There are a great many styles of “tee” fittings.
The tee fitting allows you to branch out into different directions.

Figure 1

There are a great many styles of “tee” fittings. The tee fitting allows you to branch out into different directions. They range from your most basic type, often found at the local wholesaler, to the more complex that require transitions or heavy-gauge or stainless steel.

When needed, these specialty tees can be laid out quickly, especially if you understand the different principles of developing patterns and learn when and where to use a particular method.

Often tees are made up of two separate pieces, one being the main duct and the second piece being that of the branch. The branch can be developed so it attaches to the main duct at any angle and is a good way to lessen the friction as the air or material passes from the main pipe through and into the branch. You can also develop a tee that many old timers may know as “frictionless.” Many don’t believe there can be a way to change direction without some sort of friction, but you can lessen the friction at the tee by the way it is designed.

Figure 2


Use triangulation to work your way around this tee. To develop the pattern for the tee shown in Figure 1, the first step will be to draw an elevation view as done in Figure 2, including the outlet length of the branch (Profile B). Establish the profiles A, B and C as shown in Figure 2 and divide these profiles into equal parts.

At each division along the profile, project a line perpendicular to the elevation drawing now establishing points 1 through 7 (near Profile A) and points 1a through 4a near Profile B.

Because this particular fitting is symmetrical, you only need to figure out one quarter of the tee, as shown in the gray-shaded area in Figure 3. You’ll use the same information for the other corners (see Figure 5). The red-shaded area in Figure 3 is of the pipe’s bottom side; this can easily be transferred to the pattern with a couple of different methods.

Figure 3


In Figure 3, you’ll also notice the left and right sides and how you’re able to eliminate the sharp corner of the branch.

Making the corners or gray-shaded area will be done using triangulation. All one would really need to do in this case is develop the pattern for one corner and use that same pattern for the remaining corners.

In Figure 4, we draw a true-length triangle to establish the true length lines of Point 4-3a, Point 3-2a and Point 2-1a. To do this, transfer the distance of Line 4”-3” of the end view to Line AC of the true-length triangle. Now use Point A as center and strike a mark to establish Point 3” as shown. Do this with liens 3”-2” and 2”-1”.

Figure 4


Now on Line AB of the true-length triangle, transfer the distance of points 4-3a, 3-2a and 2-1a from the elevation view in Figure 4. Connect these points on Line AB with the corresponding points on Line AC, which will be Point 3” to Point 4-3a, Point 2” to Point 3-2a and Point 1” to Point 2-1a.

They are only connected for illustration purposes; once you’re comfortable with these true-length triangles, you’ll only need to reference the points.

In Figure 4A, draw a line from points 4 to 4b. This is the length of the tee, minus any collars. Using two sets of trammel points, set one to the distance of one division on the profile such as Line 2’-3’. This one will remain the same. The other one will be adjusted as you develop the pattern for the larger distances.

Set the trammel points to the distance from Point 4 to Point 4a in Figure 4. Then using Point 4 as the center, strike an arc near Point 4a, as shown in Figure 4A. Keeping the trammel set to the same distance, using Point 4b as center, strike an intersecting arc near 4a, establishing Point 4a.

Figure 4a


If you’re going to lay out the entire pattern end as shown in Figure 4A, you’ll find it faster and more efficient if you do all four sides while your trammels are set as shown in Figure 5.

Using the smaller trammel point, draw an arc from Point 4a to near Point 3a and again from Point 4 to Point 3. Now to establish Point 3a, set your trammel points to the distance from the true-length triangle Point 3” on Figure 4’s Line AC to Point 4-3a on Line AB.

And using Point 4 in Figure 4A as center, make an intersecting arc establishing Point 3a. The distance from Point 3a to Point 3 on your pattern is transferred from Point 3 to Point 3a in your elevation view (Figure 4).

Using the smaller trammels, make two arcs. Using Point 3 as center, make one near Point 2 and again using Point 3a as center, one near Point 2a.

Before you can establish Point 2, you must establish Point 2a. To find the real length of the distance from Point 2 to Point 2a on the true-length triangle as described earlier, set your trammel points and using Point 3 in Figure 4A as center, make an intersecting arc now establishing Point 2a. The distance from Point 2a to Point 2 is found - and transferred - from the elevation view.

Figure 5


Finish the remainder of the points until you establish Line 1a-1 on all sides. In Figure 5, there is a red-shaded section; this is the same as the red-shaded section in Figure 3. This section can be developed using either triangulation or a simple parallel-line method.

No allowances have been made for seaming. You would need to add to the pattern any for your collars and the two seams.

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