Page:Sheet Metal Drafting.djvu/221

Rh '''90. Center Offset Boot.'''—The pictorial drawing shows the conditions under which a center offset boot is used. The oval wall stack or riser descending from the upper stories generally comes in the center of the stringer. Enough of the stringer is cut away at an angle of 45° to permit the boot to be connected. As in the case of the scalene cone, this fitting can be cut by a vertical plane so as to form two equal parts. Figure 295 is a plan view of one of these parts. In drawing this view, care must be taken to get the miter lines at the correct angle for a two-piece 45° elbow.

The half-profiles are then drawn in their relative positions and divided into equal parts. It has been observed that in treating oval profiles the straight sides of the oval are never divided. The divisions of the curved portions are numbered and lettered and extension fines carried from each division to the miter lines. From this view the pattern of the round end and also the oval end of the fitting can be developed according to the rules given in Chapter III.

The center or transition piece of the fitting is developed by means of sections. Figure 296 shows this transition moved to one side in order to avoid a confusion of lines. The intersections of both miter lines are also transferred. Perpendiculars are erected at each intersection and the distances from points B, C, and D, to the center line of the half-profile, Fig. 295, set off on corresponding lines at one end of Fig. 296, and distances from points 2, 3, 4, 5, and 6 to the center line of the half-profile set off on corresponding perpendiculars at the other end of Fig. 296. Curved lines traced through these points give the true sections on the miter lines.

The diagram of sections, Fig. 299, is now constructed by drawing horizontal lines equal in length to the base lines in Fig. 296. Perpendiculars are erected at each end of these lines and lettered and numbered to correspond to the base lines. Upon these perpendiculars are set off distances equal to the length of correspondingly numbered and lettered lines in the true sections of Fig. 296. The lengths of straight lines connecting these points are the true