Plasma Cutting Systems: CNC Plasma Cutters

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. Plasma exists at very high temperatures and is used by plasma cutting systems to cut metals.

Plasma Cutting Systems

Let us see how plasma cutting systems work? A gas is released through a very narrow opening, thereby increasing its high pressure. Before the gas escapes, an electrode is placed in its way that passes an electrical 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 the rest of the gas atoms to produce more ions. In no time, the gas reaches a superheated state or plasma.

Plasma cutting systems are nothing but an electrical circuit that have an electrode acting as a negative terminal (cathode), and the metal sheet to cut, acting as the positive terminal (anode). The plasma jet is attracted towards the metal sheet and at this point, in the operation, can focus as much as 30,000 degrees Fahrenheit or 16,648 degrees Celsius of heat on the metal sheet and melts it. The narrow opening of the nozzle 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 kept 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 plasma arc and keep the plasma jet directed on the metal sheet. Inert or semi-inert gases are used as shield gases to shield the area, where the plasma arc or jet touches the metal. The plasma cutter is also known as a plasma torch. Plasma cutting systems are available as manual and computer-controlled. Let's take a brief look at the latter type.

CNC Plasma Cutters

A computer numerical controlled (CNC) plasma cutter is a machine tool controlled and directed with the help of a computer. Along with the machine tool, computer-based softwares like CAD/CAM are an integral part of each CNC machine tool.

Computer-aided Design (CAD) - Computer-based software programs are used to create 3D models of the physical components to be produced. There are software tools that help in verifying the design of the component along with testing it on the computer screen to check how this design fits in the overall picture.

Computer-aided Manufacturing (CAM) - CAM 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 machine tools. The choice of CNC plasma cutter machines used by technicians depend on the type of the metal to be cut. Selection of the gas to be heated to produce the plasma jet and the inert gases to control the same depends on the metal to be cut. Precise handling of the machine and proper distance between the plasma arc and metal sheet, is necessary in precision cutting using plasma cutters. A blend of oxygen and nitrogen is used cut a metal, like stainless steel. It produces a non-oxidizing plasma jet. Different plasma gases are used for different metals.

The different types of plasma cutting techniques that are commonly used are as follows.

• 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 sheet or soft steel.
• Air Plasma Cutting: In the 1960s, this method was used on a large scale in eastern Europe. Air plasma cutting is 25% faster than conventional plasma arc cutting systems.
• 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 constriction of the arc. A constricted plasma arc is useful for precision cutting.
• Underwater Cutting: The metal sheet is kept immersed in 2-3 inches of water and the plasma arc is then used to cut it. This method reduces the noise, glare of plasma arc and smoke produced during the plasma cutting

Physical properties of the metal and the current carrying capacity of the plasma cutter, determines how deep a plasma cutter can cut. CNC plasma cutters are available as the following.

• Heavy-duty CNC plasma cutters with a current capacity of 1000 amps are used to cut metal sheets that are12-inch thick.
• Portable CNC plasma cutters using current as less as 12-25 amp are available in the market. These plasma cutters are small in size, and allow the operator freedom of movement.

CNC plasma 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 CNC plasma cutters in the manufacturing industry as a whole, is truly amazing and immeasurable.