It’s easy enough to bend sheet metal, whether it’s steel, aluminum, or brass; the trick is getting the bend where you want it and forming a sharp corner. You can clamp a sturdy piece of wood that has a crisp, straight edge at the bend point, sandwiching the metal between the block and your work surface. Pull up on the free end of the sheet and, using a mallet, tap the bend area tightly against the block. To sharpen the corner further, hold a scrap block against the metal and tap on it. For a long bend in sheet metal—say, to customize a length of flashing—replicate a metal brake (the folding tool the pros use): clamp the sheet with a full-length batten of wood or bar stock on top, overhanging the edge of a workbench at the bend point. Clamp a second batten on the underside of the metal at the same bend point, then pivot the bottom batten upward to establish the bend.
Bending flat stock
Cold-bending mild steel is easy with stock up to 3/16″thick and 11/2″ wide, although you can bend thicker material cold with enough leverage. Clamp the steel in a vise at the bend point, then pull it toward you. You can leave the bar overlong and cut it to length after bending, or slip a pipe over the end to gain greater leverage. Even Vise-Grips, locked onto the end, will add leverage. You can also apply persuasion with hammer blows close to the bend. To square up a bend to 90°, reclamp the piece sideways in the vise and hammer the outside of the bend against one end of the vise jaws.
With flat stock thicker than 3/16″, you’ll likely need some heat to bend it. (Heat also makes it easier to form a sharp corner.) Metalworkers use oxyacetylene torches, but you can make do with a propane or propylene plumbing torch. Wear leather work gloves and eye protection, and work outdoors if you can. Mark the bend point by filing a notch in the edge of the stock. Then heat the area near the bend. When it’s cherry red, quickly place it over a corner of the vise anvil and hammer to get the angle you want.
To make a radiused bend, cold or hot, clamp the stock in a vise over a piece of metal pipe with the radius you want, and pull or hammer the metal around the pipe.
Jacques Perrault
Pro Tip: To crimp the end of a downspout pipe (or steel or aluminum duct) without special crimping pliers, use ordinary needle-nosed pliers. place the jaws of the pliers as deep as they will go over the edge of the pipe and rotate them slightly, twisting the metal just a touch. Repeat all the way around, and you’ll get a neat reducing crimp that will slip easily into another length of pipe.