PARALLEL LINE DEVELOPMENT

Look at figure 8-12 as you read the following material on parallel-line development.

Figure 8-12.-Development of cylinders.

Figure 8-13.-Location of seams on elbows.

Figure 8-14.-Development of a cone.

View A shows the lateral, or curved, surface of a cylindrically shaped object, such as a tin can. It is developable since it has a single-curved surface of one constant radius. The width of the development is equal to the height of the cylinder, and the length of the development is equal to the circumference of the cylinder plus the seam allowance.

View B shows the development of a cylinder with the top truncated at a 45-degree angle (one half of a two-piece 90-degree elbow). Points of intersection are established to give the curved shape on the development. These points are derived from the intersection of a length location, representing a certain distance around the circumference from a starting point, and the height location at that same point on the circumference. The closer the points of intersection are to one another, the greater the accuracy of the development. An irregular curve is used to connect the points of intersection.

View C, shows the development of the surface of a cylinder with both the top and bottom truncated at an angle of 22.5 (the center part of a three-piece elbow). It is normal practice in sheet metal work to place the seam on the shortest side. In the development of elbows, however, the practice would result in considerable waste of material, as shown in view A. To avoid this waste and to simplify cutting the pieces, the seams are alternately placed 180 apart, as shown in figure 8-13, view B, for a two-piece elbow, and view C for a three-piece elbow.

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