When the branch pipe is smaller than the main, and the two pipes are required to lie flush against a wall, the setting out of the pattern and hole becomes somewhat complicated. We will take one typical case, which should be sufficient guide to cover most of the jobs that are likely to crop up in a practice of this character. In Fig. 31 it will be noticed that the two pipes fit together in a similar manner to those of Fig. 30. The branch pipe, however, being smaller than the main, will necessitate its being on one side, as seen in the end view.
Before it is possible to obtain the pattern for the branch pipe a proper elevation of the joint line must be found. This can be done by drawing an end view of the main pipe (Fig. 31), dropping a perpendicular 0' to 0 and making
0 3 equal to the diagonal of the branch pipe; the lines 0 1 and 1 3 representing the sides of the small pipe. The base line is, of course, equal to the diagonal of the small pipe, and the lengths 0 2 and 0 4 on this being made the same as line 2' 4 on the end view. Points on the joint line are obtained by projecting up from points 1 and 3 on to the square in end view, and then from these points running dotted lines along to meet the lines which are drawn from the points with these corresponding numbers on the base line. It will be seen that one dotted line cuts off the two points 1 and 5 on the joint line, and also that the bottom line of the top pipe determines the points 2 and 4.
The seam being up the corner of the branch pipe, four widths each equal to the side of the pipe will be set out for the girth of pattern. This width can be obtained from either 0 1 or 1 3 in the end view. Two intermediate lines 4 4 and 2 2 are required on the pattern, and for getting the correct position of these, the distances 5 4 and 2 1 on the bottom line of the pattern will be made the same as the lengths 5 4 or 2 1 on the end view. To cut off the lengths of lines on the pattern so as to give the requisite shape to form the junction of branch and main pipes, the lengths 0 0, 1 1, 2 2, 3 3, etc., on the pattern will be made the same lengths as the lines on branch pipe in the elevation, having the same numbers.
To mark out the shape of hole in the top pipe, the girth is set out to represent the four unfolded sides of the pipe, the seam being along the top. The distances 0' 1', 1' 2', etc., are taken from the lines that are marked the same in the end view of main pipe. Now, referring to the hole, it will be seen that points on its outline are obtained by projecting up from corresponding points on the side elevation. It should be observed that this method of projection cannot conveniently be used in the workshop; but the reader will probably be better able to understand how the lengths are obtained by seeing them projected in this manner. In practice, the various lengths that are used to give the width of hole should be taken with the compasses directly from the elevation, and transferred to the pattern.
To test if the hole is the correct size and shape, its lines should be measured to see if they are of exactly the same length as the lines that are figured the same at the top of pattern. The laps allowed will, of course, be such as to suit the method of jointing adopted.
Particular notice should be given as to the way in which the plates are bent, as in work such as this, where the hole is not in the centre or the pipes fitting symmetrically, it should be borne in mind that out of the two ways in which the plates can be bent, one only of them is correct. Thus, in Fig. 31 the patterns have been so set out that if the edges of the plates are bent up, the two pipes will joint together properly.