Fundamentals Laser cutting of sheet metal - how it works

Laser cutting, also known as laser beam cutting or CNC laser cutting, is a thermal cutting process that is frequently used in sheet metal processing.

Laser cutting can be used to cut various materials such as steel or aluminum sheets or three-dimensional bodies such as profiles and tubes.

Laser cutting was introduced as an industrial technology more than two decades ago. In the course of the cutting process, a high-energy and intensely focused laser beam is directed at a workpiece. This process generates metal vapor and melt, which are blown out with the aid of a high-pressure gas stream. The laser beam creates a cutting joint or cutting gap in the workpiece, the cutting edge of which can vary depending on the laser beam. Many different cutting tasks can be performed with a laser beam. The range extends from cuts in metals with a thickness of up to 30 mm to very accurate cutting kerfs in extremely thin materials. CNC laser cutting is a process in which different types of lasers are used, with fiber lasers and CO₂-lasers as the most widely used industrial applications.

Compared to other processes, CNC laser cutting can be used economically even for very small batch sizes.

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Laser cutting: The process:

Basically, laser cutting consists of two, but simultaneously running, sub-processes. On the one hand, the laser beam focused at the cutting front is absorbed, resulting in the energy required for laser cutting being applied. On the other hand, the cutting nozzle, which is arranged concentrically to the laser, provides the blowing or process gas required for laser cutting. This gas is responsible for protecting the processing head from splashes and vapors and for removing the excess material from the kerf. The aggregate state of the respective jointing material during CNC laser cutting depends on the type of process gas used as well as on the temperature reached during the effective range.

Depending on whether the material emitting from the kerf is an oxidation product, vapor or liquid and which material is involved, a distinction is made between three laser cutting processes, i.e. laser beam fusion cutting, laser beam flame cutting and laser beam sublimation cutting.

Laser cutting: Structure and components of laser cutting machines

A CNC laser cutting machine consists of several components:

• Laser beam source

• Laser beam guidance

• (usually moving) processing head or focusing optics (converging lens or concave mirror)

The laser beam emitted from the source is guided to the processing head, which is located at the processing head. In the CO₂-laser this is a deflecting mirror and in a conventional laser beam it is near infrared. There, the laser beam is bundled in a focal point by the processing head which generates the power required for laser cutting. These power requirements are between 1006 and 109 watts per square centimeter.

CNC laser cutting: Laser beam fusion cutting

Laser beam fusion cutting is a non-contact cutting process which is primarily used for laser cutting of aluminum alloys and stainless steel. The respective workpiece is melted locally by a laser beam.

With the laser beam fusion cutting process, the cutting gap of the material is continuously melted using a low-reaction process gas, whereby oxidation of the cutting surface is prevented by removing the molten material. In addition to nitrogen, argon is used as a process gas in laser cutting. Both gases prevent the material from reacting chemically. Since the cutting surfaces do not oxidize during laser beam fusion cutting and there is no burr formation at the cutting gap, the cutting edges do not have to be reworked. To ensure good cutting quality in laser beam fusion cutting, the cutting process should take place with the aid of a powerful solid-state laser.

The following factors can also influence the quality of the cut:

• Cutting gas pressure

• Feed rate

• Strength of the laser source

• Focus position

With regard to CNC laser cutting, the following can be noted: In principle, the process of laser fusion cutting can be used for all materials that can be molten.

The VdLB Verband deutscher Laseranwender – Blechbearbeitung e.V. (Association of German Laser Users - Sheet Metal Working) represents the interests of users, managing directors and company executives in the wide materials processing market - in this case, of sheet metal. Their aim is to encourage cooperation for the benefit of all parties involved. Cooperation, exchange of experience through intensive dialog, technical information and further training are in the focus of the association.

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CNC laser cutting: Laser beam flame cutting

During the laser beam flame cutting process, a laser beam heats up the workpiece locally, generating spontaneous combustion after melting. The material is largely burnt by the influx of an oxygen jet at a specific point. The resulting iron oxides are mixed with molten metal, which is remove with the aid of an oxygen jet, resulting in a kerf.

The oxygen acts as an additional source of energy during oxidation, i.e. the exothermic reaction with the material. Unlike laser beam fusion cutting, laser beam flame cutting requires mechanical post-processing. Since the cutting surfaces oxidize during CNC laser cutting, burrs form on the cutting edges. However, if the process parameters are set optimally, burr formation can be avoided.

This process is suitable for metallic, ferrous materials. Laser beam flame cutting is most widely used in sheet metal processing of steel. The high cutting speed and the fact that relatively thick workpieces up to a thickness of 30 mm can be cut are characteristic for laser beam flame cutting.

CNC laser cutting: Laser beam sublimation cutting

The characteristic feature of laser sublimation cutting is that the heated material evaporates. In other words: This laser cutting process ensures that a material changes from a solid state to a gaseous state without becoming liquid. This process is called sublimation. Due to this characteristic, laser cutting is primarily used for materials which do not have a pronounced molten state. During laser sublimation cutting, the process gas used - usually nitrogen, air, helium or argon - ensures that the vapor of the material produced during sublimation is blown out of the kerf. At the same time, condensation of the steam is prevented. The generated material vapor creates a high pressure in the kerf. This in turn is responsible for the melt being removed downwards and upwards. This CNC laser cutting process is particularly suitable for separating organic materials such as wood, textiles, leather or cardboard as well as fiber-reinforced and homogeneous plastics.

Laser cutting results in smooth and burr-free cutting edges. Therefore laser beam sublimation cutting is used to perform particularly fine cutting tasks. This is the case in medical technology, for instance.

Laser cutting: Laser-capable materials and cutting thicknesses

Currently, the maximum machinable plate thickness for steel is about 40 millimeters. For laser cutting of aluminum the maximum plate thickness is 20 millimeters. This is because cutting aluminum - and copper - is more complex. Since a large part of the radiation provided by the laser reflects at the beginning of the process, a significantly higher power or power density is required to penetrate the material. The cutting performance is significantly lower than with ferrous materials even if the power component in the cutting channel is absorbed. This is a result of the significantly higher thermal conductivity of copper and aluminum and the fact that no supportive oxidation takes place.

For laser cutting of copper, aluminum and other metals with a high thermal conductivity, a CO₂-laser cannot be used or can only be used to a limited extent. However, this is not only due to the higher thermal conductivity, but also to the reflection of a large part of the acting radiation.

Piercing is problematic in both laser fusion cutting and laser cutting because it is very time-consuming. The reason for this is that laser cutting must take place in pulsed mode with reduced average laser power. This is the only way to avoid metal splashes, which can damage the focusing optics and a cause a strong back reflection.

State-of-the-art laser machines, however, are equipped with sensors that can be used to detect the puncture. This not only saves time, but also ensures that the material is not completely pierced at the start of the cut.

Laser cutting of steel causes hardening at the cutting edges due to high temperature variations. If the material is to be subsequently machined, this can result in problems.

When flat material is cut by laser, it is positioned on a support which must meet specific requirements:

• high resistance, generating long maintenance intervals

• low back reflection, causing as little damage as possible to the workpieces

• small contact surface, ensuring that small parts and resulting waste can fall through

Basically, the laser is an extremely flexible tool, enabling laser cutting of different materials with different thicknesses. In general, the cutting gap in laser cutting is very narrow. If you compare the quality of the cutting with other cutting methods, the laser performs very well. Depending on the system used for laser cutting, almost all materials can be processed.

Laser cutting usually achieves a clean cutting edge, depending on the respective material, which does not have to be reworked. Another positive aspect of CNC laser cutting is that it is accompanied by high material utilization and is therefore very economical. Marking, engraving and cutting of the workpieces can be carried out with the same beam source and in a single operation. However, laser cutting also has disadvantages. These include high system costs, high gas and energy consumption depending on the beam source used, and strict work safety regulations.

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Advantages of laser cutting

• Economic efficiency, due to high material utilization

• Marking/engraving and cutting with the same laser beam source in one operation

• almost all materials can be machined

• high flexibility

• low minimum quantities

• narrow, clean cutting edges, depending on the material, without reworking

Disadvantages of laser cutting

• strict work safety required

• high investment costs

• high gas and energy consumption

Laser cutting: Historical background

In 1917 Albert Einstein established the theoretical basis for stimulated emissions. At that time no one expected the far-reaching effects of this theory on science and technology. The first functional laser, a ruby laser, was developed in 1960. The term "laser" is an artificial word derived from the initial letters of the English term "Light Amplification by Simulated Emission of Radiaton".

Berthold Leibinger is one of the pioneers of industrial laser applications. He has turned the machine tool manufacturer Trumpf into a global company: Berthold Leibinger died in October 2018 shortly before his 88th birthday.

As the life span of a laser was very short at the beginning and the financial means for its manufacture were enormous, it was only used in scientific applications - e.g. in experiments. Nevertheless, the laser found its way into industry, where it was used for the first time in the construction industry for measuring purposes. As a result, the technology developed rapidly: In 1962 the first semiconductor laser was developed, and in 1964 the first CO₂-Laser for laser cutting was introduced. Laser cutting was introduced in 1978, when the first industrial CNC laser cutting system was put into operation. Laser cutting for 2D materials was developed in 1984.

The Lasys trade fair in Stuttgart and the Laser World of Photonics trade fair in Munich are dedicated to lasers as tools.

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This article was first published by belchnet.

Original by Frauke Finus / Translation by Alexander Stark

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