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216 Problem 53 OVAL TO ROUND ELBOW —Figure 306 is an elevation of an elbow whose first piece is that of a five-piece round pipe elbow, whose last piece is that of a five-piece oval elbow, and whose intermediate pieces are transitions which gradually convert the round pipe to the oval pipe.

This elevation is constructed by first drawing an angle (angle AJF) equal to the required angle of the elbow. The throat radius is then laid off on the base line and the arc of the throat drawn in. This arc is then divided and the miter lines of the elbow are drawn as though the elbow were a regular round pipe fitting such as described in Chapter III. Lines AB, GH, ON, and FE are drawn at right angles to the lines AG and OF, thereby completing the elevation of the first and last pieces. The throats of the second, third, and fourth pieces are drawn tangent to the arc as described in Chapter III.

The length of miter fines CK and DM are now determined in order to complete the outline of the back. Since the reduction must be gradual, an equal amount must be subtracted from each succeeding miter line. A short and convenient method of finding these lengths is shown in Fig. 313. Four parallel lines are drawn and the lengths of lines BH and EN placed upon them so that point E is directly under point B. The difference in length between these two lines is divided into three equal parts and perpendiculars dropped from each of these points to the other parallel lines, thereby establishing the lengths of miter fines CK and DM.

These lengths are now placed on the proper miter fines in Fig. 306, and the outline of the back of the elbow is completed, by connecting points B, C, D, and E with straight lines.

Each miter line as it passes through the elevation is divided into two equal parts, thus locating center points P, R, S, and T. The center fine of the elevation may then be drawn by connecting these points.

The oval end of the elbow has two flat surfaces while the round end has none. Consequently, these flat surfaces must gradually diminish in width until they disappear upon reaching the round end of the elbow.