Plan a Better Assembly

The key to creating and planning an assembly starts before the assembly document is created. Before starting the new assembly, think about the design and capture what is currently known about the design. Understand requirements and key characteristics. This process will eliminate reworking the assembly later on, and will lead to a more robust CAD design.

The time spent creating the requirements and capturing this intent pays dividends later in the design process, during design modifications, and also when the design is reused. The ability to clearly define the requirements, function, and intent of the assembly is required to produce a good design. When starting a design, rarely are all the requirements and key characteristics known. Start with what is currently known and update the requirements and CAD reference geometry as new information is learned.

One method that can be used to capture the design intent for an assembly is a reference feature skeleton. A reference feature skeleton is a framework comprised of simple, robust features (that is, planes and axes) used to define mating surfaces, axes of rotation, common reference features. These simple features can be created and named according to their functional use. For example, two different engineers are working two cast components for an assembly. One engineer defined the requirements and created a skeleton to define the mounting location, interface points, and spatial relationships between the two components. Both parts were designed in context with the skeleton. Later in the design, the interface location and geometry had to be changed due to a supplier request. The skeleton was updated by one of the engineers and both designs were updated based on the new design intent for the interface geometry. This saved considerable time and effort in making what could have amounted to a major design change.

Planes and axes are simple, robust features that can be defined with few parent/child relationships. The ability to use a simple feature means the design intent can be clearly delineated. A reference feature skeleton can be constructed within a part or assembly. A part feature skeleton can be used to define planes or axes that will be used by more than one feature.

For major functional surfaces within a part, a well-named plane can be used by many features while creating only one parent relation to the plane. This is a better practice than using a part face for the same function.

Reference feature skeletons can also be used to simplify complex models or assemblies. When the major functional surfaces are defined, unnecessary features or components can be suppressed to help minimize the information required to rebuild the part or assembly.

Assembly with Reference Skeleton	Reference Skeleton without Assembly

The goal of a skeleton is to make simple, intelligent information available with little overhead using robust features. Sketches, planes, and axes are simple features that are robust in nature. A robust feature has few parent relationships, which means fewer problems when the design changes or other features are removed.

A key aspect in capturing design intent is to name these skeletal features. A simple name or abbreviation will help visually document the purpose of the skeletal feature. It is not a good practice to use default feature names for these key design elements.

Another feature of skeletons and assembly layouts is the necessity to define the important mating features for a design. This will help the designer better plan how a design functions and how to describe simple features to document the design intent. The simple features or sketches can also be named to help describe the purpose for the skeleton feature. This is an excellent visual aid for documenting the purpose and function for a skeletal feature.

This allows for editing a single part in the assembly with all the information required from the assembly, without the overhead required to rebuild and repaint all the other assembly components.

Another advantage of this technique is that the part will be easier to work on when the visual clutter is minimized. Too many visible assembly components can make working on the model more difficult. This also minimizes the possibility of unwanted external references.

The advantages of Reference Feature Skeletons: