Learn the important difference between sheet metal shaping and sheet metal bending.
Professional Sheet Metal Fabrication (Motorbooks, 2013) is the number one resource for sheet metal workers old and new. Join veteran metalworker Ed Barr as he walks you through the ins and outs of planning a sheet metal project, acquiring the necessary tools and resources, doing the work, and adding the perfect finishing touches for a seamless final product. The following excerpt comes from chapter five, “Beginning Sheet Metal Shaping.”
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Learning to form sheet metal by hand is the critical first step in your education in metal shaping. Machines are labor savers, but using them properly requires knowledge. Otherwise, they can transform perfectly good sheet metal into scrap with astonishing speed and efficiency. Fortunately, once you understand the basics of shaping sheet metal, its responses to your input will be less mysterious, so progress will come quickly—you will not need an arduous seven-year apprenticeship to start seeing results and finding satisfaction in your work. Furthermore, craftsmen have been shaping metal by hand for centuries, so do not be intimidated by the existence of complex and expensive machines, which simply harness electrical and/or hydraulic power to shape metal according to the very same principles you will learn here. For the average person interested in repairing rust spots and making a few patch panels for a historic vehicle, for example, a few basic shaping exercises will endow most enthusiasts with the confidence to move ahead with their intended project.
One overriding principle to keep in mind when working with sheet metal is that you often trade thickness for surface area as you shape the metal. Sometimes you increase the surface area, or stretch the metal, making it longer and thinner. Other times you will decrease the surface area, often called shrinking or upsetting, making the metal shorter and thicker. I tell students to think of their metal as a slab as you manipulate it. If you were to mash down with your thumb in the middle of a pie crust, for example, you know instinctively that the crust would get very thin under your thumb as the dough compressed. If you mashed the crust a few times in close proximity, the entire crust would spread out ever so slightly as a result. Metal doesn’t behave exactly like a crust, of course, but I think this image makes it easy to understand how to change the shape of metal by influencing its thickness.
Metal can be shaped without changing its thickness as well, such as when you bend it in a vise. Think of a bend like a fold in a piece of paper; the metal is creased along a single axis. The bend could be sharp, like when you hammer a piece of metal over at 90 degrees, or the bend could be gradual, like when you bend metal around a large pipe. Perhaps at a microscopic level the thickness of the metal is influenced very slightly, but for our purposes think of the bend/fold as a change of shape that does not change the thickness of the metal. The bend or fold is easy to understand and easy to forget. Once you start shrinking and stretching, it is easy to think only in those terms, but the concept of bending is just as important as shrinking and stretching; many shapes cannot be made without bending.
I recently learned two terms from metal shaping legend Fay Butler that perfectly embody the concepts of shaping metal by changing its thickness and forming it without changing its thickness. According to Butler, shape and form were used by men such as Scott Knight and Red Tweit at the now defunct California Metal Shaping to differentiate between two distinct modes of working. Out of respect for the tradition of shaping started at California Metal Shaping, and in an effort to develop standardized terminology among metal shapers, Butler continues to use the terms. I, too, will use shape to refer to a process involving a thickness change and form to refer to a process that does not involve a thickness change.
To illustrate the idea of the relationship between the thickness and length or surface area of a piece of metal, I have taken three identical 4-inch lengths of mild steel square stock and heated two of them to make them easier to shape. I upset one by hammering on its end. The other I stretched by hammering along its side. The third was left alone for purposes of comparison. Obviously, the upset piece got shorter and thicker, whereas the opposite is true of the stretched piece. Visualize the square stock as greatly magnified versions of your sheet metal. The same changes will take place, just within a narrower plane. Keep these simple principles in mind as you begin shaping metal; they will help you achieve the results you want and hopefully answer some of your questions as you grow in your craft. Thus, whenever metal is sandwiched forcefully between a hammer and dolly or between two hammering dies in metal shaping machine, we can expect the metal to be squeezed thinner directly at the point of contact. Likewise, we can expect an increase in surface area because that squeezed metal must go somewhere—it will compress to a degree, but any metal that does not compress will squeeze out to the sides around the point of impact. There are a few exceptions to the thickness versus surface area equation, but do not be concerned with those now. Let’s get a handle on the basics first.
Let’s apply the thickness versus surface area idea to create a crowned panel. For the demonstration, I have selected a hammer with a polished crowned face and a 7-inch diameter 20-gauge steel panel. Lay the panel exterior side down on a blemish-free hard surface, and work your way around the panel with light, overlapping hammer blows beginning in the center. Just like your finger in the hypothetical piecrust, the hammer mashes the panel so that it conforms to the profile of the hammer’s face. As you progress from the center out, you displace a miniscule amount of the unworked metal to the outside as you go. This process trades thickness for surface area—the metal gets thinner but gains shape. By the time you’ve worked your way to the edge of the panel, you may have lost your mind, your elbow may never be the same, you’ll have a curved panel like the one in the Image Gallery, and most importantly, you will forever be able to predict exactly what will happen when you hammer sheet metal against a hard surface. You would get the same result by hammering a panel with a flat hammer over a rounded stake as well, only the panel would curve away from you as you progressed rather than toward you.
You will be pleasantly surprised by the degree to which you can shape metal in a controlled manner simply by thinking in terms of its thickness versus its surface area. To explore this point further, draw a relaxed S curve 14 inches long onto a piece of cardboard or thick paper and cut it out. Write stretch on the template, as indicated in the photograph, to guide your work. Now cut a strip of mild steel or aluminum approximately 2 1/8 by 14 inches. Bend a 90-degree flange 1/2 inch wide along its length. I am using a piece of annealed aluminum sheet in the demonstration because it is easy to form. Using a light weight cross-peen hammer or a body hammer with a narrow face, called a linear stretching hammer, lay the metal strip on an anvil or the edge of a metal table and make a series of blows along the 1/2 inch flange that needs to be stretched/lengthened. The small surface of the cross-peen hammerhead or linear stretching head concentrates the force of the blow in a small area, thus squishing the metal and increasing the surface area/length of that flange. Meanwhile, the rounded edges of the head leave fewer hammer marks than would be the case with a chisel pointed hammerhead, which would mar and possibly cut through the metal. Because you have lengthened the flange you have been hammering, the metal on the adjacent leg begins to curve in response to the added length. Try to match your panel to your S-curve template. If more passes are needed, change the angle at which the hammer face meets the stretched flange to prevent over thinning the metal. As soon as you feel you understand the stretching process, turn your attention to the opposite end of your test piece, which will need shrinking.
Shrinking is always more difficult than stretching. One time-honored way of shrinking sheet metal is to create crimps—also called tucks or puckers—in the area needing to be shrunk and hammering the folds of metal flat, thereby upsetting the metal into itself. The best results are obtained when the tucks are restrained in such a way that they cannot simply unfold when they are struck. I will discuss this process further shortly. You can use pliers or a custom-made tucking tool to create crimps along your flange. In days gone by, metal workers also had special tucking tongs for creating puckers. These resembled blacksmith tongs, having one single jaw straddled by a double jaw.
Tucking tools may be hand-held or mounted in a vise, depending how resilient your metal is. For this exercise you want the tucks to rise up on the topside of the flange so that you can hammer them down against a flat surface. On future projects you may decide to make the tucks rise up on the backside of the panel. This decision will be based solely on which orientation gives you the most advantageous position for hammering the tucks flat. In this demonstration the crimps were easily created by hand with a tucking tool and then hammered flat against a metal surface with a rawhide mallet. When cold-shrinking, or shrinking without heat, you will be less likely to stretch the metal accidently if either your hammer or your work surface is softer than the metal work piece.
On the S-curve panel, create a single crimp and first try holding the metal firmly against a hard surface by hand while you hammer the pucker flat. If the crimp wants to unfold, clamp the panel to a table on each side of the crimp so that the metal has no choice but to upset when you hammer it. If you lack a suitable table, clamp a flat steel bar or piece of angle iron to your panel straddling the pucker. If your test piece is steel, your shrinking will be facilitated by applying heat. This is called heat shrinking. Clamp the piece in one of the ways just described, heat the crest of the crimp until it’s a dull red, and then gently hammer it flat with a steel hammer. The heated spot in each case will be softer than the colder surrounding metal and will readily upset or shrink. Because the metal will be soft while it is hot, you will not need to hit the metal very hard—the blow is similar to driving a tack. If you hit the metal too hard you will compress and therefore stretch it, which is the opposite of what you are trying to do. When you upset the metal by shrinking, you shorten this flange. Consequently, the adjacent leg of the panel curves toward the flange you have just shortened because of the pulling action the shrinking induces.
The process of tuck shrinking you used on the S-curve panel is useful for shrinking metal when you do not have access to more elaborate machines for the same work. Now you will have the opportunity to explore this technique a little farther. If possible, try tuck shrinking first with annealed aluminum because this soft metal responds so well to coldshrinking. Create a crimp in the edge of a test piece at least an inch long either by hand or using a homemade vicemounted tucking tool. Now lay the panel down on a hard surface and begin hammering the pucker flat with a plastic hammer, starting with one hit on the outer end of the tuck, then back to the origin of the tuck, and finally working your way out to the edge. The traditional way to shrink a tuck is to start at the origin of the tuck and move toward the edge. This works fine. Ryan Heller, a former student of mine, suggested an alternative method to me two or three years ago, and I think it works better, however. Hitting the end of the tuck first creates a tiny cul-de-sac into which you can chase the rest of the tuck. By hitting the outer end first, you work-harden the end of the tuck ever so slightly so that it is less likely, in my opinion, to unfold as you hammer the rest of the tuck. Whatever sequence of hammer blows you follow, remember that you are just flattening the raised fold of the tuck to upset the metal against the resistance offered by the wrinkled sides of the tuck and the work surface. You should not hit the metal so hard that it is compressed against the table and therefore stretched. If you use a plastic hammer stretching is unlikely. Plastic hammers don’t have a lot of uses, but their lack of mass and soft faces are easy on annealed aluminum. Experiment with tucks on the inside of the panel, which are shrunk against the table, and tucks on the outside of the panel, which may be shrunk against a stake. If your tucks try to unfold, try supporting the back side of your tuck against a hollowed out stump or concave depression in a piece of wood. The curvature of the wood offers additional support to prevent the tuck from unfolding.
If you are working with steel for your tuck-shrinking exercise, you should be prepared to try the stump technique just described or heat shrink the tuck. Cold-shrinking is certainly possible with steel, but the puckers left by your tucking tool are much more likely to unfold as you work them than if they were of aluminum. Simply heat the end of the tuck until it’s red hot and gently hammer it about halfway down. This method seems to be just right to prevent the tuck from unfolding, and yet it can be hammered completely flat once the rest of the pucker has been upset. Shrink the origin of the tuck and then work your way toward the outer edge of the panel.
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This excerpt has been reprinted with permission from Professional Sheet Metal Fabrication by Ed Barr and published by Motorbooks, 2013. Buy this book in our store: Professional Sheet Metal Fabrication.