Double-curved work in wrought iron or steel plates is, of course, much more difficult to manipulate than in the softer metals, and, on account of the greater resistance that iron or steel offers to being drawn or stretched, greater accuracy is, in consequence, required in the marking out of the plate shapes. At the best, it is only possible to approximate to the real shape of plate wanted, and, in any case, theory is not of much use in this class of work, without it is tempered with experience. Another point to remember is that the amount of stretch or contraction in any particular plate depends very much upon its treatment in working into shape. In all cases it should be aimed to hollow or raise a plate in a natural manner - that is, to work it up as near as possible to the conditions that would obtain if it were stamped or drawn in a pair of dies. We purpose giving one or two typical cases of this class of work, beginning with a
Curved Pipe - Bend.
A sketch of the bend is shown in Fig. 297, on which it will be seen that the back and throat of the curved portion is made up in two pieces, and the cheeks in three; the joints being broken as shown.
The construction lines for the templates are obtained as shown in Fig. 298. An elevation of a segment is first set out; this really showing the elevation of the pieces combined. A semicircle is described on 0 8 and divided into eight equal parts, lines square to 0 8 being run down from each division point. Then, with C as centre, arcs are run around from the foot of each perpendicular, as shown. The complete circumference of the pipe is divided into four equal parts by the longitudinal joints: hence the length of the girth line for each pattern will be equal to four divisions from the semicircle. To deal with the back pattern first. The girth line of four equal divisions is laid down, and cross lines drawn as shown, these latter being cut off equal in length to the correspondingly-numbered arc in the elevation. Thus 0 0° on the pattern equals 0 0" in the elevation, 1 1° equals 1" 1', and 2 2° equals 2" 2'. In working the plate into shape it will be found that the line 0° 0° lengthens slightly, and that the edge 2° 2° will shorten a little; hence, as it will manifestly be an advantage to be on the safe side, the best plan will be to draw the arc 2° 0° 2° to pass through the point 0°, as first found, and make it somewhat flatter than is necessary for it to run through the points numbered 2°. In this way the edge line 2° 2° will be made slightly longer. As the joints are lapped, the bottom end of the pattern will require to be made just a shade narrower than the top; this can be allowed for by deducting one and a half times the plate thickness from the width.
The throat pattern can be laid out in a similar manner to the back, ana here it may be seen that the centre line will shorten slightly in working the plate into shape, and the side lines lengthen somewhat. This difference can be allowed for by making the end arcs. 6° 8° 6°, to pass through the points 6° 6°, as first found, and slightly flatter than required to pass through the original position of the point 8°. Here again the pattern must be one and a half times the plate thickness narrower at one end than the other. It will also be an advantage to slightly curve the side edges, as shown on the pattern.
The shape of the plate for the cheek can be laid out by first making the radius O 4 on the pattern equal to C 4" on the elevation; then on each side of the point 4 setting two lengths from the semicircle, to make up the girth line, 6 2. Now, using O as centre, arcs are drawn through each point on the girth line as shown, these being cut off respectively equal in length to the corresponding arc in the elevation. Thus 4 4° equals 4" 4', 3 3° equals 3" 3', and so on for the others. It will be found that the points 2° to 6° lie practically on a straight line, hence this can be drawn in as seen. For a bend of a very sharp curvature it will be an advantage to lengthen the arc 2° 2° just a little, as this will contract somewhat in bringing the plate into shape. Allowance for joints will be added, as shown by the dotted lines.
If instead of making the throat portion in two plates, as shown in Fig. 297, it is desired to make it out of one plate, then the curves at the plate-ends will be somewhat flatter than shown on the throat pattern, as the draw will be less, on account of the longer plate.
In work of this description it is not advisable, only under exceptional circumstances, to put any holes in the plates until after they are shaped. If there are many bends a "cradle"
punching template for each plate might then very conveniently be made for marking the joint holes.
Instead of having four plates to make up the complete girth of the pipe, as in Fig. 297, two plates may be used, as in the part of which is shown in Fig. 299. Here it will be seen that the longitudinal joints run around the middle of each side of the pipe, while the transverse joints come to the middle of opposite plates.