The patterns are shown set out in Fig. 300. An elevation of a segment is first drawn, the arc A B being made equal to the width of a segment between the centre lines of the rivets on the back of the pipe-bend. The radius 0 C, of course, represents the radius of curvature of the back of the bend. A semicircle is constructed upon 0 6, and divided into six equal parts, perpendiculars from each division point then being run down as shown. The girth line, 3 3, of the back pattern will be laid out equal in length to the semicircle and divided into six equal parts, cross lines being run through each division point. These construction lines are then cut off the same length as the similarly-numbered arcs in the elevation. Thus 3 3° equals 3' 3", 2 2° equals 2' 2", and so on for the remaining lines.
In working the plate up it will be found that the line 0° 0° will lengthen somewhat: hence it will be an advantage to make the arc 3° 0° 3° slightly flatter than would be required to pass through the position of 3° as obtained by using the length of the arc 3' 3". Instead of being left straight, as in Fig. 298, it will be an advantage to curve the side edges as shown, the radius used being slightly longer than C 3'. As the girth line 3 3 on the back pattern will lengthen in hollowing, the side curves should be drawn to pass through the points 3° 3°, in this manner shortening the line 3 3 somewhat.
The pattern for the throat segment can be struck out in a similar manner to the back, the lines used being those having corresponding numbers on pattern and elevation. Here, again, it will be an advantage to curve the side edges to a radius slightly greater than the centre radius, C 3', in the elevation.
As the curves at the ends of the patterns come out practically as arcs of circles, there is really no necessity to use all the lines as shown. All that is wanted being the points 3°, 0°, 3° on the back pattern, and 3°, 6°, 3° on the throat pattern. The lengths of lines to obtain these can also be calculated if required, and thus the necessity of drawing any kind of elevation avoided. In ordinary practice, however, it is generally the safer plan to use an elevation for obtaining lengths of construction lines.
In working the plates hot, care must be taken that they are drawn or hollowed as uniformly as possible, as the plates, if worked too much in any one particular part, will be pulled out of the shape that the pattern has been designed to produce.
As the plate segments are arranged to fit alternately outside and inside, it is evident from what has been said previously that the girth of the outside plate must be 3 1/7 times the thickness longer than that of the inside plate. It will also be necessary to thin the four corners of the inner plates on the back and the four corners of the outer plates of the throat. A sketch of the method of thinning is shown at the bottom of Fig. 298.