There's probably a slim chance that you're ever going to use triangulation to fabricate octagon ductwork, even though it might offer an interesting change. However, the way homes and businesses are laid out, you may someday find yourself in a jam and need to make a square-to-triangular shape transition to get behind some wall. If you have a good understanding of how to work your way through a pattern, you'll have an advantage over the not-so-friendly areas workers are forced to place ductwork at times.

Figure 1

Draw a top and front view of the square-to-octagon fitting shown in Figure 1. It gives you all the information necessary to lay out this square-to-octagon pattern.

By referencing the two views, the true lengths of all the points necessary can be established. There is no need to make the true-length triangle that many fittings require to develop a flat pattern.

True-length lines can only be viewed in a drawing by looking at it perpendicular to its plane. Comparing two points in two separate views shows you whether a particular line is being viewed in true length. For example, in Figure 1, you would be looking at Line A-H in the top view, without the front view. At first, you might think the top view of A-H is on a downward slant. But by looking at Line A-H in the front view, you can see that from the top view you are looking at Line A-H perpendicular to its plane and seeing the true length of A-H.

You can also see in the top view that Line A-B and Line 1-2 also share the same plane. However, without the front view you don't really know if these lines are slanted downward. The front view shows what angle (if any) you're looking at when looking down from the top. In this case, it is showing that lines A-B and 1-2 are in true-length form because they're perpendicular to the plane.

Figure 2


On certain fittings, you'll have a choice of which of the three types of line development to use. You many wonder why one method was picked over another. When you have a thorough understanding of the different methods, you'll be able to make that decision without much thought. In the case of a non-tapering square-to-octagon fitting, triangulation is the only method to develop your pattern.

Before you begin the layout, it will save you time and steps if you understand that the octagon has eight equal sides, which are centered in a square that has four equal sides. Once you establish the true-length for Line 1A, this will be true for all similar lines such as 1H, 2C, 4D and 4E, etc. You'll save additional time if you use multiple sets of dividers. One divider set at the top true-length lines, one set at the (vertical) distances and the third set for the bottom lines: 1-2, 2-4, 4-3 and 3-1.

As shown in the left-side drawing in Figure 2, the pattern defines the height from the front view in Figure 1. Draw two lines parallel to each other as shown by connecting Point 1 to Point 2 and Point A to Point B. Next, establish the width of the base and mark the reference points No. 1 and No. 2, as shown. Using dividers or trammel points, set them at the distance from Point 1 to Point A on the front view shown in Figure 1. Using Point 1 as center in Figure 2, make an arc intersecting the Line AB to establishing Point A. Do this with Point 2 as well to establish Point B.

Figure 3


Turn your attention to the right-side drawing in Figure 2. It begins to establish the rest of the pattern, working to the right and left using triangulation. Setting the dividers to the distance from points A to H in the top view (Figure 1) and using Point A as center, draw a small arc near Point H in Figure 2.

Keeping the dividers set, repeat this by using Point B as center and striking an arc near Point C. With dividers now set to the length of Line 1A and using Point 1 as center, make an arc intersecting the one previously made from Point A. This intersection becomes Point H.

Now using the same dividers set to length of Line 1A, and using Point 2 as center, draw an arc intersecting with the arc made from points B to C. This intersection becomes Point C and completes two of the corners in the fitting.

In Figure 3, you establish the rest of the pattern, as you continue to work your way around the fitting. The next step in establishing the remaining sides is to continue using triangulation, using two points to determine a third point.

Set the dividers to the length of Line AB and using Point C as center, make an arc near Point D. Now with the same setting on the dividers and using Point H as center, strike an arc as shown near Point G. With dividers set to the length of Line 1B and using points 1 and 2 as centers, strike intersecting arcs at points D and G, as shown.

Figure 4
You should have used points C and 2 to establish points D and 1, and Point H to establish Point G. To establish points 3 and 4, you need dividers set to the length of Line 1-3 from the top view.

Using Point 1 as the center, draw an arc near Point 3, as shown in Figure 3. Because this fitting is square, you may keep the same setting on the dividers and using Point 2 as center, make an arc near Point 4. To establish the points for 3 and 4, set your dividers to length of Line 1A from any of the front views. Then using points G and D, draw an intersecting arc near the arc made near Point 3; repeat this to establish Point 4.

You can now proceed to establish the other points and finish developing the pattern using triangulation. It should resemble the pattern in Figure 4. Pay particular attention to the ends and notice that both contain the same reference numbers. This would be your seam. Because of the many different ways to develop finish seams, allowances were not included.

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