The Home Machinist!

(ALL),

Sheet Metal

Triangulation applied to sheet metal pattern cutting;
a comprehensive treatise for cutters, draftsmen, foremen
and students; progressing from the simplest phases of
the subject to the most complex problems employed in
the development of sheet metal patterns; with practical
solutions of numerous problems of frequent occurrence
in sheet metal shops 1917 (290p)
https://archive.org/details/triangulationap00kiddgoog

Elbow patterns for all forms of pipe, a treatise upon the
elbow pattern explaining the most simple and accurate
methods for obtaining the patterns for elbows in all forms
of pipe made from sheet metal, with useful mathematical
rules and tables 1918 (101p)
https://archive.org/details/elbowpatternsfo00kiddgoog

Enjoy,

Ken.

So that's how its done, thanks. Reading and cutting ahead.