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A Comprehensive Guide to Plasma Cutting
Plasma cutting is a beneficial process in fabrication and deconstruction. A plasma cutter can effectively cut through almost any material by utilizing electricity to create incredibly hot streams of ionized plasma. This method is known for its precision, control, and power. Understanding the functionality and components of a plasma cutting system is crucial for successful use. Therefore, it’s important to learn how plasma cutting works, familiarize yourself with the different parts of a plasma cutting system and gather essential knowledge about the process.
Definition of Plasma
We are all familiar with the three fundamental states of matter: solid, liquid, and gas. However, there is also a fourth state: plasma.
Plasma can be found in nature, primarily in the upper regions of Earth’s atmosphere. The famous auroras, or polar lights, result from solar winds composed of plasma. Lightning, high-temperature fire, and the human body also involve plasma.
Plasma comprises about 99% of the observable universe. We encounter it daily through various sources, such as fluorescent lamps, TVs, neon signs, and plasma cutters.
Plasma is a conductive ionized gas, where atoms may lack electrons, and free electrons float around.
Subjecting a gas to intense heating transforms it into plasma, which is why it is often called ionized gas.
Plasma shares similarities with gases because its atoms are not in constant contact. However, it also behaves like a liquid, which can flow when exposed to electrical and magnetic fields.
Definition of Plasma Cutting
It is a metal fabrication process that utilizes ionized gases heated to temperatures above 20,000°C to melt metal materials. The high-pressure gas melts the material and removes the cut material.
It’s important to note that this process only works on electrically conductive materials such as stainless steel, aluminum, copper, and other metals. On the other hand, plasma cutting cannot cut stone, glass, paper, and other poor conductors of electricity.
This technique is unparalleled in cost-effectiveness for cutting thick metals. Additionally, it is versatile and requires low tool maintenance costs. It also offers high cutting precision, making it ideal for cutting parts with complex geometries.
History of Plasma Cutting
Plasma cutting has existed since 1957. It started as an extension of the Gas Tungsten Arc Welding (GTAW) process. Initially, it was mainly used for cutting steel and aluminum plates with thicknesses ranging from half an inch to six inches.
The plasma cutters used during this era were unpredictable and needed to be as precise as modern-day cutters. Additionally, the electrodes and nozzles used in these cutters were prone to quick breakdowns due to the intense heat experienced during the cutting process. Consequently, the frequent replacement of nozzles and electrodes made plasma cutting an expensive undertaking during this time.
The late 1960s
This technique had a breakthrough in the late 1960s and early 1970s, creating a dual-flow torch, improving electrode and nozzle lifespan, and enhancing cut quality and precision.
1970s
In the 1970s, engineers developed a water muffler and table to address fumes and smoke produced during cutting. They also created improved nozzles to enhance the arc’s precision, allowing operators and machinists to fine-tune the process.
1980s
Engineers experimented during the 1980s as they designed and implemented several new features. These features involve oxygen-based plasma cutters, which offer better cut control by varying power levels. Engineers also focused on making the plasma cutting unit more ergonomic and portable.
From the 1990s to date
In the 1990s, high-definition plasma cutters entered the market thanks to durable oxygen processes. Combined with a new nozzle system, these processes enabled the plasma cutters of this era to quadruple the energy density of earlier models.
Since the 1990s, engineers have focused on enhancing power options, controls, and efficiency. Additionally, they have improved the precision of plasma cutters, with modern models now providing sharper edges and more precise cuts. Significant progress has also been made in improving the portability and automation of plasma cutters, increasing the availability of handheld units.
How Plasma Cutting Works?
The plasma arc is produced by electrically heating compressed air to a high temperature, causing the atoms to ionize and enabling them to conduct electricity.
The welding torch propels air through the swirl ring, where the electric arc from the electrode ionizes it. The ionized air turns into plasma, which moves from the torch to the workpiece.
The cutting torch uses electricity to produce high-velocity plasma, which melts through metal by generating intense heat. The plasma and compressed gas dissipate the molten metal, resulting in a clean cut. Plasma resembles a high-temperature gas but possesses the unique capability to conduct electricity, allowing it to cut through any electrically conductive metal.
The plasma torch is designed with a safety trigger that must be released before you can start the arc. When you squeeze the trigger, the power supply generates a DC current that flows through the connection and initiates the plasma gas flow. The DC current then switches from the electrode to the nozzle, creating a path between the electrode and the workpiece. This current and airflow continue until the trigger is released. As electricity from the cutting torch travels down the plasma, it produces enough heat to melt through the workpiece.
Three Types of Cutting Process
- High-Frequency Contact: This form is low-budget, but it needs to be more suitable for CNC plasma cutters due to the risk of interference with modern equipment caused by its high frequency. High-frequency contact cutting uses high-frequency sparks and high voltage. The spark is formed when the plasma torch comes into contact with the cut metal. This contact closes the circuit, initiates the spark, and creates the plasma used for cutting.
- Pilot Arc: During the cutting process, sparks are created in the torch by combining low current circuits and high voltage. This spark helps create a pilot arc, which produces a small amount of plasma. When the plasma cutter comes in contact with the workpiece, it makes a cutting arc, allowing the machinist or operator to start the cutting process.
- Spring Loaded Plasma Torch Head: The operator presses the torch against the workpiece to produce a short circuit. The current starts to flow when the torch is pressed against the workpiece. The operator then releases the pressure to establish the pilot arc.
Inside a Plasma Torch
Plasma cutters appear in various shapes and sizes. Some are large and use robotic arms to make precise cuts, while others are smaller and handheld, commonly found in a handyman’s shop. Despite their size, all plasma torches operate on the same principle and are built around a similar design.
Plasma cutters direct a pressurized gas, like nitrogen, argon, or oxygen, through a small channel. In the middle of this channel lies a negatively charged electrode. A circuit is created by applying power to the negative electrode and touching the metal with the tip of the plasma nozzle.
For the inert gas to pass through the channel, a powerful spark is created between the electrode and the metal, heating the gas until it becomes a stream of directed plasma at about 30,000 F (16,649 C) and moves at 20,000 feet per second (6,096 m/sec), which reduces the metal to molten material.
Plasma on the Job
CNC stands for Computer Numerically Controlled. This cutting process allows the technician to cut materials without direct contact. CNC plasma cutters have become a standard in the industry. They are often used in custom auto shops and by car manufacturers to customize and create chassis and frames, using a cutting table to aid the plasma cutting process.
Construction companies use plasma cutters for large-scale projects, cutting and fabricating massive beams or metal sheet materials. Locksmiths employ plasma cutters to drill into safes and vaults to assist customers who are locked out.
Gas Used in the Process
The type of gas used during the cutting process depends on the cutting method, the cut material, and its thickness. In addition to forming a plasma jet, the gas must facilitate the expulsion of molten material and oxide from the cut. The most commonly utilized gases for plasma cutting involve:
Argon
It is an inert gas, and its plasma arc is steady. This means that the gas scarely reacts with any metal at high temperatures. Electrodes and nozzles utilized for argon cutting often have a longer service life than those used with other gases.
Argon gas has limitations when it comes to cutting due to its low plasma arc and enthalpy. Additionally, using argon in an argon protection environment can lead to slag problems. This is mainly because the surface tension of the molten metal is about 30% higher than that in a nitrogen environment. Consequently, these issues are why argon is seldom used for plasma cutting.
Nitrogen
Nitrogen provides better plasma arc stability and a higher energy jet than argon, especially when using a higher voltage supply. It also produces minimal slag on the lower edges of the incision, even when cutting metals such as nickel-base alloy and stainless steel, which have high viscosity.
Nitrogen gas can be used alone or with other gases to enable high-speed carbon steel cutting.
Air
Air consists of 78% nitrogen and 21% oxygen by volume, making it suitable for plasma cutting. The oxygen in the air makes it one of the fastest gases for cutting low-carbon steel. Additionally, because air is readily available everywhere, it is an economical gas to work with.
The downside of this process is that the electrode and nozzle typically have a short service life, which increases cutting costs and reduces efficiency. Additionally, using air as the sole gas can lead to slag hanging and cut oxidation.
Oxygen
Oxygen, like air, enhances the speed of cutting low-carbon steel. Its speed can be increased by employing high-energy plasma arc cutting and high temperatures with oxygen. However, for the best results with oxygen, it is advisable to use high-temperature and oxidation-resistant electrodes in conjunction with it.
Hydrogen
Hydrogen is an ordinary auxiliary gas in plasma cutting applications. One popular combination involves mixing hydrogen with argon, producing a highly potent plasma cutting gas. Combined with argon, hydrogen notably raises the argon plasma jet’s arc voltage, enthalpy, and cutting capability. This combination’s cutting efficiency is further enhanced when compressed by a water jet.
Materials for Plasma Cutting
Plasma cutting can cut any conductive material. Various materials are used in this technique. Below are the most commonly used materials.
Advantages
- Able to cut all conductive materials. While suitable for cutting thick metals, flame cutting is limited to ferrous metals only.
- Maximum thickness: up to 150 mm.
- Excellent quality for materials with a thickness of up to 50 mm.
- It is the best way to cut medium-thickness stainless steel and aluminum.
- It is comparatively cheap for medium-thickness cuts.
- Smaller cutting kerf compared to flame cutting.
- CNC machines are utilized to achieve high precision and repeatability.
- Quicker cutting speed than oxyfuel.
- It can cut in water, resulting in smaller HAZ. It also reduces noise levels.
- The same machine could be used for plasma welding by manipulating the parameters.
Disadvantages
Plasma cutting offers several advantages for metal fabrication, but it also has its drawbacks.
- Only cut conductive materials.
- It creates fumes when cutting in the air.
- It is ideal for thicknesses at most 150mm. Sometimes, the operation is noisy.
- It may be cost-intensive with low-life consumables like nozzles and electrodes.
- Bright flashes produced during the process could impact human eyes negatively.
Alternatives for Plasma Cutting
For many applications, the process most similar to plasma cutting is laser cutting. These methods are closely related, as they both use directed energy to melt or evaporate the target material and a gas or air stream to sweep debris. Plasma cutting was developed to duplicate and enhance oxyfuel cutting, which is still widely used as a more straightforward but less precise alternative.
Comparison Between Plasma & CNC Plasma Cutting
The primary distinction between plasma cutting and CNC plasma cutting is the equipment used. Plasma cutting originated as a hand-torch or basic pass-jig process in the 1950s and 1960s. With the introduction of CNC machines in the 1960s and 1970s, there were attempts to attach a torch to a CNC X-Y transport mechanism, enabling precise control and repeatability without needing the same degree of operator expertise.
Conclusion
Plasma cutting is a process that utilizes the fourth state of matter to cut conductive metals. This method offers numerous benefits, including increased productivity, versatility, precision, and surface quality.
Plasma cutting can be a valuable addition to any machine shop, fabrication facility, or salvage operation. Its flexibility and usefulness mean that any shop needing cutting metal – whether for repetitive part fabrication or breaking down large pieces for easier handling – can significantly benefit from using a plasma cutter.
