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17. The Chimney Tube.—In the problem of the chimney tube, Fig. 39, the student will get a clear understanding of the idea of unrolling the envelope of an object to get the pattern.

A chimney tube is a short piece of pipe intended to be built into a chimney. If the tube is held on a level with the eye, the sides, the top, and the bottom will appear as four straight lines. The elevation of Fig. 40 shows this view. It is impossible to tell whether this elevation represents a flat or a curved surface unless another view is drawn. For this reason a profile should be drawn. This profile will show that the elevation is that of a cylinder. Extension lines are used to locate this profile properly. Fig. 41. Three dimensions are given in the elevation. The elevation shows a ⅜-inch flange on the top end of the tube. It is unnecessary to draw this flange in the profile.

Figure 43 represents a cylinder placed on its side. The profile appears on the left-hand end. Straight lines are drawn on the body from each division of the profile, parallel to the sides of the cylinder. If each line left a mark as the cylinder was rolled along a flat surface, we would obtain the stretchout as shown. The lines running from the top to the bottom are the measuring lines of the stretchout, since upon these lines any point on the surface of the cylinder can be measured (located). This illustration also makes plain the reason for drawing the line of stretchout at right angles to the view from which the pattern is to be taken.

The profile, Fig. 41, is divided and the divisions numbered as shown. The line of stretchout should be drawn and the spacing of the profile transferred to this line. The numbers should correspond. It should be remembered that it is necessary to start with the number of the profile at which the seam is to occur in the finished article. Perpendicular lines should be erected at points 1 and 1 of the stretchout. Extension lines drawn from the elevation complete the stretchout. A ⅜ inch edge is added to allow for the flange called for in the elevation. On the right and left edges of the stretchout, ½-inch edges for locks are added. These locks must be turned in the stove-pipe folder. The top of each lock is notched to reduce the thickness of the seam on the flanged end.