Modern industry requires the processing of heavy metals and alloys, and the manufacture of tools and transport vehicles necessary for daily activities cannot be separated from metals.
For example, cranes, cars, skyscrapers, robots, and suspension bridges are all made from precision-machined metal components.
The reason is simple: metallic materials are very strong and durable. For most manufacturing processes, especially those involving large and/or robust items, metallic materials are a natural choice.
Interestingly, the strength of metal materials is also their disadvantage: as metals are not easily damaged, it is very difficult to process them into specific shapes. When people need to process a component the same size and strength as an airplane wing, how can they achieve precise cutting and shaping?
In the vast majority of cases, this requires the use of a plasma cutting machine. While this may seem like something out of science fiction, in fact, since World War II, plasma cutting machines have been widely used.
In theory, the working principle of a plasma cutting machine is very simple. It is processed by manipulating one of the most common forms of matter in the known universe.
In this article, we will peel back the mysterious veil behind plasma cutting machines and see how this magical tool shapes our world.
During World War II, American factories produced armor, weapons, and planes five times faster than the Axis powers, thanks to the great innovations made by private industry in mass production.
How to cut and connect aircraft components more effectively has triggered some technological innovations.
Many factories that produce military aircraft have adopted a new welding method, which involves the use of inert gas shielded welding.
The revolutionary discovery lies in the fact that gas electrolyzed by current can form a barrier near the weld to prevent oxidation. This new method makes welds more organized and the connection structure stronger.
History of Plasma Cutting
In the early 1960s, engineers made another discovery. They found that increasing the airflow rate and reducing pore size can help improve welding temperature. The new system can reach higher temperatures than any commercial welding machine.
In fact, at such high temperatures, this tool no longer serves as a welding device. Instead, it's more like a saw that cuts through hard metal like a hot knife through butter.
Plasma Arc
The introduction of the plasma arc revolutionized the speed, precision and variety of cutting tools, and can be applied to various metals. In the next section, we will present the scientific principles behind this system.
The ease with which a plasma cutting machine can penetrate metal is due to the unique properties of the plasma state. So what is a plasma state?
There are four states of matter in the world. Most of the substances we come into contact with in our daily lives are solid, liquid or gaseous. The state of matter is determined by the interaction between molecules. Let's take water as an example:
Solid water is ice. Ice is a solid composed of electrically neutral atoms arranged in a hexagonal lattice. As the interaction between molecules is stable, it remains in a fixed form.
Liquid water is drinking water. The molecules still exert forces on each other, but they move slowly. Liquids have a fixed volume, but not a fixed shape. The shape of the liquid changes depending on the shape of the container.
Gaseous water is water vapor. In water vapor, the molecules move quickly and there is no connection between them. Because there is no force between molecules, gases have no fixed shape or volume.
The amount of heat (converted into energy) absorbed by water molecules determines their properties and state. Simply put, more heat (more energy) causes water molecules to reach a critical state where the chemical bonds between them break.
At low temperatures, the molecules are tightly bound together and the substance is in a solid state. Absorbing more heat weakens the interactions between the molecules and the substance becomes liquid.
Absorbing even more heat causes interactions between molecules to almost disappear and the substance becomes a gas. So what happens if we continue to heat the gas? This will cause it to enter the fourth state, the plasma state.
When a gas reaches extremely high temperatures, it enters the plasma state. The energy begins to completely separate the molecules from each other and the atoms begin to split.
Typically, atoms consist of protons and neutrons in the atomic nucleus (see atomic theory), as well as electrons surrounding the atomic nucleus.
In the plasma state, electrons are separated from atoms. Once thermal energy causes electrons to leave atoms, they begin to move at high speeds. The electrons carry a negative charge, while the remaining atomic nuclei carry a positive charge. These positively charged atomic nuclei are called ions.
When high-speed electrons collide with other electrons or ions, they release enormous amounts of energy. It is precisely these energies that give plasma its special properties and incredible cutting capabilities.
Common knowledge about the plasma state:
Almost 99% of matter in the universe is in a plasma state. Due to its extremely high temperature, it is not commonly found on Earth; however, it is very common on celestial bodies such as the Sun. On Earth, this state can be seen in lightning.
Plasma Cutting Machine
Plasma cutting machines are not the only devices that manipulate plasma energy. Devices like neon lights, fluorescent lights, and plasma displays work based on the state of the plasma. These devices use the “cold” plasma state. Although cold plasma cannot be used to cut metal, it still has considerable applications.
Plasma cutting machines come in many shapes and sizes. There are giant plasma cutting machines controlled by robotic arms for precise cuts, as well as simplified manual plasma cutting machines used in workshops.
Regardless of their size, all plasma cutting machines are based on the same principles and have similar structural designs.
When a plasma cutting machine is in operation, compressed gases such as nitrogen, argon or oxygen are supplied through a narrow channel. A negative electrode is placed in the middle of the channel. When power is supplied to the negative electrode and the nozzle is in contact with the metal, an electrically conductive circuit is formed, generating high-energy sparks between the electrode and the metal.
As the inert gas flows through the channel, sparks heat the gas until it reaches the fourth state of matter. This reaction produces a flow of plasma with a temperature of up to approximately 16,649 degrees Celsius and a flow rate of up to 6,096 meters per second, quickly transforming the metal into slag.
The plasma itself has a current flowing through it. As long as the electrode is continuously powered and the plasma is in contact with the metal, the arc cycle is continuous.
To prevent oxidation and damage caused by unknown properties of plasma, the plasma cutter nozzle is equipped with another set of channels that continuously release shielding gas to protect the cutting area. The pressure of the shielding gas can effectively control the radius of the columnar plasma.
Plasma Cutting
Plasma cutting machines have become a common tool in modern industry. They have been widely used in custom car shops as well as in the manufacture of custom chassis and bodies by car manufacturers.
Construction companies use plasma cutting machines on large-scale projects to cut and manufacture large metal beams and plates. Locksmiths can use plasma cutting machines to cut holes in secure areas when customers are locked out.
In a CNC (Computer Numerical Control) cutting system, it is not necessary to touch the material. Just draw the shape you want to cut on the computer and the cutting process will be automated.
1. Working principle
Plasma is a gas heated to extremely high temperatures and highly ionized. The power of the arc is transferred to the workpiece, which melts and is expelled, creating a plasma arc cutting work state.
The compressed air enters the cutting torch and is distributed in two streams through the gas chamber, forming plasma gas and auxiliary gas. The plasma gas arc melts the metal, while the auxiliary gas cools the various parts of the cutting torch and blows away the molten metal.
The cutting power supply includes a main circuit and a control circuit. The block diagram of the electrical principle is shown in the figure:
The main circuit consists of a contactor, a three-phase power transformer with high leakage resistance, a three-phase rectifier bridge, a high-frequency arc ignition coil and protection elements. It has a steep external characteristic caused by high leakage resistance. The control circuit completes the entire cutting process via the button on the cutting torch:
Pre-gas supply – Main circuit supply – High frequency arc ignition – Cutting process – Arc stop – Stop.
The main circuit power supply is controlled by the contactor; gas flow is controlled by solenoid valve; and the high-frequency oscillator ignites the arc and stops working after the arc is established.
In addition, the control circuit also has the following internal blocking functions:
Thermal control switch action, stop working.
2. Glitching
(1) Cut not completely:
a: The thickness of the board exceeds the applicable range of the equipment.
b: The cutting speed is very fast.
c: The inclination of the cutting torch is too great.
d: The compressed air pressure is too high or too low.
e: The grid voltage is too low.
(2) Plasma arc instability:
a: The cutting torch moves very slowly.
b: The power supply is provided by two phases and the operating voltage decreases.
c: The compressed air pressure is too high.
3. Installation, maintenance and replacement of cutting torch parts
When installing or replacing cutting torch parts, disassemble them in the order protective cap-lead nozzle-gas distributor-electrode-cutting torch body with the cutting torch head facing up, and assemble them in the reverse order.
When installing the nozzle, make sure it is concentric with the electrode. The protective cap must be tightened and the nozzle pressed firmly. If there is play, cutting cannot be carried out.
Use the cutting torch reasonably. Contact the nozzle with the workpiece before arc ignition. When cutting is finished, release the handle button to stop the arc, then move the cutting torch away from the surface of the workpiece to extend the life of the parts. When the nozzle center cavity affects the cutting quality, it should be replaced in time.
When the center of the electrode is recessed more than 2 millimeters or fails to ignite the arc, the electrode can be installed backwards or upgraded.
When the protective cap or gas distributor is cracked or seriously damaged, they must be replaced in time.
When cutting torch body insulation, synthetic leather jacket, cable insulation or gas pipe damage is found, it should be repaired or replaced in time.
To remove the cutting torch, retract the synthetic leather jacket, disassemble the switch connecting wires, retract the handle back, and then disassemble the connecting gaskets from the cutting torch body.
When replacing a new ceramic protective cover, apply some vaseline oil to the cutting torch body sealing ring before screwing it on to extend the sealing life.
4. Common faults and troubleshooting methods
| No. | Faults, breakdowns | Causes | Solutions |
| 1 | Turn on the power button, the power indicator light does not light | 1. The fuse in the power supply switch is broken | to replace |
| 2. The fuse behind the power box is broken | Check and replace | ||
| 3. The control transformer is defective | to replace | ||
| 4. The power button is broken | to replace | ||
| 5. The indicator light is broken | to replace | ||
| two | Cannot preset cutting gas pressure | 1. The air source is not connected or there is no air in the air source | Connect the air source |
| 2. The power switch is not in the “on” position | Trigger | ||
| 3. The pressure reducing valve is damaged | Repair or replace | ||
| 4. Poor solenoid valve wiring | Check the wiring | ||
| 5. The solenoid valve is defective | to replace | ||
| 3 | Pressing the cutting torch button during operation results in no airflow | 1. Pipe leak | Fix the leaky part |
| 2. The solenoid valve is defective | to replace | ||
| 4 | After the conductive nozzle contacts the workpiece, press the cutting torch button and the work indicator light turns on, but arc cutting has not been triggered | 1. KT1 is broken | to replace |
| 2. The high frequency transformer is defective | Check or replace | ||
| 3. Ignition rod surface oxidation or inadequate clearance distance | Polish or adjust | ||
| 4. Short circuit of high frequency capacitor C7 | to replace | ||
| 5. Air pressure is too high | Lowering | ||
| 6. The loss of the conductive nozzle is very short | to replace | ||
| 7. Open circuit or short circuit of the rectifier element of the rectifier bridge | Check and replace it | ||
| 8. Poor contact or open circuit in the cutting torch cable | Repair or replace | ||
| 9. The workpiece ground wire is not connected to the workpiece | Connected to the workpiece | ||
| 10. There is a thick layer of paint or dirt on the surface of the workpiece | Clean and make conductive | ||
| 5 | The cutting indicator light does not illuminate when the conductive nozzle contacts the workpiece and the cutting torch button is pressed | 1. Thermal control switch action | Wait for it to cool down or work again |
| 2. Cutting torch button switch is damaged | to replace | ||
| 6 | Control fuse to break after high frequency startup | 1. Damaged high frequency transformer | Check and replace |
| 2. Damaged control transformer | Check and replace | ||
| 3. Contactor coil short circuit | to replace | ||
| 7 | The main switch fuse is broken | 1. Short circuit of the rectifier element | Check and replace |
| 2 Main transformer failure | Check and replace | ||
| 3. Contactor coil short circuit | Check and replace | ||
| 8 | High frequency occurrence but no arc | 1. The rectifier component is defective (there is an abnormal sound inside the machine) | Check and replace |
| 2. The main transformer is damaged | Check and replace | ||
| 3. C1-C7 down | Check and replace | ||
| 9 | Long-term work stoppage and arc failure | 1. The temperature of the main transformer is too high and the thermal control switch operates | Wait for it to cool down before working, pay attention when lowering |
| The temperature fan is running and the wind direction |
5. Electrical schematic diagram
