A key difference between history-based and history-free CAD technologies
(the latter is sometimes now called dynamic modeling or direct geometry
editing) is in how the two systems capture and utilize design intent. In the
context of 3D mechanical CAD, the term “Design Intent” refers to things such
as parameters, constraints, dimensions, relationships and features that are
added to the 3D model. Because of the nature of the CSG tree (see the
first article in the series for details), Hybrid modelers make it easy
to capture and store information about the geometry in their history tree.
In order for a CSG or Hybrid modeler to work, it has to have this
information. Through some great marketing by a few CAD companies, we were
led to believe that history-based (hybrid) modeling is the only way to add
this “intelligence” to 3D geometry in such a way that would allow, or
disallow, modification based on our design intent. In reality, CSG and
Hybrid modelers won’t work without it. This is not to say that Hybrid is the
only technology that will allow for intelligence to be added to the
geometry. It just provides the simplest method of doing so -- but this is
quickly changing.
Features
An important element in CSG and Hybrid modelers is the “feature,” a
misleading term. For the most part, the term “feature” in the context of
Hybrid modeling refers to a primitive in the tree.
For the users of history-free systems, features or primitives are not
created with each modeling step and are not maintained automatically by the
system. A “feature” is something that a user can define at any time by
collecting faces into a group. These groups of faces can be named and are
stored with the model. The groups/features can be edited anytime to keep
up-to-date with design intent as the design matures. Features in this
context can have significant importance to the design, rather than simply
being a result of the modeling steps used to create the model.
Parent/Child Structure
Another important relationship captured in the Hybrid modeler is the
parent/child relationship. You can’t avoid this relationship in these
systems, so you must learn to manage it properly. There are a few advantages
to having a parent/child relationship that history-free systems will never
have.
There is one last thing to consider in the context of geometric
relationships. You may have varying requirements depending on your business,
your product development process, and the lifecycle of your products. For
some, it may be important for relationship to remain constant while geometry
changes (i.e. family-of-parts), for others it may be important for
relationships to change while geometry remains constant -- something a
history-based system struggles to manage. Users of history-based systems
will find themselves re-modeling the part occasionally for this very reason
-- the built-in relationships no longer fit the design intent.
Read more...
- What You See Is Not Always What You Get
- Working with a History-Free Modeler
