Top-Down Design Overview
In top-down assembly design, one or more features of a part are defined
by something in an assembly, such as a layout sketch or the geometry of
another part. The design intent (sizes of features, placement of components
in the assembly, proximity to other parts, etc.) comes from the top (the
assembly) and moves down (into the parts), hence the phrase "top-down."
For example, when creating a locating pin on a plastic part using the
Extrude command, you might choose
the Up to Surface option and select
the bottom of a circuit board (a different part). This selection would
make the locating pin exactly long enough to touch the board, even if
the board were moved in a future design change. Thus the length of the
pin is defined in the assembly, not by a static dimension in the part.
You can use some or all of these top-down methods:
can be designed top-down by referencing other parts in the assembly, as
in the case of the locating pin described above. In bottom-up design,
a part is built in a separate window where only that part is visible.
However, SolidWorks also allows you to edit parts while working in the
assembly window. This makes all of the other components' geometry available
to reference (for example, copy or dimension to). This method is helpful
for those parts that are mostly static but have certain features that
interface with other assembly components.
can be built with top-down methods by creating new components
within the context of the assembly. The component you build is actually
attached (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.
An entire assembly
can be designed from the top down as well, by first building a layout sketch that defines component
locations, key dimensions, etc. Then build 3D parts using one of the methods
above, so the 3D parts follow the sketch for their size and location.
The speed and flexibility of the sketch allows you to quickly try several
versions of the design before building any 3D geometry. Even after you
build the 3D geometry, the sketch allows you to make a large number of
changes in one central location.
Whenever you create a part or feature using top-down
references are created to the geometry you referenced.
In some cases, assemblies with large numbers of
in-context features (which form the basis of top-down design) can take
longer to rebuild than the same assembly without them.
SolidWorks is optimized to
rebuild only parts that have changed.