Page:Sheet Metal Drafting.djvu/117

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49. The Diagonal Offset.—This type of fitting is used in ventilating and heating ducts. It is also frequently encountered in running rectangular copper conductor pipes. The elevation shows a section of a lintel cornice such as is frequently seen above the first floor windows of a building. A conductor pipe running down an inside corner of a building having a lintel cornice would have to be offset diagonally in order to clear the obstruction.

The Plan.—Figure 142 shows the outline of a conductor pipe (lines AB, BC, and CD). The entire elevation is not shown because it plays no part in the development. The plan, Fig. 143, is drawn making the angle equal to 45°. If the required angle is other than 45°, it presents an entirely different problem and cannot be drawn by this method. The profiles are numbered as shown. Profile 1, 2, 3, and 4 is that of the lower part of the fitting and profile 5, 6, 7, 8, that of the upper part. While both are the same size their numbering must be different.

Diagonal Elevation.—A base line for the diagonal elevation must be drawn parallel to the line 4–8 of the plan. Extension lines are carried from each point in both profiles at right angles to this base line. Extension lines from points 1 and 3 will locate points A and E on the base line. A perpendicular is erected at point A and the distance AB set off equal to AB of the elevation, Fig. 142. The extension line from point 5, of Fig. 143, will locate points C and D in Fig. 144, heights being taken from Fig. 142. Drawing the line BC will complete the outline of that part of the fitting that rests directly upon the lintel cornice. A perpendicular is now erected at point E of Fig. 144. With B as a center and a radius equal to AE, an arc is drawn as shown in Fig. 144. Any other point F on the line CB is selected and another arc of the same radius drawn. The fine HG must be drawn tangent to both arcs. The point G occurs at the intersection of this tangent and the extension line from point 3 of Fig. 143. Line GH should be equal in length to BC. At the point D a line is, drawn at right angles to line CD. This line will intersect the extension line from point 7 of Fig. 143. The intersection should be lettered K. A straight line KH will complete the outline of the diagonal