The thickness of sheet steel, whether stainless, tin plated, or galvanized, is given in gauge number; the lower the gauge, the thicker the sheet. Steel ductwork is usually 28 or 30 gauge; sturdier items call for 26 gauge. Sheet copper is measured by weight per square foot; 16-ounce copper is used for roof flashings, 24-ounce copper for kettles.
Sheet aluminum is measured in decimal parts of an inch. Before cutting a metal sheet, mark the line with an awl or scriber and a straightedge. Wear heavy gloves and cut with tinsnips or aviation sheers. Then smooth sharp edges with a file.
To make a right-angle bend, clamp the sheet between two pieces of hardwood along the line to be bent. Gently hammer the metal over with a padded or rubber-headed mallet, working back and forth to keep the bend even. To make a curve, first clamp a length of sturdy pipe in a vise. Then gradually press and tap the metal while sliding it back and forth across the pipe.
To join two edges, make a hook on each edge and interlock them. Begin with right-angle bends; then hammer each edge over a strip of scrap metal for even spacing. Finally, hook the edges together and hammer the joint flat. For a neater, more secure joint, make a groove punch by chiseling a small channel in a block of wood and hammeringit.