Flame Cutting Sheet Metal
Flame cutting is a cutting technique that can cut sheet metal measuring up to 8m in length and above. The maximum sheet thickness is 150mm. Flame cutting machine may feature up to 6 burners for rapidly cutting thick and large-size sheet metal, but also for producing batches. This technique is suitable for both large flame-cutting projects and small cutting jobs.
Flame cutting is a method of cutting that uses one or more torches to cut sheet metal by means of combustion. This process is also known as oxy-fuel cutting, because it uses oxygen and a fuel gas. Steel is pre-heated to just under ignition temperature. This causes oxidation and slag formation, but the slag (or molten material) that is formed is burnt, while the steel product itself is not. The slag and the torch together keep the cutting process going. Because of the nature of the process, flame cutting can only be applied to mild steel. Stainless steel and aluminium are best cut using a plasma cutting machine.
An example of a flame cutting machine is the Esab Suprarex sxe-5000. It features 6 burners, which makes it particularly suitable for cutting batches. This machine can flame cut sheet metal measuring up to 8.000 x 4.000mm and up to 150mm in thickness.
Flame cutting is based on thermal and physical processes for oxidizing metal and removing the slag. The torch consists of a gas jet that preheats the steel to just under ignition temperature. A jet of pure oxygen is directed from the center of the torch. This causes a process known as oxidation. Combustion generates so much heat that further heating is not required. However, the flame is still needed for blowing away the slag and for restarting the process in case of interruptions.
The actual cutting is caused by first of all combustion and secondly removal of the slag. Just as with laser cutting, it is important that the slag remains liquid during the process so that it can easily be blown away. Materials with high levels of carbon require higher preheating temperatures.
Types of Gases and Materials
The selection of the right gas is important when using flame cutting technology. There are many different gases that each have their own cutting speed, maximum temperature and oxygen/gas ratio. Moreover, certain gases enable a higher cutting speed or lower preheat temperature. In addition, the type of material to be cut also influences the process. Certain types of alloys may facilitate or impede the cutting process. The planning engineers working at the flame cutting company know all the options and may suggest alternatives.
Cutting Bed and Torches
The torches of the flame cutting machine are positioned over the bed, using a rail. This means that one side of the cutting bed can be used for cutting while the other half is being loaded. The cutting capacity may also be increased by using flame cutting machines with several torches. This is especially practical for batch production, and reduced prices may apply to series of identical products.
Limitations on Hole Diameter
The flame cutting technique does not allow cutting holes with diameters smaller than the thickness of the sheets. The diameter of a hole cut in a 10-mm-thick sheet must therefore be at least 10 mm. If smaller holes are required, they may be drilled. It is not possible to rough-cut holes for drilling and tapping. However, rough cutting for milling is possible. The rule for minimum hole diameter applies here as well.
Limitations on the Quality of the Cut Edge
The cut edge formed by flame cutting is fairly clean. Nevertheless, the rough cut edge, the square edge and the kerf may pose problems. Flame cutting companies meeting the EN ISO 9013-221 requirements for flame cutting, produce cutting edges of good quality.
Limitations Relating to the Heat-Affected Zone
Flame cutting is a thermal process that generates a lot of heat. This heat creates a heat-affected zone that may in some cases be problematic. This is the main limitation of flame cutting. The formation of a heat-affected zone must be taken into account if further machining, welding in particular, is required.
Oxidation, Inserts and Slag Formation
Flame cutting always causes oxidation. This may be problematic if the material is subsequently welded and coated. The oxide layer is normally fairly loose and therefore easy to remove. Another limitation is slag formation. Slag may remain on the sheet metal and hinder the torch. Inserts are often applied to ensure a high-quality cut edge and to prevent slag formation. This means that an incision is made in a part of the sheet metal that is not going to be used. The advantage of using inserts is that this method creates a cleaner, tidier cut edge and slag formation is mostly on the outside of the contour. Unfortunately, it is not possible to prevent oxidation and slag formation all together, but employees at a flame cutting company can apply various techniques to make sure they deliver a high-quality product.