Knowing how to make transitional elbows gives you the flexibility to make changes in direction and elevation in one fitting. You didn't see too many "old-time" sheet metal workers making two fittings when they could get by with one.
Why? Because that's what journeyman pattern-cutters did. They knew the art. And with the use of parallel or radial lines or triangulation, they could manipulate a sheet of steel into anything. So can you. This is all relatively simple stuff. Here's how it's done.
This article explains how to develop a 90-degree transitional elbow with radius heel and radius throat. Our radius-to-width ratio is 1-to-1 and drops 6 inches throughout the 90-degree elbow. The following steps will work with just about all transitional elbows and can be used as well in transitional Y branches.
Figure 1Figure 1 shows the top view or what is known as the "cheek" of the elbow. Even though you will not be using this as part of the finished pattern, you need to draw it out so you can get the necessary information to create the transitional cheeks.
Figure 1 shows points 1 through 5. The distance from Point 1 to Point 2 is the width of the elbow on the horizontal side. Bisect points 1 and 2 to establish the center between the two points. Mark this as A' and set your trammel points the distance from Point 1 to Point 2.
With that set, place one end of the trammel points on A' and using it as center, make an arc at Point A. Extend Line 1-2 horizontally until you intersect with the arc drawn as A. This point becomes your center for the radius 2-4 and the radius 1-5. Bisect 1-5 to establish Point 3, then draw a line that connects Point 2 to Point 3 and draw another line that connects Point 4 to Point 3.
This becomes the top view of the transition. Knowing that this is a transition, you should also know then that you are not looking at this in true-length form. To see the true length, you need to be looking perpendicular to its plane surface as with Line 1-2 and Line 4-5.
In the example, you are not looking perpendicular to lines 2-3 or 4-3, or arcs drawn from 2-4 and from Point 1 to Point 5. Therefore, they are not shown as true-length lines that you need to establish to develop the cheeks.
It does, however, give the true length of the heel and throat. This will be discussed in Figure 5.
Heel and throatTo determine the true length of the cheek's heel edge and the throat edge of the cheek, a true-length triangle is needed, as shown in Figure 2. The height from Point A to Point B in the true-length triangle is the height of the offset. The offset is from the edge Line 1-2 and rises 6 inches to the edge Line 4-5.
In Figure 1, Line 1-5 was bisected to determine Point 3. Do this with the throat by drawing a line from Point 4 to Point 2 and bisecting this line. At this point, draw a line perpendicular to Line 4-2 and extend this line, intersecting the radius throat, and mark this intersection Point 6.
Draw lines that connect all the points as shown in Figure 2 and include lines from points 2 to 6 and points 4 to 6. Points 3 and 6 are half the distance of the radius and therefore are half the offset. Take half of the height of Line A-B on the true-length triangle and mark it as Point C.
Transfer to the base of the true-length triangle all the lengths from the flat-cheek pattern in Figure 2, such as 1-3, 2-3, 4-3, 5-3, 2-6 and 4-6. Also, transfer the length of Point 4 to Point 2 to the base of the true-length triangle.
Once all these lengths have been referenced on the base, draw a line from each to the proper height B (6 inches) or C (3 inches). If you look at lines that extend to Point 3, notice that they do not rise to the complete height of the offset; they rise to half the height. Only one line drops the height of the complete offset and that is Point 4 to Point 2. And on the true-length triangle, note that the only line drawn to Point B, the height of the offset, is from 2-4.
Cheek patternsUsing the triangulation method, begin to develop the pattern of the cheeks as shown in Figure 3. As shown in Figure 3A, draw a line the width of the duct, shown in true length in Figure 2, from Point 1 to Point 2. Establish the first arc near Point 3 by setting your trammel points to the true-length distance from the true-length triangle (Figure 2, C to 1-3). Using Point 1 as center, make an arc near Point 3. Now rest the trammels to the distance of Point C to Line 2-3 on the true-length triangle. Transfer this distance to your pattern using Point 2 as center and making an arc that intersects with the previous arc drawn from Point 1 to Point 3. This intersection becomes Point 3.
Now look at Figure 3B. Create Point 4 using the same method as previously shown in Figure 3A, by transferring the true-length lines from the true-length triangle.
With Figure 3C, establish Point 5. Again, the true-length distance of Point 3 to Point 5 is found on the true-length triangle at Point C to Line 3-5. Using Point 3 on the pattern as center, make an arc near Point 5. The distance of Line 4-5 is the width of the duct that was shown in true length in Figure 2. Set the trammel points to the width of the duct (Line 4-5) and using Point 4 as center, draw an arc that intersects the previous arc created from Point 3. This intersection becomes Point 5.
Establish Point 6 using the true lengths from lines 2-6 and 4-6 on the true-length triangle. With the trammel points set, draw an arc using points 2 and 4 as centers, and this intersection becomes Point 6. Point 6 is the midway point on the radius arc from Point 2 to Point 4.
Once done with the steps in Figure 3, the frame for the cheek pattern has been established. You created the true-length triangles for the same triangles that you couldn't see in true-length form in Figure 2.
ArcsNow go to Figure 4 to establish the two arcs drawn from 1 to 5 and 2 to 4.
For the two arcs needed to define the heel and throat edges of the cheek pattern, you need to establish the center point, which will be used to make an arc through three points.
Treat the heel edge and the throat edge as separate problems, even though the established points, 7 and 8, may end up very close together. They are actually two different centers for the two arcs drawn and should be treated as such.
To establish Point 7 as shown in Figure 4A, bisect lines 1-3 and 3-5, and draw a line perpendicular to 1-3 and 3-5. Now extend these lines until they intersect; this becomes Point 7.
Setting the distance of the trammel points from Point 7 to Point 1, and using Point 7 as center, create an arc from Point 1 to Point 5. If done correctly, this arc will become tangent to Point 3. Depending on the type of seam you will be using, adjust the trammel points to allow for this, and again using Point 7 as center, make the second arc.
Repeat the previous steps with lines 2-6 and 6-4 to establish Point 8. Setting your trammel points from Point 8 to Point 2 and using Point 8 as a center, draw an arc to Point 4. Take the distance from Point 8 to Point 2, and deduct the amount necessary for the seam and make another arc, using Point 8 as center. Use this developed pattern to create a duplicate pattern for the second cheek.
Developing the heel and throatWhen it comes to developing the heel and the throat as shown in Figure 5, these are merely simple transitions. The true length for the heel and throat pattern is obtained from Figure 1, by measuring the length of Point 1 to Point 5 and 2 to 4. Do not confuse the length of the heel and throat, 1 to 5 and 2 to 4, with the true length of Point 1 to Point 5 or 2 to 4 (shown in red in Figure 5) on the cheek pattern in Figure 4.
Make sure to add the necessary allowance, typically 1 inch for the S-slips and drives on the ends and 1 inch for the Pittsburgh seams, as shown.
Double check your work before you cut out the pattern. Measure the distance from Point 1 to Point 5 and points 2 to 4, shown in red in Figure 5, and compare this measurement with those from 1 to 5 and 2 to 4 in the developed pattern, shown in Figure 4C.
Run the cheek patterns through the flange before the steps in the brake. To avoid crushing the flange area at Point 3, use your folding pliers and flatten this area just enough so that it won't be crushed in the brake. Complete your Pittsburgh edge on the heel and throat and finish them in the slip-roll machine.