e-Profiles, Spring 2002: Design Using Inheritance Features

Design Using Inheritance Features

The inheritance feature, new in Release 2001 of Pro/ENGINEER®, is an enhancement to the merge geometry functionality. The merged geometry (using the merge functionality) is a single feature in the derivative model, which is fully (bi-directionally) associative with the master model. Using query select, it is possible to modify features of the master model (parent of the merge feature) from a derivative model. This is not always desirable and may not match design intent in the Pro/ENGINEER environment. The associativity of inheritance features is one way from the base to a derivative model. This is an important distinction from merge features. Inheritance features are used to merge the geometry of a master model into one or more derivative models as with the merge functionality, but they add some new flexibility and utility.

As with any feature that creates external references between databases, there is a master/slave paradigm established between a parent and child object. When discussing inheritance features, “base model” (master) refers to the model referenced by an inheritance feature. “Derivative model” (slave) refers to the model where the inheritance feature resides.

The inheritance feature enables changes to a derivative model while maintaining one-way associativity with the parent object. The inheritance feature creates a one-way associative link of geometry and feature information from the base model to the derivative model. This resolves the problem of accessing the master model from within a derivative part and also provides direct access to features within the inheritance feature.

Use Inheritance Features to:

• Replace a merge feature. While inheritance features can replace a merge feature, there are several noteworthy differences. The inheritance feature creates a one-way associative merge of geometry and feature data from the base part to the derivative part. Upon creation, all of the geometry and features in the derivative part are identical to those in the base part. The advantage over a merge feature is that, without affecting the base model, the inheritance feature allows the derivative model to have variable dimensions and feature states.

• Replace external copy geometry features. Using the inheritance feature may have advantages when references to many or all of the features comprising a skeleton model or reference part are needed in the derivative model. The inheritance feature allows you to access feature information from the base model, which a copy geometry feature does not.

• Replace family tables. A reference model can serve as the basis for multiple design variations. Using inheritance features instead of instances eliminates the need for all variations to carry the overhead of all the instances. The reference part (generic instance) serves as the base feature to create variations through varying dimensions and suppressing features. Individual variants can also be modified independently using the inheritance feature approach, making revision control much less complicated than with family tables.

• Create models representing stages in a serial manufacturing process. Inheritance features can be used to create a dependency chain between models that represent the manufacturing evolution of a part.

Consider cast parts that require some machining after the initial casting (see sidebar). In a serialmanufacturing process, it is desirable to provide a database of the “as cast” and “as machined” versions of a given part. The cast model is used to design the mold for the raw casting. This model serves as the reference in Pro/MOLDESIGN™ to design the tooling for the raw casting, and as the parent object for the “as machined” part. The “as machined” part adds features that result in modified geometry from the raw part to the “as cast” geometry. The “as cast” geometry is then captured in the machined model by an inheritance feature using the “as cast” part as the base model.

This paradigm can be applied to manufacturing stages that may require more than two models as well by nesting inheritance features to establish a dependency chain to manage downstream models using a single base model as the parent. Some examples are multi-shot molded parts and forged components that require several stages to form the final geometry. A database that represents each stage of the manufacturing process can be managed in the Pro/ENGINEER environment using nested inheritance features. This prevents anyone working on a downstream model from inadvertently altering any of the upstream models, in addition to establishing a logical dependency chain.