Before leaving gutter or moulding angles it will be as well to call attention to a special method that can be applied to square elbows, in the striking out of patterns to form the cut for any shaped section.
The shape of moulding is first set out (Fig. 184) and divided up into any convenient number of parts. Lines square to the back are drawn across through each division point. The girth line of pattern is marked down and lines drawn up square through each division point of this, these lines being cut off equal in length to the corresponding line on the section. Thus, to take one line only, the line 7 7 on the pattern will be the same length as 7 7 on the section; similarly the lengths of the other lines can be set off.
In practical sheet metal work it is most difficult to project lengths from one view or figure to the other; hence it is always the best plan to transfer the lengths with the compasses, as in the above case.