Shaping Sheet Metal with Shrinker/Stretchers and Hammerforms

Discover the basics of using shrinker/stretchers and hammerforms when shaping sheet metal.

| June 2013

  • Sheet Metal 109-A
    The jaws of the manual shrinker are on the left and stretcher jaws are on the right. In both cases, the wedge construction forces the serrated jaws to move the metal in the appropriate direction.
    Photo Courtesy Motorbooks
  • Sheet Metal Cover
    “Professional Sheet Metal Shaping,” by Ed Barr, includes clear directions for using power and pneumatic hammers and the English wheel, as well as describing specific skills like hand-forming techniques, buck building, louver punching, edge finishing and more.
    Cover Courtesy Motorbooks
  • Sheet Metal 112-A
    Here is an aluminum two-piece hammerform with a selection of soft hammers and corking tools. Soft hammers may be used to shape metal in hammerforms, but use discretion to preserve their faces. Hardwood or aluminum corking tools are safe choices.
    Photo Courtesy Motorbooks
  • Sheet Metal 114-A
    Thanks to the hammerform and the wood corking tool, the finished piece has a crisp, tidy flange.
    Photo Courtesy Motorbooks

  • Sheet Metal 109-A
  • Sheet Metal Cover
  • Sheet Metal 112-A
  • Sheet Metal 114-A

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.” 

Buy this book in the Motorcycle Classics store: Professional Sheet Metal Fabrication.

Shaping Sheet Metal: Simple Mechanical Shrinker/Stretchers

Manual shrinker/stretchers are commonly available tools that achieve the same results you have just obtained, but through different means. Their serrated jaws grip the metal and either force it laterally to shrink or stretch, depending on the type of jaws installed. The jaws are actuated by a hand lever or a foot pedal. The foot-operated machines leave both hands free for steadying a work piece in the machine, but I find they are more easily damaged by overzealous operators. Because there is only about 1/16-1/8 inch of movement in the jaws, stomping on the pedal like a professional wrestler only damages the machine. The metal carrier for the jaws would be much improved if it were made of thicker metal. I like the hand-operated machines because you can literally feel the amount of movement taking place in the metal as you operate the handle. When operating the machine, and especially when shrinking, many smaller bites give better, smoother results than a few heavy-handed ones. The latter method tends to wrinkle the flange you’re working and kinks the flange adjacent to it in the areas you’ve used the machine. Do most of your work on the outer inch of the flange, rather than deep along the 90 degree bend. The metal will move more readily along the edge. If subsequent passes are needed to increase the curvature in your piece, you may move a little deeper into the flange with each pass. If your shrinking has left wrinkles in the flange, cold-shrink them with a rawhide or plastic mallet against a hard surface.

Shrinker/stretcher machines work remarkably well and are not very expensive considering their utility. Shaping flanged pieces is their strong suit, but by no means their only use. Many times you will need a little shrinking or stretching along the edge of a crowned panel. If you decide to acquire this type of machine, I strongly recommend at least the shrinker jaws and preferably the stretcher jaws as well. As soon as possible, obtain a separate machine for each set of jaws so that you will not have to switch jaws for each round of shrinking and stretching.



Hammerforms

The hammerform is another useful tool for shaping sheet metal through controlled shrinking and stretching. A hammerform is simply a hard form over which you can shape metal by hammering. It may have multiple pieces to aid in securing the metal blank while it is hammered. Hammerforms are usually made out of wood, but aluminum is also a good choice if you anticipate needing to make multiples of something. In its most basic version, the one-piece form is shaped by cutting and sanding to match the needed part. The metal is then hammered over or hammered into the form to take its shape. The work piece is hit with a soft hammer or tool to allow cold-shrinking or stretching where needed and to avoid leaving hammer marks. Sometimes clamping is needed to keep the metal from shifting as shaping takes place. In slightly more complex hammerforms, one part of the form is cut in the shape you wish to imitate; the other part of the form holds the sheet metal blank tightly in position during hammering. With this form, the sheet metal is sandwiched between the two halves of the hammerform and clamped to a table or held in a vise. Although close-grained hardwoods are ideal for hammerforms because they shape well and their edges don’t break down, 5/8-inch-thick medium density fiberboard (MDF) works well for forms that will see limited use. MDF has no grain so it can be easily cut or sanded in any direction. It doesn’t splinter, it stands up acceptably to hammering, it can be glued and screwed into thick sections, it is inexpensive, and it is soft enough to allow cold-shrinking. For my demonstration, I will use an aluminum hammerform, which is more durable than MDF, but it is overkill unless you anticipate mass-producing something or need to machine the form to obtain a finished product of exact dimensions.

To perform this practice exercise, make a hammerform by drawing a curve on a piece of MDF or wood. If you have a band saw you can screw two pieces together and cut them simultaneously. Otherwise, cut out two identical curves from two pieces of wood for the two sides of your hammerform. Sand the edge of the piece you will hammer over if it is jagged—any  irregularities in the edge of the wood will transfer to the metal. Now cut a piece of paper about  inch larger than your form along the curved portion, insert the paper between the form halves, and place the form in a vice or clamp it to a sturdy table.






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