A Brief Overview of Plasma Cutting Systems

Plasma is the fourth state of matter. It consists of ions, positively charged atoms, negatively charged ions (electrons), and positively charged molecules. It also contains neutrally charged atoms. Plasma is a good conductor of electricity. It exists at very high temperatures, and is used by cutting systems to cut through metals.

A gas is released through a very narrow opening, thereby increasing its pressure. Before it escapes, an electrode is placed in its way that passes an electric current through the gas.

The gas atoms, on being stimulated by the electric current produce positively and negatively charged ions. These ions collide with each other, and also with the rest of the gas atoms to produce more ions. In a short time, the gas reaches a superheated state called plasma.

The cutters have an electrical circuit with an electrode, which acts as a negative terminal (cathode), and the metal sheet that is to be cut, acts as the positive terminal (anode). The plasma jet is attracted towards the metal sheet, and it can focus as much as 30,000 degrees Fahrenheit or 16,648 degrees Celsius of heat on the sheet to melt it.

The nozzle's narrow opening increases the pressure exerted by the gas, which also blows away the melted metal, thereby aiding the cutting of the metal sheet. The plasma arc is maintained thin, and controlled with the help of shield gases.

A plasma cutter is provided with channels to release shield gases, which exert pressure and control the arc, and keep the plasma jet directed on the metal sheet. Inert or semi-inert gases are used as shield gases to cover the area, where the plasma arc or jet touches the metal.

The cutter is also known as a plasma torch. They are available in two types: manual and computer-controlled.

Computer-aided Manufacturing (CAM) - It is a numerical control programming tool, which converts the design to CNC code.

Designs of components created using CAD software are converted to CNC code, which are then used by direct numerical control (DNC) programs that control CNC machines. The choice of CNC cutting machines used by technicians depend on the type of the metal that is to be cut.

Selection of the gas to be heated to produce the plasma jet, and the inert gases to control the same, also depends on the metal. Precise handling of the machine and proper distance between the plasma arc and metal sheet, is necessary for precision cutting.

A blend of oxygen and nitrogen is used for this process on metals like stainless steel. It produces a non-oxidizing plasma jet. Different plasma gases are used for different metals.

Here are the different types of plasma cutting techniques that are commonly used.

Conventional Plasma Arc Cutting: In 1957, Union Carbide introduced the 'dry arc' technique of producing plasma jet. This method uses different plasma gases to cut stainless steel, aluminum sheets, or soft steel.

Air Plasma Cutting: In the 1960s, this method was used on a large-scale in eastern Europe. It is 25% faster than the previous method.

Water Shield Plasma Cutting: Instead of inert gases, water is used to shield the area of contact between the plasma jet and metal sheet. This method increases the electrode life by providing a cooling effect.

Water-Injection Cutting: In this method, water is injected radially to the plasma arc to increase arc constriction, which is very useful to maintain precision.

Underwater Cutting: The metal sheet is kept immersed in 2-3 inches of water, and the arc is then used to cut it. This method reduces the unwanted things like noise, glare, and smoke that are produced during the actual process.

Physical properties of the metal, and the current carrying capacity of the cutter, determines how deep it can cut.

CNC plasma cutters are available as the following.

• Heavy-duty units with a current capacity of 1000 amps are used to cut metal sheets that are 12-inch thick.
• Portable cutters using current as low as 12-25 amp are available in the market. They are small in size, and allow the operator, freedom of movement.

These cutters are built for speed and precision, and are ideal to achieve the goal of mass production. They are used in airplane construction, ship building, automobile industries, etc., to cut metal sheets.

Today, artists are using small and portable CNC plasma cutters in their studios to translate their ideas into real objects. The far-reaching impact of plasma cutters in the manufacturing industry as a whole, is truly amazing and immeasurable.