The difference between metal plates and metal plates can be defined by thickness. If the metal thickness is 6mm (¼ “) or greater, it is considered a sheet. Sheet thickness is usually measured in millimeters. If less than 6mm (¼ “) thick, it is considered sheet. Boards are usually measured with gauges.
As the molten metal leaves the blast furnace, it is continuously cast into billets and slabs. Billets are usually used to form rods and wires, while slabs will be used to produce sheets and plates. Slabs usually more than 100mm thick exit the casting process. This thickness needs to be reduced in order to make a usable metal plate or sheet.
In order to reduce and, most importantly, to standardize the thickness of the full length of the metal product, the metal is rolled, black or bright. If the metal is rolled at a high temperature (above the recrystallization temperature), it is called black. The metal is fed through rollers, which gradually reduce the metal thickness until the desired thickness is reached. This thickness can be as small as 300 millimeters.
If the final metal product is more than 6mm (¼ “) thick, it is called a plate and is part of the structural metal family. Anything thinner than that is considered sheet metal.
Bright drawing is when the material needs increased strength, or very precise thickness. The amount of thinning per pass is lower than that for producing black steel, so the process takes longer than black steel.
It is important to note that when measuring sheet products, gauges are neither standard nor metric, but independent values. The gauge shall be converted to mm using a conversion table.
There are a variety of gauge systems in use today, with different gauge systems for different metal types. For example, an 18 Ga. Aluminum plate is 1.02 mm thick. The size of the other 18 Ga steel plate is 1.21mm. The two metals have different gauge systems, resulting in different thickness of the same gauge. Visit this article for sheet metal gauge diagrams.