Two basic design goals for any master part should be robustness and ease of use. Parameters (described fully in Parameters) help you achieve both goals. By defining parametric values for some or all of the appropriate constraints on your part, you can reduce the number of constraints that the end user needs to modify directly in order to produce the variation. Other constraints can be parameterized to depend on the values supplied by the user. In complex situations, expressions and macros can be supplied to check or filter the user's input before acting on it.

If the constraints that must be modified by the user are themselves parameterized, the user need not reassign these constraints. Instead, they will appear in the parameter value table, which provides a much simpler interface for the occasional user.

For example, consider the following master part, which was used to generate the variations shown in the following figure:

For this part, we want to generate variations in which only the configuration of the large arm changes. In all variations, we know that the outer radius of the arm should depend on the inner radius. Rather than require the end user to calculate this relationship and manually Assign both radii for each variation, we use parameters to do most of the work.

After constraining the part normally, we assign a dimensional constraint to the inner radius and give it the parameter name I_rad The outer radius also gets a dimensional constraint, named O_rad. Figure 229 shows the constraints we supplied for this master part. The remainder were generated by Complete.

Using the parameter definition table, we define the constraint represented by the parameter O_rad to be (1.65 * I_rad). This defines the relationship between the two dimensions. As a result, the user only needs to give a value for I_rad to generate the variation. The I_rad parameter is available in the parameter value table, so the user can easily change it and then Solve for variations.

Another advantage of parameterization is that the parameter values can be stored and recovered for later use. The Save and Input functions in the Current Constraints dialog box allow you to write out the current parameter value table to a file and later restore it. This feature lets you build a "library" of value tables that can be used to drive the master part. The advantage of this is that a series of standard parts based on a single master part can be stored as a single MI file (the constrained master part) and a collection of parameter value files. This arrangement takes up much less disk space than if an individual MI file were stored for each part in the series, and is much easier to update if the geometry of the master part needs to be changed. To recreate any member of the series, the user need only load the master part, Input the desired parameter value table, and Solve. Generating Variations describes this functionality.