That's when it would be good to know how to develop a simple "parker" fitting. As illustrated in Figure 1, it is not much more than an extended plenum with a top takeoff - except this one has a radius heel that helps get the air moving with less resistance.
ShortcutThere is a shortcut to this layout that shows similar results. However, first study Figure 2 and learn how to develop this pattern using and understanding line projection to obtain two radius bends.
Drawing A in Figure 2 illustrates a transition without the radius heel. The layout for this fitting in Figure 2-A is similar to a typical reducing transition because the height of the two sides is the same. This can be easily created as shown in Figure 2-A by combining the two sides and top together.
Also shown in Figure 2-A is the outline of the section that will be cut out for the takeoff. This cutout should be done before forming the small, 90-degree flange part of the seam. If you're not concerned about airflow resistance, you can make an end cap for the small end of the transition and the fitting is completed. But for those who are concerned about the resistance from one piece of duct to the next, you can lay out a radius heel that will assist airflow as shown in Figure 2-B.
For clarity, the cutout will not be in Figure 2-B or C as it was shown in Figure 2-A.
To take the fitting as shown in Figure 2-A and lay out a radius heel as shown in Figure 2-B, set your dividers to the width of Side 1. On Line AB, using B as center, make an arc that intersects Line AB at R, as shown.
Keeping the dividers set to the width of Side 1 and using R as center, now make an arc from Point 1 to Point B as shown in Figure 2-B. Divide the arc into six equal spaces and reference them as shown. Project each of these points, one through six, perpendicular to Line AB until they all intersect Line CD as shown in Figure 2-B.
Where points 1 through 6 intersect on Line CD, draw a line perpendicular to Line CD as shown in Figure 2-C. Using your dividers, transfer the length of each line from Side 1 to Side 2, as with Line 2 shown in Figure 2-C. By projecting these lines this way, you'll be sure to have your radius bend run parallel throughout the curve from 1 to B's Side 1 and 1a to D on Side 2.
Developing the heelWhen developing the piece for the heel, the length will be Point A1 to Point 1, plus the length (stretch-out) of the curve from Point 1 to Point B in Figure 2-C and shown in Figure 3-A.
In Figure 3-A, the distance from C1 to D is the true length of C to D in Figure 2-C. The distance from Point 1a to Point D in Figure 3-A is the true length Point 1a to Point D in Figure 2-C.
Although there are no allowances for seams in prior drawings in this article, it does show that an allowance has been added for the Pittsburgh seam, only to demonstrate that the curved stretch-out from Point C1 to Point D is on the same plane and allows you to use a Pittsburgh seam.
A question that often comes up is, "How can you feed this part of the fitting through a Pittsburgh machine if the curve is on an angle?" The answer is that the curve, as shown in Figure 3, is still on the same plane.
A shortcut to obtaining the radius from Point 1a to Point D in Figure 2-B is knowing how to make a circle that will connect three random points. This is basic geometry shown in Figure 4.
Figure 4Start by making three dots similar to A, B and C in Figure 4. Draw a line that connects Point A to Point B and another one that connects B to C. Bisect Line AB and BC. To bisect the line properly, the arc shown by a red dotted line drawn using Point A as center, must be the same measurement as when you use B as center to make the intersection near points 1 and 2.
Draw a line from Point 1 to Point 2 and extend this line as shown near Point D. Repeat these steps with bisecting Line BC and so forth. The intersection of these two lines at D becomes your center. Setting your dividers the distance from Point D to Point A, make an arc to Point C. Your arc will include Point B, if done correctly.
Finishing upNow to apply this method to the fitting as shown in Figure 5 (A and B). Starting with Figure 5-A, establish Point 2 by bisecting the arc drawn from Point 1 to Point B. Now extend a line as shown from Point 2 to Point 2a, perpendicular to Line AB, which establishes the intersection Point 2a on Line CD.
Now again from Point 2a, extend from this point, a line perpendicular to CD. The distance from 2a to 2b is the same distance from Point 2 to Line AB. With having points 1a, 2b and D, there are now three points that you can connect as shown in Figure 5-B. Use the method described for Figure 4.
No allowance has been added for seams. However the small flange for the Pittsburgh seam would be added to Side 1 and Side 2 in Figure 2. And the lock side of the Pittsburgh seam will be added as shown in Figure 3.
(For more information on manual sheet metal layout and exercises, visit www.thesheetmetalshop.com).