Top-down 3D Assembly Design |
  
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Top-down 3D Assembly Design |
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When you use "Top-down" design, you can create parts in an assembly based on the geometry of other parts of the assembly. This is a fundamental difference from "Bottom-up" design, where parts are edited separately, independently of other parts in the assembly. That's why we will see how parts in the assembly will contact with each other only when we have finished editing the part and updating the assembly. In "Top-down" design, an assembly defines the dimensions, position, and contact elements of its parts and subassemblies, which means that top-down design requires top-down communication: the relation between an assembly and assembly fragments. The relation between the assembly and the assembly fragments is performed by the Reference Element. Reference Element borrows geometry from assembly to part. The Reference Element may borrow major elements of the model and topological elements. Control of relations between fragments and their relations with the assembly, the fragments variations, navigation through the assembly are performed in "Assembly Structure" window.
The Top-down design principle and the Reference Element mechanism allow you to solve problems that arise in the development of complex assemblies. For example, the user can borrow the geometry of elements from the elements of higher-level fragments in the assembly structure, or from fragments located in "parallel" subassemblies. The user can receive changes of the uploaded geometry from the source document or update the source document in accordance with the changes in the uploaded file at any time.
Thanks to the Reference Element mechanism, it is possible to work effectively in a team, where each user performs his task. For example, when designing an excavator cab, the cab body parts are developed by one user, and other users do not have access to this file. The geometry of the cabin can be uploaded to a separate document so that several users can work in parallel.
On the basis of these unloaded elements another user can create equipment in the cabin, having full data on the current dimensions of the cabin. In this case, it is possible to snap to the unloaded geometry and update the unloaded geometry in accordance with the changes in the source file of the cabin. Then the changes made in the cab may be transferred to the general assembly.
The "Top-down" design method cannot be considered suitable for all assembly design cases. It has a number of limitations:
•a more complex scheme of the organization in comparison with "Bottom-Up" method;
•less resistance to topological changes in the 3D model;
•parts are less convenient when using the same 3D model in different assemblies;
•higher requirements to computer resources.
"Top-down" and "Bottom-up" design methods can be combined. The "Bottom-up" design method is useful when adding standard parts to an assembly, parts with strictly defined geometry. The "Top-down" design method is preferably used in the design of parts and assemblies that provide interconnection and fastening in the assembly of structural elements that have a strictly specified geometry.
Create Assembly Features Using Top-down Design
The commands for creating assembly elements.
•Insert 3D Fragment. Allows you to insert a fragment by LCS. It is used during "Bottom-up" designing.
•Create 3D Fragment. Allows you to create a fragment in the structure of an existing assembly. It is used during "Top-down" designing.
•Detail. Detail > Create and Detail > Unload. Allows you to create a fragment on the basis of a body in the assembly. Creates a detail file that depends on the body or fragment in the assembly. The command is available in the context menu of the fragment or body.
•Reference Element. Allows a fragment to borrow geometry from an assembly or another fragment. Reference elements carry out the relationship on which the principle of "Top-down" design is based. You can only create reference elements in the context of an assembly (except for internal references to resolve recursions – see the "Reference element" subsection).
•Open in Context: The command is available in the context menu of the fragment or body. Edit in Context: the command is available in the Create 3D Fragment command dialog.
•Assembly Structure. The command opens the "Assembly Structure" window. The window is required to navigate the assembly and control assembly changes.
Various options of these commands, their combination when creating assembly elements, allow you to create assemblies with complex hierarchical relationships. Fragments can depend on both the assembly and other fragments in different ways, you can combine the "Top-down" design principle with the "Bottom-up" design principle. The types of fragments and how they relate to each other and to the assembly are described in the section "Variations of fragments relations with assembly when designing ’Top-down’".
Two Basic Ways to Create Parts in Assembly
Creating a primary hierarchy of subassemblies and parts is already considered above. The recommended and convenient way to work is to create an empty detail file using the Create 3D Fragment command, and then fill it with geometry and create Reference Elements in the context of the assembly. In T-FLEX CAD terminology, in General, any subassembly and any part is called a fragment. Therefore, the creation of the fragment may mean the creation of subassemblies and the creation of the part (if there are no other parts in the part).
It may happen that you need to create a fragment based on the body in the fragment for the correct assembly structure. The fragment with the body will become an assembly, the created fragment - a part in the assembly. The fragment with the body may already contain other fragments, i.e. already be an assembly. To create a part based on a body in a fragment, there is a set of options for the Detail > Create command. This is the second way to create parts in "Top-down" design. Consider these two ways to create parts in a simple example.
Topics in this section:
•Create 3D Assembly Structure in «Assembly Structure» Window
•Working in Context of 3D Assembly
•Filling a 3D Assembly with Geometry
•Parts That Do Not Depend on 3D Assembly
•Repeat Insertion of Part into 3D Assembly
•Dependence of 3D Fragments on Assembly in Top-down Design
See Also: