Plasma cutting technology is extremely rarely used in everyday life, but in the industrial sphere it has become very widespread. Due to the fact that with the help of a plasma cutter, you can easily, quickly and efficiently cut almost any conductive metal, as well as other materials – stone and plastic, it is used in mechanical engineering, shipbuilding, public utilities, advertising production, for repairing equipment and much more. The cut always turns out to be smooth, neat and beautiful. Those who are just going to master this technology may be interested in a reasonable question, what is a plasma cutting machine, what is the principle of its operation, as well as what types of plasma cutters are and what each of them is used for. All this will give a general understanding of the plasma cutting technology, will allow you to make the right choice when buying and master the operation of the device..
How does a plasma cutter work? And what is meant by the word “plasma”? For the plasma cutter to work, only two things are needed – electricity and air. The energy source supplies high-frequency currents to the torch (plasmatron), due to which an electric arc arises in the plasmatron, the temperature of which is 6000 – 8000 ° C. Then compressed air is directed to the plasmatron, which at high speed is ejected from the branch pipe, passes through the electric arc, heats up to a temperature of 20,000 – 30,000 ° C and is ionized. The air, which is ionized, loses its dielectric properties and becomes a conductor of electricity. Plasma is precisely this air..
Escaping from the nozzle, the plasma locally heats the workpiece in which it is necessary to cut, the metal melts. Particles of molten metal formed on the frontal surface of the cut are blown away by a stream of air escaping at great speed. This is how metal is cut.
The speed of the plasma flow (heated ionized air) increases if the air flow rate is increased. If we increase the diameter of the nozzle through which the plasma escapes, then the speed will decrease. The plasma velocity parameters are approximately as follows: at a current of 250 A, it can be 800 m / s.
To make the cut even, the plasmatron must be held perpendicular to the cutting plane, the maximum permissible deviation is 10 – 50 °. Cutting speed is also of great importance. The smaller it is, the wider the cut becomes, and the cut surfaces become parallel. The same happens when the amperage increases..
If you increase the air consumption, the cutting width will decrease, but the cutting edges will become non-parallel..
Plasma cutting machine consists of power supply, plasmatron and cable-hosepack, through which the power supply is connected and compressor with plasmatron.
The power source for the plasma cutting machine can be a transformer or inverter, which supplies a large current to the plasma torch..
Plasmatron, in fact, it is the main element of the apparatus – a plasma cutter. Sometimes, by mistake, the entire apparatus is called a plasma torch. Perhaps this is due to the fact that the power source for the plasma cutter is not unique in any way, but can be used together with a welding machine. And the only element that distinguishes the plasma cutter from another device is the plasma torch.
The main components of a plasmatron are an electrode, a nozzle and an insulator between them..
Inside the body of the plasmatron there is a cylindrical chamber of small diameter, the outlet channel from which is rather small and allows the formation of a compressed arc. An electrode is located in the back of the arc chamber, which serves to excite an electric arc..
Electrodes for air plasma cutting can be made from beryllium, hafnium, thorium or zirconium. Refractory oxides form on the surface of these metals, preventing the destruction of the electrode. But for the formation of these oxides, certain conditions are required. The most common are hafnium electrodes. But they are not made from beryllium and thorium, and the very oxides are to blame: beryllium oxide is extremely radioactive, and thorium oxide is toxic. All this can have an extremely negative effect on the operator’s work..
Since the initiation of an electric arc between the electrode and the workpiece of the metal being processed is directly difficult, first the so-called pilot arc is ignited – between the electrode and the tip of the plasmatron. The column of this arc fills the entire channel. After that, compressed air begins to flow into the chamber, which, passing through the electric arc, heats up, ionizes and increases in volume 50-100 times. The nozzle of the plasmatron is narrowed downward and forms a plasma flow from the heated ionized gas / air, which is ejected from the nozzle at a speed of 2 – 3 km / s. In this case, the plasma temperature can reach 25-30 thousand ° C. Under such conditions, the electrical conductivity of the plasma becomes approximately the same as that of the metal being processed..
When the plasma is blown out of the nozzle and touches the workpiece with the torch, a cutting plasma arc is formed – the working one, and the pilot arc is extinguished. If suddenly, for some reason, the working arc also went out, it is necessary to stop the air supply, turn on the plasma torch again and form a pilot arc, and then start up the compressed air.
Plasmatron nozzle can have different sizes and the capabilities of the entire plasmatron and the technology of working with it depend on this. For example, the amount of air that can pass through this diameter per unit of time depends on the diameter of the plasma torch nozzle. The cutting width, operating speed and cooling rate of the plasma torch depend on the amount of air consumption. Plasma cutters use nozzles no more than 3 mm in diameter, but rather long ones – 9 – 12 mm. The length of the nozzle affects the quality of the cut, the longer the nozzle, the better the cut. But here you need to be careful, measure is important everywhere, since too large a nozzle will wear out and collapse faster. The optimal length is considered to be 1.5 – 1.8 times the diameter of the nozzle.
It is imperative that the cathode spot is focused strictly to the center of the cathode (electrode). For this, a vortex supply of compressed air / gas is used. If the vortex (tangential) air supply is disturbed, then the cathode spot will move relative to the center of the cathode together with the arc. All this can lead to unstable burning of the plasma arc, the formation of a double arc and even the failure of the plasma torch..
The plasma cutting process uses plasma-forming and shielding gases. Plasma cutting devices with amperage up to 200 A (you can cut metal up to 50 mm thick) use only air. In this case, air is a plasma-forming gas and a protective one, as well as a cooling one. In complex industrial gantries, other gases are used – nitrogen, argon, hydrogen, helium, oxygen and their mixtures.
The nozzle and electrode in the plasma cutting machine are consumables that must be replaced in a timely manner, without waiting for their complete wear..
Basically, it is customary to buy plasma cutters ready-made, the main thing is to choose the right unit correctly, then you will not have to “finish with a file”. Although in our country there are “Kulibins” who can make a plasma cutting machine with their own hands, purchasing some parts separately.
Plasma cutters are distinguished by several different parameters. Plasma cutting devices can be portable installations, gantry systems, articulated cantilever machines, specialized structures and installations with a coordinate drive. Particularly highlighted are CNC (Computer Numerical Control) plasma cutting machines, which minimize human intervention in the cutting process. But besides these, there are other gradations..
Manual Plasma Cutting Machine It is used for manual cutting of metal when the plasma torch is held by a human operator and guides it along the cutting line. Due to the fact that the plasmatron is always suspended above the workpiece being processed, the human hand may tremble slightly even during normal breathing, all this affects the quality of the cut. It may have sagging, uneven cut, jerk marks, etc. To facilitate the operator’s work, there are special stops that are put on the plasma torch nozzle. With the help of it, you can put the plasma torch directly on the workpiece and carefully guide it. The gap between the nozzle and the workpiece will always be the same and meet the requirements.
Machine cutting devices are portal-type plasma cutters and devices for automatic cutting of parts and pipes. Such devices are used in production. The quality of the cut with such a plasma cutter is ideal; additional edge processing is not required. And software control allows you to make cuts of various shaped shapes in accordance with the drawing without fear of jerking your hand at the wrong moment. The cut is precise and smooth. For such plasma cutting machines, the price is an order of magnitude higher than for manual machines.
There are transformer and inverter plasma cutters.
Transformer plasma cutters heavier than inverter ones and larger in size, but they are more reliable, since they do not fail in the event of voltage surges. The operating time of such devices is longer than that of inverter ones, and can reach 100%. Such a parameter as the duration of inclusion directly affects the specifics of working with the device. For example, if the duty cycle is 40%, this means that the torch can run for 4 minutes without interruption, and then it needs 6 minutes of rest to cool down. Duty cycle 100% is used in production, where the device lasts the whole working day. The disadvantage of a transformer plasma cutter is high power consumption.
Transformer plasma cutters can handle thicker workpieces. For a similar air plasma cutting machine, the price is higher than for an inverter one. Yes, and it is a box on wheels.
Inverter plasma cutting machines used more often in everyday life and in small industries. They are much more economical in energy consumption, have less weight and dimensions, and most often represent a hand-held apparatus. The advantage of the inverter plasma cutter is the stable burning of the arc and the efficiency is 30% higher, compactness and the ability to work in hard-to-reach places.
It is worth noting that there are not only air-plasma cutting machines, the principle of operation of which and the device were described above, but also water-plasma cutting machines.
If in air plasma cutters air acts as a plasma-forming, and as a protective, and as a cooling gas, then in water-plasma cutters water acts as a cooler, and water vapor of the plasma former.
The advantages of air plasma cutting are low price and low weight, but the disadvantage is that the thickness of the cut workpiece is limited, often no more than 80 mm.
The power of water-plasma cutters allows you to cut thick workpieces, but their price is slightly higher.
The principle of operation of the water-plasma cutting machine is that instead of compressed air, it uses water vapor. This makes it possible to dispense with the use of a compressor for air or gas cylinders. Water vapor is more viscous than air, so much less is needed, the supply in the can is enough for about a month or two. When an electric arc flows in the plasma torch, water is supplied to it, which evaporates. At the same time, the working fluid lifts the negative pole cathode away from the positive pole cathode of the nozzle. As a result, an electric arc ignites, the vapor is ionized. Even before the plasma torch approaches the workpiece being processed, the plasma arc ignites, which performs cutting. A striking representative of this category of plasma cutters is the Gorynych device, for such a plasma cutting device the price is about 800 USD.
Depending on whether the material being cut is included in the electrical circuit of plasma cutting or not, the type of cutting depends – contact and non-contact.
Contact plasma cutting or plasma arc cutting looks like this: the arc burns between the plasma torch electrode and the workpiece. This is also called a direct arc. The arc column is aligned with a plasma jet that escapes from the nozzle at high speed. The air blown through the plasma torch nozzle compresses the arc and gives it penetrating properties. Due to the high air temperature of 30,000 ° C, the rate of its outflow increases and the plasma has a strong mechanical effect on the blown metal.
Contact cutting is used when working with metals that can conduct electricity. This is the manufacture of parts with straight and curved contours, cutting pipes, strips and rods, making holes in workpieces and much more..
Non-contact plasma cutting or cutting with a plasma jet looks like this: an electric arc burns between the electrode and the forming tip of the plasma torch, a part of the plasma column is carried out of the plasma torch through the nozzle and represents a high-speed plasma jet. It is this jet that is the cutting element..
Non-contact cutting is used when working with non-conductive materials (non-metals), such as stone.
Working with a plasma cutting machine and air plasma cutting technology is a whole art that requires knowledge, patience and adherence to all rules and recommendations. Knowledge and understanding of the plasma cutter device helps to perform work efficiently and accurately, since the operator understands what processes occur in the plasma torch and beyond at one time or another, and can control them. It is also important to observe all safety precautions and safety measures, for example, it is necessary to work with a plasma cutter in a welder’s suit, in a shield, gloves, in closed shoes and tight pants made of natural fabric. Some oxides released during metal cutting can cause irreparable harm to human lungs, therefore it is necessary to work in a protective mask or at least ensure good ventilation in the work area..