This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
As the metal is procurable only in flat sheets of various dimensions and thicknesses, some knowledge of geometry is required to determine how the flat piece is to be marked and cut in order to produce the shape decided on for the finished article.
There is scarcely any end to the variety and intricacy of pattern which may be introduced into sheet-metal goods; but when the surface is very irregular it becomes necessary to employ machines for stamping out the design, or rolls for impressing it on the metal. Apparatus designed for these purposes will be described further on; but many simple articles can be constructed without such aid. In measuring the metal in sheet to make an article of any desired dimensions, allowance must be made for the amount of metal used up in forming the joint, when that is to be of the lapped kind. Where the edges only abut against each other, no such allowance is needed. It is generally between 1/8 and 1/4 in. per joint, according to the thickness of the metal used and the strength required in the joint. Before cutting out the piece of sheet metal corresponding to the dimensions aimed at, it is well to make a pattern in stout brown paper, and fold it up so as to make a counterpart of the article in view. Unforeseen errors can then easily be rectified, and the metal cut exactly to the corrected pattern, without risk of waste.