In Fig. 468 let ABCD represent a side elevation of the cover of which EGFH is the plan or shape of the vessel it is to fit. Various constructions may be employed in making such a cover as this; that is, the joints, at the option of the mechanic, may be placed at other points than shown here; the principle used in obtaining the shape, however, is the same, whatever may be the location of the joints. By inspection of the elevation and plan it will be seen that the shape consists of the two halves of the envelope of a right cone, joined by a straight piece. Therefore, for the pattern proceed as follows: At any convenient point lay off B2 C2, in length equal to B1 C1 of the plan. From B2 and C2 as centers, with radius equal to A B or C D of the elevation, describe arcs, as shown by O N and P M. Upon these arcs, measured from O and P, respectively, set off the stretchout of the semicircular ends, as shown in plan, thus obtaining the points M and N. From N draw N B2, and from M draw M C2 From B2 and C2, at right angles to the line B2 C2, draw B2 K and C2 L, in length equal to A B of the elevation, which represents the slant hight of the article. Connect K and L, as shown. Then ONKLMP will be the required pattern.
Fig. 468. - Elevation, Plan and Pattern of an Oblong Raised Cover with Semicircular Ends.