A ready way of jointing the two pipes of an elbow together is to slip one inside the other, first having turned down the edge inside the throat, and then turn the edge at back over the inside pipe. The patterns for this kind of a joint are shown in Fig 6. The elevation is drawn in the usual way, and the lengths A D and 0 C made a little greater in length than the required lap. In setting out the pattern for the pipe with the outside lap, the lengths of lines are measured up to the line 0 D, and marked up on pattern on the corresponding lines. This will give the curve 0D0. In developing the pattern for the pipe with inside lap, lengths will be measured along to the line C A, and set up on pattern, and these will give the curve C A C. Hence, it will be seen that' the curve for net pattern is 0 A 0, for bottom pipe 0 D 0, and for top pipe C A C. This is the way in which two pipes of exactly the same diameter can be jointed with a lap joint, one pipe fitting inside the other. The length of the curve 0 D 0 is, of course, greater than that of C A C, and the difference in the lengths of these two curves can be made anything we please by arranging the lengths of A D and 0 C in the elevation. The ellipse at the end of one pipe will be less in circumference than the ellipse at the end of the other; consequently, the smaller will go inside the larger. If no inside lap is required, as in the case of a galvanised sheet-iron rain-pipe elbow with soldered joint, then the length C 0 in top pipe will be made considerably shorter than in the figure. In every case the lengths that A D and C 0 are made will depend upon the thickness of the metal used.

Fig. 6.