Plasma cutting is a metal cutting process where a jet of hot plasma is used to cut electrically conductive materials. Plasma is basically a collection of charged particles in form of a jet of ionized gas that can generate temperatures above 200000C that melt the material to perform the cutting operation. During the plasma-cutting process, the plasma is constricted through a small orifice which increases the velocity to form a jet and blow through the metals under consideration. The plasma cutting process is extensively used to cut materials like carbon steel, aluminum, stainless steel, copper, brass, Monel, cast iron, titanium, etc. The use of plasma cutting is often found in fabrication shops, industrial construction, automotive repair and restoration, and salvage and scrapping operations. Due to the high precision cutting capability with very high speed, plasma cutting finds widespread applications worldwide.
What is Plasma?
Plasma is an electrically conductive ionized gas consisting of free electrons and positive ions. When any gas is subjected to intense heating, it can transform into plasma. On the influence of the electrical and magnetic fields, plasma can flow easily. Natural lightening is an example of plasma.
How does Plasma Cutting work?
As plasma cutting is a thermal cutting method, it uses high heat to melt the metal. A Plasma cutter works by sending an electric arc through the plasma gas over a constricted opening. Common gases that are used as plasma gas are air, nitrogen, argon, oxygen, etc. The metal under consideration is made part of the circuit and the electrical conductivity of the plasma transfer the arc to the workpiece.
A plasma cutting torch uses a copper nozzle through which the gas stream with the arc is transferred at a very high velocity to the workpiece. The metal cutting operation is achieved by the high temperature and velocity. Actually, the heat generated by the arc melts the metal, while the gas jet removes the molten metal from the cut. Shielding gases are used for protection. Common shielding gases are helium, nitrogen, argon, or a mixture of these gases. Refer to Fig. 1 below that shows a schematic representation of the plasma arc-cutting process.
Plasma cutting is one of the best options for cutting thin and thick conductive materials. Hand-held plasma torches are used to cut metals up to 1.5 in thick; while stronger computer-controlled plasma torches have the ability to cut up to 6 in thick metals and alloys. Plasma cutters generate a very hot and very localized “cone” to cut the metal and are characterized by fast cutting speeds. However, the process usually generates high noise.
Plasma Cutter Temperature
The plasma cutter temperature is very high. The temperature of the plasma cutter can be as high as 200000C to 250000C. So, the safety of the people at the workplace is of prime importance. Due to these high plasma cutting temperatures, the operation must be handled with proper care using all the PPEs.
Cutting Speeds for Plasma Arc Cutting Process
For economic reasons, the cutting speed for the plasma cutting process should be as high as possible. However, for a specific electric power and gas mixture, there is an optimum speed range for each type and thickness of the material. Excess speed causes a decreased kerf width. 1 m/s is a reasonable speed for manual control and complicated cuts.
Depth of cut for Plasma Cutting Process
The cutting ability of the plasma cutting machine depends on the amperage of the plasma cutter. A low amperage plasma can easily cut up to a depth of 6mm. While high amperage machines are required for cutting higher thicknesses.
In general, for high-quality plasma cutting the material thickness range from 15 to 50 mm.
Plasma Cutting gas selection
The selection of plasma-cutting gas depends on the speed and quality of cut required for specific metals. Plasma cutting can be performed using several gases. However, Nitrogen is widely used due to its inexpensive nature and easy availability. Also, Nitrogen can be used on many materials and thicknesses. A mixture of argon and hydrogen gases improves cutting speed and quality on thicker metals except for carbon steel. Oxygen as plasma-cutting gas improves cut quality by increasing heat, improving cutting speed, and/or reducing power requirements. For thicknesses below 25 mm, compressed shop air is widely used for many applications as it is inexpensive and provides good-quality cuts on carbon steel.
For proper operation of the plasma cutting system, plasma gas quality plays an important role. Contaminates such as gas impurities, moisture, dirt, improper gases, etc can cause misfiring, poor cut quality, or poor consumable life.
For plasma cutting of aluminum and stainless steel, non-oxidizing gases are used. Argon and hydrogen mixtures permit good cuts and high cutting rates because the hydrogen increases the arc voltage and thermal conductivity of the mixture. Parallel kerfs, little dross, oxide-free cut faces, and minimal fumes result from the use of Argon and hydrogen mixtures.
Plasma cutting of carbon steel requires an oxidizing gas for the best results; the exothermic iron-oxygen reaction provides additional heat at the cutting point and so reduces the amount of electric power required. Air is a highly efficient plasma-cutting gas for carbon steel.
How to use a Plasma Cutter
A plasma cutter is a machine that helps in the metal-cutting process. It takes electric power to create the plasma and arc required for the cutting. The use of a plasma cutter is fairly straightforward. The steps for using a typical plasma cutter is mentioned below:
- Connect the plasma cutter to a suitable air compressor.
- Depending on the specification of the cutter, connect the machine to a 240V or 415V supply.
- Connect the earth lead to your workpiece.
- Next, connect the plasma cutting torch to the machine.
- Now, set the cutting amperage depending on the material thickness.
- Finally, press the switch of the torch handle to start cutting and move it across the workpiece to get desired cut profile.
CNC Plasma Cutting
A CNC plasma cutting system is a machine with a plasma torch whose cutting movement is controlled by a computer. The term “CNC” means “Computer Numerical Control”, which clarifies the motion is controlled by numerical codes entered into a computer program. As compared to handheld manual plasma cutters, CNC plasma cutting machine is used for complex geometries and for critical jobs with clean, sharp, and high-quality cuts.
Depending on the forms of material and flexibility of the cutting head, there are usually three main configurations of a CNC plasma cutting. They are:
- 2-dimensional / 2-axis plasma cutting: The simplest form of CNC plasma cutting produces flat profiles with cut edges 90 degrees to the material surface.
- 3-dimensional / 3+ Axis plasma cutting: Complex form of CNC plasma cutting creating flat profiles with cut edges at any angle.
- Tube and section plasma cutting: Used for the processing of pipe, tube, and any other form of long sections.
Advantages of Plasma Cutting
Plasma cutting is one of the best methods for cutting conductive metals and alloys. The main advantages that plasma cutting offers are:
- Ability to cut a material thickness up to 150 mm with good quality.
- Cost is comparatively less, value for money cutting.
- Curves, odd shapes, and even intersecting cuts can be easily performed using plasma cutting.
- Very quick and efficient process.
- Can cut in water, resulting in smaller HAZ (Heat Affected Zone). Also reduces noise levels.
- Versatile and suitable for a wide range of conductive materials.
- Best way to cut medium-thickness stainless steel and aluminum.
- CNC plasma cutters are available to provide high precision and repeatability.
- Smaller cutting kerf compared to flame cutting.
Disadvantages of Plasma Cutting
However, there are some disadvantages of plasma cutting as provided below:
- Noisy operation.
- Application is limited to conductive materials only.
- Not suitable for thicknesses in excess of 150 mm. Does not reach thicknesses like waterjet or flame cutting.
- The bright flashes generated in plasma cutting negatively impact human eyes.
- For thinner sheets and plates, quality is not as good as laser cutting.
- High power consumption.