In the previous chapter we dealt with the setting out of patterns for the various kinds of elbows used in connection with square pipe work. We now give a few examples that may be useful for rectangular pipes.
Fig. 33 shows a sketch of a square elbow, the broad sides of the pipe being at the back and throat. The elbow is made up by two pieces of pipe, each being cut at 45°, and mitred as shown in the sketch. A pattern for one of the branches is shown set out in Fig. 31. A side elevation is first drawn, and before attempting to strike out the pattern, the position of the seam should be decided.
In Fig. 34 we have assumed the seam runs up the middle of the back and along the centre of the top. The girth line of the pattern is drawn, its total length being made up by the pipe dimensions, as marked on the figure. Thus, suppose the section of the pipe is 11 in. by 6 in., then these sizes will be used in obtaining the total length of the girth-line. The heights 11 and 22 on the pattern will be measured from the respective lines with the same number in the elevation. Allowance for seams must be added on to the sides of patterns for both arms of the elbow. If the joint is a simple lap, and riveted or soldered, it will be necessary to add laps on to the end of one pattern only.
The elbow, of course, could be constructed in the same manner as explained in Chapter VI (Square Pipe Elbows And Tee-Pieces. Square Pipe Elbow)., Fig. 27, or it may be formed of four pieces, two sides and back and throat, and jointed at the corners by knocking up. This latter method gives a very rigid form of pipe elbow, but has the disadvantage of costing more to make.
The sketch shown in Fig. 35 represents a similar kind of pipe worked into an elbow, with the broad sides forming the cheeks. In this again the seam is taken up the middle of the back. The size of pipe being 7 in. by 4 in., the length of pattern will be made up as seen by the dimensions in Fig. 36. The heights are shown projected; but these, in practice, would of course be taken directly from the elevation.
The heights of lines to form the cut, both in the case of square elbows and also offsets, can of course be calculated as explained in previous chapters. This would then do away with the necessity of drawing an elevation, the shape of pattern being marked directly on the plate.