Page:Sheet Metal Drafting.djvu/97

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39. The Pitch Top Cover.—The cover of any receptacle should be made with a pitched top, such as considered in this problem, in order to obtain the necessary rigidity.

The Elevation.—The elevation of the pitch top cover appears as shown in Fig. 119. This cover consists of a cone top, joined to a cylindrical rim by a "clinched" seam. The rim has a No. 12 wire rolled into the bottom edge. A semicircular wired handle is drawn with a 2-inch radius by using the apex of the cone as a center. The distances C to B, and J to K, are straight lines connecting the semicircle and the slant height lines. The distance from A to B is 1 inch. The handle is joined to the cover by 1½ lb. rivets.

The Profile.—A half-profile should be drawn, using extension lines to locate the view properly. The half-profile is divided into equal parts and each division numbered. The profile is equal in diameter to that of the rim inside of the wire. Extension lines are carried upwards from each division of the half-profile, to the base of the cone, and thence to the apex.

Drawing the Pattern.—With a radius equal to the slant height of the cone, and any point as a center, the arc of stretchout is drawn. The spacing of the half-profile is transferred to the arc of stretchout, doubling the number of spaces in order to obtain the whole pattern. The divisions are numbered as shown in Fig. 120. A ⅜-inch edge parallel to the arc of stretchout is added to allow for joining the rim. The locks are drawn parallel to lines 1 and 1, Fig. 120. These locks are notched as indicated. The rivet holes may be located on any two elements that are opposite each other, viz., on 4 and 4, 3 and 5, 2 and 6, etc., but in shop practice they are generally placed 90° from the lock seam. This would bring them on lines 4 and 4 as in Fig. 120. The distance from the center of the pattern to the center of the holes is found by measuring downwards on the slant height. Fig. 120, from the apex of the cone to the center of the rivet as shown in the elevation.

Pattern for the Rim.—The pattern for the rim, Fig. 121, is a straight piece of metal the length of which is equal to D×π (diameter of profile×3.1416), and the width of which is equal to the