Putting a twist fitting in ductwork
Use basic triangulation to make a twisted fitting
In this article, we will explain how to make a twisted ductwork fitting using basic triangulation. As more building interiors incorporate open spaces and angled ceiling lines, exposing ductwork, putting a twist in fittings becomes more important.
How can just a little twist make something so complicated? For the most part, twisted fittings are not complicated to make using basic triangulation.
Too many contractors make laying out the pattern complicated and fail to see what it’s supposed look like and that’s where they get into trouble. It’s hard - but not that hard - because too many can’t picture what it’s supposed to look like, they don’t know where to start.
Figure 1 shows a twisted fitting in both the plan and elevation views, twisted horizontally 45 degrees on center. Keeping the width and length the same on each end of the fitting, you can see in the plan view - and because the top and bottom planes are parallel to each other - you would only need to establish the height of the fitting. The elevation view has been included only to give you a better understanding of what you’re looking at.
By triangulating the pattern, one must establish several true-length lines. Begin this by drawing the plan view of the fitting as shown in Figure 2 and making references to the corners also shown. It would also help to understand why the corners are numbered. So if you take a moment and glance at Figure 2 and Figure 4, you should be able to see the relationship between the two; this will be clearer as you near the end of this article.
Beginnings
In this particular fitting, there are several places one can begin to lay out the pattern. Beginning with Line 1-3, because it’s in its true length, you can draw this to start and then continue to establish Point 2 and work your way across the pattern as shown in Figure 4.
Before that can be done, all the true-length lines need to be established. This can be done as shown in Figure 3 by creating a true-length diagram. To keep the drawing in Figure 3 clear and uncluttered, create two true-length diagrams. Both should include a horizontal line drawn from Line AB that represents the height of the fitting. Both also include a line from Line AC perpendicular to Line AB that will allow you to reference the distance between each corner now on the plan view.
You now need to establish the true-length line from Point 1 to Point 2 on the pattern as well as the others, but keep in mind that if the twist is rotated on center, your rectangular openings are the same length and width. In other words, the distance from 1 to 2 will be the same as points 5 to 6.
It is often mentioned to keep things symmetrical. Take the distance of 1 to 2 from the plan view of Figure 2 and transfer this to the true-length diagram as shown from A to a” in Figure 3A. Now take the distance from 2 to 3, also from the plan view, and transfer this distance to the true-length diagram as shown from A to a’ in Figure 3A. Again, this is the same for points 6 to 7.
Distances
The established distance from Point a” to point B in Figure 3A is the true-length line from 1 to 2, as well as the established distance from Point a’ to Point B. This is the true-length line from Point 3 to Point 2.
Continue around the fitting and establish the true- length lines with the use of the true-length diagram as shown in 3B.
Once all your true-length lines have been established on the true-length diagrams, you can begin to set your trammel points and transfer the distances of each true-length line to the pattern. If you look at Figure 4, you’ll see the blue lines, which are the true-length lines from the true-length diagram 3A and 3B, and you’ll see the red lines that are transferred directly from the plan view as they are already shown in their true length on the plan.
It would be to your benefit to have three trammel points on hand. Set one to the short side (X) of the rectangle and one to the longer side (Y) of the rectangle, as these will remain constant throughout developing the pattern. The third trammel point may be used to transfer the true-length lines from the true-length diagram, shown as the blue lines in Figure 3 and Figure 4.
The trammel will need to be reset as you work through the pattern.
Begin to lay out the pattern. On the next few steps, you’ll be using your dividers or trammel to transfer the true-length lines necessary to develop the pattern. To begin, start by taking the distance of Line 1-3 from the plan view in Figure 2 and draw a straight line this length on the material you’re using for the pattern, as shown in Figure 4. Reference these points as 1 and 3, as shown in Figure 4.
Arcs
Set your trammel points to a” and B on the true-line diagram in Figure 3A and transfer this length to the pattern by setting one end of the trammel on Point 1. Using Point 1 as a center pivot, make an arc near Point 2, as shown in Figure 4.
Reset your trammel points to a’ to B on the 3A diagram and transfer the points to the pattern, using Point 3 as center and making an arc that intersects the previous arc drawn near 2.
This intersection establishes Point 2 on the pattern. Line 2-4 is true length in plan view; using the trammel points called X and place one end on Point 2. Using 2 as a center pivot, strike an arc near Point 4.
To establish the exact location of Point 4, set your trammel points from b’ to B in the diagram in Figure 3B. This is the true length of Line 3-4, and using Point 3 on the pattern as a center pivot, draw another arc that intersects the previous arc drawn from Point 2. This intersection is Point 4.
The next step would be to establish Point 5, shown in Figure 4. Using the X trammels and using Point 3 as center pivot, strike an arc near 5 as shown. To establish Point 5, the trammels must be set to the true length of Line 4-5, which can be taken from b” to B from Figure 3B. Using Point 4 as center pivot, strike an arc that intersects with the previous arc drawn from Point 3 to Line 5. This intersection is Point 5.
Continue through the points in a similar fashion to establish points 6, 7, 8 and 1a. Remember that the distance from 1a to 2a is the same as 1 to 2. This is why it was stated that it’s somewhat important how you decide to reference the points. Following the fitting around and numbering the points in some order allowed you to do the same as you developed the pattern.
This pattern does not allow for any seams or flanges. All folds are valley folds and the actual blue lines from point to point become the bend lines. There will be a bend upward on all the bend lines with the exception of 1 to 2 and 1a to 2a.
These same steps can be used with just about any degree of rotation as well as the rectangular openings can be different in size. The only difference will be that you’ll need to establish more true-length lines on a true-length diagram.
Visit Wisconsin contractor Bud Goodman’s Web site, www.TheSheetMetalShop.com, for free worksheets that you can use to brush up on your pattern-development skills.
Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!
