Roller Milling (variable sheet
    metal thickness)

Assignment

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 condition cannot be fulfilled. With new milling technology, there is an improvement. The advantages are, further weight-saving and at the same time, an increased stability. Sheet metal with a continuously variable thickness has already found it's way into the mass production.

Function

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 an extremely heavy coil. 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 increases this number even more. Through chemical alteration, e.g., the addition of alloy substances, one has succeeded in increasing the hardness of steel-plating of the same thickness, without compromising the milling quality and the weldability. One speaks of 'high tensile' steels.
If you take notice of the above figure, and enlarge it with one or two mouse clicks, you can see the plating thicknesses which are printed on the sheet metal. This means that while the plating was slowly going through 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). 11/10