Page:Sheet Metal Drafting.djvu/69

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28. The Four-piece 90° Elbow.—In laying out this elbow, Fig. 82, an angle of 90° should first be drawn. A distance equal to the sum of the throat radius and the diameter of the elbow should be laid off upon the horizontal side of this angle. The arcs of the throat and the back are then drawn in. A four-piece elbow has six backsets. Consequently, the arc of the back should be divided into six equal parts. Miter lines are next drawn through the first, third, and fifth divisions of the arc of the back, above the base line. This gives the first piece of the elbow one backset, the second piece two, the third piece two, and the fourth, or last piece, one.

Perpendiculars from the starting point of each arc are erected until they meet the first miter line. From these points straight lines are drawn so that they just touch the arc at one point and continue on until they meet the next miter line. In like manner straight lines representing the third piece of the elbow are drawn. The elevation is completed by straight lines drawn from the intersection of the third miter line to the ends of the arcs. The length of each miter line thus established can be tested with the dividers. If all are not of equal length, the drawing is incorrect. The elevation of an elbow is always drawn around the outside of the arcs. The straight lines of the throat and back are never drawn inside of the arcs. Many students make this mistake in the elevation and thereby produce an elbow wholly different from the one intended. The profile, Fig. 83, should be drawn and divided into sixteen equal parts. Extension lines are carried upwards from each division until they meet the miter line. The line of stretchout and the measuring lines of the stretchout. Fig. 84, are drawn. The spacings of the profile are transferred to the line of stretchout with numbers to correspond. Extension lines from each intersection of the miter line are carried over into the stretchout. Each division of the profile should be traced by means of the extension lines, first to the miter line, and thence to the correspondingly numbered line in the stretchout. Each point thus located in the stretchout should be marked with small circles. A curve drawn through these points will give the miter cut of the big end.