When developing fittings make them simple and symmetrical.

Many people have heard the old acronym "KISS," or "Keep it simple, stupid."

For sheet metal workers, perhaps a more appropriate meaning might be "Keep it simple (and) symmetrical."

A big part of most workers' jobs is developing fittings and laying them out as efficiently as possible. Making patterns that can be used repeatedly will save plenty of labor later.

Another way to save labor is to keep your fittings symmetrical. That means you have only one part of the fitting to create and the others can be drawn off the same blank.

The illustration in Figure 1 is of a symmetrical fitting, a right-angled transition from large, round duct to a smaller round duct. Maybe you would see this on larger spiral jobs, but it is also used on smaller ductwork. By using symmetrical layout on large fittings, it would be easy to break the fitting up into several smaller pieces for easier fabricating and assembly.

Figure 1

Right angles

To develop this right-angle transition using triangulation, begin by drawing two views, a top view and an elevation view. This is shown in Figure 2. The objective is to establish the points, which will be the ends of bend lines and help obtain the necessary measurements in which to create a true-length triangle, which is needed to develop the final pattern.

For you to see exactly where the bend lines are located, you must draw the necessary profiles. The profile is the end view of the fitting in its true form. Divide all profiles into equal spaces, as shown in Figure 2. Reference each point, as shown in the profile, as Nos. 2, 4, 6 and 8 and project these points to the top and elevation views from Profile 1 and Profile 2. Do the same with Profile 3 and project down to the elevation view, as shown. This now establishes where the bend lines will be. However, you are not seeing them in their true length, so you must develop a true-length triangle using this information.

Now that all points are referenced on both the top view and elevation view as shown in Figure 3, draw a line connecting the points as shown: 1 to 2, 2 to 3, 3 to 4, 4 to 5, 5 to 6, 6 to 7, and 7 to 8, in the top view. Do the same with the points in the elevation view: 1a to 2a, 2a to 3a, 3a to 4a, 4a to 5a, 5a to 6a, 6a to 7a and 7a to 8a.

Figure 2

Obstructed views

Although you cannot see the true length of lines in either view, in the top view you can use the distance from Point 1 to Point 2 through Point 7 to Point 8 as the base of the triangles you are about to make.

Using trammels or dividers, transfer all the lengths of 1 to 2, 2 to 3, 3 to 4, etc., from the top view to the horizontal base of the triangle, as shown in Figure 4. From the elevation view shown in Figure 3, and setting the trammels from Point C to Point 2a, strike the height on the vertical bar of the triangle. Do this with Point C to Point 4a, C to 6a and C to 8a as well.

Figure 3

True length

Also in Figure 4, connect the corresponding numbers in the triangle as shown to establish the true length of each line. For instance, if you have reference mark 1-2 on the base of the triangle (Figure 3), this is connected to 2a on the vertical bar. That's because if you were to look at Line 1-2 in the top view of Figure 3, the elevation view shows the height of this triangle at 2a.

Take another look at 3-4 (Figure 4) on the base of the triangle. This is the same as Line 3-4 in the top view of Figure 3. Point 4a in the elevation view of Figure 3 represents the height of that particular triangle, the same as in Figure 4 by drawing a line from Point 3-4 to Point 4a.

Continue to connect the lines in the true-length triangle. This will give you all the true-length lines necessary to complete the pattern.

Figure 4

Using triangulation

Draw the flat pattern by triangulation, as shown in Figure 4. Transfer the true-length lines from the triangle to the pattern. Using Profile 3 from Figure 2, set a second set of dividers to the distance from Point 1 to Point 3 - call this divider A. Keep these dividers (A) set to this distance for the top of the pattern to establish the distance from Point 1 to Point 3, 3 to 5 and 5 to 7.

Set a third set of dividers to the distance of Point 2 to Point 4 from Profile 1 in Figure 2 - call this divider B. Keep these dividers (B) set to this distance for the bottom of the pattern to establish Point 2 to Point 4, 4 to 6 and 6 to 8. Begin by drawing the Line 1-2.

Using Point 1 as center and divider A, make an arc near Point 3, as shown in Figure 4. Repeat this on both sides of Point 1.

Now using the third set of dividers, set them to the true-length distance of Line 2-3 from your triangle. Using Point 2 as center, make an arc that intersects with the arc drawn from Point 1 to establish Point 3. Repeat this on both sides of the fitting, as shown in Figure 4.

Using divider B and Point 2 as center, draw an arc near Point 4, as shown. Continue to transfer the true-length lines from the triangle and pay close attention to which dividers you are using.

Figure 5


Put tape on one of the dividers and above the pattern under development to keep track of which dividers to use. The distance from Point 8 to Point 9 is already shown in true length in both the top and elevation views in Figure 2.

Once you finish establishing the points, use a steel rule to make a line through all the points, as shown in Figure 5. Do not cut out the pattern. Depending on the seam you choose, you will need to make a line parallel to Line 1-7 and Line 2-8. You will also need to draw a line parallel to Line 7-9.

Figure 6

Seperate sides

Figure 5 is only half of the pattern. Two are needed to complete the right-angle transition. The advantage of remaining symmetrical is that you can draw the second half using this pattern you just developed, remembering of course to allow for the seams. Figure 6 illustrates the fitting with separate sides. Two of each piece would be needed. This can make for easier assembly if the fitting is large.

Figure 7

Figure 7

If you look at Figure 7, keeping the fitting symmetrical, there are still only two patterns that need to be developed: one triangular in shape for the sides and a second pattern that will remain the same for all four corners of the transition. Again, you will need to add any allowance for seams.

(For more information on manual sheet metal layout and exercises, visit www.thesheetmetalshop.com).