Discussion on the system of installing digital zone by moving standard parts
According to the requirements of the standardization theory, a fixed data table and several floating data tables were established for each part. Among them, the fixed data table includes the main key size (nominal size) and all auxiliary dimensions that determine the trunk structure of the part. Considering that the dimensional deviation in the original data sheet in the design manual has little effect on driving the 3D part prototype, it is only attached as a feature to the corresponding model, and no data table is created specifically for this purpose. Taking the part as an example, the standard size data of the lead bushing after normalization is as shown in the figure. Wherein, the length of the guide sleeve length L should satisfy the Lmin MDT is an excellent feature-based mechanical CAD system developed by Autodesk. It supports three forms of user parametric modeling mechanisms, namely local variable driving mechanism, global variable driving mechanism and table driving mechanism. MDT3 parametric modeling mechanism introduction table drive mechanism table drive mechanism is actually an extension of the local variable drive mechanism. It uses Microsoft's spreadsheet Excel as the medium to store the entire size data of the driven parts. The user specifies the corresponding version (size combination) of the parts according to the design requirements, and then the remaining work is done by the MTB built-in VBA utility. Automatically extracts dimensional data, assigns values ​​to design variables, and uses assigned design variables to drive standard or generic prototypes to ultimately obtain the feature entities of the desired part. Since the mechanism for the organization of dimensional data is denormalized, the workload of building a size table is quite large and lacks flexibility. Local Variable Drive Mechanism This mechanism records the parameter sizes of the currently active part in the MDT built-in model database in the form of variable names, expressions, and values. Among them, the variable name is the identifier of the parameter size, the variable name in the same part must be unique, the expression determines the value of the parameter size, which can be a single constant, a variable, a mathematical function or their four arithmetic expressions, and the value is an expression operation. result. Changing the value of some expressions in the parameter size record set, you can achieve the parameter size to drive the currently activated part structure, but the value must be changed in the MDT environment (or directly manipulate the design variable dialog box, or use the table driver mechanism to achieve ). Global Variable Drive Mechanism The global variable drive mechanism works basically the same as the local variable drive, except that the former can drive both the currently active part and other inactive parts with the same name parameter size. Therefore, the global variable drive mechanism is particularly suitable for the assembly design (styling) of injection molds. In addition, the mechanism can output a global parameter size record set to the extension. In prm's external text file, it is convenient for users to edit these recordsets outside the MDT system, and then use the edited recordset to drive related parts, parts or the entire assembly. The idea of ​​building a library is based on the MDT global variable driving mechanism, respectively establishing a 3D prototype of each mold part, and recording all the size driving elements related to the prototype in the open part index table (formally equivalent to establishing a standard) Part 3D prototype sub-library). Considering that the global variable mechanism can drive all the parts with the same parameter size at the moment, when building the same type but different specifications, the existing parts of the same model and different specifications must be localized, that is, the part and global parameters are canceled. The link between the size records so that it can no longer be driven by the new parameter size record set. System Openness The openness of the system is mainly reflected in the user's customary standard and common part prototype and its driving size can be added to the corresponding sub-library according to the agreed rules, without programming, and can be on the MDT platform. Freely edit the parts and drive size tables in the library. The specific implementation process is as follows: establish a 3D prototype of an injection mold standard part or a general part, and design a global variable for the prototype, and then store it in a dimensionally driven 3D part prototype sub-library; according to the relational database normalization theory, in the part prototype size A size-driven data table related to the new part is created in the library, and a record reflecting the driving property of the part (the structure of the open part index table and the meaning of the data item) is added to the open part index table; Dynamically construct SQL statements to access and edit the dimension tables of new or specified parts; The prm file passes global variable values ​​to the 3D part prototype to drive the prototype to generate specific standard or generic parts.
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