Plasma cutting procedures

Plasma metal cutting was developed in the 50s of the last century. Since then plasma cutting has been developing more and more with new techniques and plasma cutting processes. Each of the procedures brings different advantages and disadvantages with it. Techniques or plasma cutting processes are reduced to:

• Plasma cutting without secondary media
• Plasma cutting with the presence of a secondary media
• Plasma cutting with a narrowed bow
• Plasma cutting without a secondary media

The plasma cutting procedure without secondary media is used for:

• Manual plasma cutting
• Flush with plasma cutter
• Plasma cutting of sheets of less thickness
• Cutting a smaller number of parts
• Less need for cutting quality

Plasma cutting without the use of an auxiliary gas is most commonly used in shipbuilding where cutting is preceded by welding. In addition, such a plasma cutting process is applied to steel, or during automated (robotized) cutting of profiles. In practice, such a cutting process is often used in combination with CNC technology.

However, a special feature of this cutting process is the use of only one gas having a dual role – cooling and cutting. Because of its simplicity, most handheld plasma cutters work on such a principle. What is even more important is that plasma cutting without an auxiliary gas is used for metals with a maximum thickness of up to 16 mm.

Underwater plasma cutting / Foto: © ECVV

Plasma cutting with the presence of secondary gas

Equally, as the previously described plasma cutting process, this process also has typical situations in which it is used, these are:

• Metal drilling
• The beginning of plasma cutting
• High-quality cut demand
• Plasma cutting of grids
• Underwater plasma cutting

This plasma cutting procedure is specific thanks to a special nozzle that serves to provide the rotating gas. Its main role is the protection of the plasma gas from potentially harmful effects of the atmosphere (environment). The fact that plasma gas is protected, makes this plasma cutting procedure ideal for underwater plasma cutting.

Underwater plasma cutting brings along a number of advantages related to the filtering of potential radiation as well as to the protection of the operator. Underwater plasma cutting also affects noise and dust reduction.

Plasma cutting with injected water

Plasma cutting using injected water uses only one gas to make the plasma. Water injection is radiated or vortexed. This procedure narrows the bow and directly leads to an increase in plasma density. In addition to the mentioned narrowing of the bow, the use of water also affects the cooling.

Nitrogen is most commonly used to generate plasma, but in practice, different, cheaper gases are used, which don’t reduce the quality of the cut. Such a cutting procedure is primarily used for cutting materials where the requirements for cutting quality are exceptionally high.

Plasma metal cutting / Foto: © American Machinist

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The importance of quality control

Quality control has an irreplaceable role in business. Through sampling and process control, organizations can improve their performance. The same is valid for the laser cutting process. However, the question arises as to how to measure the quality of laser metal cutting?

In the previous texts, we emphasized the importance of optimizing the laser cutting parameters, ie the implementation of preventive maintenance of the laser cutter. When it comes to good and high-quality laser cutters, the possibility of anomalies in the cutting process is reduced to a minimum. However, attention should be paid to other important key performance indicators (KPI) that provide feedback on the laser cutting quality level.

Laser cutting norms

As a big part of the business and processes in today’s organizations is standardized and normed, standards that prescribe quality laser cutting parameters have been developed. ISO standard DIN EN ISO 9013: 2002 defines the requirements for laser and other types of thermal cutting. The standard prescribes the control of the following cutting parameters:

•  molten metal
• the incision of the cut
• the puncture of the cut
• cut lines
• roughness

The quality of thermal cutting is defined by the norm / Foto: © Bystronic

Quality control parameters

1. Formation of the molten metal – the control of this parameter is usually done by visual inspection of the metal being sliced. The information obtained by checking refers to the focus of the laser beam or the laser cutter optic
2. The incision of the cut– a parameter that depends on the material being cut and is determined by the tolerance for each metal differently. The information obtained by inspecting the incision tells us about the size of the laser beam penetration into the metal
3. The puncture–The puncture of the metal is created due to corrosive action and it can greatly change the quality of the cut. This parameter is usually visually inspected and qualitatively described
4. Cut lines– the cutting lines during laser cutting are determined by the speed at which each position is cut. This parameter is also visually inspected, and the information provided by the inspection speaks about the need to adjust the cut parameters in terms of power and speed
5. Roughness – a parameter that influences the need for additional material handling after laser cutting. The roughness is defined depending on the thickness of the material being cut

How to improve the quality of the cut?

The quality of the cut can be improved by the aforementioned permanent maintenance of the laser cutter, or by optimizing the cut parameters. Quality control and these parameters are just a mere lead to better optimization of the cutting parameters. The quality control alone depends on the series of cuts that are being performed. If several dozens of identical positions are executed, these parameters must be controlled to prevent a series of identical errors in the executed positions.

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Reflective metals laser cutting

Laser cutting companies in practice often encounter high-reflective metals such as aluminum. The cutting of these metals requires special attention and the preparation of a laser cutter. Namely, due to the reflexive properties of such metals, careless cutting, or non-preparation of the sanding surface, may result in damage to the lens of the laser. In addition to aluminum, laser cutting of stainless steel that is further processed by polishing can also be a major problem.

Why are there any cutting difficulties?

Co2 laser cutters function on the principle of directing the laser beam through the mirrors and lenses on the small surface of the cutting material. Since the laser beam is actually a light beam of high strength, the reflexive properties of metal can cause laser beam rejection. In this case, the reversed laser beam enters through the head of the laser cutter  on the lenses and the mirror system, which can cause damage.

Reflective metals laser cutting can be a problem

In order to prevent potential rejection of the laser beam, several actions are required. Reflective metal must be covered with a layer or a device that absorbs the laser beam. This approach to cutting doesn’t affect the quality and the precision of the cut, and the laser cutter is protected from destruction.

In addition to the above-mentioned processing, most modern laser cutting machines come with an implemented self-protection system. This system in the case of a laser beam reflection shuts down the laser cutter and thus prevents the lens from being destroyed. The whole system works on the principle of radiation measurement, that is, its monitoring when cutting. However, the advancement of technology has developed laser cutters that are resistant to such occurrences, and these are fiber lasers.

Fiber laser cutter

Today, in addition to the standard CO2 laser cutters, when it comes to laser metal cutting the use of fiber laser is also practiced. Fiber laser technology is one of the latest cutting techniques that gives significantly better performance than CO2 lasers.

Laser metal cutter – Bystar Fiber

Fiber lasers use optic fibers that guide the laser beam, instead of using a complicated mirror system. This type of laser is the fastest and most cost-effective alternative to the CO2 reflective metals laser cutting.

In addition to the fiber laser cutter, another technique used for reflective metals is water jet cutting. The main reason for this is the fact that fiber lasers lose their efficiency at a metal thickness larger than 5 millimeters.

Reflective metals list

The table gives a list of all reflective metals and recommendations for their cutting.

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Regular maintenance is the key to laser cutting quality

To make the cut as good as possible, the laser cutter requires regular service and cleaning.

However, many companies forget about the importance of regular maintenance. One of the leading experts for laser metal cutting, Brent Donner, brings a few questions that need to be asked before the cutting begins, in order to ensure satisfactory cutting quality.

Otherwise, Donner is a long-time consultant with great experience in laser cutting projects. During the visits to numerous companies that asked for his opinion, he noted the three major failures that laser cutter operators are doing.

Before each laser cutting project, three most important questions must be asked, in order to ensure that the quality of the project implemented is as good as it can be.

Are the laser cutter lenses cleaned in the prescribed manner?

Cutting lenses are one of the most important parameters that affect the quality of the cut. Since during the cutting, processes that can lead to the formation of layers on the lens, which reduces the quality of the cut occur, the operator has to clean the lens after a certain number of cutting cycles. However, during cleaning, special attention should be paid to the way the lens is being cleaned. By improper cleaning, the cutting lens may be damaged or not cleaned entirely, leaving a thin layer of dirt on it.

There are specially developed means for cleansing the lens. However, Donner states that operators often don’t know how to use a cleaning agent. What is needed is to apply a reasonable amount of cleaning agent. After that, the lens polishing follows, which is done by pressing your hand on it very gently. With regular maintenance and cleaning, the life span of the laser lens can be extended to as long as 8 months, with the inevitable improvement in cutting quality.

Is the laser jet centered?

The jet in  a laser cutting process has a very important and irreplaceable role. In addition to conducting the laser beam and directing it to the metal, an auxiliary gas used for cutting passes through it as well. For this reason, after a certain number of cutting cycles, the operator must check the centering of the jet by directing the laser beam on the tape for about 1 second. After this, one should make sure the hole created in the tape is well centered.

Since the hole is of very small dimensions, the operator is recommended to use the magnifying glass. In addition to increasing the precision of the cut, a good positioning of the jet can increase the speed of the cut by as much as 40%.

Is the laser cutter optic focused?

In addition to regular and duly maintenance of the cutting optics, it is necessary to focus it after a certain number of cutting cycles. Modern laser cutting machines have a system that focuses it automatically without the need for any kind of intervention done by the operator. However, in older laser cutters, the operator must be the one to complete the focusing process.

Different methods that give the same result have been developed for laser optic focusing. The focusing is done by the operator who releases a low-power laser beam and manages the focusing system located on the laser cutter. It is necessary to observe the color of the laser beam. Once it becomes blue, the operator must write down the values and repeat the whole procedure three times. The average result obtained by this process is entered by the operator into the laser cutter system, thus focusing the optic.

However, the focusing process depends largely on the type and laser cutter manufacturer. Therefore, it is recommended that you consult with the manufacturer, so the focusing would be done right.

Regular maintenance is the key to laser cutting quality

These questions are just some of the guidelines that lead to laser quality improvement. However, the key to improving quality and extending the lifespan of laser cutters is regular maintenance. Regular maintenance, in addition to increasing speed, increases the quality of laser cutting at the same cost.

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Technological advancement of laser cutting

The development of the laser cutting technology brings the ability to cut all the slim materials. In addition, the advancement of the laser cutter components increases the precision as well as the possibility of performing more complicated cuts.

However, laser cutting had a completely different application in the past. This applies primarily to the most used CO2 lasers. Laser technology has provided almost an ideal way to produce prototype tools and various other components that required the manufacture of larger sized mechanical cutters.

Optics as key to success

By the increase of the demand when it comes to quality and cutting in general, a need for innovation development that would make the laser optics more efficient appeared. At the very beginning, laser cutters used one lens with only one focal length, which greatly restricted laser cutting.

However, after the appearance of multi-lens laser technology, laser cutting took place at higher speeds and its quality has increased as well. Although initially, the lens replacement was a complicated process in which the cutter head needed to be dismantled and it lasted for a relatively long time, many companies decided to install additional lenses into their laser cutter.

The need for better laser optics resulted in the development of the autofocusing system. With this system, the operator no longer had to manually adjust the focus of the lens, but the focusing system would do it automatically. This increased the speed and the quality of the cut with minimal waste of time. In addition, by developing and enhancing the laser cutter head, lens replacement has become simpler and today it lasts for an average of 20 minutes.

Laser cutting automatization

The development of automatization has caused the increase in cutting speed. The CNC technology has been further upgraded so the laser cutter could run many more complex positions without any problems. Additionally, the automatic cutting of metal plates on the laser cutter’s work desk accelerated the entire cutting process, and the operator could focus exclusively on laser cutting parameters.

Apart from the automatization of the modification of the metal plate being cut, automatic changes of the cutting head were done as well. Due to the need for the change of the laser lens during cutting, a system is designed to automatically adjust the lens, or physically replace the head of the laser cutter. With this kind of system, the laser cutting process is further accelerated and becomes more efficient.

The evolution with the appearance of the fiber laser

With the emergence of a new type of laser technology, fiber laser, which has enabled the creation of laser beams of greater intensity or strength, previous CO2 lasers have been replaced by considerably better fiber lasers, which for creating laser beams use a special type of diode instead of gas.

Fiber lasers have also led to increased cutting efficiency. CO2 cutters invest only 10% of the energy they use into the cutting process, whereas with fiber lasers, this figure increases to as much as 35%. Nonetheless, CO2 is still being used because of its advantages in laser metal cutting.

What does the future bring?

Today, laser technology is at a turning point. Will the industry see the benefits of fiber laser or will it continue to use CO2 lasers? Numerous engineers trying to answer this question are pointing to new innovations in the field of laser optics or laser cutter heads.

he latest innovations bring the diminution of any need for major operator interventions in the laser cutting process. In the future, it is to be expected that laser cutting will remain the primate, ie synonym for quality and the best price and quality ratio when it comes to cutting both metal and other materials.

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The comparison of different metal cutting techniques

Laser metal cutting

Laser metal cutting has been in use for 25 years, and cutting technology is continually improving. From the initial use of lasers, modern technology, in particular CNC, has enabled increased precision and economical laser cutting, making it one of the most affordable cutting techniques. Depending on the strength, laser technology can cut soft metal up to a 12.7 millimeters of thickness, inox of 10 millimeters maximum thickness and aluminum of 5 millimeters maximum thickness. The maximum metal thickness that can be cut with laser is 25 mm for structural and stainless steel, and 15 mm for aluminum.

Laser metal cutting is most commonly used for cutting homogeneous structure metals, while impurities and admixtures greatly reduce the quality of the cut. In addition to reduced quality, fused metal resulting as an inevitable cutting product can damage the optical lens of the laser. Today, in practice, there is a growing need for cutting very thin materials requiring additional operations during cutting. Because of the low material thickness, it is necessary to drain the heat resulting from the cutting process itself.

Therefore, laser cutting is being used in cutting various metal types and maximum thickness depending on the power of the laser. With economy and quality; the combination of laser and CNC technology enables the performing of the cut and very complex positions.

Laser Cutting Machine by Bystronic

Plasma cutting

Plasma cutting has been in practice since 1960 when it was first used. In the last five years this cutting technique has experienced a revolution. New cutting methods have been developed that have increased precision, metal thickness and cutting quality.

Plasma cutting as such consists of directing negatively charged gas ions almost instantaneously at the speed of sound, into the metal that is being cut. It should be noted that the metal during plasma cutting is positively charged. After the plasma beam penetration into the metal, temperatures up to 28,000 Co are developed. Due to high temperature, there is a corrosion risk, so the auxiliary gas is also applied when cutting. The choice of auxiliary gas in plasma cutting depends largely on the metal being cut. For example, when cutting stainless steel, air, oxygen or argon and hydrogen blend can be used, and when it comes to aluminum cutting only air can be used as an auxiliary gas.

Plasma cutting is commonly used for metals of thickness of 8 up to 31.75 mm. With the advantage of high cutting speed, plasma cutting is also characterized by high temperature development which can damage the material that is being cut.

Water jet metal cutting

Water jet metal cutting is the same in application and practice as the previous two cutting techniques. The principle of water jet cutting operation consists of accelerating the mix of water and abrasive medium to the sound velocity. At such speed, the water penetrates into the metal that is being cut and just like the previous cutting techniques, cuts the metal.

Water jet cutting parameters are mostly equal for all types of metal being cut. One of the main advantages of such metal cutting is high precision and low temperature developing when cutting. These advantages make water cutting the most acceptable technique for metals up to 100 mm thick. Above 100 mm, water cutter loses the precision and exits from precision ranges of about 0.0076 to 0.381 mm.

Which cutting technique should you choose?

When it comes to choosing the best metal cutting technique, it is first necessary to consider the required precision of the cutting and the properties of the material being cut. In addition, it is necessary to include in the calculation the speed or position where the metal being processed is going to be incorporated.

Laser cutting is therefore recommended for metals that require more complex and precise processing with high precision and cutting efficiency. In practice, this would mean that laser cutting is most economical for metal thicknesses up to 10 mm, which do not have high reflexive properties and medium thicknesses as well. Likewise, plasma cutting is most economical for metals up to 10 mm thick. Most often, plasma cutting is used for sheets of larger dimensions and thicker section. Water cutting is recommended for metals up to 460 mm thick, or temperature-sensitive metals.

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The term laser is an acronym that comes from the English-speaking area, meaning “Light Amplification by Stimulated Emission of Radiation”. The laser is nothing but a light oscillator that allows focusing on a point of a very small diameter, smaller than 1 mm.

The laser cutting alone is the thermal material cutting process, in which the laser source, i.e., the resonator creates a laser beam that is, by means of a mirror, carried out in the cutter head of the machine, focusing on a point of very small diameter.

Laser metal cutting– application and benefits

Due to its high precision, laser cutting has been used in many branches of the industry, some of which are metallurgy, electronics, shipbuilding etc.

Laser metal cutting offers many benefits, and some of them are the following:

• low heat input on metal
• small deformation of the metal
• high speed
• high quality cut
• flexibility
• cost-effectiveness
• precision

The principles of laser metal cutting

Laser metal cutting can be done by:

• CO2 lasers: used when drilling, cutting, marking, engraving
• Nd lasers: most commonly used for drilling. They are characterized by high energy impulses and low speed repetitions
• Nd – YAG lasers: used for drilling, engraving and adjustment. They use very high energy impuls

The illustration of the work of a laser cutting machine

Laser cutting is a thermal process that starts by heating and focusing laser beam (density ranges around 104 Wmm-2) in combination with gas (active or inert). The laser beam melts metal that is being sliced, and gas with its current eliminates the liquefied metal. Laser metal cutting begins by drilling a hole in the metal, and hole drilling can take up to 15 seconds depending on the thickness of the metal. The laser beam applied during the laser cutting process is parallel to a thickness of 1.5 to 12.5 mm.

Laser metal cutting is one of the variations in laser cutting.

An example of a metal cut by a laser is shown below.

Characteristics of laser metal cutting

The precision that can be achieved by laser metal cutting is up to 0.01. The amount of heat that is being introduced during the cutting process, can be seen in the table. The amounts are given in watts [W].

Speed is one of the benefits of laser cutting.

The table below shows the time in minutes needed to cut an inch of material.

Laser metal cutting has several parameters that can be changed, and one of the most important is impulse, that is, continuous beam. Peak power at impulse laser cutting or average power at continuous laser cutting determines penetration. Continuous beam is applied to smooth and thicker material, while impulse beam is used for precise cutting. Comparison of the continuous and the impulse beam cuts can be seen in the photo.

Conclusion

One of the most modern methods of machine metal cutting is laser metal cutting.

The price of laser metal cutting depends on the specific requirements, ie, on the type, the quality and the thickness of the material being cut.

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