Parametrics Or Free-Form Modeling: When to choose which

Feb. 22, 2001
There are times to design with parametric features and times to work with free-form versions.

John Wright McCullough
Product Manager

André Beaupre
Senior Application Engineer
Marlborough, Mass.

Edited by Paul J. Dvorak

A fixture, version one
The simple fixture provides an example with duplicate features, an indicator that parametric modeling methods would work well if changes are needed.

Version two
The designer could write parametric features into the model when it's known how and what details are likely to change. In this case, a single equation could assign a height value for the middle posts.

Version three
The third version of the fixture shows that each post has a different height. If these were anticipated, free-form solid modeling may be the better tactic. If the model were created with parameters, an engineer using Cadkey could recover by saving it as a B-rep version, and then making alterations.

Each dimension on the part could be a parameter, governed by another parameter. Such a complicated setup would make unanticipated yet required changes difficult to produce.

A curved cover
The cover has several complex curves that would make free-form modeling methods difficult to apply. A better approach would be to break the model into smaller, more manageable operations that can be modified with parametric methods.

A pin
The pin with a hex head is made of four primitive solid shapes and a Boolean add and subtract operation. Because it's made of planar and a full cylinder face, it is completely editable using free-form functions. Modelers such as Cadkey come with free-form editing features and allow adding parametric operations as needed.

A part with snap fits
The cylinder has a gap and repeated features, the snap fits. These indicate a model more easily modified with parametric features, assuming the prongs are evenly spaced.

A few advantages of designing in 3D solids include associative detail drawings, creating complex features, and making quick changes to a design. What is often lost in promotions for the technology is that modelers come in two flavors: parametric and free-form. Each has advantages.

Parametric solids for example, are defined by a series of geometric operations and relationships. In most cases, designing parametric models takes more training. But the effort pays off when a model needs changing. Free-form solids, on the other hand, store only topology — no rules or equations describe geometry.

Both offer powerful capabilities. The hazard is that designers may use one when the other would work better. Examining several design situations that call for one tool or the other can help decide which is more practical.

Parametric solid modeling, for example, is generally more practical than free-form techniques when constructing iterative designs, families of parts, multiple mating parts, and duplicate features within a part. Iterative designs are those in which parameters must be cycled and tested through several variants. Properly constructing a single parametric model and its associated drawing allows the designer to quickly create and propagate changes through an entire family of drawings.

Multiple mating parts are another application. Changes to one mate often introduces the need for changes to the other. They can be linked to a single parameter to correctly rebuild the part models and drawings of an entire assembly by adjusting, in some cases, a single variable. Parametric solids also allow for the timesaving association of design features, such as a group of holes that behaves together as a bolt circle.

The basic construction technique in para-metric solid modeling is to first create a 2D closed sketch and drive it along a path in space to define a 3D shape. This is also the most common technique in free-form solid modeling. Constructing a model like the accompanying blue fixtures can be similar with either approach. Decisions at this early stage about what profiles should be used are critical to the flexibility of the parametric solid model. Proper selection requires training, experience, the ability to anticipate potential changes, and time to think them through.

Problems arise when an unanticipated change violates design assumptions built into the model. A designer might, for example, anticipate only height changes while change requests include width adjustments. In such a case, the original parametric model might be no use at all, leaving users to start over. Many designers using parametric solid-modeling tools don't realize that free-form solid-editing tools can move and extend faces, remove blends, and quickly change the size of simple holes and bosses. Because free-form solids don't depend on design assumptions, it's possible to quickly recover from unanticipated changes. The original profiles are not related to future edits. Free-form modeling requires only that designers anticipate which profiles will create the part in the least number of steps.

The first of three blue fixtures is built assuming constant symmetry. A quick way to construct the model with either approach would be to extrude the three profiles needed to define a single quadrant of the model and mirror it horizontally and vertically. The parametric model easily handles the changes shown in Version Two but the changes in Version Three would require starting from scratch.

Free-form solid editing, however, easily makes the changes in Version Three, but Version Two would require a significant number of steps involving trimming and rebuilding of the widened tabs. The lesson is that parametric solids are not necessarily the better choice for handling design changes when they cannot be predicted.

This simple case also shows it is possible, with a little extra thought, to create a para-metric model from a different set of profiles that will easily handle the changes in Versions Two and Three. But if the design does not change, the extra time put into the steps will not pay off.

Constructing the curved cover is a similar operation using either modeling approach. But unlike the blue fixtures, selecting appropriate profiles for this model is straightforward and will not take extra planning using parametric solids.

However the part has moderately complex topology that excludes practical use of free-form solids editing. Only several of the part's more than 20 features can be easily modified using those tools. Free-form modeling techniques are not well suited to edit the complex interrelationships between faces that often result from semicircular and tangent profile entities, or those formed by shelling and blended intersections of drafted nonperpendicular faces. The cover has all those conditions. While such complications may slow the reprocessing of similar para-metric models, the approach can break the model into separate and manageable operations, and easily make most changes.

The discontinuous cylindrical snap fit provides a good example of duplicate features. One way to create the six prongs of this design feature would be to rotate a profile 30° to form a single prong and evenly space it five more times about the part. Suppose the designer knows that spacing will remain even. A parametric solid could track the copy operation and allow adjusting the diameter, number of prongs, or included angle at any time while keeping an even spacing. Free-form editing of the included angle is not difficult but would require selecting 12 separate faces that cannot all be visualized well from any single viewing perspective. In addition, changing the diameter or number of prongs using a free-form approach is time consuming. Many systems would involve additional manual trigonometric calculations to determine proper offset distances, something that's easily written into parametric rules.

A simple adjusting pin provides a case in which the free-form solid approach is clearly more practical than parametric methods. Constructing the pin with free-form operations involves creating and positioning just four primitive solid shapes (one polygon and three cylinders) and two Boolean functions to add (join) and subtract them. Because the solid is made entirely of planar and full-cylinder faces (unlike the previous example), it is 100% editable using free-form face-manipulation functions. Parametrically creating the same part would require creating one to three offset planes, three to four sketches, three to four features, eight dimensions, and 15 to 20 constraints. Neither parametric nor free-form solid modeling should be the automatic tool of choice for designing a part. Understanding the likely changes to a part are important when selecting a design tool. Many everyday parts such as the pin and Version Three of the fixture are best handled using solid modeling's free-form approach. The method has limits and requires the use of more advanced hybrid solid/sur-face functions to handle more complex cases. Understanding your parts and the choice between free-form and parametric solids helps complete original designs and design changes more quickly and efficiently.

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