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03-10-2009, 05:38 PM
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Fender Bender: A Lesson in Metal Shaping.
After just one pass with light press in both directions, you can no longer lay the panel flat, it is beginning to take some shape.
After a few passes like this, it's starting to really take some shape, but just stretching doesn't get a compound shape. Creating a compound curve in sheet metal is about more than stretching the sheet metal and making it thinner, it also takes shrinking it and making it thicker. Thinner in one area and thicker in another is what it's all about. The next picture I'm using to illustrate what I mean by this.
If you only stretched the panel, you would eventually get no farther than a piece that looked like a canoe or something to that affect. To bring the panel down around the tire in two directions, down both the side walls and around the diameter, you need to gather a lot of extra material on the sides. I placed a piece of paper on the tire to simulate a flat piece of sheet metal bent around the diameter of the tire. As you can see from the folds on the sides, to get the panel to wrap around the sidewall as well as the top to follow the diameter of the tire there is a lot of extra material that has to go somewhere. Many times you see someone that cuts slits down the entire side of a piece they are fabricating, or cuts pies out of it and then welds all these back up. This is material that needs shrinking. There is more than one method of shrinking sheet metal, be it tuck shrinking, a hand or kick shrinker (used here), or a set of "thumb nail" dies in a power hammer. I have a hand shrinker that has been converted to a foot operated unit. It is not always the best tool for the job, but it is what I have to work with at this time. Take a look at the jaws.
The jaws in a shrinker like this are four separate dies, two top and two bottom. The dies have a machined surface that act to grab hold of the sheet metal. When at rest, the dies have a space between the upper and lower as well as between the separate sides. When you put the edge of the sheet metal in and step on the pedal, the dies first close on the metal by the top two coming down and pinching it. As this happens the left and right sides (both top and bottom) pull together and gather the sheet metal, pulling it from both sides toward the center. To better understand what's going on here, make your bed (your wife will be proud of you!) No really, picture this. You stand at the edge of your bed, with the sheet laid out nice and tight with no creases in it... the sheet is the sheet metal. Now grab a hand full of sheet up by the headboard, and down by the foot board with each hand, and pull to the center. What you now have is way more sheet at the center in front of you all wrinkled up where it came together. This is how a hand/kick shrinker works. Your gathering metal together and making that material thicker. Stretching thins, shrinking thickens.
So back to the fender, in order to get the compound curve that will be the fender (by compound I mean it has shape one direction, as well as the perpendicular direction), I start shrinking the sides as I stretch the rest.
Here you see the start of the shrinking. This will gather all the extra material on the sides together to eliminate all the folds you saw in the paper, and help to pull the shape of the fender around the tire length wise. The more you do, the more "stuff" you find yourself acquiring. To help in the shape of the sides, I looked to my assortment of homemade "T" dollies and came up with a piece of 1 5/8" tubing (scrap from a roll cage) that had the correct shape that matched what the curve of the fender sides would be.
By tipping the edges of the fender over it (my hands aren't quite strong enough to bend it over by themselves) I put some of the profile of the sides in. I use a nylon or rubber hammer to do this kind of work, because the hammer will not stretch the steel back out where you have just shrunk it. If you were to shrink it, then hit it too hard with a body hammer and a steel dolly under it, you would be just stretching the metal back out again, effectively working against yourself. By shrinking along the edge that has now been tipped down, you begin to affect more than just the material in the shrinking dies. Every time you push down on the pedal and the dies come together, you can see the panel ends moving downward, beginning to look like a fender.
Last edited by Ace of Spades; 04-28-2009 at 06:38 PM..
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http://i.imgur.com/FyVxoJq.gif
"I go to bed at 10 now".
Just kidding about the suckage... very nicely don sir!
I now have more of an appreciation of just how specialised those skills are.
Cheers