If it is desired to join more than two branch pipes on to the main pipe, then the above method will still hold good. The first thing to do is to obtain the plan of a joint line; thus in Fig. 147 the line 3 C on the semicircle, it will be readily seen, is the plan of the joint line when there are two branch pipes. To obtain the position of the plan of one of the joint lines when there are more branch pipes than two, a line will have to be drawn through C, making an angle with 0 C equal to -
180 degrees / number of branch pipes
Thus, suppose there are three branch pipes, then the angle of the joint line will be -
100 / 3 = 60 degrees.
In this case the plan of the joint will be the line 2 C (Fig. 147). Now for the pattern. Where the line 2 C crosses the lines B 3, B 4, and B 5, swing on to the base line with B as centre. From the base line project up on to the correspondingly numbered lines in the elevation, thus obtaining the points 3°, 4°, and 5°. Now with T as centre swing these points around on to the pattern and draw in the curves. The thick dotted curve thus shows the cut for the toe of pattern when three branches are required to be jointed to one main pipe.
In the same way as above, after fixing the position of the joint-line plan, a pattern for a connecting pipe for any number of branches can be set out. On examining Fig. 146, it will be seen that all the joints are paned down. They may, of course, be knocked up if the material is sufficiently malleable to stand the operation. The allowances for this method of jointing will be a double edge on the end of each straight pipe, a single edge on the ends of each tapered pipe, and a double edge on one and a single edge on the other to form the middle joint of the connecting pipes.