Shaping Sheet Metal with the Stump and the Shot Bag

While they may seem inadequate for shaping sheet metal, the stump and the shot bag are extremely effective tools.

Sheet Metal 116-B

For the first round of stretching with the plastic hammer, address the deepest areas of the part, delineated by the black marker line.

Photo Courtesy Motorbooks

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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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Stretching and cold-shrinking wrinkled metal comes into play with the next tool you will want to add to your repertoire—the stump. This magnificent tool has probably been used for shaping sheet metal for as long as metal has been shaped. Stump enthusiasts inevitably modify their stumps to suit their tastes, but all agree on the economy and efficacy of this old workhorse. Some people prefer a stump that stands in one place in the shop at working height, complete with tool holders attached and even a steel band that can be tightened to prevent splitting as the stump dries out over time. For people with limited space, or perhaps for someone just starting out, a smaller block of wood, such as a slice of a tree trunk, makes perfect sense because it can be moved about the shop and stashed under a workbench when not in use. How big should your stump be? You will want at least a cereal bowl–size depression and a flat area a few inches wide for smoothing wrinkles. The block we use regularly has one cereal bowl–size depression, a second smaller bowl depression on one side, a dinner plate–size crescent shape cut into one side, and a concave curved side for making reverse curves. All of these cuts were made with a chainsaw, and very little time was spent cleaning up the wood afterward. The end grain of a hardwood stump would be the logical ideal choice for your stump, but I’ve hammered a lot of metal into all kinds of random pieces of wood, including treated landscape timbers, and it seems like anything will work for a while. Try whatever is available to you locally, and develop a feel for shaping with the stump. Then you can go about upgrading and refining your equipment. Our city’s waste management facility accepts yard waste, so at any time there is pile of stumps 30 feet across free for the taking. If only I could offer a free stump with the purchase of this book.

You can bend, stretch, shrink, and smooth metal with your stump. You can also stand on it and give a speech, sit on it and have lunch while pondering your misspent youth, or carve into it a heart with your sweetheart’s name. The edges of your stump can be cleaned up for bending metal or sharpening up flanges. Stretch metal over the stump by hammering a flat sheet over one of the depressions. You need not hammer until the metal meets the bottom of the depression. Remember, your goal is to stretch the metal, not hammer metal to follow the contour of the stump in every case. You can stretch very subtly in this way with aluminum. Steel stretches, but not smoothly. Hammering over the depression will stretch the metal where you hammer it and wrinkle, or pucker, the surrounding metal as it is pulled toward the stretch. In this instance, the metal will behave like a napkin being pulled through a napkin ring. Tug the edge of your bedspread from a single point and you’ll get a similar effect. The wrinkles or puckers that form as a result of your stretching can be shrunk by nestling them down snugly into one of the concave spaces cut into the wood and hammering down each pucker, thereby upsetting it. You can also hammer a flat sheet against a shallow dished portion of your stump to force the metal to take that contour. You can shape metal exclusively with the stump, by stretching over a hollow and shrinking the resulting folds, or you can stretch into a nearby shot bag and cold-shrink into the stump. Need the metal to shrink in one spot even though there isn’t a wrinkle? Make a pucker with the tucking tool and shrink it into the stump. As you can see, few tools will take you as far as fast and at such low cost as the stump.

I strongly recommend annealed aluminum sheet for your first stump exercise because it is simple to stretch and, more importantly, easy to shrink with hand tools. For this project you will create a shape resembling one half of a motorcycle gas tank. The piece does not need to fit a real bike or even be functional. The purpose is to learn the techniques involved in sheet metal shaping to build your skills.

Make several sketches on a piece of paper of your proposed tank silhouette. Don’t forget to make the pattern a little larger than the finished size because the metal will be curving around the shape like a chic salad bowl. Begin by hammering the most deeply shaped portion of the tank over a hollowed out portion of the stump with overlapping blows. As wrinkles form along the edges, hammer them into the stump to flatten them or hammer them against a stake held in a vise to shrink them. If the tank edges need to curl around more, hand tuck and shrink them or stretch the deepest part of the tank more and then shrink the folds along the edges. The crescent-shaped recess in the stump is handy in this instance because you can form against it. In about 30 minutes you will have a slightly lumpy version of one half of a motorcycle gas tank. Hammer out the lumps in the panel by laying it over a stake that matches the profile of the shape you want and hit it with a soft hammer or spoon. You can also lay the panel across a flat portion of your old friend the stump and smooth out the bumps. Define the shoulder that runs along the top edge of the tank by shaping the blank over a suitable stake with a soft hammer. This is how the tank half will look after shaping.

The Shot Bag

Now that your eyes have been opened to the lowly stump’s rich possibilities, you may be less likely to underrate the next meager piece of equipment, the shot bag. At first thought, the act of hammering a piece of metal over a leather bag filled with sand or lead shot may seem primitive. Newcomers may wonder how this ritual could possibly lead to anything besides a hideous, misshapen mess. Well, after you have completed one project you will wonder no longer. Regardless of the filler material used, lead shot or sand, the bag provides a stable support for hammering, and thereby stretching, the metal into the desired shape. Shot bags and sand bags are typically lumped together whenever they are discussed in the metal-shaping literature, but they are not interchangeable. In fact, they have very different characteristics. Sand is dramatically less expensive than is lead shot, but also less effective, in my opinion—unless, of course, you have a dike to repair or bullets to dodge. The sand bag is like an elusive politician; it seems to offer the desired support when you need it, but fails to live up to your expectations. The sand bag inevitably gives way just when you want it to hold firmly in place. The shot bag, in contrast, anticipates your every whim, providing exactly the kind of support you desire with each swing of the hammer. After only a few hammer blows into a shot bag you will marvel at how technologically advanced this homely leather bag actually is. The shot bag forces the metal to take the exact shape of your hammer’s face, but only in direct proportion to the power of your swing. As a result, you can move metal rapidly or with surgical precision, if necessary, simply by changing your swing. The shot bag provides the control necessary for successful hand shaping, and it provides predictable, repeatable results.

More from Professional Sheet Metal Fabrication:  

Sheet Metal Shaping and Bending
Shaping Sheet Metal with Shrinker/Stretchers and Hammerforms 

More from Ed Barr: 

• How to Build a Motorcycle Fairing 

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.