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Roller Milling (variable sheet metal thickness)

On the car body, only a little sheet metal of a constant thickness is used. At certain points reinforcing is necessary, other areas should absorb e.g., impact energy. To achieve this, it is important that the sheet metal thickness continuously varies. As long as plating in different thicknesses or gusset plating is used, this requirement cannot be fulfilled. Using new milling technologies, there is an improvement. The advantages are, further weight-saving and at the same time, an increased stability. Even sheet metal with a continuously variable thickness has already found it's way into the mass production.

Roller milling itself is a well-known process. Steel- or aluminium blocks which are obtained by continuous casting are brought to a temperature of approx. 1250°C, then, as so called slabs, they are repeatedly sent through the rolling mill, first in the one- then in the other direction. They are then sent through a (e.g., 500 meter long) roller stretch (with e.g., 7 roller frames) and are made thinner and thinner while being sprayed with water as a protection against tinder, finaly they are wound into extremely heavy coils. Then the steel for body-part plating goes into an approx 800°C zinc bath. This process is called 'hot-dip galvanizing' and offers a very good protection against corrosion.

The, up to 2 meters wide metal coils, are brought to the press shop of a vehicle manufacturer. The milling direction must be taken into consideration when punching out, vacuum forming and bending. There are, at the moment (2007), approx. 200 different types of steel. Intensive research is increasing this number even more. Through chemical alteration, e.g., the addition of alloy substances, one has succeeded in increasing the hardness of steel-plating while maintaining the same thickness and without compromising the milling quality and the weldability. One speaks of 'high tensile' steels.

While the plating (picture above) was slowly going through the mill, the distance between the rollers was being changed. Thereby of course, the expansion of the material and the position of the part later to be punched out, must be precisely calculated. Also, this process, up to now (2005), is only possible up to a width of 40 cm and only in one direction. All in all, these measures lead to a distinct reduction of the car body weight, at the same time, increasing the stability (by 40% and more).

'High-tensile steels' have in the meantime, been used in the car body field for some time. However, just how high tensile they are, seems at the moment, to be changing again. Apart from the generally used 'cold- working' shaping process, there is now also the hot working of body parts. When speaking of 'hot-working' approx. forging temperature is meant, which of course, saves the use of the high pressure needed for cold- working. This, in the meantime, is hardly possible when using certain steels, because due to the particular alloying elements and the refinements during the cooling process, they are almost too hard.

Right now, steel manufacturing is competing against aluminium and CFRP (carbon-fiber-reinforced plastic or simply carbon) and is offering fierce resistance. In fact, the outlook is not all that bad if one considers the total energy resources needed for the manufacture and operation of an automobile. It would take a great deal of diligent aluminium recycling, otherwise steel will gain the upper hand. One does not necessarily need varying material thicknesses to insert a part which is laid out for greater stress at a certain place. This can also be done by combining various materials.

In this respect, what is repeatedly found is the sandwich-technology. Here e.g., the thinnest surfaces are made from steel plate combined with somewhat thicker layers of synthetic material, whereby the thickness still remains a good bit less than 1 millimeter. One cannot see the layers from the outside, but weight is saved. As mentioned earlier, the steel branch is putting up resistance. Not long ago, the existence of the manufacturers of timing belts was threatened. They seem to have prevailed. We'll have to wait and see if the steel producers can also succeed. 03/12

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