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45. The Three-piece Rectangular Elbow.—This problem deals with a three-piece, 90° elbow, Fig. 131, having a throat radius of 4½ in. Looking at the elevation of this elbow, Fig. 132, one would be unable to tell whether the fitting was round or rectangular piping. The profile shows that the elevation is of a rectangular pipe elbow.

The Elevation.—First, an angle of 90°, one side of which is to be used as the base line of the elbow, is drawn. From the vertex of the angle, distances of 4½ in. and 2½ in., as shown in Fig. 132, are set off. The arc of the throat and the arc of the back are drawn, using the vertex of the angle as the center of the elbow. The arc of the back is divided into four equal parts. Miter lines are drawn through the first and third divisions of the arc, above the base line. The elevation is completed by drawing straight lines tangent to the arcs. The detailed description of an elbow elevation is given in Problem 10, Chapter III.

The Profile.—A profile, Fig. 133, is drawn, using extension lines to locate the view properly. Each corner of the profile is numbered. It should be remembered that the seam always occurs at number 1 in the profile. In this case the seam comes at one corner of the elbow. Many prefer to have the seam at the center of one of the sides, or faces, of the elbow.

The Patterns.—Three lines of stretchout, one at right angles to each piece of the elbow, must be drawn. The spacing of the profile is transferred to each line of stretchout and is numbered to correspond. Measuring lines are drawn through each point in the lines of stretchout. Starting at point 1 of the profile an extension line should be traced upwards to the miter lines, and from there an extension line to lines 1 of the stretchouts should be drawn. Notice that two stretchouts are served by each intersection of the miter line. Extension lines from the elevation into any stretchout must always be drawn at right angles to the sides of the pipe. In like manner intersections of the stretchouts can be located and the patterns completed by drawing straight lines between these points as shown in Figs. 134, 135, and 136. Three-sixteenths inch single edges and ⅜-inch double edges, as shown, join the pieces of the elbow by double seaming.