Top-down design starts with the broad project specification in mind and putting that information in a centralized location. The design process progresses by communicating the centralized information to the individual parts.

A top-down design system consists of following basic elements

  1. Central location for design data
  2. Data communication tools

Central Location for Design Data

In top-down design methodology, the known project specification and information is placed at a centralized location. This information can be copied from this location to any component in the project assembly. This makes sure that all the components are based on same design criteria. So if some specification or information related to the design is changed in the central location, all the components referring it will also update.

Placing information in a central location is not a one-time process. You start a project by placing known information in central location and then work on individual components and assemblies. As the design develops you get more insight into the project and more information and constraints become known. Then you put this information and constraints into central location and refine your components and assemblies. So this is an iterative process.

Creo provides us two types of objects to capture the design data at a central location.

  1. Notebook
  2. Skeleton Model


A notebook can be described as an engineering notebook where we can define parameters and relations related to a project. It maintains design intent in a central location. Parts and assembly models can access these parameters and information directly.

A notebook can also contain sketches to elaborate the parameters

Skeleton Model

 A skeleton model is a special type of model that acts as a central location for storing design information relating to a project. The design criteria is represented by defining surface geometry, curves, and datum features. When the skeleton changes, the components connected to it also change.

A skeleton model usually contains the geometry that affects or crosses more than one component

A skeleton model, like any other assembly component, has features, layers, relations, views, and so on. However the functional characteristics of skeletons are different from other components. Therefore a skeleton is identified by a different icon in the Model Tree. Skeleton models do not show up in the Bill Of Material (unless you specifically include them) and they do not contribute to mass properties.

There are two types of skeleton models:

  1. Standard Skeleton
  2. Motion Skeleton

Motion Skeleton is used when we want to incorporate motion into the design project.

Data Communication Tools

Creo provides us following tools to communicate the design references from the skeleton to individual components

  1. Copy Geometry Feature
  2. Publish Geometry Feature

Copy Geometry Feature

A Copy Geometry feature is used to pass any type of geometric reference information to and from parts, skeleton models, and assemblies. You select individual references that may include curves, surfaces and datum features.

When you create Copy Geometry feature, you can reference either individual references like curve, surface and datum features or a Publish Geometry feature. The part from which reference geometry is copied is called Source and to which is copied is called Target.

Copy Geometry feature

There are two types of copy geometry features.

  1. Internal Copy Geometry
  2. External Copy Geometry

Internal Copy Geometry

The internal Copy Geometry feature copies geometry from model to model in the context of the assembly. It follows the entire path within the assembly between the target part and the source part. We need all the assemblies, in the path of source and target parts, in session to regenerate the internal copy geometry feature.

Internal Copy Geometry Feature is usually called only the “Copy Geometry Feature”.

External Copy Geometry

The External Copy Geometry functionality copies geometry from model to model without copying the geometry in the context of the assembly. Dependency on the assembly and all models along the path between the two components is avoided. Source and target components must be relatively positioned, but are independent of the assembly context.

We need only source and target parts in session to regenerate the external copy geometry feature.

Publish Geometry Feature

Publish geometry feature allows to mark the references, in a source part, that can be referenced by a copy geometry feature altogether. So it is a consolidation of multiple local references that can be copied to other models.

This method makes it easy for the designer working on a model (usually skeleton) to identify which features are stable and appropriate to use as a reference in the downstream models.

When you create an Internal Copy Geometry or External Copy Geometry feature, you can reference a Publish Geometry feature.

It is a good practice to give unique names to Publish Geometry features. This will make it easier to determine which model or lower level skeleton, a Publish Geometry feature is intended for.