Shaping Sheet Metal with Shrinker/Stretchers and Hammerforms

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

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

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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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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.

The paper blank in your form will serve as a template for your metal to minimize waste. It will also clarify where shrinking and stretching will take place on your panel. In fact, the easiest way to foresee the work you will need to do on any panel is to lay a piece of lightweight paper over the intended shape, if possible, and see how the paper drapes over it. If you must create folds in the excess paper to get it to lie flat against the form, the metal will have to shrink here. Likewise, if you must cut slits or tear the paper to get it to lie flat, the metal will have to stretch there. In this hammerform exercise, the paper tells us that stretching will take place on the tight curve, while shrinking will take place on the sweeping curve. This may seem a little counterintuitive at first, but it makes sense if you imagine running a tape measure along the extreme edge of the panel before we bend a flange in it and then measuring again after we bend the flange. The metal along the extreme edge of the panel will be shorter on the large curve and longer on the small curve after the flange is added.

Now that you have seen what will need to happen to your metal, cut out a metal blank using the paper blank as a template. Adjust the halves of the form so that the top half is just slightly below the lower half of the form. This step will give you better access to bend the flange. On future projects, consider making the clamping piece of a hammerform slightly smaller than the piece you are hammering over. Now gradually begin bending over the flange with a rawhide or plastic hammer or with a steel hammer and a wood corking or caulking tool. These are simply homemade driving tools used to direct the hammer blow accurately on the blank. Soft tools do not leave hammer marks and make cold-shrinking easier. Start hammering out on the edge of the intended flange to get it to start bending. Gradually work your way along the entire flange so that progress proceeds evenly along the entire length of the piece. Do not bend a small section completely and expect to move along to an adjacent area; doing this will lead to troublesome wrinkles.

Once the flange is bent, work your way up next to the bend itself, but do not mar the pristine edge formed by the crest of the bend. Flat spots here will stick out like a sore thumb. By working the flange slowly and evenly with the wooden corking tool, I’ve achieved a nice finish on the demonstration piece. In addition, take into account the thickness of the metal when cutting the size of your hammerform, especially if the piece you are forming has to fit around or over something exactly, such as an air cleaner or metal container. If you make your hammerform the exact size of the object you are copying, the new piece will not fit. A good rule of thumb for containers is to increase the size of the cover by twice the thickness of the metal.

More from Professional Sheet Metal Fabrication:  

Sheet Metal Shaping and Bending
Shaping Sheet Metal with the Stump and the Shot Bag 

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.