I am certainly happy to know that there are some people out there who may find this interesting. Because of all the comments regarding final finish (paint vs. polish) I am going to mostly focus on that aspect. I am always happy to talk metal, so if anyone has any questions don’t hesitate to ask. ***I want to stress that this is entirely my opinion only*** Some may disagree, or approach things differently, and that is perfectly fine.
Project Planning & Material Selection
When making your own tins there are several factors to take into account when selecting your material. Importance of weight, potential abuse (vibrations/impacts), structural integrity (etc. rear fender with passenger seat) are all obvious factors. For example; if you are building a rigid chopper with a solid mounted v-twin, thin aluminum tanks and fenders are probably not the best choice. Proper mounting, varied material weights, welding techniques, and overall build techniques also play a big role, but we won’t get into that right now. Another factor which can be very important, but may not seem so obvious is final finish. Especially when working with a customer. In order to explain why final finish plays a role in material selection, and why switching finishes mid-project is a big deal, I first need to talk a little about the shaping and metal finishing process.
So what is metal-shaping? When metalshaping you are using various techniques to shrink or stretch the material in different areas to put the desired shape into the panel. When stretching the material you are increasing surface area, but decreasing thickness. When shrinking it is the opposite. A good way to think of it is if you were to take a flat sheet of steel and bend up the four sides to form a box, you could bend the sides back and have a relatively flat sheet again. It is pretty much the same as it started. In contrast, if you were to take the same sheet of steel, place it on a sand bag and begin pounding the center until if formed a bowl shape, you could not flatten it back out. It is not the same sheet anymore. By pounding on it you’ve stretched the center of the sheet and increased the surface area, at the expense of thinning material in that area. Make sense?
There are many different tools and techniques to shape metal. Some shrinking, some stretching. I am not really going to get into tools and processes here, but for this topic it’s important to note the consequences of your approach to shaping. Some techniques will move material fast, but leave a harsh, inconsistent surface. Think about a ball peen hammer striking a sheet over a sandbag. Other processes will move material more slowly, but leave a nice, even surface. Think about an english wheel with nice polished dies. The techniques and tools you use will generally be dictated by the shape you’re making. For example; a very tight-radius panel may not fit into an english wheel, and will need to be shaped entirely with hand tools like hammers and dollies.
The last last stage of shaping a panel is referred to as “metal finishing”. This is where you deal with the imperfections left behind by whatever techniques you used to shape your panel. Essentially, you are bringing up any low spots, and bringing down any high spots. Just like in your initial shaping, the panel design will determine what approach you take with this process. Maybe you can run your panel through the english wheel with very low pressure to smooth it out. Or, you may be using a polished slapper and dollies to make a consistent surface. Often you will coat the panel with a die, and skim it with a straight file or sanding block to reveal the high and low areas. There is a lot to cover in this process alone, and you can continue the metal finishing process until your panel looks like chrome. It is tedious and time consuming. What is important to note for this topic, is that you will generally be further thinning the material as you go through this process.
How perfect does a panel surface need to be? This is a topic that a lot of people with a little knowledge have a lot of opinions about. The truth is, it depends. Obviously if its polished aluminum panel, it needs to be pretty perfect. If it’s going to paint, does it need to be flawless? No, it certainly doesn’t need to be. You can create a panel that can be painted without any body-filler, and some people like to boast about that. But, let’s think about what that means to the metalshaper and the customer. One option is to metal-finish the panel to a level where 1/16” or less filler is used in the lowest spots. (For motorcycle tanks and small parts it’s generally 1/16” or less, but on large panels it’s not uncommon for top quality shops to go up to 3/16”). From a technical standpoint this is perfectly fine, faster, and less labor cost to your customer. On the other hand, you can spend hours creating a perfect surface, have zero filler, and your panel will look exactly the same as the panel with a little filler. If you’re making panels for yourself, or have a customer who wants to pay for flawless panels, perfect. But, is it good business to create a flawless panel to go off to paint?
Now, using the above knowledge we can circle back to the the beginning topic of material selection, and how it relates final finish. Let’s look at the two common aluminum thickness used in shaping, .063” and .080”. Let’s say that I am going to be shaping an aluminum fuel tank which will have a lot of shape. If it will have a final polished finish I am going to consider some factors; thinning in spots during shaping, continued thinning during metal finishing, and even more thinning during the polishing process. So for a polished tank I am going to use the .080” material, which will be more difficult to work, but after all of the finishing/sanding/polishing will still be strong. In contrast if the tank will be painted, I will use the .063” and metal-finish to a surface where an acceptable amount of filler can be used. In the end both tanks will be practically equal thickness, and the customer can be charged appropriately for the product they recieve. This is also why I charge less for tank ready for paint vs. a polished tank.