This is Bob Haverstock's press...
This is Bob Haverstock's press forming setup for an arbor press.
In the last couple of years, I've met three people who are experimenting with a process that is so new that it doesn't have an "official" name yet. I'm calling it Low Pressure Press Forming until a better term is developed. For years, production sheetmetal parts have been made by various processes, including stretch forming, stamping, deep drawing, and some others, but all these processes require a huge machine, usually exerting hundreds of tons of force, and the Kirksite or tool steel dies most often used in these machines can cost $10,000 for even a small die, or millions for one large enough to make a car fender or roof skin. Obviously, this is beyond what a home hobbyist or small shop owner can afford. Recently, some clever people have discovered how to use the same principles, but they scale the process down so it can be done on relatively inexpensive presses using simple, shop-built tooling.
Bob Haverstock, from Sullivan, Illinois, has a passion for vintage motorcycles, and his background as a blacksmith has given him some great ideas that he applies to sheetmetal fabrication. Bob has made many fenders by roughing-out the shape with a mallet and sandbag, then refining the shape with either hand tools or an English wheel. He was looking for a shaping technique that would not be so punishing to his body.
He hit on the idea of using an arbor press to push the metal into a sandbag, rather than beating the metal with a mallet. As soon as he tried this technique, he was amazed at how well it worked. Over many months, Bob has refined every step of the process, so now he can shape panels very efficiently. Bob makes his own sandbags, about 4 inches in diameter and 2 inches thick, which constrain the sand much better than a larger bag. He has fitted the sandbags inside a ring cut from a steel tube, so the tube takes the brunt of the expanding force, and the stitching on the bag isn't stressed nearly as much.
Bob experimented with the size and shape of the nose pieces he puts on the end of the ram, the part that actually pushes the metal into the sandbag. Currently Bob uses two nosepieces, one with a tight radius for roughing, and a lower-crown version for smoothing.
He's using a two-ton arbor press, primarily because it has a larger throat depth, although a one-ton press would generate more than enough force for shaping light-gauge sheetmetal. Bob found that one of the most important considerations is carefully placing each of the "pushes," and regulating how much force each is given. In his first trials he used a lot of force, which moves the metal quickly, but over time he has learned to work the metal in smaller increments, which speeds the final smoothing steps and keeps the metal thickness more uniform.
Bob starts working in the center of the fender, making a series of small pushes from one end to the other, and then makes some passes slightly off-center on both sides. This domes up the center of the fender, and also forms small ruffles on the edges of the metal. He learned that controlling the size of these ruffles is very important. The metal is shrunk by pressing these ruffles flat, but the process is most efficient when the ruffles are small, and closely spaced. As you'll see in the photos, the process works exceptionally well, and can produce a part that needs little cleanup work. Any small discrepancies that remain can be smoothed with a hammer and dolly, an English wheel, or a planishing hammer. Bob sells a starter kit for those who would like to try press-forming sheetmetal, but do not want to make their own tooling. Bob's freehand method works great, as you can see, but we're going to look next at a slightly more sophisticated version of the same process.
 The low-crown and medium-crown...  The low-crown and medium-crown forming tools are shown in the foreground, and the special sand-filled pad is behind. |  Bob makes a series of pushes...  Bob makes a series of pushes down the center of a strip of sheetmetal. |  The next rows of pushes are...  The next rows of pushes are made in a pattern that gradually approaches the edge of the panel. |
 By working slowly and carefully,...  By working slowly and carefully, the forming on the center of the panel is quite smooth, and there are a series of small folds intentionally left near the edge. |  With a lower-crown tool on...  With a lower-crown tool on the ram, Bob starts carefully working the edge against a flat piece of composite material. |  You can see that the small...  You can see that the small folds are nearly perfectly flattened, which requires only moderate pressure. |