Figure 2. Patterns for two-piece transition – flat on top, shown in Figure 1. (Courtesy of LAMA Books / Leo Meyer)

Figure 2 shows the layout for the two-piece transition shown in Figure 1. The finished fitting is 14 inches long, but the bottom piece is longer than this. This is because it must be long enough to fit the slanted side of the fitting. Use dividers or a rule to transfer dimension X from the slanted side of the pattern to the straight bottom piece pattern.

Of course, allowances for seams and connections have to be added to make a complete pattern.
 

Figure 3. Four-piece transition. (Courtesy of LAMA Books / Leo Meyer)

Four-Piece Transition

More often, transitions are made in four pieces in order to save both time and sheet metal. Figure 3 shows the plan and elevation of a typical four-piece transition. Each side of this transition is different. You must learn how to lay out these transitions and how to check your layouts by matching sides.

Always lay out the patterns for each side so that the inside is the side you see. This is so the marking instructions for forming will be on the inside of the fitting when it is finished. Also, it is less confusing if you see all the patterns from the same side. To lay out the patterns with the inside up, picture yourself as being inside the fitting and looking at the side. You can then visualize whether an edge slants to the left or to the right.

The length of the finished fitting in Figure 3 is 6-1/2 inches, but none of the patterns will be 6-1/2 inches long. Because each side slants, the length of each pattern piece will be determined by the true length of the centerline of that side.

Figure 4 shows a plan view of the fitting marked to explain the layout. Each side is identified by a number. The corners are identified with letters.
 

Figure 4. Plan view used for layout for four-piece transition. (Courtesy of LAMA Books / Leo Meyer)

Lay Out Side 1

To lay out side 1:

To start the layout, draw the bottom of side 1 at 11 inches (Figure 5A). This is line DC in the plan view (Figure 4). Note that D is marked on the left end of the line, since you want to see the inside of the pattern.

Figure 5. Patterns for four-piece transition in Figure 3. (Courtesy of LAMA Books / Leo Meyer)

An X near line DH and an XX near line CG. (You will see in a bit how they are helpful in matching up corners.)

In actual shop work, you usually won't have a plan view to work from. And once you learn the method, you won't need the letters. They are only part of the learning process. What you will generally have is a shop ticket or a sketch to work from like the one in Figure 6. As you gain experience, this is all you will need. If you have trouble visualizing the fitting, make a freehand sketch of the plan view and letter the corners.
 

Figure 6. Shop ticket for Figure 3 transition. (Courtesy of LAMA Books / Leo Meyer)

Lay Out Side 2

Lay out side 2 (Figure 5B) as you laid out side 1: