Page:Sheet Metal Drafting.djvu/130

tern is: Find the total surface area of the finished piece and convert this area into disc inches. Applying this rule,

$\scriptstyle {\text{Diameter}}^{2}\times .7854=(18'')^{2}\times .7854={\text{area of bottom}}=254.47\ {\text{sq. in.}}$

$\scriptstyle {\text{Circumference}}\times {\text{Height}}=18''\times \pi \times 7/8={\text{area of flange}}=49.48\ {\text{sq. in.}}$

$\scriptstyle {\text{Total or combined area}}=303.95{\text{sq. in.}}$

$\scriptstyle {\sqrt {{\text{Area}}\div .7854}}={\sqrt {303.95\div .7854}}={\sqrt {387}}=19.67'',{\text{Diam. of pattern.}}$

The nearest fraction to . 67 in. to be used would be $\scriptstyle {\frac {43}{64}}$ in.; therefore, the diameter of the pattern for the bottom, Fig. 162, would be 19 $\scriptstyle {\frac {43}{64}}$ in.

Pattern for Slat.—The top and bottom of the slat are "cut back" on an angle of 60° as shown in Fig. 163. An elevation showing the miter cut at a 60° angle should be drawn. Extension lines are carried from the profile to the miter line. A line of stretchout is drawn and upon it the spacings of the profile are set off. The measuring lines are drawn in. The intersections from each point in the profile are traced to the miter line, and thence to the corresponding line of the stretchout. These intersections are connected by straight lines to obtain the miter cut. If the wooden core is to be used, some means for closing the end must be provided to prevent the core from slipping out. If the proper machine is available, an end may be "pressed on" the metal slat. Another method is to provide laps as shown by the dotted fines on the pattern. These laps may be fastened by one rivet. Holes for riveting the slats to the barrel must be laid out to correspond exactly to the spacing of holes in the body pattern, Fig. 159.

Assembling the Barrel.—The body blanks are cut from sheets of No. 24 galvanized steel 24 in. wide by 120 in. long. Rivet hole centers are transferred from the master pattern, and holes are punched in each blank. Locks are then turned in the stovepipe folder, after which the body blanks are formed in the rolls and grooved in the grooving machine. The slats are riveted to the body. The upper hoop is then riveted on. The bottom and the lower hoop are then placed in the barrel and riveted in place. The drop