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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Historic development of the CNC technology

CNC technology began to develop in the 40s and 50s of the last century. The then-known technology of numerical control was based on existing tools and machines used in the processing of materials.

The servomotors that controlled their movements were added to the mentioned. The milestone in the development of the CNC machines, as well as in the change of the production process, is the development of computers and electronics in general.

Such innovations and technology allow you to send program commands from your computer directly to the machine that directly executes them.

The first CNC machine was manufactured in America at the Massachusetts Institute of Technology (M.I.T) and it marked a major milestone in the manufacturing process. Unlike today’s computer-controlled machines, this machine was operated with a drilled paper ribbon. One of the curiosities associated with them is related to the management unit that was larger than the CNC machine. From then on, the CNC technology is progressing and becoming more and more advanced so it is almost impossible to imagine a modern machine park without the CNC.

The advantages of using the CNC technology

Today’s production of metal parts without the use of the CNC technology is unthinkable. The benefits of using the CNC machines are many. Some of them are reflected in the flexibility of the machining process (especially with CNC milling and drilling), precision and (CNC cutting) processing speed.

However, one of the most important and greatest advantages of using the CNC machine in metal processing is the fact that the CNC can perform multiple operations in a single work cycle.

That’s how for example you can program the CNC center to perform flat grinding, drilling, and milling, and everything can be integrated into one program.

How CNC changed the metal industry?

Time is a very im0portant component in business. By applying the CNC machine metal processing , the time required to make individual positions is greatly reduced. In addition, CNC processing increases the processing speed, and repetitive processing requires a simple launch of the program.

On the other hand, classical processing without the use of the CNC technology requires a complete adjustment of both the tools and the machines during rework. In addition, by using the CNC technology, the size of a machine park is reduced but the quality of the end product is increased, and one machine can now be used to create a single position, while in the past several were needed.

However, with the obvious advantages of the CNC technology and CNC machining, there are some drawbacks as well. The disadvantages are linked to the high cost of such technology so only large and developed companies can obtain a CNC system.

Another disadvantage is the need for specific knowledge of employees related to programming knowledge.

What is the CNC technology applied for?

CNC technology can be used in various operations related to the processing of metals as well as other materials.

The most famous and most commonly used application of this kind of technology is turning or milling.

The CNC milling application in metal processing provides high precision and speed; which on the other hand is not provided by the conventional turning machine.Apart from that, the CNC technology is being applied during  laser metal cutting, plasma cutting, water jet cutting, grinding, drilling etc.

In addition to its application in engineering or metallurgy, CNC technology is applied in a number of other industries. One example is the wood industry where the CNC is used for the same or similar purposes as with metal processing.

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