In the Top-down assembly modeling design approach, one or more features of the part are defined by something in the assembly. Example: A layout sketch or the geometry of another part. The design intent of the model, examples: the size of the features, location of the components in the assembly, etc. take place front the top level of the assembly and translates downward from the assembly to the component.
A few advantages of the Top-down modeling approach are that design details of all components are not required and much less rework is required when a design change is needed. The model requires individual relationships between components. The parts know how to update themselves based on the way you created them.
Designers usually use the Top-down assembly modeling approach to lay their assemblies out and to capture key design aspects of custom parts specific in the assemblies. There are three key methods to use for the Top-down assembly modeling approach. They are:
Individual features method: The Individual features method provides the ability to reference the various components and sub-components in an existing assembly. Example- Creating a structural brace in a box by using the Extruded Boss/Base feature tool. You might use the Up to Surface option for the End Condition and select the bottom of the box, which is a different part The Individual features method maintains the correct support brace length, even if you modify the box in the future. The length of the structural brace is defined in the assembly. The length is not defined by a static dimension in the part. The Individual features method is useful for parts that are typically static but have various features which interface with other assembly components in the model.
Entire assembly method: The Entire assembly method provides the ability to create an assembly from a layout sketch. The layout sketch defines the component locations, key dimensions, etc. A major advantage of designing an assembly using a layout sketch is that if you modify the layout sketch, the assembly and its related parts are automatically updated The entire assembly method is useful when you create changes quickly, and in a single location.
Complete Parts method: The Complete parts method provides the ability to build your model by creating new components In-Context of the assembly. The component you build is actually mated to another existing component in the assembly. The geometry for the component you build is based upon the existing component.
This method is useful for parts like brackets and fixtures, which are mostly or completely dependent on other parts to define their shape and size.