For some simple stereotyped products and improved products, the traditional bottom-up design idea can be adopted. First design each part, and then assemble the designed individual parts step by step. In the design process, there is only a simple assembly relationship between parts, and there is no correlation between design parameters. This design idea and method is easy to master, and it is the most widely used design idea, but at the same time, it also has many shortcomings, such as the design data in each part are not related, and only one part can be modified at a time, which leads to the inconvenience of design modification, the interference caused by repeated modification, and the assembly operation of parts is relatively cumbersome, so the design efficiency is low.
At present, the design cycle of some new products is accelerating, and the traditional bottom-up design idea can no longer meet all the needs. At this time, we can use the idea of top-down design to design products. In fact, the idea based on top-down design is to plan the hierarchical structure of products and gradually refine it, including product layout design, hierarchical structure design and detail design. Based on the design results and following the design intent, the design of each part of the product is finally realized. This design idea is conducive to the gradual transformation of products from abstract to concrete. We can model the whole product from the appearance of the product, and then separate the models of parts at all levels according to the matching relationship between parts, and design the detailed structure of parts with reference to the relationship between parts, and finally complete the design of the whole product. Its design data is highly correlated and product modification is convenient. The typical application is to use skeleton model to ensure the design intention, or to use the main control design method. Usually, skeleton models or main controls are used to create the appearance of products, and objects such as surfaces and sketches for drawing parts at all levels are constructed according to the matching relationship between parts. Of course, it is not limited to this, and the main part of the appearance modeling data can be transmitted from the skeleton model or the master model.
Compared with the traditional bottom-up design, the top-down design has the following obvious advantages.
1) The top-down design idea is more in line with the development process of modern products and more suitable for designers' innovative design thinking. It is based on the "total-sub" design under the design intention. In the design, the function to be realized and the final appearance of the product are given priority, and then the corresponding structural elements are designed according to the function and appearance, so that the structure and function can always be effectively coordinated and unified, and deviation is not easy to occur.
2) Through the application of skeleton model or master control, there can be data association between bottom-level parts and top-level design information, which is convenient for concurrent design and design modification. If you change the top-level design by modifying the skeleton model or the master control, these changes can be automatically transferred to the controlled bottom-level parts.
The Creo assembly model tree shown in figure 1 shows the design of applying skeleton model.
Figure 1 skeleton model design
The function phone in Figure 2 adopts the main control design. In fact, the main control is to establish the shape of the product, the parting surface of the main parts and the corresponding curves according to the requirements of product function and appearance modeling in a separate part design mode.
Figure 2 Application main control design (function phone design)
3) After designing the basic model of the product, other series design forms of similar products can be obtained by modifying the top skeleton model or master control. Top-down design is especially suitable for the serial design of products, maintaining the same style and similar structure of the same series of products, with high design efficiency and greatly shortening the development cycle.
Taking the product structure design in Creo parameterization as an example, the design process is briefly described.
1) In many consumer electronic products, the internal hardware layout of the product roughly determines the external size and internal key structure position of the product. Therefore, in the early stage of design, it is very important to obtain the hardware layout model of the product, as shown in Figure 33 (taking a functional mobile phone as an example). The model can be established according to the hardware layout size and the hardware size in Creo Parametric, and the corresponding data can also be derived from the third-party hardware layout software for generation or auxiliary generation.
Figure 3? Hardware layout model
2) Establish an assembly file, import a hardware layout model, and then create a solid part and activate it during assembly. According to the hardware layout model, function and design requirements, determine the appearance model of the function phone and the parting surface of the main parts, as shown in Figure 4 (taking the function phone as an example). At this time, it is necessary to fully consider the disassembly requirements of the product and determine the overall framework structure of the product, that is, to clarify the structural level of the whole product. You can also create a skeleton model to replace the master control in an assembly. Skeleton model can be understood as an important three-dimensional parametric layout. In the skeleton model, according to the hardware layout requirements and structural hierarchical planning of the whole machine, the shape information such as surfaces and curves and the position information such as datum plane, datum coordinate system and datum point required for assembly at all levels can be designed as * * * shared data to influence the design of subsequent parts.
Figure 4? Master control design/skeleton model
3) Create corresponding parts in the assembly, and model the whole machine structure in detail. After creating a new part in an assembly, there are many ways to share the data in the master control model or skeleton model with the new part. The main methods are "merge/inherit" and "publish geometry+copy geometry". Among them, "published geometry" and "copied geometry" features are top-down design tools, which are used together to transfer design standards and data to each other. In some large designs, each design group can create skeleton models in its subassemblies by using the Copy Geometry feature that references the top-level product skeleton, without accessing the top-level assembly. Because each group's framework contains copied references, everyone uses the same design standards and maintains the same relevance.
L "Merge/Inherit": You can copy, merge/inherit all geometric features in the master and skeleton models.
L "Published Geometry": This feature contains independent local geometric references, and external references are not allowed. It is actually a synthesis of multiple local references that can be copied to other models. Published geometric features can be created in part, skeleton and assembly models, and the selected reference geometry must be selected in the source model. For example, using the "Publish Geometry" function, you can select several geometric features in the skeleton model as a whole and provide them to the target parts for * * * to enjoy.
L Copy geometry: This function is used to propagate geometric references and user-defined parameters between models. Using Copy Geometry on the target part, you can copy the skeleton model or the geometry published in the master control to the target part as the basis and reference for the modeling of the target part, and at the same time reduce the amount of data in the session without retrieving the whole reference source model.
4) Detailed structural design is carried out in each activated component, and some structures need to refer to other components in the modeling process. The design reference effect is shown in Figure 5.
Figure 5
5) Activate the top-level assembly, and conduct relevant inspections such as "global interference" and "clearance" to check whether there is unnecessary interference. For some motion mechanisms, we can also do motion simulation and other operations. If static interference and dynamic interference are found, carefully analyze them and try to correct and optimize them.
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