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Posted on Nov 4, 2020
A metal laser cutter is a useful piece of equipment and many workshops use it for a multitude of operations. The main applications are cutting materials such as sheet metal and plates.
This also works not only on steel plates, but also on aluminum plates, mild steel, stainless steel, to name a few. The laser cutting process is very precise and delivers excellent cutting quality. There is a very small width of cut and a small heat affected zone, and it makes the cuts into extremely complicated geometries, shapes, and small holes.
Many may not know that the word "LASER" is the acronym for light amplification through stimulated emission of radiation. The real question is how can just light slice right through a steel plate? In this article we will answer this and other important questions regarding metal laser cutters.
A laser beam is a column of light with very high intensity, wavelength, or color. In a typical CO2 laser, this wavelength is in the infrared part of the light spectrum and is therefore invisible to the human eye. The light beam is very narrow and almost 10mm in diameter when passed through the machine's beam path.
This is via the laser resonator which makes the beam. There are many ways to reflect it in the multitude of paths by many mirrors or beam flexors. Then the final way is it is virtually focused on the steel workpiece. This highly intense laser beam moves in a path passing the nozzle and then aims at the plate. Compressed gas such as oxygen or nitrogen also flows through this nozzle opening.
A special lens or a curved mirror can be used for the focusing of the laser beam, and this is done in the laser cutting head. The beam must be precisely focused so that the focal point shape and the energy density are perfectly round and coherent at that point and centered in the nozzle.
The heat density at that point is intense when you concentrate a large beam on a single point. Remember to focus the sun's rays on a leaf using a magnifying glass and how this can trigger a fire. Now, think about concentrating 6 kW of energy in one place and you can imagine how hot that place is going to get.
Fast heating, melting and partial or complete evaporation of the material is caused by a high power density. The heat from the laser beam is sufficient to start a traditional "oxy-fuel" combustion process while cutting mild steel, and the laser cutting gas is pure oxygen, much like an oxy-fuel torch.
The laser beam actually melts the material while cutting stainless steel or aluminum, and nitrogen blasts the molten metal under high pressure out of the gap.
The cutting head is shifted over a metal plate in the shape of the desired part in a CNC laser cutter, thereby cutting a part off the plate. A very precise distance between the tip of the nozzle and the cutting plate is maintained by the capacitive height control system.
This distance is important as it determines where the focus is in relation to the plate surface. The quality of the cut can be influenced by raising or lowering the focus just above the surface of the plate, on the surface, or just below the surface. There are many, many other parameters that also affect cut quality. However, when all are properly controlled, laser cutting is a stable, reliable, and high-precision cutting process.
There are many ways to cut mild steel sheets that are suitable for automation. However, not all methods are useful for thinner sheets, others for thicker sheets. Some are fast, some are slow. Some are cheap, others are expensive. There are precise ways and then there are a few ways that are imprecise.
Let us now compare the strengths and weaknesses of each process, and then list some criteria that can be used to decide which process is best for your application.
Cutting with an oxygen or flame torch is by far the oldest cutting process that can be used on mild steel. It is generally believed that it is a simple procedure and that the equipment and accessories are relatively inexpensive. The oxy-fuel burner can cut through a very thick plate, which is mainly limited by the amount of oxygen supplied.
It's not uncommon to use 36 and even 48 inches of steel when using an oxygen torch. However, when it comes to profiling steel plate, the majority of the work is done on a plate that is 12 inches thick and thinner. A correctly adjusted oxy-fuel burner ensures a smooth, square cut surface. There is little slag at the bottom and the preheating flame is only slightly rounded at the top. This surface is ideal for many applications without further treatment.
Plasma or gas cutting is ideal for panels greater than 1 inch thick, but can be used up to about 1/4 inch thick with some difficulty. This is a relatively slow process that takes about 20 inches per minute on 1 inch material. Another great thing about oxygen cutting is that you can easily cut with multiple torches at the same time, which increases your productivity.
Plasma cutting is a wonderful process for cutting mild steel sheet that offers much higher speeds than oxygen cutting, but requires a certain edge quality. Plasma is difficult here. Edge quality has a good point, typically between about 1/4 "and 1.5" depending on the cutting current.
The general squareness of the edges starts to deteriorate as the board becomes very thin or very thick although the edge smoothness and slag performance can still be quite good.
Plasma equipment can be expensive compared to an oxy-fuel torch because a complete system requires a power supply, a water cooler (in systems above about 100A), a gas regulator, torch cables, combined hoses and cables, and the torch itself.
However, the increased productivity of plasma compared to oxygen fuel will quickly pay off in system costs. You can cut plasma with multiple torches at the same time, but this usually comes with additional costs limited to two torches. However, some customers choose to have up to three or four plasma systems on a single machine.
However, these are usually high-end manufacturers who cut large quantities of the same parts to power the production line.
The laser cutting process is suitable for cutting mild steel up to 1.25 inches thick. With the exception of the 1-inch barrier, everything must be in order for it to function reliably, including the material (laser beam), gas purity, nozzle condition, and beam quality.
The laser is not a very fast process as mild steel is basically just a burning process that uses the extreme heat of a focused laser beam instead of a preheated flame. Therefore the speed is limited by the speed of the chemical reaction between iron and oxygen. However, the laser is a very precise process.
Creates a very narrow cutting width and can therefore cut very precise contours and exact small holes. Edge quality is usually very, very good, with very little tooth and lag lines, very square edges, and little or no scabies.
Another great benefit of the laser process is its reliability. The life of the consumables is very long and the automation of the machine is very good, so that many laser cutting operations can be performed "with the lights off". Imagine loading a huge piece of 250 by 1000 mm steel plate 1/2 inch thick onto the table, pressing the start button, and then driving home for the evening. When you get back in the morning you can cut hundreds of pieces and be ready to unload.
Due to the complexity of the beam delivery, CO2 lasers are not suitable for cutting with multiple heads on the same machine. However, with fiber lasers it is possible to cut with several heads.
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