The sheet-metal worker's guide : a practical handbook for tinsmiths, coppersmiths, zincworkers, etc., comprising numerous geometrical diagrams and working patterns, with descriptive text (1888)
Crane, W. J. E. (W. J. Eden) · London : Lockwood and Son · leaf 97
ELBOW PIPES. 85 If the workman cliooses to draw another curve Fig. 85. line within the edge of inside course, as shown in … rig. 86. Fig. 86, he can safely divide off his holes, and
Sheet-metal work
Internet Archive · Getty Research Institute · Public domain (published before 1931). Free to use for anything.
