The forms and shapes of breeches-pieces are numerous. Those of the oblique cone order and coppersmith's kind are dealt with in Chapter XXIX (Worked-Up Pipe Bends, Breeches Pieces, Etc. Pipe Bends).; but there are still many, that do not come under the above names, which can be formed of portions of cylindrical and conical pipes, or the latter alone. We shall now give two examples of this class of work - one regular in form, and the other irregular - and this should suffice for all practical purposes.
An elevation of the above is shown in Fig. 21. The centre lines are first laid out at the required angle, and a circle described about their meeting-point of the same diameter as the cylindrical pipe. The ends of the conical pipes are then marked down in their proper positions and correct diameters. Lines are now drawn to touch the circle, and where required produced until they meet. The intersection points of these tangential lines will give points on the joint lines or joint lines produced. Thus, the line of connection, d e, between the pipes "A" and "C" drawn by joining d to f, and where this line cuts g h will give the point e. It should be observed that this latter point does not coincide with the centre of the circle.
The girth line of the pattern for the pipe "B" is obtained by taking the end line of the pipe as a cone-base, and on this describing a semicircle, from which the girth line lengths can be measured and the radial lines drawn. Having projected the radial lines on to the outside line of the cone, the striking out of the pattern will be the same as in the former cases. There is, however, one little detail to which it is, perhaps, worth while calling attention. It will be noticed that the points E E do not He on the regularly-spaced radial lines, but in between the lines passing through the points 3 and 4 on the girth line. To obtain the former points accurately, extra construction lines must be put in. To do this, join e to b, and from where the line crosses the cone-base run up a perpendicular to the semicircle, so obtaining the point a. Now measure the arc 3 a, and set along the girth line from the point 3. Join b to a, and produce the line to meet the outside curve, which is swung around from e', in E.
The finding of the intermediate point has, in the above case, been explained at some length; and, as it is occasionally necessary to use this construction, it is worth while taking notice of the method followed.
The pattern for the cylindrical pipe "A" (Fig. 22) is laid out in the usual manner, the right-hand upper quarter-circle being used in this case from which to project the lengths of the construction lines. The girth line will, of course, be equal to four times the length of the quarter-circle 0 to 3. The lengths of the cross lines are shown projected from the elevation. For a pane-down or knock-up joint a double lap is put upon the pattern for "A,"' a single lap for "B," and double lap along the middle pact E E, and a single lap at the ends for "C."