Figure 1

There are a few simple steps to developing this square-to-round Y branch or what some know as a two-pronged fitting using triangulation. This fitting is symmetrical in shape, as shown in Figure 1. If you have ever developed a square-to-round fitting you should find this fitting uncomplicated to complete. The challenge here and main focus will be developing the area of the two branches creating the intersection.

Keeping the height of the intersection equal to the radius of the round part of the base will make developing the pattern much easier.

To begin, first develop an elevation drawing, as illustrated in Figure 2. Draw a random-length line as shown from A to B. From this line, measure up and reference the desired height of the fitting and draw a second line from Point C to Point D, parallel to Line AB.

Near the center of Line AB, draw a perpendicular line as shown from Point 1 to Point 3 establishing the plane line of the intersection.

Set the trammels to half the diameter of the round section and using Point 1 as center, make an arc from 2 to 3b. This establishes the diameter of the round section or what will be the base of the fitting.

Figure 2


At the same time and with the same trammel setting, using 1 as center, draw an arc from Point 2 to 1a to establish profile No. 1 of the round section. Strike another using Point 1 as center from 3b to 3a establishing  profile No. 2, which also becomes the height of the intersection between the two branches.

Divide each profile into equal sections,  remembering that if you divide the distance of Point 1a to Point 2 into four equal sections, you must also divide 3a to 3b into four equal sections. Mark these and reference them as shown: 2b, 2c and 2d and also 1c, 1b and 1d. You can choose to divide these into any amount of sections; however, the more sections the easier it will be to define your pattern along these lines that we are establishing the points for.

From the plane Line AB, draw perpendicular lines to each 2b, 2c and 2d in profile No. 1 and from the plane line, Point 1 to 3, draw perpendicular lines to each 1b, 1c and 1d in profile No. 2, also shown in Figure 2.

The last step in Figure 2 will be to establish the location and top plane line for the square branches. On Line CD, measure over from Point 3 to Point 4 one-half the desired distance between the two branches and then from Point 4 to Point 5 will be the width of the square opening.

Figure 3


Figure 3 is about establishing all the necessary true-length lines for the pattern. There are two sets of true-length lines to make for each half of this pattern as shown in figures 3B and 3C.

Figure 3B shows the true length for lines connecting Point 5 to all the points on the profile drawn from 2 to 1a as shown in Figure 3A.

Figure 3C shows the true length lines connecting Point 4 to all the points on the profile drawn from 3a to 3b, also shown in Figure 3A.

To develop the true-length diagram as shown in Figure 3B, draw a horizontal line at any random length. Draw a line perpendicular to the horizontal line as shown from (E) to 5”. The length of this line, (E) 5”, is transferred from the plan view of Figure 3A.

Using the trammels transfer the distances of 5 to 2, 5 to 2b, 5 to 2c, 5 to 2d and 5 to 1 of the elevation drawing in Figure 3A to the horizontal line in Figure 3B and reference them accordingly, using Point 5 as the fixed point for all.

Now from each point on the horizontal line in Figure 3B: points 1, 2d, 2c, 2b and 2, extend lines perpendicular to the horizontal line. The lengths of these lines are transferred from the respective profile section (profile No. 1) in the elevation view of Figure 3A as shown by the blue dividers.

The last step is to connect Point 5” of Figure 3B with all the sectional lines as shown, these last lines are the true-length lines you’ll use to develop the pattern.

Repeat this last part to develop the true length lines as shown in Figure 3C.

Figure 4

Half patterns

To begin the half pattern as in the shaded area of Figure 4, draw a vertical line (shown as a dashed line in Figure 4). The length of this line, E2, will be found as the length of E to 2 in the true-length diagram. At Point E, draw a random-length line perpendicular to Line E2. Set your trammel to the distance of E to 5” from the true-length diagram, and using E as center, draw an arc to establish Point 5 on the pattern. If you were developing the complete pattern, you would strike a complete circle that would also establish Point 6 as well as repeating the same steps from side to side as you make your way through developing the pattern.

Draw a line from 5 to 2 on the pattern; this can also be checked for accuracy from the true length diagram by measuring against the length of 5” to 2.

Set your trammels from 5” to 2b of the true-length diagram and using Point 5 on the pattern, strike an arc near Point 2b on the pattern. Set a second set of dividers or trammels fixed to the distance of X to X1 in Figure 3A. Using 2 as center strike an arc intersecting the previous arc drawn from Point 5; this establishes Point 2b. You can use the fixed trammels and draw another arc as shown near 2c. Transfer the distance of 5” to 2c of the true-length diagram using 5 as center and draw your arc intersecting with the previous arc drawn from 2b. Repeat this for 5 to 2d and 5 to 1 in the pattern.

The next step is to establish Point 4 in the pattern. Because your branches for this fitting are square, you can set the trammels from 5 to 6 in the pattern and using Point 5 as center, draw a partial arc as shown near Point 4 in the pattern. Understand that you don’t know the exact location of Point 4 just yet.

Figure 5

Reset, repeat

Reset the trammels to the distance of 4” to 1” of the diagram. Now using Point 1 of the pattern as center, draw an arc that intersects the arc drawn from Point 5 to establish Point 4. From here in the pattern, repeat the same steps used to establish lines 5 to 2b, 2c and 2d.

When points 4 to 1d, 4 to 1c, 4 to 1b and 4 to 3a have been established, set your trammels to the distance of Point 4 to Point F from the true-length diagram in Figure 4. Using Point 4 on the pattern, draw a partial arc near F as shown. From the true length diagram, set your trammels to the distance of  (F) to 3a and using 3a in the pattern as center, draw an arc that intersects the previous arc drawn for F.

This intersection becomes point F. Line F to 3a in Figure 4 is the seam edge.

Figure 5 shows the completed pattern; two pieces are required. All the lines are brake lines and lightly bent upward until the half fitting is formed. This pattern does not allow for any seams or flanges.

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