Plasma Arc Cutting: Equipment, Setup, and Process Principles

What Plasma Arc Cutting Is

Plasma arc cutting (PAC) uses a constricted, high-temperature column of ionized gas to melt and expel metal from a kerf. The word plasma refers to matter in a fourth state—beyond solid, liquid, and gas—in which gas molecules are stripped of electrons and become electrically conductive. In a plasma torch, a pilot arc or high-frequency spark ionizes the gas passing through a small orifice (the nozzle). The resulting plasma jet reaches temperatures in the range of tens of thousands of degrees Fahrenheit, far exceeding the melting point of any structural metal. That extreme heat melts the base metal, and the high-velocity gas stream blows the molten material out the bottom of the kerf.

Unlike oxy-fuel cutting, plasma cutting does not rely on an oxidation reaction. It is therefore capable of cutting any electrically conductive metal, including stainless steel, aluminum, copper, and carbon steel.

Major Components of a Plasma Cutting System

Power supply. A plasma power supply converts shop AC current to a smooth DC output. The output polarity is typically DC electrode negative (DCEN): the torch (electrode) is negative and the workpiece is positive. Machines in the 40–60 A class used in training shops can sever mild steel up to about 5/8 in. and are suitable for stainless steel and aluminum in the thicknesses covered in this course. The power supply also contains the pilot-arc circuit, the gas solenoid, and the control logic.

Torch. The torch body routes current, gas, and sometimes cooling water to the cutting tip. For air-plasma units in the training shop, the torch is air-cooled. Inside the torch, the electrode (typically a copper body with a hafnium or tungsten insert) is centered within the nozzle. The gas flows around the electrode, is constricted by the nozzle orifice, and exits as the plasma jet.

Consumables. The electrode and nozzle are wear items and must be inspected and replaced regularly. A worn hafnium insert produces a ragged, off-angle arc that degrades cut quality and can damage the nozzle. A damaged nozzle orifice widens the kerf and increases dross. The swirl ring (or gas distributor) meters gas flow and promotes a rotating gas pattern. Some torches also use a shield cap that protects the nozzle and can improve cut quality at lower speeds.

Gas supply. Shop-class air-plasma units use clean, dry compressed air at the pressure specified by the manufacturer—typically in the range of 60–100 psi (consult the operator manual for the exact value for your unit). Industrial systems may use argon-hydrogen or nitrogen for stainless steel and aluminum to reduce oxidation and produce a cleaner cut face. For training purposes, air is used for all three materials in this course.

Kerf and Cut Quality

The kerf is the slot left by the cutting arc. Plasma kerfs are narrow but wider than laser kerfs. Cut quality is described by squareness of the cut face, surface roughness, and the presence or absence of dross (also called slag)—solidified molten metal that adheres to the bottom edge of the cut. Several variables control cut quality:

• Amperage. Higher amperage increases heat input and cutting capacity. Running too low an amperage for the material thickness leaves uncut metal or heavy dross. Running too high can widen the kerf and increase the heat-affected zone (HAZ).
• Travel speed. Too slow a travel speed produces excessive dross on the bottom edge and a wide HAZ. Too fast a speed produces a beveled or incomplete cut and heavy top-edge rounding. The manufacturer's cutting chart lists a recommended speed range for each material type and thickness at a given amperage.
• Torch standoff (arc voltage). On units with voltage control, arc voltage is a proxy for the distance between the nozzle and the work surface. Correct standoff keeps the plasma column focused. If a unit does not display voltage, standoff is set by hand at the distance specified in the manual.
• Gas pressure. Insufficient pressure starves the arc and causes erratic cutting. Excessive pressure can extinguish the plasma column.

Reading the Manufacturer's Cutting Chart

Every plasma unit ships with an operator manual that includes a cutting-parameter chart organized by material type, material thickness, amperage setting, gas pressure, and travel speed range. To use the chart correctly:

1. Identify the material type (carbon steel, stainless steel, or aluminum).

2. Find the row that matches the material thickness.

3. Read across for amperage, gas type/pressure, and travel speed.

4. Set the machine to those values before striking the arc.

5. Make a test cut on scrap of the same material and thickness; adjust travel speed within the listed range to eliminate dross.

Comparing Plasma and Oxy-Fuel Cutting

Oxy-fuel cutting (OFC) relies on preheating carbon steel to ignition temperature, then directing a pure oxygen stream to oxidize the metal. It cannot cut stainless steel or aluminum because those metals form refractory oxide layers. Plasma cuts all conductive metals. OFC equipment costs less up front; plasma consumables add ongoing cost. Plasma produces a narrower HAZ than OFC, which matters when welding procedures limit HAZ width or when distortion must be controlled—a point relevant to AWS D1.1 structural work and ASME Section IX weld qualification where base-metal condition affects test results.

Safety

• Eye protection. Use a lens shade recommended by the manufacturer for the amperage in use; AWS Z49.1 provides shade guidance. For 40–60 A plasma, a shade 8–9 is a common starting point—verify with your manufacturer's manual.
• Hearing protection. Plasma cutting generates significant noise. Wear earplugs or earmuffs rated for the exposure level.
• Fume and gas hazards. Ensure local exhaust ventilation or general ventilation is adequate. Cutting galvanized, coated, or plated metals produces toxic fumes; additional precautions and respiratory protection are required.
• Electrical hazard. The open-circuit voltage of plasma power supplies can be lethal. Never touch the electrode or nozzle while the unit is energized. Follow lockout/tagout (LOTO) procedures before changing consumables.
• Fire and burn hazard. The plasma jet and ejected dross ignite combustibles. Clear the work area; use a fire-resistant welding blanket where needed.
• Compressed gas. Inspect air hoses and fittings for damage before each use.