If you've ever found yourself in a modeling situation where you have no alternative but to edit someone else's parametric equation, chances are you spent a lot of time, possibly hours, investigating how the equation works and how a change will affect the assembly. While parametric equations provide a powerful way to incorporate design intent into your assemblies, the equations are often complex and intimidating, making them difficult to create and modify. Often, parametric equations can be so complex that only the person who created the equation is able to modify it. Due to this level of difficulty, many designers and engineers avoid parametric relationships altogether, viewing the concept as too cumbersome and time-consuming, and, as a result, they have avoided moving from 2D to 3D design.

Fortunately there is an alternative to difficult and complex parametric design. Autodesk Inventor 3D-mechanical-design software uses adaptive technology, which allows you to seamlessly incorporate design intent into your assemblies without the headaches associated with parametric equations or cross-part parametrics. This month, we'll examine the benefits of adaptive design over parametrics, and walk through an example on how to use adaptivity.

Let's start by looking at the various and sometimes over complicated terms used to describe many of today's modern CAD systems.

Parametric stands for a directed relationship that defines an object in terms of objects created before it. For example, line B is parametrically located and offset from line A. If you move A, then B moves due to the parametric offset. However, because of the parametric offset, you cannot move B directly. Parametric methods are reliable and solve quickly, but they often incorrectly capture design intent.

Variational is an undirected relationship that exists between two or more objects. The order of creation does not impact editing. For example, lines A and B are defined as parallel and offset. If you move either line, the other line follows. Variational relationships more accurately capture design intent.

Features make it possible for models to capture design intent. Prior to features, models were just a collection of edges and surfaces. Feature-based models are composed of design-oriented objects, such as pockets and ribs, rather than generic surfaces.

Cross-Part Parametric is another directed relationship that defines an object in terms of objects created before it. A cross-part relationship allows the features of one part to drive the size and shape of another part. For example, a notch on part A defines the size of a boss on part B. If the notch changes size, the boss updates to reflect the change, but not vice versa.

Adaptive Modeling is a new design paradigm in Autodesk Inventor that allows the definition of cross-part relationships without the need for parameters or equations. Adaptive modeling is order-independent and defines relationship without the limitations of directed parametric or cross-parametric relationships.

Parametric vs. Adaptive
Today, modern systems use variational relationships to position parts, and parametric relationships to define the shape and size of parts. Using these two methodologies in the same assembly can often cause problems because the order imposed by parametric relationships limits flexibility when making design changes. In addition, the order of feature creation is often arbitrary and not part of design intent, and reorder operations are restricted by existing relationships, thus requiring you to be intimate with how the part was built.

This simple example will help put things into perspective. Since parametric equations are so complex, we'll use a cross-part parametric approach to add design intent to a design. Part A is defined with a hole, as shown in Figure 1. Shaft B is created with a cross-part relationship to the hole via an offset circle. If the position and size of the hole is moved or changed, the shaft adjusts accordingly. However, as the design matures, it turns out the size of the shaft is more critically defined by part C.

Figure 1. Problems arise with parametric methodologies as a design matures and intent changes, forcing you to rework your designs. Conversely, adaptivity enables assembly components to easily adjust to major design-intent changes.

In theory this change should be easy, yet it proves too difficult with the cross-part parametric approach. The shaft must be rebuilt in terms of part C, and the hole in part A must be redefined. Multiply the situation by 20 parts, each with 20 features, and you have a cross-part parametric nightmare. Many companies avoid problems with assembly relationships by outlawing the use of parametric equations and cross-part parametrics.

An easier alternative to parametric design is adaptive technology, which allows the designer to simply define part size and shape in the assembly context without creating any unexpected assembly-relationship problems. This is accomplished by eliminating parametric equations and cross-part parametrics. Autodesk Inventor uses variational relationships to define part size, shape, and position. Using this adaptive approach, the shaft is sized and positioned using an assembly-mate relationship. If the size or position of the hole changes, the shaft will update to reflect the changes.

So when it does turn out that the location of the shaft is more critically defined by part C, adaptive technology provides the flexibility to rapidly change a component from being a design-intent driver to design-intent driven. To accomplish this, you change which parts are marked as adaptive and add the appropriate mating relationship. As a result, shaft B moves to fit part C and the hole in part A moves in response. The order of events has no impact on the flexibility of design change, hence there are no confusing relationship issues.

Controlling Flexibility
So if parts adapt automatically in Autodesk Inventor, how does one control what's adapting to what? A simple right mouse-click will straighten this out.

In Figure 2, you can see that a sheet-metal bracket originally from another design is being used. The desired design intent involves having the metering valve's location control the length of the bracket. Before the bracket adapts to follow the desired design intent, you must indicate what feature on the bracket is to adapt. To mark a feature adaptive, first edit the component, then select the feature(s) to adapt. Now you right mouse-click and then select the adaptive option. This will instruct the feature (for example, extrude, revolve, and so on) as well as undefined or driven sketch geometry that causes the feature to behave adaptively. As the feature labeled Flange1 in our example is marked adaptive, the bracket is simultaneously marked adaptive. Finally, by placing an assembly relationship (constraint) between the bracket and the valve, the bracket adapts to fit the assembly, adopting proper design intent.

Figure 2. By marking specific sketches, features, and parts adaptive, you have total control over the behavior of adaptive models. Flange1, belonging to this sheet-metal bracket, has been marked adaptive. As a mate relationship is added to the bracket, the length of the adaptive flange will adjust to fit the design.

Parts are not built in isolation; they are virtually always designed to operate in the context of an assembly. The features of one part define the shape and size of another. Traditional CAD systems provide parametric and cross-part parametric mechanisms for defining such relationships. While these mechanisms work well for simple cases, they often fail in the real world of mechanistic subassemblies and unpredictable layout changes.

Autodesk Inventor software's adaptive technology eliminates the assembly relationship problems created by parametric design. You can adaptively define parts in the context of their assembly, using simple parts interfaces to drive part shape, size, and position. Adaptive technology enables you to freely design and edit parts in an assembly-centric environment.