Unveiling the Art of Precision: A Comprehensive Guide to Plasma Cutting

Plasma cutting stands as a revolutionary method in metal fabrication. It utilises high-velocity ionised gas to precisely cut through conductive materials. Rooted in the principles of plasma physics, this process harnesses the power of extreme heat, forming a conductive arc that transforms gas into plasma. By directing a controlled, electrically charged state of matter through a focused nozzle, you can easily cut materials such as steel, aluminium, and other metals. In this comprehensive guide, we will look at the intricacies of plasma cutting, exploring its underlying physics, working principles, essential components, and a comparison with other popular cutting methods.

Understanding Plasma:

Plasma distinguishes itself by its ionised composition of positively charged ions and free electrons. Plasma is a fourth state of matter distinct from solid, liquid, and gas. Like other states, plasma is formed once the gases that make steam are exposed to high energy and become ionised. Created when high energy transforms gases into charged particles, plasma is a conductive, reactive substance capable of conducting electricity, responding to magnetic fields, and emitting light. Examples of natural plasma include lightning, auroras, and stars, while technological applications range from TVs to plasma cutting.

How Plasma Cutting Works:

Plasma cutting harnesses the power of plasma by generating an electric arc between the electrode and the base metal. A controlled gas, such as oxygen, nitrogen, argon, or shop air, is forced through a nozzle, creating a plasma jet reaching temperatures of up to 40,000°F. This intense heat easily pierces through metals, making plasma cutting up to ten times faster than traditional methods. The system comprises a power supply (plasma cutter), arc starting console, plasma torch, and external gas.

Parts of the Plasma Cutting System:

1. Power Supply (Plasma Cutter): 

The plasma cutter is a central piece of every plasma-cutting process. The plasma cutter works by converting a single or three-phase AC line voltage into a smooth, constant DC voltage required to form and maintain a plasma arc throughout the process. Plasma cutters are compact and reasonably priced machines that will provide enough stable energy to create and sustain the plasma-cutting process from start to finish.

2. Arc Starting Console: 

The arc starting console produces the spark inside the plasma torch to create the electric arc. As the electric arc passes through gas, it becomes plasma, which is fired onto the base metal. Based on the design and internal components, there are several ways to start a plasma arc. 

High-frequency start: high-frequency contact is the oldest and cheapest method to start a plasma arc. It is typical for conventional plasma cutters and requires a high voltage and high-frequency spark. High-frequency spark provides enough energy to ionise the compressed gas and form a plasma arc, but it creates “electrical noise.” The high frequency can interfere with nearby electronic devices such as phones, computers, and, most importantly, a CNC plasma cutting table, which makes them unsuitable for most applications.

Pilot arc start: The pilot arc method is an advanced technology that allows a spark to form at the torch tip without touching the material. The ability to start an arc without touching the base metal will prolong consumable life when cutting rusty or painted metals. In addition, you don’t have to worry about interfering with other electrical devices in your surroundings, which makes it an essential feature for CNC plasma cutters.

Spring loaded start: this plasma arc start is similar to lift TIG. To start an arc, you press your torch into the base metal, and the short circuit and electron flow are established. Once you lift the torch and release the pressure, the spark initiates the pilot arc (non-contact arc). So, there is initial contact with base metal but no contact during the cutting operation.

2. Plasma Torch: 

A plasma torch is your primary cutting accessory.. The torch features several consumable pieces that, in conjunction, start and control the arc through the process while supplying compressed air. In traditional plasma cutting torches, the consumable parts are in contact with one another before initiating an electrical arc. Still, once enough pressure is built up, they are forced apart and create a spark that ionises the pressured air. The essential parts of a plasma torch are:

Electrode:

Plasma torch electrodes are small, narrow pieces that conduct current to create and maintain an arc for plasma-cutting tasks. They receive the electrical current from a cathode block inside the torch and focus the charge through its tip. Electrodes in plasma cutting are considered consumable since they wear over time, and it is recommended to change them simultaneously as a nozzle.

Swirl ring:

A swirl ring is a vital consumable that swirls the gas surrounding the plasma arc. Spinning the gas around the nozzle has two essential effects. Firstly, the spinning gas is gyroscopically stabilised like a rifle bullet, making the plasma column less prone to deflect. Secondly, a spinning action centrifugally places the cooler gases on the outside of the nozzle, which protects it from extremely high heat that can damage it.

Nozzle:

A nozzle is a consumable with a small opening that controls and directs the plasma arc. Similar to welding torches, there are different sizes, so a nozzle with a larger opening is used for gouging. In comparison, a nozzle with a smaller opening can better direct the gas and so is used for fine, detailed work.

Retaining cap:

The retaining cap essentially holds all of the consumable parts of the torch together. Most importantly, the internal cap keeps all the consumable pieces aligned. This gives you a reliable and focused arc each time. Keep in mind that this cap can also degrade over time, causing coolant leaks, gas errors, and or poor cut quality.

Shield cap:

A shield cap protects the other pieces and internal components of the torch from molten metal and sparks. The shield takes the brunt of the fallout so that wear to other parts is minimised as much as possible.

Plasma Cutting Gases:

To create plasma, you must supply an external gas, which is then pressurised inside the nozzle and heated by an arc. Plasma cutters can work with various gases. These include compressed air, nitrogen, argon, hydrogen, and oxygen, or blends of two or three components. Similar to shielding gas, each of these shows different results when cutting.

Compressed air: Air is one of the most popular and widely used plasma-cutting gases, which you are likely to use in your various applications. You’ll need a compressor or a plasma cutter with a built-in compressor, an all-in-one welder and cutter. This gas is highly versatile and inexpensive, and it will work well with mild steel, stainless steel, and aluminium on sheet metal and thin gauge up to 1 inch. 

Oxygen: Oxygen is another prevalent plasma gas that is widely used on mild steel in applications that require clean cuts and fast cutting speeds. Cutting arc is hotter than air, so that you can cut mild steel up to 1 1/4″ thick. However, higher heat means more damage to consumable parts. Additionally, oxygen is not friendly to shiny surfaces of stainless steel and aluminium.

Nitrogen: Nitrogen is a plasma gas used in heavy-duty applications and thick metals up to 3″. It produces quality cuts on most materials, including stainless, mild steel, and aluminium. Nitrogen is often used with several secondary gases, such as air, carbon dioxide, and argon, for thicker material.

Argon: Argon is an inert gas, which means it doesn’t react with the metal surface you are about to cut, which makes it suitable for specific applications. However, argon itself has low conductivity, which makes it rarely used as a standalone plasma-cutting gas. Typically, argon is mixed with hydrogen to create the hottest plasma-cutting flame and some of the cleanest cut.

How Plasma Cutters Work:

Firstly, you will have to assemble your torch with all the pieces, including the electrode, swirl ring, nozzle, retaining cap, and shield. Next, connect the plasma cutter to your outlet and gas supply, and ground the piece you are about to cut. Set the amperage according to the metal thickness you are about to weld.

Once everything is set up, it is time to cut. Take your torch, and depending on the start type, place it near the base metal or touch it. Once you press the trigger, the entire process is initiated. The gas flows through the torch into the nozzle, and a swirling ring spins it around to promote cooling and direct the gas. Simultaneously, the arc starting console starts an arc by creating the spark inside the torch (high-frequency contact) or on top of the torch (pilot arc).

Once the arc passes through the pressurised gas, it gives it the energy to form a plasma, which is conductive, ionised gas. The plasma jet reaches exceptionally high temperatures, and it is fired through a narrow plasma nozzle opening. The concentrated, hot plasma arc melts the base surface and easily cuts through conductive material. The process stops once you release the torch trigger, including the gas supply and arc. 

Types of Plasma Cutting Systems:

Now, this is the fundamental explanation of how plasma cutting works, but keep in mind that there are two different plasma cutting systems that are based on the same theory. The components are modified to produce different results so we can divide the systems into two groups:

• Conventional plasma cutting systems,
• Precision plasma cutting systems.

Conventional plasma cutters, or handheld cutters, are the ones you are likely to deal with as a beginner or hobby metal fabrication enthusiast. They are built to provide versatile and economical results with decent cutting capability at lower currents. As a result, these are cheaper cutters, which often include high-frequency contact starts and cutting, with a maximum cutting power of approximately 1 inch and a moderate amount of dross and kerf.

A precision plasma cutter work, and it is designed for the sharpest, highest quality cuts that are achievable with plasma. They are designed for metal fabrication processes that require the highest precision with minimal kerf. This is done by introducing higher-end elements and parts. They are often part of computer numerically controlled (CNC) plasma cutters and tables that can be programmed to provide the best results, but they come with a price.

What Can You Cut With a Plasma Cutter?

A plasma cutter can cut all electrically conductive materials, including mild steel, stainless steel, aluminium, brass, copper, etc. Conductive metals allow the formation of an electrical arc between the electrode and the ground, which provides the energy to turn the pressurised gas into a plasma.

As a beginner or hobby metal fabricator, you will likely use your plasma cutter to weld carbon steel. Low carbon steel, also known as mild steel, is one of the most common materials widely used across industries, and you can easily weld it or cut it with an air gas supply.

Stainless steel and aluminium might be more challenging to weld, but you can easily cut them with a plasma cutter. You will want to avoid oxygen gas, but you can do it with an air compressor and pieces around the workshop. Brass, copper, and cast iron are used in particular applications, so you probably won’t deal with them that often. Still, knowing that you can cut them with a plasma cutter is always a good option. 

Pros and Cons of Plasma Cutting:

As noted, plasma cutting is a versatile and cost-efficient process that provides a wide list of advantages, but it also has some drawbacks.

The advantages of plasma cutting are:

• High versatility: you can cut all conductive metals of various thicknesses, from sheet metal up to several inches.
• High-quality cuts: due to highly focused and small arc, plasma cutters produce high-quality cuts with low levels of residual scum on the edge of the metal.
• High-speed cutting: Plasma cutting is a fast-cutting process that will allow you to finish your work quickly, which is essential for mass production and large-scale industrial applications.
• Reliable and repetitive results: plasma cutters can be programmed with a CNC table to produce highly reliable and redundant results with minimal distortions each time.
• Budget-friendly: The latest handheld plasma cutters are relatively affordable yet highly functional machines that you simply must have if you are into metal fabrication or own a small garage shop.

The disadvantages of plasma cutting are:

• You can only cut conductive metals that will allow electric arc formation.
• Cutting thick metals over a few inches is not recommended.
• Consumable pieces can be worn quickly.
• It can produce fumes, radiation, noise, and sparks while cutting.

Plasma Cutting vs. Flame Cutting vs. Laser Cutting:

As a recap of our article, we’ll make a comprehensive comparison of the three most popular cutting methods among metal fabricators – plasma cutting, flame cutting, and laser cutting. Understanding the ups and downs of each cutting method will significantly help you choose the most suitable one for your cutting projects.

Final Thoughts

In the realm of metal fabrication, plasma cutting stands out as a dynamic and efficient process, offering precision and versatility. Understanding the science, components, and applications of plasma cutting equips metalworkers with the knowledge needed to harness this transformative technology. Whether working on a DIY project or in an industrial setting, plasma cutting stands as a powerful tool in the hands of those seeking precision in metalwork.

Welding Nickel-based Alloys to Steel:

Common Nickel-base alloys, such as Monel and Inconel, can be successfully joined with low-alloy steel by different arc welding processes. You should use the Inconel base electrode when welding Inconel to mild or low-alloy steel. Likewise, welding Inconel or Monel to stainless steels will require a proper Inconel or Monel-type electrode.

Welding Low-Carbon Steel to High-Strength Steel:

Repairing or welding structural or heavy equipment will often require welding low-carbon steel to high-strength steel. The high-strength steel, such as A514, typically offers a yield strength of 100,000 psi, while standard low-carbon steels have a yield strength of 70,000 psi.

If you remember the first part of the text, to successfully weld these two, you will need a filler material that matches the strength of the weaker metal.

Final Thoughts:

Welding dissimilar metals demands a comprehensive understanding of the materials involved. By carefully considering physical and mechanical properties, selecting the right equipment and filler materials, and employing proper techniques, you can successfully join dissimilar metals for various applications. Mastering the art of welding dissimilar metals opens up possibilities for tackling diverse projects with confidence.

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