Many sheet metal workers have probably used this method in one form or another during their careers. It may have been to make a simple rectangular piece of duct or a complex, ornate conductor head, sometimes called a “scupper box” or “collector head,” which are often fabricated by sheet metal artisans the same way today as they were a hundred years ago.

You’re going to use the parallel-line method to develop the pattern of a straight piece of round duct that intersects another round duct of a larger diameter, as shown in figures 1 and 2.

The red-shaded areas in Figure 1 and Figure 2 are all straight pieces of duct that intersect another duct. The only difference between Figure 1 and Figure 2 is the angle or miter of intersection, however you can clearly see that similar steps will also work for Figure 2.

As shown in Figure 1, draw both the elevation view and the end view of the two pieces that intersect one another. When the end view is completed, you will have also established the profile for the No. 2 section, shown as Profile 2.

Division

Draw a profile of the No. 1 section shown in Figure 1 as Profile 1 and Profile 1a. Divide the circumference of the Profile 1 and the circumference of Profile 1a into equal sections and reference as shown.

For instruction purposes, you’ll typically divide the circumference of the duct into 12 equal parts or three sections per quarter. Pay particular attention to how you reference these points. Make sure when you’re looking at the elevation view in Figure 1, you’re looking at the right side of the end view in Figure 1.

The reference numbers in Profile 1 must correspond to the correct section of Profile 1a. Draw lines from points 1’, 2’, 3’ and 4’ of Profile 1 perpendicular to the plane Line XX’ and extend these lines into the elevation view near a’, b’, c’ and d’ as shown.

Symmetry

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Because this fitting is symmetrical, the points a’, b’, c’ and d’ when extended from points a, b, c and d also answer for points e, f and g, as shown on the established miter line in the elevation view of Figure 1a.

Now on to laying out or stretching out the pattern for No. 1. Draw a line as shown from Point 1” to Point 1’” perpendicular to the piece being developed in the elevation view, shaded in red. Using your dividers set to the distance of one section measured from the Profile 1 or 1a and along the line extended from Point 1” to Point 1”’.

Make the same number of references as your circumference was divided into (12 equal sections in this drawing).

The lengths of each line, 1” a”, 2” b”, 3” c” etc., can be established by either projecting lines from the view in the elevation drawing or by using your dividers and transferring the distance from the piece in elevation view to the corresponding line of the pattern. This pattern is rolled and seamed. There are no allowances added for seams.

To lay out the pattern for the section of duct in Figure 2, you would repeat the same steps as done for the similar section of duct in Figure 1. However, understand that as the drawing is shown in Figure 2a, the pattern would still have to project perpendicular to the drawing in the elevation.

Reduce this layout to only what is needed and then add the rest.

When you become comfortable with these fittings, you might save some time and space to only develop the pattern that is absolutely necessary as in the shaded areas of Figure 3. Once this is completed you then have the correct length and all you would need to do is extend the height to whatever the requirement should be. This is also a good pattern to save; it will probably be used again.

Visit www.TheSheetMetalShop.com for printable pattern worksheets and to learn how to make the matching pattern necessary for the opening in the horizontal piece of the elevation section No. 2.