Page:Sheet Metal Drafting.djvu/77

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31. The Backset Method.—The Backset Method is a short, but accurate, method of developing an elbow pattern. Figures 91 and 92 show the elevation and profile of an elbow, the pattern of which is developed in the manner previously described. It should be noticed that the pattern has been moved over to the right to allow a half circle to be drawn between it and the elevation. The extension lines cut this half circle at points A, B, C, D, E, F, G, H, and J. These views should be carefully drawn and placed in the position shown.

The distances between A and B, B and C, etc., should be exactly equal if the drawing is carefully made. The diameter of this half circle is equal to the height of the backset. Because of the foregoing, the elevation and profile are not necessary if the backset height in inches is known. The half circle divided into equal spaces can be used instead.

Figure 94 shows a five-piece, 90° long radius elbow laid out by this Backset Method. A piece of metal is cut of sufficient size to make the whole elbow. A horizontal line is drawn to represent the height in the throat of the first piece. Above this, another line is drawn to represent the height of the backset of the elbow. Lock lines are next drawn ½ in. in from the right and left-hand edges of the blank. The half circle is next drawn in the backset as shown. This half circle is divided into eight equal parts. The girth (distance between the lock lines) is next divided into sixteen equal parts. The girth in Fig. 94 is 28 in. and each space would equal 28÷16 or 1¾ in. The dividers can be set 1¾ in. and the spacing performed without repeated trials. Measuring lines are then drawn through each division at right angles to the bottom of the blank. A rectangular piece of metal with one edge turned to a right angle is a convenient tool for drawing these lines. Extension lines are carried over from each division of the half circle, and points of intersection determined. A curved line through these intersections will complete the pattern of the first piece. Experienced men can locate the intersections with four settings of the compass because point B is the same distance from the top as point H is from the bottom line. This