Final Methodology

Following the creation of the third and fourth families, the ``Creating a Family of Parts'' document was expanded to cover the issues encountered while building those families. The following items were added to the ``Modeling Considerations'' section of the document.

• The size of the master part files is important. The model should be created so that it is as small as possible, without sacrificing necessary flexibility. Models created using solid primitives and boolean operations generally use more disk space than the same models created using wireframe profiles, extrusions, and sweeps. [Ault]
• If there are many variables that are the same among a group of the members, but differ between different groups, consider including only something to identify the group in the table, and use conditional constraints in the model to set the variables.
• When creating a family that is going to have a variable number of features, the order of creation becomes very important. The terminal strip is an important example of this. The fillets on the fins have to be added before the fin is copied as the Fillet Between command can only be used on one face at a time. Originally, the bosses were created as holes using the Insert Feature Hole_Blind command. This feature can only be inserted at one location at a time, so the part had to be made by creating small segments with the holes and duplicating them. Insert Hole (and later Insert Boss) turned out to be a much better option, as it allows multiple locations to be selected. However, the command only allows one entry face, so the bosses had to be added before the fins were created and unioned to the rest of the part. Unioning the fins to the body of the part splits up the top face into several smaller ones. The locations of the bosses were selected using a rectangular pattern, located by the corner.
• Another important point that comes up when creating parts that have a variable number of entities is how to properly select all of those entities. The group selection modifier is the only one that is retained in the history. Selecting the items using the window, layer, or other modifiers simply puts one selection in the history for each item selected. If the number of items later changes, then the command will either fail if the number is reduced or will not select all of the necessary items if the number is increased. When performing the final union, first the main body was selected, then the first fin, and then the rest of the fins using the group modifier and selecting the first one.
• There is no way to select the last, next-to-last, etc. item of a group of items created using Duplicate Entity, or when the locations of the entities were specified using a pattern. The selection is recorded as the first item, second item, etc. This can cause problems when the number of entities changes. For example, the command could fail because the entity no longer exists. Therefore, avoid using references to entities that were created using a pattern or the Duplicate Entity command if the number of entities created can change. One exception to this is that when using a rectangular pattern located by the corner, it is safe to reference the entity at that corner.
• When naming your part, you must keep in mind that CADDS has a limit of 20 characters on any one component of the name. (Components of the name are separated by periods.) When the parts are generated, seven characters are added to the last component of the master part's name, so master part names can't be longer than 13 characters.

The following section was added to the document, detailing the steps necessary to design a master part.

1. Determine which geometric features are necessary.
2. Determine the given dimensions of the part.
3. Determine an origin and orientation for the model.
4. Construct a modeling strategy for the part.
5. Determine how each entity in the model will be referenced.
6. Determine which parameters in the model will be controlled by variables.
7. Determine which variables will control each of these parameters.
8. Determine which of these variables will be controlled from the family table and which will be computed by constraint equations.
9. Create equations for all of the computed variables.
10. Review the modeling strategy, variables and equations to determine if the modeling strategy can be improved to use fewer commands, CSG modeling techniques, variables, and/or equations.

This methodology can be used to create all but one of the parts identified during the user survey at the start of the project. The only part that can't be made with this method is a family of rack panels. These panels have a variable number of mounting holes. These holes are not evenly spaced, so they can not be created in the same manner as the holes from the terminal strips. Overall, the methodology has been very successful.

Last Modified: Wed Aug 28 14:41:29 EDT 1996