Shrinking and Stretching Sheet Metal
Shrinking and Stretching Metal
Many different tools have been invented for the manipulation of sheet metal. In addition to the normal bending brakes, shears and other cutting tools, there are tools for stretching and shrinking sheet metal. The planishing hammer, hammer and dolly, mallet and shot bag or English wheel all work to stretch metal, usually in the center of the work piece. The shrinker and stretcher combination can be used to shrink or stretch the edge of the work piece. These tools are used to create compound curves or crowned (bowl shaped) pieces. At Flying Hammer we use all of the mentioned tools to fabricate hot rod body parts and interior sheet metal.
Most of us have used or seen this pair of shrinker stretchers sold by Eastwood, Irvan- Smith and many of the sheet metal tool sources online or at the auto fairs. They have shallow jaws and only the edge of a piece of metal is gripped by serrations. As the handle is depressed, the metal is either pushed together (shrunk) or pulled apart (stretched). If you do this to the edge of piece of bent sheet metal, a curved flange can be created. You can also use this tool along with an English wheel, planishing hammer or shot bag and mallets to create dish shaped metal pieces. By shrinking the edges together, a crowned shape can be made deeper and more pronounced. I have used this pair of tools for many years and made some nice pieces with them. They are limited by the reach of the jaws can reach and they are not effective on thinker gauge material. These tools can be purchased for $250-$350 and some sources have added pneumatic cylinder equipped pair with a foot pedal to do the work for you and speed up the shrinking or stretching. this frees up both hands to control the work piece.
Both the shrinker and stretcher are necessary. Sometimes you have to bring the deformation of the metal back a little and when creating inside and outside curves, both are needed.
A few years ago I was attending some of the race shop auctions around Charlotte. At the liquidation auction for A. J. Foyt’s shop in Mooresville there was a shrinker stretcher being sold. This one was different and considerably more advanced than the models listed above. The tool is a blue colored, pedestal based tool with a round handle extending out of the top. This tool is from Eckold, a Swiss company. The jaw assemblies are round and held into the machine with a sort of cam locking lever system. This lever system allows very quick exchange of dies. Because of the higher cost and size of this tool, usually a shop will have one, with several sets of jaws. The gripping teeth on the jaws are knurled. There is a diamond pattern to the teeth, rather than the serrated jaws in the economy models. Notable features of this shrinker/ stretcher are the depth of reach and the power it produces. The jaws are supported by the heavy gauge steel side plates which form the main body. These side plates are notched so the work piece can extend beyond the gripping part of the jaws. This allows the jaws to do their metal manipulation away from the edge of the piece. In other words, you can stretch or shrink the work piece a few inches in from the edge and use multiple patterns of pushing or pulling to get the desired result. I have seen these notches lengthened, without compromising strength, as deep as 5 inches. This allows reaching deep into the work piece. The strength of the design is strong and this modification did not appear to cause any operational or structural issues.
This tool has certain feel to it as you work the metal. When the lever is pushed there is a springy feel to it. Of course by controlling the amount of pressure, the amount of change is controlled. I like to mark my piece with evenly spaced lines and work side to side using smaller changes at first. The more smaller bites you take the smoother the result. By marking lines parallel with edge, several rows of stretching or shrinking can be done and a curve can be be pulled into the metal.
One of the first things I made was a fender for my mini chopper. I first used the English wheel along the center of the rectangular piece of aluminum using multiple parallel passes from end to end. I then used the Eckold along the edges to pull the shaped into an arch. A little more wheel, then more shrinking and more wheel. The English wheel is often used to smooth out the rough spots. Because of the hard smooth wheels, any imperfections can be wheeled away. This included smoothing some of the tooth marks and ripples from the shrinking dies. Trying to make metal thicker by pushing it together always leaves some ripples where the metal is bunched up. The English wheel can smooth allot of this back out. Since the English wheel can only stretch metal, most times you have to introduce more shrinking to produce the desired end result. Practice makes perfect or at least close enough to minimize the addition of body filler.
Irvan- Smith has it listed for $2995. Complete die sets are $875. The 4 replacement jaw plates are $150 (shrinker) or $155 (stretcher) per set of 4. I won the bid at Foyt’s and got the tool for $850. It was missing one of the jaws sets. Irvan- Smith provided it for $875. This tool is expensive but really worth the cost for serious metal worker.
Here are pictures of the grill shell for the hot rod M37 project truck. Because the design, the truck has a grill shell completely reshaped from the original, we used 5 pieces of metal to form the shape needed. There are two sides, 2 corners and one top piece. Each piece has compound contours on it, especially the corners. These required a lot of hits on a shot bag with a mallet, some planishing hammer time, shrinking along the edges and finishing on the English wheel. Once the 5 pieces were made and fitted over a wooden buck, they were trimmed, tacked together, then tig welded and hammered to flatten the welds and remove distortion. The Eckold shrinker worked very well for pulling the edges together adding more shape needed for these pieces. The frame around the opening was made from additional pieces and finishes the opening. Those pieces were bent and hemmed on a box and pan brake and then shaped with the Eckold and and the economy shrinker and finished with hammer and dolly. The grill shell is not finished but will require a small amount of bondo and some light hammer and dolly work to make it ready for primer. The second picture shows the cross welded into the center of the framed opening. It is a Dodge and this represents the 2 x 2 matrix of grill openings on the Dodge models. The slats are stainless steel strips welded to a flange for mounting in the openings. In the first picture, the part shown behind the grill shell is the inner cooler for the turbo. We are using a 24" tall x 18" wide x 4½" thick core. It consumes all available space in the grill shell. There are two 14 x 14 radiators mounted next to the four link bars in the rear frame of the truck with stainless steel piping to feed them from the engine.
There are more pictures of the metal work on the M37 power wagon project at www.flyinghammerllc.com. The photo gallery at the bottom of the home page can be viewed as a slide show.