessary to rotate the pipe joint at a certain angle in the processing position shown in FIG. 4 , and mark the three positioning surfaces and the clamping surface of the component according to the requirements of the part structure and the processing technology. Positioning surface 1 restricts three degrees of freedom of the part (Z-axis movement, X-axis rotation, and Y-axis rotation), positioning surface 2 restricts two degrees of freedom of the part (X-axis movement and Y-axis movement), and positioning surface 3 restricts one-degree-of-freedom of the part. (Z-axis rotation) to achieve complete positioning of the part.
Figure 5 Call of the fixture component graphics library
Figure 6 Positioning element design
Figure 7 Clamping mechanism design
Fig. 8 Design of fixture design for fixture design. First call the positioning element in the fixture component graphics library and select the parameter specification according to the actual needs. See Figure 5. The positioning member 1 completes the design of the positioning surface 1, and the positioning member 2 completes the design of the positioning surface 2. The positioning member 3 completes the design of the positioning surface 3 . The main dimension of the positioning member 1 is designed according to the sum of the outer circle size of the flange and the outer circle size of the positioning member 2. The positioning member 2 and the positioning member 3 are designed by reference to the positioning surface 2 and the positioning surface 3, as shown in FIG. 6. Guide design. The center distance of the guide is from the center hole distance of the flange of the pipe joint. The size of the guide is determined by the size of the hole on the flange and the standard structure of the fixture. According to the processing requirements, the guide member axis is perpendicular to the positioning surface 1, and the guide member is mounted on the positioning member 1. Clamping device design. The clamping element is shown in FIG. 7, the clamping element 1 and the clamping element 2 complete the clamping of the clamping surface. The pins 1 and 2 are used as supporting members, the clamping member 1 and the clamping member 2 rotate around the pin axis, and the adjusting nut of the clamping member pushes the ball pin, the flat pin, and the steel ball as a force transmission mechanism to clamp the workpiece. , Return to position under the action of elastic force. The above parts are designed with new internal parts and their initial structure and size. If there are fixture components in the fixture component graphics library, they can be retrieved from the graphics library and can also be transferred into similar components, which can be edited and modified as required. Components. Considering the installation and positioning of the clamping force transmission mechanism, the mounting holes for the ball, the ball stud and the flat head pin are provided on the positioning member 1 , and the spring mounting holes are designed on the clamping member to complete the preliminary design of the clamping mechanism. Clip design. Based on the preliminary design, taking into account the installation and positioning of the positioning member and the clamping member, the design of the clip is concrete. In order to install the other parts on the clip and install the clip specifically on the drill press, screw holes, pin holes, and the like are required on the clip concrete. The position size of these holes does not need to be calculated, and is obtained by using MDT's unique function of dividing the parts directly from the fixture parts that have been designed. The holes and shafts of the positioning pins and pins 1, 2 are designed based on the generated parts, clamp specific parts, as shown in FIG. 8. Assembly design. The fixture components such as screws and nuts are transferred from the fixture component library. The components that have been designed and the components that are transferred in are assembled with MDT assembly constraints. At this time, the three-dimensional assembly design of the fixture is completed. After the design is completed, all fixture components and standard parts are externalized to make them part documents. 5 Concluding remarks
The universal fixture CAD system is a parametric graphic information management system for fixture designers. It integrates a database operating system and a parametric graphic system. This article describes in detail the design process of a universal fixture CAD system based on solid modeling in the MDT 3D software environment. A specific example provides a design method for fixture design under this system.
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