To obtain the exact shape of a plate or sheet in the flat, for an article whose surface has a double curvature, is generally almost impossible. In practice, however, very good approximations can nearly always be found; the degree of accuracy in working up into the finished article depending more or less upon the treatment that the metal receives at the hands of the workman. In all good work, especially that which has to be under pressure, such as steampipes, the object aimed at should be to keep the plate in the finished article the same thickness all over, or, at any rate, to mind that it is not unduly thinned at any particular part.
As one workman will stretch or draw the plate more than another, it is obviously impossible in this class of work to set out a pattern that will suit all manipulators. Two things should be kept in view in marking out patterns for hollowed work - one is to make sure that the plate is not too small, and the other to mind not to waste metal. It is an easy matter to get a plate large enough, and then to shear and cut away in working up until the right size of object is obtained; but this manifestly is a most expensive method to follow, especially in the dearer metals such as brass and copper.
Generally, in hollow wort, a good guide to follow is to try to set out the net pattern so as to have the same area as the surface of the finished article. After this, allowance can be made, if required, for any undue contraction or draw, and also for trimming and jointing.