Bottom-up design method
Bottom-up design method is a more traditional method. You design and shape the part, insert it into the assembly, and use mates to position the part. To change parts, you must edit the parts individually. These changes can then be seen in the assembly.
Bottom-up design is the preferred technology for previously built, off-the-shelf parts, or for standard components such as hardware, pulleys, motors, etc. These parts do not change their shape and size according to your design unless you select a different component.
Top-Down Design Method
Top-down design method is also known as “Associative Design” in the SOLIDWORKS Help.
In top-down design, the shape, size, and location of parts can be designed in the assembly. For example:
you can model a motor holder so that it is always the correct size to hold the motor, even if you move it. SOLIDWORKS automatically resizes the motor bracket. This feature is especially helpful for parts such as brackets, appliances, and housings, whose purpose is primarily to hold other parts in their correct positions. You can also use top-down design for some features that would otherwise use bottom-up design, such as dowel pins.
A copier design can be laid out in a layout sketch whose components represent the copier’s pulleys, reels, belts, and other components. You create 3D components from this sketch. SOLIDWORKS automatically moves or resizes 3D components in the assembly as you move or resize elements in the sketch. The speed and flexibility of sketching allows you to try out various versions of the design and make many types of changes in one central location before constructing the 3D geometry.
The advantage of the top-down design approach is that less rework is required when design changes occur. Parts know how to update themselves based on the method you create.
You can use top-down design techniques on certain features of a part, on a complete part, or on an entire assembly. In practice, designers typically use a top-down design approach to lay out their assemblies and capture key aspects of custom parts specific to their assemblies.
Top-down design
In top-down assembly design, one or more features of a part are defined by something in the assembly, such as a layout sketch or the geometry of another part.
Design intent comes from the top level, the assembly, and moves down to the part. Examples of design intent include feature size, displacement of components in an assembly, and proximity to other parts.
For example, you use the Extrude command to create dowels on a plastic part. Select the Form to Face option, then select the bottom of the board, which is another part. This selection will allow the dowel length to just touch the board, even if the board is moved in future design changes. The length of the pin is defined in the assembly, not by static dimensions in the part.
Methods
You can use these top-down methods:
individual features can be designed top-down by referencing other parts in the assembly, as in the dowel example.
In bottom-up design, parts are built in a separate window where only parts are visible. However, you can edit parts while working in the assembly window. This makes the geometry of other components available for reference. Examples of geometry to reference include replicas or dimensions.
This method is helpful for parts that are mostly static but have features that interface with other assembly components.
Complete parts can be built using a top-down approach by creating new components in the context of the assembly. The component you build is attached or mates to another existing component in the assembly. The geometry of the components you build is based on existing components.
This method is useful for parts like brackets and appliances that rely mostly or entirely on other parts to define their shape and size.
The entire assembly can also be designed top-down, starting with a layout sketch that defines component locations, critical dimensions, and more. Then use one of the above methods to build the 3D part so that the 3D part follows the size and position of the sketch.
The speed and flexibility of sketching lets you quickly try out several versions of your design before building any 3D geometry. Sketches allow you to make a lot of changes in one central location, even after you’ve built the 3D geometry.
Considerations
Whenever you create a part or feature using top-down techniques, external references are created for the geometry you reference.
In some cases, an assembly with a large number of associative features (which forms the basis of top-down design) may take longer to rebuild than the same assembly without associative features.
Optimize SOLIDWORKS to rebuild changed parts.
When creating associative features, do not create conflicting mates, as such mates can cause long rebuild times and unpredictable geometry behavior. You can generally avoid these conflicts by not creating mates for geometry created by associative features.
