Machine Design

Streamlining assembly modeling with design-based patterning

Design-based patterning adds intelligence to model creation and modification.

Kris Kasprzak
Program Manager Unigraphic Solutions
Huntsville, Ala.

Edited by Stephen Mraz

The model of a conveyor table can use patterning to fix the number of legs, their relative positions, and accurately place mounting holes for them. Patterning can also assure there will be enough rollers to span the entire length of the table, even if the length is changed.

Engineers and designers are always looking for ways to squeeze the last drop of performance from their CAD systems. One way is to make design-based patterning a regular part of your modeling routine. Used properly, it lets engineers model mechanical assemblies faster and more accurately. In general, patterning lets CAD software calculate the number of features, along with spacing and hardware requirements, after users provide certain dimensions for the finished product. Any changes in those dimensions cues the software to recalculate the number and placement of features.

In fixed patterns, the most common type, users specify the number of identical features for the model and the distance between them. For example, an engineer might want support legs placed every 2 ft. The software appropriately places the legs, along with attachment holes and hardware. For circular patterns or assemblies, users determine both the overall arc angle and the angle between each feature. For example, an assembly might arc around 180° and have a radial spoke every 10°; the computer would place 18 spokes on the half circle.

The example in this article, a conveyor table, uses a fixed pattern to generate a series of mounting holes for support legs. Support legs will be attached to the frame with a series of uniformly placed bolts. Because all the legs will be similarly mounted, a fixed pattern is well suited for this task.

This simple type of patterning has a major limitation — it cannot handle variations. For instance, a new conveyor that requires heavy-duty legs may also need a new mounting hole pattern. When many changes are expected, fixed patterns may inhibit productivity. Although some equations could be established to control the number of holes, supports, and spacing, manual placement may be the method used.

Design-based patterning, such as that incorporated in Solid Edge, offers some solutions to these problems. Let's look at the situation in which spacing between features is constant but the number of features may vary. An example would be the mounting holes for rollers in our conveyor assembly. The software defines the number of rollers needed based on a boundary that encompasses the entire conveyor table. In this case, the complete length of the support channel defines the boundary. The software fills the area with a computed number of rollers at user-defined spacings. If the conveyor's overall length changes, the number of rollers automatically adjusts. For this method to work seamlessly, the overall boundary size needs to be a multiple of the spacing or there will be unequal spacing at one end.

Another method of patterning deals with fitting a specific number of identical features to a specified area. The system fills the design boundary with the defined number of features and adjusts the distance between each occurrence. The number of legs in the conveyor assembly provides a need for this class of pattern. It may be desirable to have several legs supporting the conveyor frame, two at either end and several at the center. As the conveyor length increases or decreases, the number of legs stays constant, but the spacing between them changes. While this could be done by simply placing dimensioned holes at the proper locations, design-based patterning computes these locations automatically, and updates them if there is a change.

In our example, the conveyor table, a fixed pattern placed mounting holes for the legs, three on each side. A filled pattern positioned the rollers, and a fit pattern controlled the number of legs. Increasing the length of the side rail length, for instance, does not change the relative position of the support legs or their mounting holes, and the number of support legs is still three. However, spacing between the legs increases to accommodate the new length, and the number of rollers increases as do the number of mounting holes for the rollers. Design-based patterning adds the required mounting holes while maintaining the same distance between each. Some systems recognize this type of pattern for part placement at the assembly level. This reduces modeling time and improves model integrity despite changes.

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