If you are a little more adventurous or you are using your CycleKart build to learn new skills you might be considering using traditional metal shaping techniques to form your bodywork.
In days gone by many automotive bodies were shaped entirely by hand. This was especially true of coach-built marques and one off prototypes. Tools such as the english wheel and planishing machines made shaping metal easier, but are not technically required. You can achieve the same results by hand, it just takes a lot more work. The following gives a brief overview of metal shaping techniques. Many of these skills are becoming lost over time and whilst the internet and Youtube do have lots of videos showing people shaping metal they often miss out on the very basics. So this is a quick primer on how to metal shape for the complete beginner.
Of course practice makes perfect so make sure you have a few (or lots of) practice runs before getting stuck in, and if you have any questions feel free to post in the forums and I will be happy to help out if I can.
I learned learned a majority of these techniques from Peter Tommasini. If you are serious about learning metal shaping techniques I definitely recommend attending one of his workshops.
Fundamentals
The first thing to get to grips with before you pick up a hammer is to understand how metal can be shaped. Having a basic understanding of the way that metal moves will help you to determine the best way to make your bodywork. If you think of the metal in its un-shaped form, it is much like a piece of paper - flat and without shape. You can use this simply fact to your advantage.
Take a sheet of paper and go to your family car. Find a suitable part with a reverse curve maybe the bottom of the A-Pillar as it curves up to the screen in one direction and then down over the wing in the other direction. Now try to lay the paper over the bodywork, you will no doubt find that it doesn't easily conform to the shape. It wrinkles and creases or in some cases may even want to stretch or tear.
In those places where the paper creases, fold some small 'darts' along the edge of the paper so that it better conforms to the shape of the panel. Now in those places where the paper wanted to tear, cut some small slots along the edge to allow the paper to better conform to the panel shape. Do this until the paper conforms to the shape of the panel.
What you have done is identify where the metal needs to be shrunk and where the metal needs to be stretched. In those areas that you folded darts into the paper, you will need to shrink the metal, whereas where you had to cut slots into the paper, you will need to stretch it.
When you are using some kind of pattern or buck, or you are copying the shape of any existing form, just like you did with the car body, you can use this method to 'transfer' the required shape onto a flat piece of metal, by laying the paper on top of the metal sheet and then using a scriber to mark through the paper along the edges of the folds. You can then highlight those areas with a sharpie so that you can see where and how you need to move the metal.
If you don't have anything from which to take the form from, you can make a wireframe buck. This is a wire-form of the body shape usually made from thick wire and attached to the chassis or existing bodywork so that you can shape the form by eye until it looks something like your plans. You could also go one stage further and make a wooden buck by splitting the profile of the bodywork into sections and then cutting those profiles out of wood and assembling them into a buck. You can then use the buck to take patterns from and as a gauge on which you can test the metal part that you make. In all cases when shaping metal it helps to have a point of reference.
Stretching metal
Whilst this sounds like you might pull the metal sheet much like you would an elastic band, stretching metal is generally achieved using a hammer and dolly. When you hit metal between a hammer and a dolly you are making the metal at the point that you struck it thinner. Of course, the metal that was present does not actually get thinner, it simply gets displaced. It moves out from between the hammer and dolly into the surrounding area. This movement causes the sheet to grow, effectively 'stretching' the metal. The higher the crown on the hammer, the more metal will be moved.
The same principle applies to using an English wheel. Except in this case instead of a hammer and dolly, the metal is literally squashed between two wheels. The immense pressure between the two wheels and the curved shape of the bottom anvil wheel causes the metal to move out from the contact point, along the curve of the wheel in much the same way that it does when using a hammer and dolly. The smaller the radius of the anvil wheel, the more metal is moved.
Following this technique further, you can use hammers with a very high crown such as a blocking or raising hammer or, even using forming mallets, and instead of using a hard dolly you can use a sandbag. In this case a lot of metal is moved very quickly. This allows a lot of shape to be put into a panel relatively easily. However, when raising large curves using this technique you should be aware that extreme stretching may leave the metal much thinner than is ideal. A similar technique can be used on an english wheel by covering the top wheel in a Go-Kart inner tube.
Shrinking metal.
Shrinking metal is not so simple and is generally the biggest barrier to amateur metal shaping. You may see special shrinking hammers online that have curves or grids formed into the face surface, however none of these actually shrink metal. It's not the hammer that causes the shrink, it is the technique.
The most rudimentary method for shrinking metal, and is one worth learning, is how to use a tucking fork. A tucking fork is very easy to make yourself. It's basically two tapered bits or metal rod welded together with a small gap between them, and a handle of some sort at the other end. You can even make a really basic version by cutting a slot into a length of flat bar but it is worth making a proper version as it will form the root of the tuck properly and be a lot easier to use. The technique is to fold the edge of the metal first one way so that it raises a ridge and then on each side of the ridge, fold the metal back the other way to make similar ridges underneath. what you should notice is that at the 'root' of the tuck / fold, which is the end furthest from the edge of the metal sheet, looks a little like the metal is pinched.
Then using a dolly, or bench stake, you need to hammer the root to 'trap' the pinch by flattening the root without allowing the fold to flatten out. You do this by striking it sharply directly on the root. Once the pinch is trapped you can then dolly along the fold to drive the metal together. Essentially the trapped root holds the metal in position and so when you work back out to the edge of the sheet flattening the fold out, instead of the fold flattening out, the excess metal from the fold is pushed together. You will notice that the metal afterwards is thicker than before and the panel will start to take on a curved shape. This is as a result of the metal being 'shrunk'
The big drawback with this technique, which is also the same for shrinking machines is that they only really work along the edge of panels. You cannot use this technique if you want to shrink an area in the middle of a panel. This is why it is sometimes easier to divide a panel up into smaller sections and weld them together at the end. You can to a degree shrink metal using a wheeling machine by pushing downward (or pulling upwards) on the panel against the wheels to force the metal to move where you need it to go. Practice is key to making this technique work.
Shrinking and stretching machines
Shrinking and stretching machines as their name implies do exactly that. However, they do this by using metal jaws that grip the metal and forcing it together or pulling it apart. Those metal jaws unfortunately leave lots of marks on the metal surface which are not that great for skin panels as they can be hard to planish or file out. These machines are generally best suited to use on edge flanges and areas that will not be seen.
Planishing machines
Planishing machines are generally based on pneumatic chisels mounted in a frame that are fitted with basic hammer type anvils. They can be used quite effectively to planish panels and also stretch metal but generally due to the way that they are made do not have shrinking dies available and so cannot be used to shrink metal in the middle of panels
Pullmax style machines / Power hammers
Pullmax style machines are a big step up from planishing hammers and generally incorporate a guided and sprung head that uses interchangeable dies. Custom dies can be made to help form panels into custom profiles. This kind of machine is not really accessible to the hobbyist level due to the cost, but is worth mentioning as it is this type of machine that is required to be able to use shrinking dies, which are otherwise unavailable for machines such as the planishing machine mentioned previously. Shrinking dies will allow you to shrink metal in the middle of panels
Swaging Machines / Bead rollers
Swaging machines generally come with a variety of different dies that allow you to form different profile swages in a panel. These can be flanges or steps or beads or any number of custom profiles. You can use them to add raised beads to panels to provide strength, and you can use them to add steps to panels where you need panels to overlap.
Making your panel
- MIG welding produces a very hard HAZ that is often full of impurities and inclusions. It is less than optimal for joining skin panels
- TIG welding produces a much more ductile HAZ that can be more easily planished.
- Gas welding / brazing produces a HAZ that is as ductile as the parent metal and is preferred to all other methods.
To address panel distortion the panel must be welded in a very methodical manner. It is clamped at one end only and stitched together a tack at a time. The weld is allowed to cool, which will cause the HAZ to contract and the unclamped part of the panel to move. Using a hammer and dolly the weld is then planished with a few short sharp taps - just enough to stretch the weld back and align the unclamped part of the panel. The next tack is then undertaken and so on.
Of course this kind of process is not for the impatient, but it is the only way to produce distortion free welds. Once the panel is completely tacked it can then be fully welded. The welds can then be filed out and planished smooth. Note that it is possible to file gas and TIG welded welds, but MIG welds cannot be filed and need to be ground. Filed and planished welds if undertaken correctly will be almost indistinguishable from the rest of the panel.