Getting Started with Sheet MetalBy Fay Butler April 1999
riting about metalworking is very difficult because of the need to transfer some basic "non-verbal" concepts, that means they are not easy to write down. As amateurs and hobbyists we look for tricks to solve our problems. Tricks give us tools for immediate use, which are often specific for individual materials, tools and personalities. As we move into a true professional or a serious amateur, through experience, it is the basic principles that give us the foundation to retrace our steps more accurately when we are problem solving. The principles will also give us the ability to think up the unique "tricks," as we need them, to suit our individual tasks, tools, materials and personalities. So what I would like to talk about is the basics of "how to get started."
One of the first steps that I do, whether fabricating a repair patch or an entire new piece is to get the "surface development" of the part. This can be done using inexpensive medium-weight drafting paper or packaging paper.
First, cut a generous section to represent the metal part you will be making. Note: This should be one piece of paper as taping more than one together can give you an incorrect reading as the flat paper is truly representative of the flat sheet of metal we will be starting with. Leave some extra for trimming while making the pattern.
Proceed by getting the paper to lay flat on the original metal part. I generally start by locating the "neutral zone." This is the area on the part where there is no (or very little) compound shape indicated on the paper — which will allow the paper to lay flat on your original part. You may actually be doing some shrinking, stretching or even folding of the metal in this area, but the paper should lay flat with no cuts or folds in this neutral zone.
The pattern paper can be held in place with magnets if you are working on steel. If you're working with a non-ferrous metal like aluminum or copper, use magnets on both sides — stack them up if they are weak. You can also cut small, 1/2- to 3/4-inch square holes in the paper, and tape over the hole to hold the paper to the metal.
Continue working to get the paper to lay flat by working around the edges of the part. Where the paper bunches up, distribute the excess paper evenly into tucks. Fold the tucks over and hold them in place with more magnets (or whatever you are using). If you have an area with a stretched edge or a reverse curve, (reverse curves are generated from a stretched edge) cut slits in the paper from the edge in until the paper lays flat. In this case the slits will open up progressively more towards the edge. Now the paper should lay completely flat on the part.
Before you take the paper off, look carefully to see if you can add any other information that could help later. One thing that comes to mind is the "highlight line" on some parts. This highlight line can be difficult to explain. Technically, it is the area on a part where two imaginary intersecting sweeps or arcs would accelerate over some lead in, reflecting light, to show the outline of the part. To understand this, you've got to remember that a curved surface, such as a wheel pant, for instance, is usually made up of several curves that intersect at some point. If the curves on the major surfaces of the part intersected where they were supposed to, many aircraft and automobiles would have sharp edges (referred to as a true-edge design). In order to make things more aerodynamic, however, the sharp edges are knocked off and rounded. In order to round off these edges, a gradual change in the arc is made. As the arc changes, it is said to "accellerate." And as with any type of acceleration, it changes over a given distance. The two accelerated curves then meet somewhere in the middle where it again forms a smaller true radius. This point is where the highlight line forms. If you have that part painted, and you shine a light on it, the light would reflect off of this highlight line. Whether or not you realize it, when you look at a curved surface, the shape of what what your mind perceives is based on these highlight lines. This is important for a painter because they would get the surfaces to show these highlights. The fabricator is actually making these highlights and if the fabricator doesn't make these correctly, the part won't look right, and the people looking at this won't even know why.
A large fuselage panel probably would not have a highlight line, although a wheel pant around the outside radius edge surfaces would. Engine cowlings might also, to help fair into the body and to show the design shape. You can use this information later on, to track your shaping, you can mark these highlight lines using the paper pattern as a guide.
Once you have this surface development completed you can tell how the part needs to be shrunk or stretched to build the part. If the paper lays flat on the part, with only folds and no tucks or slits, (a cylinder or traditional box with straight turned edges are common examples), then the part can be made out of flat metal without any shrinking or stretching — only some folding required. Very subtle compound curves might be difficult to distinguish from actual flat sheets.
As to the question of whether you generate the compound curve by shrinking or stretching, or a combination or both, that would depend on what your skill level is and what equipment/ labor saving devises you have access to. Remember, high quality parts can still be made efficiently with very simple, common hand tools. If the pattern has lots of tucks in it, then you might choose to shrink in those areas. Shrinking can be done in two ways: hot and cold, but the techniques used are wide and varied.
With a limited amount of tucks, you may choose to stretch the areas outside of the tucks that appear "flat" on your paper pattern — blending progressively into your tuck area. Where there are any slits in the pattern, the metal will need to be stretched there, maybe using a "linear stretch" contact area which is a method of cold working the material with specialized equipment.
Experience will help you determine how much shrinking and stretching the sheet metal can handle. If you think it is too much then re-pattern it to get the minimum amount of shrinking and stretching required per piece. For example, if there is too much shrinking or stretching, you may have to break the part into several pieces and weld them together. To understand the concept of repaterning, it helps to understand that you can build any complex curve without having to shape metal by cutting a part into 1-inch squares — but this would be impractical. On the other hand, making the entire aicraft out of one sheet of metal would also be impractical. What you have to find is some reasonable balance of size of the pieces verses workability of these pieces when determining how to construct the part.
Sometimes I may paper pattern a part three or four different ways. If the part is to be welded, I use the paper to see how the different placement of the proposed weld seams affect the amount of shrinking and stretching required and the size of each piece. Of course, some of the "heat treatable" aluminum alloys are not weldable so this may restrict you from breaking the design into pieces. Consult a weldability chart, with compatible welding rods. The Aluminum Association has such a chart set-up. In any event you can repattern, to "even" the amount of shape required over the panel as opposed to just having it in one area or edge.
Now you have a road map to get you started making the part. Lay the paper pattern flat on your new metal sheet to trace the outline of the blank (This will give you the size and shape of the initial blank and this is the reason I don't "tape" the tucks or the slits on the initial surface development). You can leave extra metal around the edge although I prefer only the bare minimum from zero to 1" max. Extra material means more weight on larger pieces and perhaps more shrinking and stretching if your part requires that around the edges. For those of you that are more cautious, you can always leave extra metal and trim it off later.
Before I remove the pattern from the blank, I check to see that I have transferred all of the information from the pattern to the blank. Details can be transferred with a punch, if that part is going to be trimmed, or by placing carbon paper between the pattern and the blank, and tracing over the detail lines, the same for highlight lines, etc.
By strategically placing prick punch marks around the edge, you can create registration marks so that you can replace the pattern on the partially shaped part later to check your progress (if the prick punch mark is an an area that is not acceptable, put it in the "extra metal" area, to be cut off).
Often I will key "profile templates" to specific planes on the original part and mark the location of these on the paper pattern. I then transfer this location to the part being shaped for checking my progress.
These templates are made from scraps of poster board, taped, and/or stapled together. The staples work as "dowel pins" between the pieces of poster board which prevent them from moving. If I want to make sure they stay flat I will tape heavy (3/32 diameter), "Z" bent welding rods to the surface of the template with tape for rigidity. In this case, you are substituting the templates for a wooden buck (a ribbed representation of the piece). If more accuracy if required, a wooden buck will help. In any event, the cardboard templates are needed anyway if you are going to make a wooden buck.
This is only a guide to the use of paper patterns for the surface development. The better pattern you make, the more you will be able to read off of it, and the more accurate the part will be. The paper pattern will allow you to experiment with different sizes of parts, direct you where you need to concentrate or highlight the areas to be shaped, and placement of weld seams. It can help you to get started.
I use this concept of surface development, whether making large or small parts. In the case where you don't have the original part, paper patterning a wooden station buck can be difficult because you do not have full surfaces to pattern, although it can give you a rough idea of how the metal will need to be shaped, and the rough size of the metal blanks.
What is important is that you just get started. We all start with insufficient information, and problem solve as we go. Making mistakes is all right. If we can learn from them, and not repeat the same mistakes again, that is even better. Remember, no one was born with these skills.