This section is from the book "Practical Sheet And Plate Metal Work", by Evan A. Atkins. Also available from Amazon: Practical Sheet And Plate Metal Work.

A rounded corner-plate for a tank, motor-car hood, or other object can be set out in the flat very much the same as explained in connection with Fig. 302. A sketch of the corner-plate is shown in Fig. 303, and, on consideration, it will be seen that its surface can be imagined to be that of one-eighth of a complete sphere. In practical plate work it would, of course, be an advantage to break the joints somewhat differently to that shown in the sketch; but this example, as the joints are arranged, will serve to illustrate the setting out of patterns for objects that come out as a part of a spherical surface.

Fig. 304.

In Fig. 304 the necessary setting-out required for the pattern for one-eighth of a sphere of 14 in. diameter is shown. The circumference of a 14 in. sphere will be 44 in., one quarter of this, of course, being 11 in. First construct an equilateral triangle of 11 in. side, lettered A B C, in the figure. It is found from experiment that the radius giving the best curve for the sides of the pattern is 2¼ times the radius of the sphere, which in this case will be -

2¼ in. X 7 in. = 15¾ in.

Bisect A B in D, and join to C, producing the line D C outwards. Now, using A as centre and 15| in. as radius, mark the point E. This will give the centre from which the arc, A F B, can be described. In the same way the other two arcs can be constructed. A set-square should now be put upon each corner, and two tangential lines, mutually perpendicular, drawn from the arcs. This is best shown by the enlarged corner at the bottom of Fig. 304. Here B G and B H represent the side arcs, and K G, K H the pair of mutually-square lines. It will thus be seen that the small shaded area is added on to the pattern to make it work up correctly. Allowance for laps will be added as shown.

As mentioned in the last example, it is always advisable for the inexperienced in this kind of work to make up a model sector before proceeding to the larger job. A pattern for this is shown at the bottom of Fig. 304.

If the pattern A B C is to be one of the four gores to make up into a hemisphere, with a pipe fitting centrally, as shown in the elevation, Fig. 304, then the part to be cut away on the pattern can be determined by drawing s t square to o s, and using the former line as radius for the pattern cut; that is, the radius C T on the pattern will be equal in length to the line s t in the elevation.

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