Some designs—for instance, a waveguide for electrical and RF signals, require 90º twists between two rectangular cross-sections. Although a smooth blend seems to work, the cross- section changes throughout the feature and doesn’t really create the required geometry.

The solution can be found with the variable section sweep, using the trajpar variable. Trajpar is a Pro/ENGINEER-controlled parameter that varies from 0-1 along the trajectory of the variable section sweep (i.e., it increases from 0 at the start, to 0.5 at the midpoint, to 1 at the end). It can be used directly in the sketcher to govern sketcher relations and dimensions.

To create the twist, it first helps to create datum curves that define the origin trajectory (the path the section origin will follow throughout the sweep) and the trajectory that defines the horizontal section vector.

1. Start a part with default datums and coordinate systems, and sketch two straight parallel datum curves 5.0” long (Fig. 1).

Figure 1. Trajectories for variable section sweep.

2. Choose Feature, Create, Protrusion, Advanced, Variable Section Sweep.

3. From the Sweep Opts menu, choose NrmToOriginTraj.

4. Select one of the curves as the origin trajectory. Choose Done.

5. Select the second curve as the trajectory defining the horizontal vector. Since this example only needs one horizontal vector curve, choose Done.

6. We want to make a twist of a rectangular cross-section through 90º. Using intent manager and constraints, create the following sketch (Fig. 2).

Figure 2. Twist section.

7. To create a sketcher relation to govern the angular dimension, select Sketch, Relations and enter

sd17=90*trajpar

where sd17 is the angular dimension.

8. Choose OK. You now have a twist with a constant cross-section (Fig. 3).

Figure 3. Constant cross-section twist.

9. To control the tangency at the start and end of the twist, you can modify the sketcher relation something like this:

sd17=45*(1-cos(180*trajpar))

Figure 4. Constant cross-section twist with tangency at both ends.

John McDougall is a design engineer at EMS Technologies in Montreal, Canada. He can be reached by e-mail at mcdougall.j@ems-t.ca.