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-   -   Fender Bender: A Lesson in Metal Shaping. (http://www.clubchopper.com/viewarticle.php?articleid=60)

Ace of Spades 03-10-2009 04:38 PM

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.






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.






So now I'm into the thick of shaping. I'm working back and forth between stretching the center of the panel to get the shape and shrinking the sides to gather all the extra material. If you could check the metal thickness, you would be finding the sides to be getting thicker and the middle of the fender to be getting thinner. Metal is definitely moving, like if you were rolling pizza dough with a roller. So while your working with the english wheel and the shrinker, you need to be checking your shape along the way so that it does not get out of control. I keep the radius gauge close by and after a shrink pass and a stretch pass I check the surface with the gauge.




In this picture you can see that I'm close to the #4 radius that I want but not there yet, as light can be seen between the gauge and the fender surface. The gauge will show you where you have not stretched enough, or where you have done too much. In some places it may be good and in others it may need more, to keep the surface even you must constantly check it or it can get ahead of you and cause a mess.



Another concern is the shrinking you do on the sides. You MUST shrink evenly, or the fender will twist, spiral, tweak out of shape, or just generally become deformed. If your shrinks are not evenly dispersed both down each side, and the same on both sides of the fender, you will not get a nice even curve. Pushing harder on the shrinker in one place compared to a place even just an inch away will give you hard peaks instead of a gradual curve as it follows the line of the tire around. Shrinking more material on the left of the fender than the right will pull that side harder, both making one side a different diameter then the other, and giving the fender a twist. Basically it will not fit as a fender should.




At this point it has really come around and is starting to look like a fender. I have got another dolly out which has a really nice smooth shape to use to blend the transition from the shape of the top of the fender into the sides. I used the nylon and rubber hammer to bump it over the dolly, but at this point I'm starting to smooth out the shape of the sides as I'm working them so the body tools come out. Also the shrinker will leave some puckers here and there from pulling the material, so I use a flat faced body hammer and a slapper to even it out. You have to remember that if you strike hard and here the tink of the dolly solid under it, you will be stretching some. The slapper helps to not stretch so much. So basically this is how the shaping goes, stretching, shrinking, checking the surfaces, always trying to stay ahead of the metal. You need to understand what it is doing so that you can properly adjust your actions to how it is acting. I'll fast forward now to a fender that is ready for all the details.




So why would anyone go to the trouble of scratch building a fender anyhow with all the fenders you could buy on the market of just about any size and shape you ask? Well I now do work primarily for one person, Shane Gatto. You may know him on Club Chopper as Rootbeer Float, and if there is one thing I have learned about him, he likes to really challenge people for their best work. We could have just modified an existing fender, but it makes the bike all the more special and interesting when it has a few hand made parts with character. So now that I had a good shaped fender, we worked out the details of the fender that will set it apart from any others. The fender will not only have a rib up the center, but also a bead all the way around it, with strut mounts integrated into the details. So after trimming the edges to the fenders final dimensions using tin snips I start laying out the lines for the rib.




Now the problem with needing to run a rib down the center of the fender is a good one. I only have at this point in my equipment collection a bead roller. The bead roller has a deep throat to it, but no depth at all. Which means that the sides of the fender will prevent the fender from passing through the machine. After a lot of thought, I made the decision to cut the fender in two pieces length wise off center, run the rib down the center, then weld the other half back on. So after I marked the center line of the fender and the off-centered cut line.




I cut the fender in two using a very thin cut off wheel. The thinnest cut off wheel you can get along with light pressure will give you a nice clean cut when it's the only thing that will get the job done. Let the wheel do the cutting at the speed it likes, don't try to force it to cut fast, that's when you end up with a cut edge that looks like a lightening bolt...not good.





I already had the center line of the fender marked to position the rib before I cut it in two, so next I marked out the rib actual size on the fender. You can not go straight to the bead roller however and just put the rib in. If you were to just start forming the rib in the bead roller, it would deform the fender shape, the forming of the rib would pull material from outside of the rib and would change the shape of the middle of the fender. Once the rib is formed, the shape of the fender is pretty much locked in. So, A. the fender shape must be finished completely before going any farther, and , B. the material that you will be forming the rib with should be pre-stretched before the rib is formed. So here you can see the rib marked out and it is being pre-stretched in the english wheel.




By pre-stretching the rib you are giving the dies in the bead roller more material to pull up before it starts pulling from outside of the rib. You should pre-stretch enough to see the raise in the material. After the pre-stretching, the rib can be put in.




This is my bead roller, bought on sale at Harbor Freight for a hundred bucks. The bead roller itself needs work out of the box, the frame flexes incredibly bad when even trying to form light gauge sheet, but the set of dies that come with it are well worth the money and take the sting out of having to weld up extra bracing to your new purchase before ever even using it. On the back of the roller frame I have welded 1"X2" square tubing to rid the machine of the flex, and removed the hand cranking handle to replace it with the steering wheel. With the hand crank it was a two person operation as you couldn't possibly crank it yourself and keep your face in front of the dies at the same time. The steering wheel allows you to stand directly in front of the dies to follow your lines and direct the sheet metal with one hand while cranking with the other. The rib is not put in all in one pass, I actually took four passes to do it, going pretty slow. With no material protruding beyond the back of the dies, a fence, or back guide, was not usable to keep the rib straight, so I took the slow approach and steered it straight the whole way.




The first pass was a light one, just to set in the rib. Once I had some form started, I progressively tightened the dies raising the rib more each pass, until I was satisfied with the rib on the forth pass.




Once the rib was in, I fired up the TIG welder, my trusty Miller Syncrowave 180, and dashed the two halves back together. If your careful to make your cut (when it was cut in two) slow and clean, you should have two pieces that butt back together without a gap. If you get the fit up tight like that, then you can easily tack it up using no filler wire. I start at one end, the TIG is normally set at 110 when I do sheet metal, and with the tungsten hovering about a 1/16" above the metal I stab the peddle to the floor for about a second (a fast second) and get a nice tiny, clean little pin tack. I go the entire length of the seam like this. The seam will pull together as you go, and when the metal pulls together too hard and wants to bind or begin to overlap, a hammer and dolly and a couple hits on the tacks stretch it back out and relax the seam again. Once the weld seam is tacked together the entire length about every inch, it's ready to weld.






Once the fender was a tacked together I fully welded it. This is another important area to know what your doing. Most times you see pictures of something someone has welded up, you see a bunch of welds all about an inch or so long one after the other the length of the panel. If your MIG welding, then yes, you should use small stitch welds every six inches or so until the whole seam is welded up to keep the heat down. With TIG welding, this is not necessary. The weld should be one long even weld, with a heat affected zone (the blued area) about an inch or less wide. It should not get wider and narrower over it's length, it should stay constant. The reasoning behind all this is this... When you keep the weld all one long, constant, weld, it all shrinks evenly. It WILL warp the panel. The idea is not to keep it from warping, but to be able to get the warping smoothed out. Obviously, the more shape or "crown" in the panel, the less it will warp. After you make your weld, and it is all even and constant, you can pass over it and stretch it back out with a planishing hammer, your english wheel, or your hammer and dolly. Just like you did a nice, even, and constant weld that shrunk evenly, when you stretch it with your tool of choice, it will stretch back out evenly. In other words, just like everything from the beginning, when you wheel it, you do it evenly, when you use the shrinker, you use it evenly, when you weld, you keep it constant and even, and when you planish out your weld... Yep, you got it, you stretch it evenly. Don't hit one area of the weld harder than the other, and go the entire length before hitting one spot more than another. After your first pass over the weld stretching it back out, you should be able to feel with you hand what still needs more work to bring it up the same as the rest.




You can see one spot where I had to stop welding and start again, sometimes you just can't make it all in one weld just because of the space you have to position it on your welding surface, but the less stop and starts you have the easier it will be to planish out.



I have no room for my air compressor or planishing hammer in my garage/shop at home right now, so all my weld planishing lately has been with hammer and dolly and the e-wheel. I planished out the weld and when I was done I went over it with a blending wheel, more or less a coarse scotch brite type wheel on a grinder. It removes material, but not near as aggressively as a hard grinding wheel, and leaves a nice semi polished surface.



Next was a small bead around the outer edge of the fender, not quite a wired edge, but an edge detail to give it more character. I laid out the lines on the edge to follow, and again pre-stretched the material and then used the bead roller with the smallest bead die I had to add the edge detail.





Basically all that was left at this point were mounts, but at this point, after placing the fender over the tire and standing back, we decided that we didn't want any fender below the seat, soooo... we cut the fender almost in half!




Now we could move on to the mounts, I I had a nice short fender piece to hang on my wall. I made the strut tabs with the bead roller also. After drawing a couple different shapes on card board and holding it up to the fender on the bike, I decided on one and set the dead roller up with a set of stepping dies that were the same depth as the small bead I formed around the edge, as they would both transition together.






I marked the exact location where the tabs would be welded to the fender. With a hammer and a home made dolly ( a table top would work the same) I flattened out the bead where the tab would be welded in, not the entire weld, just the bottom half.




Once the two pieces fit the way I wanted them two, I welded the tabs to the fender, and finished the weld out the same as the long weld down the fender earlier.





The only thing missing at this point was some type of mount for the bottom front. We decided on a single mount in the center at the lowest point, it would be underneath the seat and clean. I bent up probably one of the smallest boxes I have ever made on the box brake, cut a notch out of the fender and fit it in place, welded it in and drilled a hole in the center for the bolt, slightly over sized for fender alignment adjustment.




The mount on the frame is tapped, a bolt from underneath will secure it.




And there you have it, a scratch built fender from start to finish. I'm not able to show the rest of the bike yet, but Rootbeer Float will be starting a build thread on it shortly, I did the tank and some other misc. fab work for it also, so keep looking for it.






I hope I've done a decent job at making metal shaping a little less of a mystery to those who read this article. I've always been upset that most all the articles I've ever seen in magazines tell you little about the science behind it, they just seem to show someone using an english wheel and saying "it was shaped on this". Now you know what it's actually doing. Maybe you'll have the confidence to try a project on your own, or at least now maybe you know where to begin your next project. If your genuinely interested in learning and becoming a professional, or have a shop and want to send yourself or employees to someone to learn or improve your skills, there are a few people out there teaching. I recommend one above all, my mentor and friend Fay Butler, www.FayButler.com ; What you will learn from his seminar in three days will change your life forever. Browse his sight for more information and helpful tid bits, you'll be surprised what you can find, and while your there, browse through the pictures of "past seminar participants, you may be very surprised to see some of the names that have been to him, including others who now teach! Now, get some sheet metal and start experimenting!

Mark Usyk (Oilburner)


Andrew C 05-12-2009 09:01 AM

Awesome!
 
Awesome Oilburner! thanks for all the info and pictures too. For me this has always been interesting and your article took alot of the mystery out of it. --Drew

->Bullet<- 05-12-2009 11:12 AM


 
fuckin cool ! bravo !

oilburner 05-12-2009 06:43 PM


 
Glad to see some feed back posted up, thanks guys. I wanted to really explain what was going on so maybe people would get something out of it more than just "he used an english wheel and this shrinker thing". I'll answer any questions anyone has about the process if something isn't clear. Mark.

mechdesign2k4 10-07-2010 04:06 AM


 
OMG This post sucks! now I have to go invest in an english wheel, bead roller, and shrinker!!!! LOL

Just kidding about the suckage... very nicely don sir!

skins61 06-06-2012 10:45 PM


 
Mark that's a great write up and pictures. I appreciate how long that must have taken besides actually making the fender. Thanks lots. I know a bit about machining and welding after 25yrs in the game but sheet metal work is out there.
I now have more of an appreciation of just how specialised those skills are.
Cheers

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