1. Definition of Sheet Metal
What is sheet metal? There is no clear definition for it. As a rule, it is understood to be a flat piece of metal whose width is significantly greater than its thickness. If the sheet is less than 3 millimeters thick, it is referred to as thin sheet; if it is 3 millimeters thick or more, it is referred to as heavy sheet. Another important distinction is made by the manufacturing process (see also item 4). There are cold-rolled and hot-rolled sheets:
- Hot rolled material is typical for thicker plates. The surface is more coarse-pored than cold-rolled material and carries a rolling skin / scale layer. If the mill scale is not removed, these plates do not require oiling to protect them from corrosion.
- Thinner plates are usually cold rolled. The tolerances are smaller and the surfaces finer. Steel sheet in particular is protected from corrosion by pickling and oiling before it goes from the steel mill to the sheet processor.
In addition, there is a wide range of sheet grades, formats and material thicknesses. From the material composition and the manufacturing and processing procedures to fabrication and storage, there are various factors that influence the properties and quality of the sheet.
2. The Formats of Sheets
When sheets are delivered for processing, there are usually two formats: Coils and blanks.
In the coil, the sheet is coiled, usually going up to a thickness of 15 millimeters. Its weight can reach 20, 30 or even more tons from the rolling mill. The coil makes it possible to transport large quantities of sheet in a compact form comparatively easily and safely. However, it must be uncoiled again before further processing, which requires a coiler. In addition, because the sheet is curved on the coil, it must be straightened to remove the curvature. When uncoiling from the coil, you can cut off exactly the length of sheet you need.
Blanks are understood to be sheets of metal. They are either already cut from the coil, or in the case of particularly thick sheets, already produced in this form. To simplify trading, there are standardized dimensions for them. The small format 1000 mm × 2000 mm (standard sheet), the medium format 1250 mm × 2500 mm (medium sheet) and the large format 1500 mm × 3000 mm (large sheet) are particularly common. Above the large panel, the maximum format 2000 mm × 4000 mm and the super format 2000 mm × 6000 mm are still quite widespread.
3. The Materials for Sheet Metal
A sheet can be made from almost any metal, it just depends on the formability. So in jewelry manufacturing there are certainly gold or silver sheets. Much more common, however, are sheets made of industrial metals: Steel, stainless steel, aluminum or copper are in the first place.
In order to adjust the properties of the sheets more precisely, various elements are added to the basic material. The composite material is then called an alloy. As a result, sheets have a higher tensile strength or are less susceptible to corrosion, for example.
4. How Sheet Metal is Made
Before the Industrial Revolution, sheet metal had to be hammered out of castings by hand. This was very time-consuming, which is why sheet metal was correspondingly expensive back then. Today, sheet is rolled from steel blocks cast in the shape of a cuboid, so-called slabs. During hot rolling in the steel mill or rolling mill, these are rolled into sheets of the desired thickness by reversing or continuous processes. The temperature is above the re-crystallization temperature of the metal. Plate thicknesses of at least 0.8 millimeters can be achieved.
Because cold rolling requires more force than hot rolling, it is not used for all sheets. As a result, however, thin sheets can only be produced using this process. Sheet just 0.1 millimeters thick can be rolled from steel, and as much as 0.0065 millimeters is possible for aluminum. In addition, lower tolerances can be maintained with cold rolling than with hot rolling.
5. Tolerances and Possible Defects of the Sheets
Important for working with sheet metal is: Any processing leads to a mechanical load or heat development. Since sheet metal is generally easily deformable, stresses and unevenness quickly arise. DIN EN 10029 specifies the flatness tolerances for the individual sheets. For example, a sheet with a thickness of 20 mm must have a minimum thickness of 19.4 mm and a maximum thickness of 21.3 mm. Other flatness defects include various waviness and curvature.
For all downstream manufacturing processes in the sheet metal processing industry, however, sheets that are virtually stress-free and as flat as possible are very important. In addition, on the one hand the large number of different sheet metals, and on the other hand the wide variety of manufacturing and processing methods often make the behavior of sheet metal during processing unpredictable. Therefore, it is necessary to straighten and deburr a sheet after its processing.
6. How to Machine Sheet Metal
In order to turn sheet metal into a usable finished product, there are various processing methods. The following techniques are most commonly used:
- Punching: In punching, the sheet metal is punched out with a manually or automatically operated press, punching machine or punching tool and thus separated. Punching is mainly used in the production of small components from flat metal strips.
- Laser cutting: In this method of cutting, the heat of a laser beam ensures that the sheet metal is divided. As a very flexible tool, the laser is particularly suitable for processing thin sheets. If fine kerfs and high quality are required, laser cutting is the most common method in flexible sheet metal processing.
- Plasma cutting: This process belongs to the methods of thermal cutting. A plasma volume flow is used here, the heat of which leads to local material liquefaction. The high kinetic energy of the plasma volume flow blows out the liquefied material. The process is usually used for sheet thicknesses between 10 and 50 millimeters.
- Flame cutting (oxyacetylene cutting): In this cutting process, the sheet is heated locally to ignition temperature by a fuel gas-oxygen flame. The supply of oxygen creates a kerf at which the sheet is then cut. If metals with large material thicknesses are to be cut, flame cutting is the most common method.
- Bending: Press braking or bending describes the forming of sheet metal. In this process, the sheet metal is placed on the die of a so-called press brake. A tool then moves down onto the workpiece from above and bends the desired angles into the component.
- Round rolling: Round rolling is also a frequently used forming process. In this process, the sheet is usually plastically deformed over three rolls in order to bend it round.
- Roll forming: This process is also one of the methods of forming. In this process, the sheet is unwound from a coil and passed through several pairs of rolls arranged in a row until it is bent into the desired shape. This process is generally used for elongated profiles in high volumes.
- Deep drawing: In deep drawing, the sheet metal is specifically pressed into the desired shape by a tool. As a rule, tools act on the sheet metal through automatic punching machines or forming presses. As the tooling costs often incur considerable one-off costs in production, the processes are often used for high volumes.
If two or more sheets are to be joined together, hemming, riveting, soldering and welding are the usual methods of joining.
7. Special Types of Sheets
Sheet metal comes in very different shapes and with very different properties required to meet specific needs. Among the special features are:
- Blue sheets, also known as blue-gloss sheets, are cold-rolled thin sheets with a firmly adhering iron oxide layer. This layer is formed by annealing at 800 to 900 °C in an oxygen atmosphere and produces a special visual effect due to its dark blue appearance. It also protects against corrosion and serves as a base for coatings.
- Black plate consists of unalloyed steel and is annealed in an open fire or box. It gets its dark appearance by reacting with the ambient air. It is in demand as a design element, which is why it is not further coated.
- Tinplate is a thin cold-rolled steel sheet whose surface is coated with tin. It is mainly used for (food) packaging.
- Laser sheets do not have an official definition. As a rule, it is understood to mean sheets that exhibit particularly good properties when lasered. However, these are usually not specially produced sheets, but rather a particularly careful selection of sheets that is offered specifically for laser customers at a corresponding surcharge.
- Chequer plates are plates with diagonal ribbed structure. This is primarily used to improve slip resistance, and it is also used as a decoration. The corrugation is applied by high force of forming rolls during the production of the sheet and cannot be produced by subsequent embossing.
- Corrugated sheets are metal sheets with a wave-shaped profile. This geometry significantly improves stiffness and load-bearing capacity in the longitudinal direction. This allows corrugated sheets to be used as roofing, for example.
