Since the introduction of multibody modeling in Creo Parametric 7.0, I’ve seen and heard a lot of comments and responses along the lines of:

• Multibody modeling isn’t needed because you have skeletons and data-sharing features in top-down design.
• Now that you have multibody modeling, you no longer need to use skeletons, data-sharing features, or top-down design.

Some responses have been even more visceral: Multibody is a betrayal for Creo’s most experienced users. The other side says users shouldn’t have been forced to use top-down design for so long, when Creo could have implemented multibody.

Which, if any, of these views are correct? Let’s look at top down and multibody to figure out how they compare and contrast.

Overview of Top-Down Design

Top-down design is a methodology for developing products with complex interdependencies and relationships between systems and components. Consider designing a new printer, or an electric car, or a medical scanner. The exteriors of these products consist of multiple components with precise fits. You have to control mating surfaces, lines of action, mechanisms, and space claims for the interior components.

Top-down design can be broken down into three areas:

• Consolidating important design information about geometry in skeletons and about dimensions in notebooks.
• Sharing that information to the individual components and then developing the components and their features.
• Managing the interrelationships between components.

These tools allow you to implement changes easier. Imagine having to make one of these products a few inches bigger or smaller, or changing the overall shape. Without top-down design, a “simple” change could involve modifying dozens or hundreds of components individually, at great risk of making a mistake.

Overview of Multibody Modeling

Bodies are containers in part models for solid geometry. Prior to Creo Parametric 7.0, when solids from a new feature intersected existing solid in the part, the exterior surfaces and interior volumes were automatically merged. This was actually a huge breakthrough in the development of 3D solid modeling in the early days of CAD.

Multibody modeling allows you to control whether the solids are merged. If so, they are part of the same body. If not, the new solid goes into different bodies. A part model now consists of multiple bodies of solid geometry.

This is an easy way of developing an assembly inside of a part, and then splitting up the part and extracting individual parts later.

Which Is Better, Multibody or Top Down?

Or more to the point, which should you use?

This is a false exclusive choice.

You can use multibody modeling and top-down design together or separately.

The master model methodology for multibody combines multibody with top-down design. Also, when you extract a new part from a multibody part, it has an External Copy Geometry feature. Other ways in which the two can be used together include:

• Skeletons can contain multiple bodies.
• Multibody parts can have data-sharing features like Copy Geometry, Shrinkwrap, Merge / Inheritance, and Publish Geometry that reference higher-level assemblies and share geometry with other components.
• A part model with multiple bodies can be converted into a skeleton.

Situations in which the multibody process can be easier and more convenient than top-down design include:

• Fast concepting in part mode.
• Simpler assemblies with interdependencies. Designing with Copy Geometry features between parts in assemblies of a dozen components or so is perfectly fine. In these situations, skeletons can feel like overkill.
• Designing injection-molded parts with multiple materials and multiple shots.

Top-down design can be performed, as it has for decades, without multibody functionality if desired.

Multibody and top down are both powerful design methodologies that should be in your toolbox and used together. The incorporation of multibody into Creo Parametric design represents an exciting advance for product design. If you haven’t tried using multibody with top-down design, upgrade to Creo Parametric 7.0 and give it a shot today!