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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Quality management implies all the procedures that thoroughly review the quality of all the factors in the production.

The ISO 9000 standard defines quality control as “quality management focused on achieving the required level of quality ”.

Introduction to quality control

Control or quality management is an activity without which today’s organizations can’t operate and be competitive. As the word itself implies, quality control is an organization’s activity related to controlling manufactured products and comparing their properties with quality standards, to ensure that the product meets customers requirements.

However, quality is a relative term and what is good for one customer is not good for the other.

That is why every organization needs to reduce the quality of its products and services to a common denominator. Also, quality for organization means cost.

In other words, increasing the quality level increases the cost needed to achieve this quality level as well. Accordingly, the organization must find the optimal level of quality so that the end product doesn’t have a high price customers won’t able to pay.

Quality Management Philosophies

Today, there are several approaches in the world that define the activities the organization should take to increase the quality of its products and services. The most famous philosophies for quality improvement come from Japan where the quality tradition has been developed for many years.

One of such quality improvement philosophies is Kaizen. 

Kaizen methodology is implemented in most major manufacturing organizations in Japan, and relates to continuous improvement of the process. The main goal of implementing this method in business is to reduce costs that can lead to new costs being generated. Continuous improvement is at the same time the requirement of the ISO standard which will be discussed later.

6 sigma philosophy is a quality improvement method which emphasizes that an organization must eliminate from its business all the possible causes that lead to the production of scrap or non-conforming products. This philosophy is fully geared towards the customer and fulfilling his requirements. It was developed in Motorola in 1986, and it has been applied in all branches of business since.

The next method that can contribute to raising product quality is Quality Circles. Quality circles require that workers who care about quality after, or during work time gather in one place and under the guidance of a moderator discuss how to increase the quality of a particular product. The method also envisages the use of various mathematical methods of quality control. It was first developed in the US and later expanded into most organizations in Japan.

Total Quality Management (TQM)

Total quality management is such a quality approach that has a long-term orientation towards product and service quality. This approach requires all employees in the organization to be involved in the improvement of the quality of their products. It is considered that TQM is actually a way of organizational culture and organizational life.

TQM’s core features are continuous training of all employees to enhance their competences, thereby improving quality, active participation of all employees at all levels of organization, teamwork, engaging suppliers in the production process, and gathering information from the market.

ISO system

ISO is an international organization that deals with standards and standardization, and is at the top of the quality infrastructure.

In its norms ISO gives recommendations, ie instructions, by adhering to which, the organization improves the quality of its products, services and general business. There are a number of related, family-friendly norms that ISO provides guidelines for, related to preventing and removing non-conforming products, conducting audits, and raising the satisfaction of the customers to a higher level. Some of the standards prescribed by the ISO are : ISO 9000, ISO 9001, ISO 14001, ISO 19011 etc.

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