Taking Advantage of the Variable Section Sweep–It's Easier Than You Think

By Keith Carew, BMW Oracle Racing

One of the feature types I find myself coming back to time and again is the variable section sweep (VSS). Although one of the most flexible and powerful features that Pro/ENGINEER offers, it remains sadly overlooked by users because it can seem complex and troublesome to use.

The variable section sweep creates solids or surfaces that are non-ruled surfaces, yet it can be similar to lofting through many sections like a ruled surface.   By incorporating relations and graph features into the section definition, it is possible to create extremely complex geometry in only one or two features.  

A Better Way to Create Surfaces

In traditional lofting, you create a surface by joining sections with splines. The splines form a latticework and the surface is controlled by the edges of the sections, the relative position of each section, and the stiffness of the splines bridging the gap between sections. As the lattice becomes denser, a surface emerges.

While this is one way to create surfaces, Pro/ENGINEER provides something much more powerful in the VSS.   It can be described as a lofted surface using an infinite number of sections. Instead of the space between sections being bridged with splines, the section spacing gets smaller and smaller until a surface emerges. The advantage is that you control every one of those sections along the way.  

Perhaps the most common way to control the shape of the section is to use multiple trajectory references to define the edges of the section being swept. But by using relations to control the section shape, you can create more powerful features. Even better, you can use a graph feature with the “evalgraph” relation function to control the section shape along the length of the sweep.

Here's an example demonstrating all three of these techniques.

The VSS in Action

Let's start with a datum graph feature that has a coordinate system at the lower left.   In this case, it is simply a sloping line with a value of 50 at the left and 25 at the right. The default name for this feature is GRAPH1. The x-value goes from 0 to 50. This feature is essentially a sketched graph of a function and must follow the same rules, such as only one y-value for each x-value.   In addition, a y-value must exist for all x-values out to the extent of the graph.

Next, I have created this simple sketched datum curve. These two curves serve as sweep trajectories for our feature. Note that all segments must be tangent. (As shown below, however, this is not a restriction for graph features that control the variable section sweep cross-sections.)

To make a variable section sweep feature, I select the straight line for the origin and the curved line for an additional trajectory.

I accept the defaults for the section plane control and so on, making the sketch/section plane perpendicular to the origin trajectory. When the sketcher UI appears, Pro/ENGINEER has created reference points where the sketch plane intersects the guide rails.

I will use those points to help me create my sketch. The sketch will be a rectangle with a radiused corner in the upper right. The upper left of the rectangle is constrained to be fixed to the curved guide rail, while the lower left is connected to the straight guide rail.  

Now, if I left it at that, I would have created a box that changes along its length only in that the height is controlled by the two trajectories. However, I have added relations to control both the width and the size of the radius to show what else you can do.  

Recall that the length of the graph feature was 50. The relation to control the width of the sketch is:

Sd4=evalgraph (“GRAPH1”, trajpar*50)

The function is evalgraph and the variables passed to it are the name of the graph feature to evaluate and trajpar, or an internal “trajectory parameter” that varies from 0 at the beginning of the origin trajectory and increases linearly to a value of 1 at the end. The trajpar parameter is multiplied by 50 to adjust its scale to that of the graph feature's maximum x-value.  

The control for the size of the radius is also affected by the trajpar function:

Sd5=trajpar*5+1

As this is evaluated , the radius increases from 1 to 6 linearly along the length of the sweep. The resulting geometry appears below.

Now to demonstrate how you can create geometry with sharp edges, I will change the graph feature.  

The geometry changes accordingly. Note that the sweep trajectories must be tangent or curvature-continuous along their lengths, but the graph feature does not have this restriction.   As a result, I would not be able to create this shape using a sweep trajectory to describe the edge with the kink in it.

Now that you have seen a few new tricks, play around with the variable section sweep to discover what else you can create.   Keep in mind the following restrictions:

• All sweep trajectories must be smooth and tangent or curvature-continuous.
• The ends of the trajectories control the extent of the sweep, so if you lengthen any trajectory, the shortest one limits the extent of the geometry in that direction and vice versa.
• You can use existing geometry for the trajectories.

See what else you can come up with!

Keith Carew is a mechanical design engineer at BMW Oracle Racing in Auckland, New Zealand.