Armour Making Tutorial - How to roll the edges of armour
Metal Forming Concepts
I believe that it is important to have an understanding of what is happening to the metal when it is being formed. Knowing this helps one anticipate trouble areas and better understand metal smithing on the whole. Metal forming can be broken down into three basic processes;
- Bending or folding, where the metal is changing its shape on only one plane. This is generally the easiest method of forming.
- Stretching or "drawing." Metal can take a different shape on 2 or 3 planes. The surface area grows larger while the plate metal becomes thinner.
- Compressing also can form metal on 2 or 3 planes. When compressing the surface area becomes smaller and the thickness of the plate usually increases. I find this is to be the most difficult process of metal forming.
In armour smithing most properly shaped pieces will require a combination of all three of these methods. To better relate to these processes... Bending can be used to make a tube or a simple vambrace. Dishing is, for the most part, stretching. When "raising" you are actually compressing the metal. For more practical application of these processes of metal forming please see the knee raising tutorial. The amateur physics nut in me feels compelled to point out that when bending metal the outside surface does stretch while the inner surface compresses but they pretty much cancel each other out so forget I mentioned it ;)
Okay, with those 3 processes in mind let's look at what forming is truly being done to the metal as the edge is being rolled over.
Rolling the edge of a straight plate.
For the most part the easiest edge to roll is one on a straight line or one with a very slight outward curve. This is because the metal is simply being bent on one plane.
Figure 1-1 shows that the length of the edge before the roll (A) is the same as the outermost area of the roll (B) and as the inner most part of the roll (C). No stretching or compressing is needed to achieve this shape. Visualize, if you will, rolling the edge of a piece of paper... Paper is not very malleable, it does not stretch or compress easily, but to roll it on one plane is not a hard task. When working with metal more effort is needed to get it to bend but the idea of bending is the same.
Figure 1-2 Illustrates a common problem most people face when first attempting rolled edges. If one area of the edge is bent much further than a near by area the metal between those two points will stretch. Notice the drastic difference in the length of the plate at line (A) compared to the outer edge (C). In order for the edge to roll back inward it will need to be re-compressed. Since compressing metal is more difficult than stretching fixing this area and maintaining a nice even edge would be a major task. Most often the finished roll would have wobbles in it or the main plate would curve to compensate.
Figure 1-3 When making armour it is going to be necessary to use multiple hammer strikes to bend the plate. Some stretching is inevitable but it is best to try to keep it to a minimum. A good technique is to make a number of passes, bending the plate incrementally.
Note that due to the inevitable slight stretching that will occur, the plate will most likely curve slightly, depending on its thickness and other factors. This is why I said above that a straight line or a "slight outward" curve is easiest to roll.
Rolling the edge of a outward curved plane.
Rolling the edge of this shape is technically more complex as it involves stretching and compressing. Even though it has more processes I sometimes find this shape easier to roll because of that inevitable stretching thing I mentioned above.
Figure 2-1. Because the circumference of the metal at (B) and (C) are greater than at (A) those areas must stretch. If this flange was simply bent the plate would straighten out to compensate to a degree depending on its overall size, thickness and many other factors. In order to preserve the contour of the plate one may need to strike the top of the flange while it is touching the anvil to stretch the metal. Do not over do this though, I will explain further below.
Figure 2-2. The outer edge is now curled back in to the main plate. Now the radius of (A) and (C) are nearly identical while the radius of (B) is longer than the other two. In order to achieve this the metal between (B) and (C) must be compressed. Because compression of most sheet metal is difficult the plate will tend to curve a little rather than compress fully. This is why I previously warned not to overly stretch the metal.
In practical application you will probably find that the curve of the plate will straighten a bit when first hammering out the flange then return back to its original curve once the edge is rolled back inward.
Rolling the edge of an inward curved plate.
When rolling edges of an outward curve there is little or no stretching and only compression is used. These types of rolled edges give me grief hehe. Other than the fact that compression is harder than stretching there is also the issue of trying to find just the right shape stake to support the piece when returning the edge back in on itself as seen on Page 7. These shapes often make it difficult to find a comfortable angle from which to hammer and strike the right area.
Figure 3-1. Notice that the circumference of the metal at (B) and (C) are shorter than the plane of the main body piece (A). This means that the metal must be compressed in order to form the flange. Because metal is difficult to compress it often tends to straighten out the curve or buckle along the outer edge (C).
Figure 3-2. When the outer edge (C) is rolled back into the main body of the plate some stretching occurs in the metal between (B) and (C). This portion of the roll is not particularly difficult other than finding a good angle to hammer from as mentioned above.
Figure 3-3 shows the roll of a small flat disk, also known as the edge I try to avoid rolling hehe. In this case there is no stretching. The metal must only be compressed in order to preserve the shape of the plate. Those of you who have made flat topped Great helms will be familiar with what is known as "Potato chipping." This is where a flat, circular (or oval) plate distorts when the edges are turned downward without compressing the metal to compensate for the, now reduced circumference. Because of the pressure (from too little compression) of the flange the, previously flat, plate distorts into the shape of a Pringles potato chip. It is a bit difficult to explain without many more diagrams but try putting a 1/2" flange on a small 4" round piece of 18g scrap and the problem should quickly become apparent.
I hope my sketches help give an understanding of what is happening to the metal when it is formed. I know my text is a bit convoluted. I welcome any suggestions you may have about how to better explain the subject.