Plasma Cutter

What a Plasma Cutter Is

A plasma cutter is a thermal fabrication tool that severs electrically conductive metal by forcing a gas through a small nozzle while an electric arc heats it to the plasma state, a fourth state of matter in which the gas is partly ionized, electrically conductive, and extremely hot. The constricted plasma jet reaches temperatures on the order of 20,000 degrees Celsius, far above the melting point of any commercial metal. The jet melts a narrow channel through the workpiece, and the high gas velocity blows the molten metal out of the bottom of the cut, leaving a kerf.

The defining requirement is electrical conductivity. Because the cutting current must flow from the torch electrode, through the plasma, into the metal, and back to the power supply through a work clamp, plasma cuts only conductors: carbon and alloy steels, stainless steel, aluminum, copper, brass, titanium, and similar. It cannot cut glass, stone, wood, or plastics, which is the line that separates it from laser and waterjet processes that work on non-conductors.

A complete plasma cutting system has four functional parts. First, the power supply, a constant-current DC inverter or chopper that delivers a stable arc at a set amperage. Second, the torch, which holds the consumable electrode and nozzle and directs the gas and arc. Third, the gas supply: clean compressed air, or bottled oxygen, nitrogen, or argon-hydrogen for higher quality. Fourth, the consumables, chiefly the electrode (a copper body with a hafnium or zirconium emitter) and the nozzle, both of which wear with use and define a running cost.

The process matured commercially through the 1950s and 1960s. Plasma arc cutting grew out of plasma arc welding work, and was developed to cut stainless steel and aluminum, metals that oxyfuel cutting cannot sever because they do not oxidize the way mild steel does. Early systems used inert gas and high current. Over the following decades the introduction of air plasma, the hafnium electrode for cutting with oxidizing gases, contact-start torches, inverter power supplies, and finally CNC high-definition systems progressively raised cut quality and lowered cost.

Today plasma covers an enormous span of work, from a 110 V hobby and maintenance unit cutting 6 mm sheet on a job site, to a 400 A mechanized bevel head on a shipyard plate line cutting 50 mm structural steel, to CNC high-definition tables producing near-laser edges on 12 mm stainless. No single machine spans that range. The core of selection, covered in the chapters that follow, is matching the metal, thickness, edge quality, and production volume to a specific machine class, amperage, and gas.

System Types and Classes

Plasma cutting equipment splits along two axes: how the torch is guided (handheld versus mechanized or CNC), and how finely the arc is constricted (conventional versus precision or high-definition). These two axes, not the brand, determine what a machine can realistically do. The table below summarizes the four main classes a buyer will encounter, with representative amperage and use.

Conventional handheld systems are the entry point. A single-phase inverter, an air-cooled torch with a contact-start or pilot-arc trigger, and a built-in or external air supply. They are portable, simple, and forgiving, and they dominate the maintenance, repair, agricultural, and small-shop market. Cut quality is acceptable for parts that will be welded or ground, with a visible bevel and some dross. Output typically runs from about 25 A up to 105 A, mapping to mild-steel cut capacity from roughly 6 mm to 32 mm.

Conventional mechanized systems put a similar single-gas or dual-gas torch on a CNC table, track burner, or pipe machine. Mechanized guidance removes the hand-motion variation, so even a conventional torch produces straighter, more repeatable edges than handheld. This class covers a large share of general fabrication: cutting brackets, gussets, base plates, and structural members where the edge will be welded.

Precision or high-definition plasma is the quality tier. A specialized nozzle, swirl ring, and tightly controlled gas flow constrict the arc to a higher energy density, producing a near-square edge with minimal bevel and dross. High-definition systems narrow the gap to laser on mid-thickness conductive plate and can reach ISO 9013 quality range 2 on thinner material and range 3 on thicker plate. They run only on CNC tables, use richer multi-gas consumable stacks, and cost considerably more, but they cut faster and cleaner in high-volume production.

Heavy mechanized and bevel systems use high current, often 200 A to 800 A, for thick plate and weld-prep bevels in shipbuilding, pressure-vessel, and heavy-structural work. These are water-cooled, gantry-mounted, and frequently fitted with rotating bevel heads that cut V, Y, K, and X weld preparations in a single pass. Edge quality is secondary to capacity and productivity on heavy plate.

A separate distinction is the arc-start method. High-frequency (HF) start uses a high-voltage, high-frequency spark to ionize the gas; it is robust but the HF energy can disturb nearby electronics and CNC controls. Contact (blowback) start uses internal parts that separate to draw the pilot arc; it is HF-free and CNC-friendly, which is why most modern handheld and CNC-ready machines use it.

The Plasma Arc Process and Gases

Understanding the two-stage arc explains most of what a plasma cutter does and why the gas choice matters. The cut begins with a pilot arc and finishes with a transferred arc, and the gas serves as both the medium that becomes plasma and the force that clears the molten metal. The sequence below is the same on a hobby unit and a high-definition table; only the precision of control differs.

When the trigger is pulled, gas flows through the torch and the power supply establishes a pilot arc between the electrode and the nozzle, ionizing the gas into a small plasma jet at the nozzle orifice. The pilot arc lets the torch start without touching the plate. As the torch approaches the grounded workpiece, the conductive plasma jet bridges the gap; the current finds a lower-resistance path to the work and the arc transfers from the nozzle to the workpiece. The power supply senses the transfer and ramps to the full cutting current the operator set, switching to the optimal cutting gas. The transferred arc carries the heat that melts the metal, while the high-velocity gas blows the melt out the bottom, forming the kerf.

Two start mechanisms create the pilot arc. A high-frequency spark ionizes the gas without contact, or a contact-start torch briefly separates internal components to draw the arc. Both end at the same place: a stable pilot jet ready to transfer. A pilot-only (non-transferred) arc can also be used deliberately to cut very thin or marginally conductive material where transferring to the work is impractical.

The plasma gas is selected to match the metal. Air is the universal, low-cost default. Oxygen gives the cleanest, fastest cut on mild steel. Nitrogen is preferred on stainless and aluminum for an oxide-free edge, and an argon-hydrogen mix is used for thick non-ferrous work on high-definition systems. A separate shield gas (air, nitrogen, oxygen, or water) cools and further constricts the arc. The table below summarizes typical plasma and shield gas pairings.

Air plasma is the most common configuration because compressed air is free at the point of use and a single gas serves plasma and shield. It cuts mild steel, stainless, and aluminum, with edge quality below oxygen on steel and below nitrogen on stainless, but it is the right choice for general handheld and light mechanized work. The cost of air is consumable life: oxygen in the air mildly oxidizes the hafnium electrode, which is why air-plasma electrodes use hafnium emitters and wear faster than inert-gas electrodes.

Oxygen plasma adds an exothermic reaction with the iron in mild steel, raising cut speed and giving a cleaner, near-dross-free, more weldable edge. It is the standard for high-quality mild-steel production cutting. Oxygen is aggressive on consumables, so oxygen systems use specially designed electrodes and a controlled preflow and cutflow sequence to protect them.

Nitrogen and argon-hydrogen serve the non-oxidizing metals. Nitrogen produces a clean edge on stainless and aluminum and gives long consumable life. The argon-hydrogen H35 mix (about 35 percent hydrogen) carries more heat and is used on thick stainless and aluminum on high-definition systems, where it produces the squarest edge and best color, at a higher gas cost.

Materials, Standards and Cut Quality

Plasma cuts any electrically conductive metal, but cut speed, edge quality, and consumable life vary by material and depend on choosing the right gas and amperage. Mild and low-alloy steel is the easiest and most common work, especially with oxygen plasma. Stainless steel and aluminum cut well with nitrogen or air but tend to leave more dross than steel because the molten oxides are more viscous. Copper and brass are highly conductive and thermally demanding, so they cut more slowly and pull more current. Titanium cuts cleanly but should use inert gas to avoid a brittle, oxygen-rich edge.

The cut edge is never perfectly square. A plasma cut has a kerf (the width of material removed), a bevel or angularity (the deviation of the edge from vertical), dross (resolidified metal clinging to the bottom edge), and a heat-affected zone (HAZ, the band of metal whose microstructure changed from the heat). Conventional plasma produces a bevel of roughly 3 to 8 degrees on the off side of the cut, while high-precision plasma can hold the bevel near 0 to 3 degrees. Plasma's kerf and HAZ are wider than laser but much narrower than oxyfuel, and it produces less distortion than oxyfuel on the same plate.

These edge characteristics are graded objectively by ISO 9013:2017, the international standard for thermal cut classification covering oxyfuel, plasma, and laser. It applies to plasma cuts from about 0.5 mm to 150 mm and grades the edge chiefly by perpendicularity or angularity tolerance (u) and mean height of profile roughness (Rz5), sorting cuts into quality ranges where range 1 is tightest. AWS C4.6M is the US adoption of the same standard. Specifying an ISO 9013 range on a drawing replaces vague terms like "clean cut" with a measurable, contractible tolerance.

The table below compares plasma against the two adjacent thermal processes a buyer weighs against it, oxyfuel and laser, on the dimensions that drive process choice. The figures are typical engineering ranges, not absolute limits, and depend on machine class and material.

The practical reading of the table: oxyfuel still wins on very thick mild-steel plate and on cost per machine, but it cannot touch stainless or aluminum. Laser wins on thin-sheet edge quality and tight features, but its capital cost is much higher and its thickness ceiling on conductive metal is low. Plasma owns the broad middle, especially 6 mm to 50 mm conductive plate at high productivity, which is why it is the default cutting process in general metal fabrication.

Key Specification Parameters

A plasma cutter spec sheet lists many numbers, but only a handful drive the buying decision: output amperage, the cut, pierce, and severance capacities, duty cycle, input power, gas requirement, and torch type. Reading them correctly prevents the two classic mistakes, buying too little machine for the plate, or buying a high amperage that the shop air supply cannot feed. Each parameter is explained below.

Output amperage is the primary capacity figure. Cut capacity scales with current, so amperage is the first number to match to your thickest routine plate. As a working map: roughly 25 to 30 A cuts mild steel to about 6 to 10 mm, 45 A to 12 to 16 mm, 65 A to 20 to 25 mm, 85 A to about 25 mm, and 105 to 125 A to 38 to 50 mm. Stainless and aluminum cut at somewhat lower thickness for the same amperage because they conduct heat away faster.

Cut, pierce, and severance capacities are three different numbers that buyers frequently confuse, so a machine's specs list all three:

• Recommended (production) cut: the thickness the machine cuts cleanly at a usable travel speed, with an acceptable edge. This is the number to design around.
• Pierce capacity: the maximum thickness the torch can pierce from the top to start an internal cut. It is always less than the cut capacity, because piercing throws molten metal back at the nozzle. For example a 65 A class machine may cut 20 to 25 mm but pierce only about 16 mm; thicker plate must be edge-started.
• Severance (maximum) cut: the absolute thickest the machine can separate at very slow speed. The edge carries heavy dross, a wide bevel, and is rough, so severance is a one-off capability, not a production rate.

Duty cycle is the percentage of a ten-minute period the machine can cut continuously at a stated amperage before thermal protection forces a cool-down, always tied to a specific current and ambient temperature. A unit rated 50 percent at 65 A cuts 65 A for five minutes per ten, but the same unit may be rated 100 percent at a lower current such as 46 A. Long mechanized and CNC runs need a high duty cycle at full output; short handheld cuts tolerate less.

Input power must match the site. Light handheld units run single-phase, often 110 V to 240 V, with dual-voltage models common; mid and heavy machines run three-phase 208 V to 600 V. A unit that draws more current than the available circuit or generator can deliver will trip or sag and lose cut quality, so the input current rating and circuit-breaker requirement on the plate must be checked against the supply.

Gas and air requirement sets the support equipment. Handheld air systems typically want clean, dry, oil-free air around 5.5 to 6.2 bar (80 to 90 psi) at a stated flow, and the compressor must sustain that flow continuously, not just briefly. Inadequate or contaminated air is the leading cause of poor cuts and short consumable life. Multi-gas precision systems add oxygen, nitrogen, or argon-hydrogen bottles and a gas console.

Torch type, consumables, and safety marks round out the sheet. Note whether the torch is air- or water-cooled, the consumable family and its published life, and the arc-start method (HF versus contact). Confirm the machine carries the relevant arc-equipment safety conformity, typically the IEC 60974-1 series for arc welding and cutting power sources, and CE or equivalent regional marking. These determine running cost and whether the machine is accepted on regulated sites.

Selection Decision Factors

To turn the preceding chapters into a specific model, work through the decision sequence below in order. Most selection errors come not from one wrong number but from deciding amperage before defining the material, thickness, and edge quality the parts actually require. These steps double as an RFQ template.

1. Material and thickness: List the metals (mild steel, stainless, aluminum, copper) and the thickest plate you cut routinely, not the rare maximum. This pair fixes the minimum amperage and the gas. Remember stainless and aluminum derate capacity versus mild steel at the same current.
2. Edge quality required: Decide the target ISO 9013 range. Weld-prep and structural edges tolerate range 3 to 4 (conventional plasma); visible or tight-tolerance parts need range 2 to 3 (precision or high-definition). Edge quality, not just thickness, separates a conventional from a high-def system.
3. Handheld versus mechanized: Manual torches suit maintenance and low volume; CNC tables suit repeatable production and complex profiles. The guidance method changes both the machine and the budget far more than amperage alone.
4. Amperage and capacity margin: Choose output so your thickest routine plate sits within the recommended production-cut rating, with margin, and verify pierce capacity separately if you start internal holes. Do not size to the severance number.
5. Duty cycle versus workload: Match the duty cycle at full output to how long your cuts actually run. Long CNC sheets need high duty cycle; short handheld work does not. Undersizing duty cycle means waiting on cool-downs and losing throughput.
6. Gas strategy: Air for general work; oxygen for quality mild steel; nitrogen for stainless and aluminum; argon-hydrogen for thick non-ferrous on high-def. Confirm the gas console, bottles, and a clean, dry, oil-free supply with inline filtration and drying.
7. Input power and site: Verify single- versus three-phase, voltage, input current, and breaker size against the actual supply or generator. Confirm compressor flow sustains the machine's continuous air demand.
8. Total cost of ownership (TCO): Add consumable cost per meter (electrode and nozzle life at your amperage and pierce count), gas cost, and downtime to the purchase price. A cheaper machine with short consumable life and a low duty cycle often costs more per cut part over a year of production.

One last and commonly overlooked dimension is serviceability and consumable supply: local availability of genuine electrodes, nozzles, swirl rings, and shields; the published cut charts that set current and standoff; torch-part lead time; and local service and warranty support. Plasma is a consumable-driven process, so a machine whose parts are hard to source or whose cut charts are unavailable will cost more in downtime than its purchase price saved. Established makers such as Hypertherm (Powermax and X-Definition lines), ESAB (including Thermal Dynamics Cutmaster), Lincoln Electric (Tomahawk), and Miller (Spectrum) maintain documented consumables and cut charts and broad distribution, which is why they are common defaults for production fabrication. Always verify the exact model's current specifications against the manufacturer datasheet before purchase.