A peculiar case of an elbow is that shown in Fig. 324, where one arm is twisted so that the elbow would not lie flat on a plane surface, or geometrically, when the centre lines of the arms are not in the same plane.
This is usually made up with the middle piece telescopic, so that the elbow can be twisted into its proper position. In any case the correct angles that the arms make with the middle pipe must be determined, and it will perhaps be an advantage, also, to show how the pattern for the middle part may be set out in one piece.
In Fig. 325 a plan (a b) and elevation (a' b') of the centre line of the middle pipe is shown. The angle for the bottom elbow can be found by drawing b B square to a b and making it equal to c' b'. If a line then be drawn through a perpendicular to a b the bottom angle will be determined as indicated. The top angle can be set out by making a B' equal to a B, and drawing a line through B' square to B' b Having found the angles the patterns can be struck out as shown in Chapter II (Elbows For Round Pipes. Pattern For Round Pipe Cut On The Slant).
To make the pattern for the middle pipe in one piece it will be necessary to find the true length of one line on the pipe and use this to set out the length of the pattern. Draw m n parallel to a b, and project down from n' to determine the point n. Now draw n N perpendicular to n m, and so fixing the point N on a B. The length a N will be the true length of the side line of the pipe. Having set out the pattern cut for the bottom elbow (shown passing through A) the girth line should be divided into four equal parts, and lines run along as shown. The side line A N is made equal to a N from the plan. Next draw the line G G to pass through N and mark the distance N O equal to the length of the arc n' o in the elevation. The point O will be the throat part of the top elbow, hence the curve must be drawn as shown passing through the point N.
The patterns for the arms and the construction for the curves are not shown, as these will come out as explained in the early chapters.