Parallel line development is one of the simplest types of metal layout.

Figure 1
One of the simplest forms of sheet metal layout is known as parallel-line development. It can be used in laying out rectangular ductwork, round ductwork, tees, intersecting tees and other fittings.

In addition, parallel-line development is often used in some of the more intricate pattern work used in architecture.

There is one condition to developing your pattern using the parallel-line method: all sides or bend lines must run parallel to each other or all must be perpendicular to the same plane surface. This allows the creation of patterns along stretch-out lines. The stretch-out line is the total length of an object if you were to unfold it into a flat piece. It makes no difference if the piece you're developing for the pattern is square or round. It can even be a bit of both, as in cornice moldings with profiles that include concave or convex shapes.

Parallel-line development is the simplest of all line developments; many sheet metal workers use it in their everyday layout work. Look at the isometric drawing in Figure 1, an open-end rectangular box or piece of ductwork. The red line represents the perimeter or edge. Points 1, 2, 3 and 4 are included on one single plane, all lines projecting (1A, 2B, 3C and 4D) do so perpendicular to this plane.

If this piece is unfolded, this line becomes a straight line or otherwise referred to as the stretch-out line, shown in red in the flat pattern of Figure 1. In the isometric drawing, lines 1A, 2B, 3C and 4D are all parallel with each other. The distance between 1 and 2, 2 and 3, 3 and 4, and 4 and 1 (top of isometric drawing), are all transferred, in that order, to the red line (stretch-out line) in the flat pattern. Lines 2B, 3C and 4D are the bend lines.

Figure 2

Flat patterns

In Figure 2, we develop the flat pattern for round duct. To do this we divide the circumference of the duct into equal spaces as shown and use these spaces to lay out a stretch-out line, as we did in Figure 1. Once again, lines 1A through 12L are parallel to each other and perpendicular to the base; the height of lines 1A through 12L would be the duct's height.

The stretch-out line becomes more important when you have ends that are no longer parallel to one another, such as with round or rectangular duct cut on an angle, as shown in Figure 3. You will still divide the circumference the same and because the base is a flat plane, draw the stretch-out line along the bottom, as shown.

Then complete the next step of marking off the stretch-out line to the equal spaces, as in the Figure 3 top view. At reference marks 1 through 12, draw random-length lines perpendicular to the base (stretch-out line).

Using your dividers, set the distance of Point 1 to Point A from the elevation drawing and transfer this distance to the pattern by placing one end of the dividers on the stretch-out line and drawing a slight arc to establish Point A. Continue this until points A through G have been established as shown in the pattern view.

This is all there is to parallel-line development: Establishing a stretch-out line and using it as a base to create the rest of the pattern. Many patterns have a profile - the stretch-out line represents the true length of that profile as if it were laid flat on the bench.

Figure 3

Use in architecture

Many architectural pieces are developed using the parallel-line method, such as conductor heads, finials and cornice materials. A simple profile of a conductor head is shown in Figure 4. The two areas to pay particular attention to are the red line on the front view with numbers 1 through 8, and the shaded area of the top view.

The red line in the front view is the profile of the side of the conductor head or shaded area of the top view. In this particular drawing, similar profiles for both the front and the sides are used, allowing you to only develop one pattern and then use it for the remaining miters. Dissimilar profiles are seldom used and not discussed in this article.

Figure 4

Top View

From the top view, you are looking at the top edge of the conductor head's back plane, which is against the wall. The reference points 1 through 8 along the profile line represent the side of the conductor head, which is perpendicular to the back plane, making them true-length lines. You will take the length of these lines and transfer them to a stretch-out line as shown in Figure 5.

For an accurate pattern, the distance from Point 7 to Point 8 needs to be divided into equal spaces, as shown from Point a to Point i in Figure 5. To properly lay out the pattern, all the points along the profile must be dropped to the miter line, as shown.

Figure 5

Stretch-out line

Draw a stretch-out line and transfer the length of each line from point to point, beginning at No. 1 until all, including, No. 8, is on this line. From each of the numbers along the stretch-out line, draw a perpendicular line to the center, as shown in Figure 6. From where the line is drawn from the profile and where it intersects the miter line, draw a line parallel to the stretch-out line, as also shown in Figure 6.

Figure 6

Creating a grid

Doing this creates a grid with which you can now line up corresponding numbers and mark them off. Connect these points on the grid and you will have a pattern for the miter.

Use this one miter pattern for all of the pieces. Save the pattern. How you connect the miters will determine the additional material that needs to be added for the seams.

It's important to understand how to develop the miter line and why it is important to use the method shown in figures 5 and 6. In most architectural pattern work, you will need a thorough understanding on how to obtain a pattern of a miter.