Assembly Structure

Assembly Structure

Overview: Understanding the following concepts will make it much easier for you to build and maintain optimal assembly structures.

The Assembly File

The assembly file describes which part files are used, where the parts are positioned in space, and how they fit together. Assembly files are created by selecting File | New. Think of an assembly file as a collection of components and other sub-assemblies with their own components. Assemblies can have many levels where each level contains components of some upper-level assembly.

Components and Instances

Components are the individual part files or sub-assemblies making up an assembly. Adding a part file to an assembly structure creates a component. Components can have unique names, colors, layers, and attributes. Use the properties to identify different piece parts in an assembly.
For example, a single plate can be designed for use as both an upper and lower plate in an assembly, such as a coupling. The plate is added to the assembly twice. One plate can have the component name upper_plate and the other lower_plate. Although they have different component names, there is only one single plate part file defining them. Changes to that part file from within the assembly, or outside of the assembly, causes both the upper and lower plates to update.
Component geometry is not copied into the assembly file when creating or adding components. When the assembly structure is defined and components created or added into the assembly, instances are created.
Instances are not a duplication of geometry, rather they are objects linked to the original object. Since the original geometry exists in the component part file, any changes to the original geometry made from inside the assembly or outside of the assembly, are automatically reflected in all of the instances. Therefore, it is possible for an assembly file to only contain instances, constraints and manufacturing information.
When an assembly component is loaded, you can use these instances to locate and load all of the component parts into memory. The component instances are displayed in the assembly product file. They allow the assembly files to be quite small, while maintaining automatic associability between the assembly structure and the individual component parts.
If a single component is used multiple times within the same assembly, there are multiple occurrences of the instance. Again, a change made to any occurrence is automatically made in all of the instance occurrences.
For example, in the design of a machine, many bolts of the same size are used. If a machine design uses 200 1/4 inch x 1-inch bolts, you create a single component part file to define the bolt. When you add a bolt into the machine assembly structure, you establish the instance. CATIA adds the rest of the bolts as instance occurrences of the bolt component part. Since geometry is not copied, the size of the assembly is much smaller and changing the single bolt component part file updates all 200 bolts. It does not matter if you select the first or the 200th occurrence; all 200 bolts will automatically be updated. Even if you change the bolt part file outside of the assembly, all 200 bolts will update.

Active Component vs. Displayed Component

When you load an assembly, the active component is the same as the displayed component. It is important to understand that all additions and changes are always made in the active component. You can change the active component to another sub-assembly part or an individual component. The displayed component can be the whole assembly, a sub-assembly, or an individual component.

Tips

• When you select File | Save, only the current active component and the modified components underneath are saved. Therefore, if you save the top-level assembly, you automatically save all of the modified components.
• Select File | Save Management to look at which assemblies and components will be saved and their file path. At this time, you can specify whether to save the parts and where to save them. We recommend using Save Management as a best practice, instead of Save, Save As, or Save All.