In sheet- metal development work, some fabrication or repair jobs can be laid out directly on sheet metal. This development procedure, known as SCRATCHING, is used when the object to be made requires little or no duplication. When a single part is to be produced in quantity, a different development procedure is used. Instead of laying out directly on the metal, you will develop a PATTERN, or TEMPLATE, of the piece to be fabricated and then transfer the development to the metal sheet. The second development procedure is what we are primarily concerned with in this section. Special attention is given to the three primary procedures commonly used in developing sheet-metal patterns. They are parallel line, radial line, and triangular development. We will also discuss the fabrication of edges, joints, seams, and notches. PARALLEL LINE DEVELOPMENT Parallel line development is based upon the fact that a line that is parallel to another line is an equal distance horn that line at all points. Objects that have opposite lines parallel to each other or that have the same crosssectional shape throughout their length are developed by this method To gain a clear understanding of the parallel line method, we will develop, step by step, a layout of a truncated cylinder (fig. 2-50). Such apiece can be used
Figure 2-50.—Truncated cylinder
as one half of a two-piece 0 degree elbow. This piece of sheet metal is developed in the following procedure: 1. First, draw a front and bottom view by orthographic projection (fig. 2-51, view A). 2. Divide half the circumference of the circle (fig. 2-51, view A) into a number of equal parts. The parts should be small enough so that when straight lines are drawn on the development or layout between division points, they will approximate the length of the arc. Project lines from these points to the front view, as shown in figure 2-51, view B. These resulting parallel lines of the front view are called ELEMENTS. 3. Lay off the baseline, called the STRETCH-OUT LINE, of the development to the right of the front view, as shown in figure 2-51, view C.
Figure 2-51.—Development of a truncated cylinder
4. Divide the stretch-outline into twice the number of equal parts equal to each division of the circumference on the half circle of the orthographic view (fig. 2-51, view C). 5. Erect perpendicular lines at each point, as shown in figure 2-51, view C. 6. Using a T-square edge, project the lengths of the elements on the front view to the development (fig. 251, View D). 7. Using a curve (french or other type), join the resulting points of intersection in a smooth curve. When the development is finished, add necessary allowances for warns and joints, then cut out your patterns.
RADIAL LINE DEVELOPMENT The radial line method of pattern development is used to develop patterns of objects that have a tapering form with lines converging at a common center. The radial line method is similar in some respects to the parallel line method. Evenly spaced reference lines are necessary in both of these methods. But, in parallel line development, the reference lines are parallel—like a picket fence. In radial line development, the reference lines radiate from the APEX of a cone—like the spokes of a wheel. The reference lines in parallel line development project horizontally. In radial line development, the reference lines are transferred from the front view to the development with the dividers. Developing a pattern for the frustum of a right cone is a typical practice project that will help you get the feel of the radial line method. You are familiar with the shape of a cone. A right cone is one that, if set big-side-down on a flat surface, would stand straight up. In other words, a centerline drawn from the point, or vertex, to the base line would form right angles with that line. The frustum of a cone is that part that remains after the point, or top, has been removed. The procedure for...