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03-10-2009, 04:38 PM
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Fender Bender: A Lesson in Metal Shaping.
"What kinds of tools and equipment do you have?" "What does that tool do... Yea, but how does it do it?" "So how would you make this part then" These are questions I get all the time, and more often than not, it's very hard to explain something to someone without doing it right in front of them and explaining each step as I go. I've shaped a good deal of sheet metal over the past 10 years, but to this point I have never gotten a project in pictures every step of the way. I was always missing a couple key steps in photos, or didn't think to start taking pictures until it was to late. So when I was asked if I was interested in doing an article for Club Chopper the gears started turning, and I waited for a real good job to come around. I hope by the time we get to the finished product at the end that it will shed some light on the whole sheet metal shaping thing for some. Everyone has at-least seen pictures of dozens of scratch built gas tanks, but I think a scratch built fender will teach just as much.
So where do you start with something from scratch? Well, on this particular project, I had a tire that I wanted to build a tight fitting fender too, so what I did was start by looking at the tire itself. I didn't want the fender to just wrap around the tire, I also wanted it to have the same profile as the tire. In other words, I didn't want a flat fender on top of a round tire. So for starters I used a tool called a profile gauge, not one of the adjustable ones (which would work fine except I always seems to drop them and loose the shape) but I have one cut out of aluminum with 12 radius's cut in it. This helps me in a couple diff ways.
By finding the radius of the tire's profile, I can match up an english wheel die that has the same radius, and also as I'm shaping the fender, I can keep checking it's shape with the gauge to not only know when to stop, but also the find high and low spots.
So once I have the profile gauge matched up to the tire, and the matching lower die in the english wheel, I take a measurement of how long and how wide the fender needs to be. Then it's sheet metal time. I shape mostly with 18ga cold rolled steel, which is what I used for this fender. I cut out my sheet metal blank using small 36" jump shear and a set of electric shears, but what ever you have to get the job done is what gets it done. There was a time when I would cut the entire piece out with a cut off wheel, it worked, took a little more time and clean up, but it got me there. I file the edges before I go any further because nobody likes bloody finger prints all over their work!
Once I have the piece all cut out, well, it's time to start shaping! What does the english wheel actually do? Simply put it stretches the metal. To try to explain it a little better try this. Put a piece of sheet metal on your welding table, and with the rounded end of a ball peen hammer give it one good whack! Now look at that spot you just hit. See the little dimple? You just stretched the metal. Now imagine if you had the ability to whack the sheet-metal, only instead of the hammer bouncing off, you could make it stop at the point just before the rebound and stay there. Then grab hold of the sheet-metal and pull it toward you. What you would see instead of a little dimple was a line. You could feel the line raised up on the back side, because it was stretched out. When you put a piece of sheet metal between the anvils in the english wheel (or in a planishing hammer) this is exactly what your doing. You tighten down the upper wheel on the lower with the sheet metal between, and it squeezes, stretching the metal at that contact point. When you move the sheet metal forward and backward the anvils stay put and the sheet-metal stretches to squeeze between them. Like a crowd of people 20 wide that suddenly hits a hallway only wide enough for 4 people... they thin out. So in the next picture you can see that I have started with the flat sheet and made one pass down the entire length of the sheet. I spray the sheet down with WD40 first, it helps me to see my "tracking" marks (the marks left from the pass of the anvils). If you look closely, you'll notice that I'm only stretching the middle of the panel, I stay a good 1/4" to 1/2" away from the sides. If I was to begin stretching and went completely off the sides, the panel would grow out in size (it's perimeter would increase) more than it would start to take any shape. Not stretching all the way out traps the material inside the perimeter and gives it no choice but to lift up where it is stretched.
After a pass down the panel in one direction, you cross over the opposite direction. By keeping each push or pull just overlapping the previous (the tracking) and going both directions across the panel, you keep your stretching even, which keeps your work more predictable. If you were to stretch it more in one place than another, miss areas, and so on, you have a much harder time keeping the surface constant. So in the next picture I have now gone across it the opposite direction.
Last edited by Ace of Spades; 04-28-2009 at 05:38 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