Page:Sheet Metal Drafting.djvu/129

Rh must be made for the thickness of the metal. Figure 156 shows a "dot and dash" line. This line is called the neutral axis, and takes its name from the fact that the metal at this point remains stationary while that on either side stretches or shrinks as the hoop is formed up. It will also be noticed that this line is not in the center as it would be if the cross-section were rectangular in shape. According to Fig. 156, this line passes the "square corner," against which the top of the body rests, at a distance of &Prime;–&Prime;, or &Prime;. The rule is to double this quantity and add one thickness of the metal body. Following this rule would give $$\scriptstyle{\frac{1}{16}+\frac{1}{16}+.025=.150}$$ or (nearly). This should be subtracted from the diameter of the body (18&Prime;−.15&Prime;=17.85&Prime;) and the remainder multiplied by $$\scriptstyle{\pi}$$, in order to get the length of the blank for the upper hoop, Fig. 160. This would give 17.85&Prime;×3.1416=56.077&Prime;, for the length of Fig. 160, The pattern for the lower hoop, Fig. 161, must be shorter than that for the upper hoop, because the lower hoop must go inside of the bottom of the barrel as well as the body, Fig. 157. Using the rule given above:

Since the ends of the hoops are to be butt-welded no allowance need be made for joining. There are to be four rivets in each hoop; therefore, the distance between the rivet holes on centers would be equal to one-fourth of the circumference. Placing half of this space, or ⅛ of the circumference, at each end would avoid a rivet hole through the weld. The spacing of the rivet holes in Fig. 160 will not be the same as that in Fig. 161, because of the difference in length of the blanks.

Pattern for the Bottom.—The bottom of the barrel has a ⅞-inch flange turned for riveting to the body. This flange can be worked up by hand, but it is generally pressed in a machine. Figure 162 shows a section of the bottom, and the pattern with allowance for flanging. If ⅞ in. were added to each side of the diameter of the finished bottom, the machine would turn a flange deeper than ⅞ in. The rule for finding the correct diameter of the