The steps required to make the model shown in Figure 1 would have been reduced and simplified by using profile sketches that had an interior loop representing the inside wall of the duct. Incidentally I added the flanges on this model after creating the duct itself. Solids from 3D Helix Paths Since Release 3, Mechanical Desktop has used splines to make helix shaped 3D wireframe paths that can be used to sweep a profile sketch in creating a spring or, as shown in Figure 2, grooves in a cylinder. You will select a cylinder if you intend to make screw threads or a worm gear, and you will select a work axis if you intend to make a spring. You can also use this option to create a spiral.
Spirals grow laterally away from, rather than along, the axis with each revolution.
The centerline for both helixes and spirals is in the z axis direction. Once you have established the axis of the helix sweep path, Mechanical Desktop will display a dialog box for you to use in entering the curve's parameters. Since pitch, length and number of turns are interrelated, you can specify any two of these parameters to set the third. You can also specify that the helix will change diameter along its length. Changes in helix diameter are specified by an angle from the axis, rather than by a diameter increase per revolution.