Plasma Cutting

How plasma cutting works

Start with the fourth state of matter. Heat a gas hot enough and its atoms shed electrons; the gas becomes a soup of free electrons and positive ions called plasma. That soup is electrically conductive and ferociously hot. Plasma cutting is the trick of generating a small, controlled, knife-thin column of this plasma and dragging it through metal.

The torch does four things in sequence. First a pilot arc jumps from a central electrode to the surrounding copper nozzle (or is struck by a high-frequency spark or a "blowback" contact-start), ionizing the gas inside the torch. Second, the now-conductive gas lets the main current "transfer" to the workpiece — the part itself becomes part of the circuit. Third, the gas is forced through a tiny constricting orifice in the nozzle, which squeezes the arc into a narrow, high-velocity jet: constriction is the whole game, because it raises both the temperature and the velocity of the plasma. Fourth, that jet melts a spot of metal and its own momentum, helped by the gas flow, blows the molten metal out the bottom of the cut.

The key insight is that a free-burning, unconstricted arc (like a welding arc) spreads out and runs "cool" — a few thousand degrees over a wide, soft cone. Force the same arc through a 1–3 mm orifice and you concentrate the current into a tiny cross-section. Current density skyrockets, the gas is heated far hotter, and it exits at speeds that can approach the local sound speed. The same electrical power becomes a cutting tool instead of a welding tool.

The four functional pieces:

  Electrode (cathode)  →  hafnium-tipped copper; emits electrons
  Constricting nozzle  →  copper orifice that squeezes the arc
  Plasma / cut gas     →  air, O2, N2, or Ar-H2 (ionized to the jet)
  Shield gas + cap     →  surrounds the jet, sharpens the cut edge

Circuit (transferred arc):
  power supply (+) → workpiece → plasma column → electrode (−) → supply

The governing physics: power, current density, kerf

Plasma cutting is, at heart, an energy-balance problem. The plasma delivers power; the metal needs a certain amount of energy per unit volume to be heated to melting and ejected; cut speed is set by how those two balance.

The electrical power dumped into the arc is simply:

P = V × I

  V = arc voltage   (≈ 100–200 V across the constricted arc)
  I = arc current   (20 A handheld → 400+ A industrial)

Example: a 130 A machine at ~140 V  →  P ≈ 18 kW into the arc.

Not all of that reaches the cut — a chunk is lost to the nozzle, radiation, and the kerf walls. The fraction that actually does cutting work, the cutting efficiency, is typically 40–60%. What matters at the metal is the energy needed to remove material:

Energy to melt & eject a volume of steel:

  E/volume = ρ × [ c_p × (T_melt − T_0) + L_f ]

  ρ      ≈ 7,850 kg/m³        (density of steel)
  c_p    ≈ 0.49 kJ/(kg·K)     (specific heat)
  T_melt ≈ 1,510 °C, T_0 ≈ 20 °C
  L_f    ≈ 247 kJ/kg          (latent heat of fusion)

  E/volume ≈ 7,850 × (0.49×1490 + 247) ≈ 7.7 GJ/m³ ≈ 7.7 J/mm³

The material removal rate (kerf cross-section × cut speed):
  MRR = w × t × v
    w = kerf width, t = plate thickness, v = cut speed

So:  v ≈ (η · P) / (E/volume · w · t)

Plug in numbers. A 130 A torch (P ≈ 18 kW) with η ≈ 0.5 puts ~9 kW into a 10 mm steel plate. With a 2 mm kerf, the steel removal demands ~7.7 J/mm³ × 2 mm × 10 mm = 154 J per mm of travel; 9,000 W ÷ 154 J/mm ≈ 58 mm/s, or about 3.5 m/min. Real machine charts for 130 A on 10 mm mild steel land near 2–3 m/min once you account for the melt-ejection inefficiency — a good sanity check on the simplified model.

Two more design levers fall out of this. Kerf width scales with the orifice diameter: a wider orifice cuts a wider, more wasteful kerf but tolerates more current. And the arc's swirl — the gas is injected tangentially to spin around the electrode — stabilizes the column and centers the hot cathode spot, at the cost of depositing slightly more heat on one side of the kerf (the source of the characteristic bevel).

Variants: conventional, dual-gas, water-injection, HD

• Conventional (single-gas) plasma. One gas does everything — usually shop air. Cheapest, simplest, handheld and light-industrial. Air plasma cuts mild steel fast but nitrides the cut edge (air is ~78% nitrogen), which can cause porosity if you weld over it without grinding.
• Dual-gas plasma. A separate plasma gas and shield gas. Oxygen plasma on mild steel adds an exothermic oxidation boost (like oxy-fuel) for faster, cleaner cuts; nitrogen or argon-hydrogen on stainless and aluminum avoids oxidation and improves edge color. The shield gas tunes the kerf and reduces top-edge dross.
• Water-injection plasma. Radial water injected at the nozzle constricts the arc further (the water vaporizes and pinches the column), giving a squarer cut and cooling the nozzle for longer consumable life. The underside of the table is often a water table that catches fume and dross and dampens noise.
• High-definition / high-precision (HD) plasma. Tighter orifice geometry, vortex shield gas, and tightly regulated standoff produce kerfs near 1 mm and bevel under 1–2° — approaching laser quality on medium plate. Hypertherm's HPR/XPR and Kjellberg's HiFocus are the canonical product families. The trade is more expensive consumables and a narrower process window.
• Underwater / submerged-arc plasma. Cutting with the plate submerged 50–75 mm under water drastically cuts noise, UV, and fume — common on large CNC tables for stainless and aluminum.

Plasma vs other thermal & abrasive cutting

	Plasma	Oxy-fuel	Laser (fiber)	Waterjet	Wire EDM
Cutting mechanism	Melt + blow (electric arc)	Oxidation + melt (flame)	Melt/vaporize (light)	Erosion (abrasive + water)	Spark erosion
Materials	Any conductive metal	Ferrous (steel) only	Metals + some non-metals	Anything	Conductive only
Sweet-spot thickness	6 to 50 mm	10 to 300+ mm	0.5 to 25 mm	1 to 200 mm	0.1 to 300 mm
Kerf width	1 to 4 mm	1 to 5 mm	0.1 to 0.4 mm	0.5 to 1.5 mm	0.02 to 0.4 mm
Edge squareness	3 to 5° bevel (HD <2°)	~2° bevel	Near square	Slight taper	Excellent
Cut speed (10 mm steel)	~2 to 3 m/min	~0.4 to 0.6 m/min	~1.5 to 3 m/min	~0.3 to 0.8 m/min	~2 to 5 mm/min
Heat-affected zone	Moderate	Large	Small	None (cold cut)	Tiny recast layer
Capital cost	Low to medium	Lowest	High	High	Medium to high

Real-world specs and consumable economics

System class	Current	Pierce / cut capacity (steel)	Typical use
Handheld / hobby air plasma	20 to 45 A	Cut ≤12 mm, sever ≤16 mm	Repair, farm, light fabrication
Light industrial	60 to 105 A	Cut ≤25 mm, pierce ≤16 mm	Job shops, ductwork, art tables
CNC mid-range	130 to 200 A	Cut ≤38 mm, pierce ≤25 mm	Structural steel, general fab
Heavy industrial	300 to 400 A	Cut ≤50 mm, pierce ≤38 mm, sever ≤80 mm	Shipyards, heavy plate, pressure vessels
Special high-current	600 to 1000 A	Sever 150 to 160 mm	Scrap reclamation, mill gouging

The hidden running cost is consumables. The hafnium- or zirconium-tipped electrode and the copper nozzle erode every time the arc starts and stops. The hafnium emitter recedes a fraction of a millimetre per pierce; once the pit exceeds ~1.5 mm depth the electrode is scrap. A mid-range CNC torch consumes an electrode-and-nozzle set roughly every few hours of arc-on time, and each set can cost a few to several dollars. Because most wear happens at arc start and stop, a nesting program that minimizes pierces (chaining contours, using common cut lines) directly lengthens consumable life — a real-money optimization, not a nicety. Gas is the other line item: shop air is nearly free, but oxygen, nitrogen, and argon-hydrogen mixes add cost and need clean, dry, regulated supply.

Standoff height is tightly controlled in CNC: the torch rides at a fixed height (often via an arc-voltage height control, since arc voltage rises as the gap grows) typically 1.5–4 mm above the plate. Too low and the nozzle double-arcs and dies; too high and the cut bevels and loses energy.

Where plasma cutting is used

• CNC cutting tables. The dominant use. A gantry drives the torch over a slat or water table, cutting nested parts from sheet and plate at production speed. Pairs with arc-voltage height control and often a marking/etching mode.
• Shipbuilding and heavy structural steel. 300–400 A systems slice 25–50 mm plate for hulls, bridges, and pressure-vessel blanks faster than oxy-fuel and on stainless where oxy-fuel can't go.
• HVAC and ductwork. Fast cutting of thin galvanized and stainless sheet; air plasma's speed on thin material is its strong suit.
• Gouging. A flatter torch angle and a gouging nozzle let plasma carve out weld defects, remove old welds, or back-gouge a root pass — replacing carbon-arc gouging with less smoke and noise.
• Demolition, scrap, and field repair. Portable inverter units cut rebar, beams, and machinery on site. High-current systems chop scrap for recycling.
• Robotic 3D plasma. A torch on a 6-axis robot bevel-cuts pipe ends and structural members for weld prep, where the controlled bevel that's a defect on flat plate becomes a feature.

Common failure modes and pitfalls

• Dross on the underside. The most common complaint. Low-speed dross (rounded, easily knocked off) means cutting too slowly; high-speed dross (thin, hard bead on the top edge) means too fast. The fix is matching cut speed, current, and standoff to the thickness chart — there is a sweet-speed window for each combination.
• Excessive bevel. Some bevel is inherent to the swirling arc (the "square" side is to the right of cut direction). Excess bevel signals wrong standoff, a worn nozzle, or cutting too fast/slow. HD plasma and proper consumable replacement bring it back in spec.
• Double-arcing. If the torch dips too close to the plate or the nozzle is worn, the current finds a second path — electrode-to-nozzle and nozzle-to-work — instead of one clean transferred arc. It instantly melts the nozzle bore. Arc-voltage height control and not letting the torch crash prevent it.
• Consumable burn-out from bad start/stop. The hafnium emitter erodes mostly at arc ignition and termination. A worn-electrode "ramp-down" gas post-flow protects the emitter as the arc dies; skipping it, or hot-starting on a worn electrode, blows the consumable early.
• Fume, UV, and noise. Plasma generates intense UV (welding-shade eye protection required), ozone, metal-oxide fume (hexavalent chromium when cutting stainless — a regulated carcinogen), and noise above 100 dB at high current. Water tables, downdraft extraction, and shade-rated PPE are not optional in production.
• Trying to cut non-conductors. Plasma needs the workpiece in the transferred-arc circuit. Aim it at glass, stone, or plastic and there's no return path — no cut. (Non-transferred plasma exists for spraying, but not for cutting these.)