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27. The Two-piece 60° Elbow.—Figure 79 shows the elevation of a two-piece 60° elbow having a throat radius of 3 in. to fit over a pipe 4½ in. in diameter, and to be made of No. 24 galvanized steel. The elevation is started by drawing a base line 7½ in. long. The base line of an elbow is always equal in length to the sum of the diameter of the elbow and the throat radius. Using this base line as one side, an angle of 60° must be laid off. A distance equal to the throat radius (3 in.) is set off from the vertex (point) of the angle. The arcs of the throat and back are drawn, using the vertex of the angle as a center. The number of backsets in the elbow is equal to (No. of pieces-1)&times;2. For this elbow it will be (2-l)&times;2=2 backsets. The arc of the back is divided into as many equal spaces as there are backsets in the elbow; in this case, two equal parts. The miter line is drawn from the center of the elbow through the first division of the arc, above the base line. Perpendiculars (lines drawn at right angles) to the base line are erected from each end of the diameter of the big end. These perpendiculars must stop at the miter line. Straight lines drawn from these intersections to the extremities of the arcs complete the elevation of the small end.

After the profile, Fig. 80, is drawn, it should be divided into sixteen equal parts, and extension fines carried from each division up to the miter line of the elevation. The fine of stretchout. Fig. 81, is next drawn. The divisions of the profile are transferred to the fine of stretchout and numbered to correspond. Number 1 of the profile must be so placed that it will bring the seam of the big end in the throat. The measuring lines of the stretchout are drawn and extension lines from the intersections of the miter line carried over into the stretchout. Each extension line should be traced from its starting point in the profile, up to the miter line of the elevation, and thence to a correspondingly numbered line of the stretchout. A small circle marks each intersection thus obtained. A curved line drawn through these intersections will be the miter cut of the first piece (big end) of the elbow. An extension line from the base line of the elbow carried over into the stretchout completes the pattern for the big end. Lines 1 and 1