CNC Plasma Cutting: What Is It and How Does the Arc Cut Metal?
CNC plasma cutting uses a computer to move a plasma torch along a programmed path while a constricted arc melts metal and gas blows the molten material out. This page explains the arc, the kerf, the heat-affected zone and the thickness limits, so you can tell whether a part belongs on a plasma table or on a mill.
How the Arc Forms and Why It Cuts
Plasma is a fourth state of matter: a gas heated until electrons break free of their atoms, leaving ions and free electrons that carry current. Inside a plasma torch, a pilot arc jumps between the electrode and the nozzle. Once the torch moves close enough to the workpiece, the arc transfers to the plate and the pilot circuit drops out.
The nozzle bore constricts the arc, so current density stays high. Compressed air, oxygen, nitrogen or argon-hydrogen flows through that constriction at high speed and becomes the plasma jet. Temperatures at the arc core reach roughly 15,000–30,000 °C, far above the melting point of any common structural metal.
Melting alone does not make a cut. The gas stream must push the molten pool out of the kerf before it resolidifies on the bottom edge. That is why gas flow rate, nozzle condition and standoff distance matter as much as current. A worn nozzle widens the jet, and the dross returns.
The cut is a thermal process, not a machining process. There is no cutter touching the plate, so hardness does not limit you the way it does with a saw. Conductivity does. The workpiece must close the circuit, which rules out plastics, glass and most composites.
- 1Pilot arcStarts the torch in air before the plate is contacted.
- 2Transfer arcCarries the cutting current once the plate closes the circuit.
- 3Constricting nozzleRaises current density and sets the kerf width.
- 4Gas jetBlows molten metal clear of the cut.
What the CNC Controller Actually Does
A CAD file defines the outline of the part. CAM software turns that outline into G-code: torch on and off commands, rapid moves between features, and the feed rate along each contour. The controller then drives the gantry or the boom to follow those coordinates.
Good CAM output does more than trace the drawing. It adds lead-in and lead-out moves so the pierce point sits off the finished edge. It sets kerf compensation, offsetting the path by half the kerf width so the part comes out to nominal size. It also sequences the cuts so the plate does not shift as internal features drop free.
Torch height control is the part that separates a clean cut from a scrappy one. Arc voltage rises with standoff distance, so the controller reads voltage and drives the Z axis to hold a constant gap, typically 1.0–2.5 mm on mild steel. Without that loop, a warping plate will pull away from the torch and the cut will taper.
Modern tables also manage pierce delay, initial pierce height and cut height as separate parameters. Piercing at full cut height blows spatter back into the nozzle. Piercing too low welds the tip to the plate. The controller handles the sequence automatically once the values are set.
- 1Kerf compensationOffsets the toolpath by half the measured kerf width.
- 2Lead-in / lead-outKeeps the pierce crater off the finished contour.
- 3Voltage height controlHolds a constant standoff as the plate warps.
- 4Pierce delayLets the arc punch through before motion starts.
Kerf, Taper and the Heat-Affected Zone
The kerf is the slot the arc removes. On a 6 mm mild steel plate cut at 100–130 A, expect roughly 1.5–2.5 mm of kerf. The top of the kerf is wider than the bottom because the arc diverges as it travels through the plate. That difference is taper, and it grows with thickness.
Taper is the reason plasma is a roughing or plate-cutting process rather than a finishing one. On a 20 mm plate the edge can lean 1–3° from vertical. If a bracket sits flat against that edge, the gap shows. If the edge is welded, the bevel is usually an advantage because it forms part of the weld prep.
The heat-affected zone is the band of metal whose microstructure the arc has changed. On mild steel it runs about 0.2–1.0 mm deep. The edge hardens slightly, and on a machined-then-plasma part that hardness will fight a HSS cutter. Cutting oversize and milling the edge removes it in one pass.
Dross is the resolidified metal clinging to the bottom edge. Slow cutting produces a heavy, weldable dross that is hard to remove. Fast cutting produces a light, brittle dross that flakes off with a scraper. Speed that is too high leaves a rounded top edge and an incomplete cut, which is worse because the part is scrap.
- 1Kerf widthGrows with current and thickness; measure it per setup.
- 2TaperWorst on thick plate and on inside corners.
- 3HAZ depthRoughly 0.2–1.0 mm on mild steel.
- 4Dross typeTells you whether speed is too low or too high.
Thickness, Tolerance and Material Limits
Handheld and light mechanized plasma torches cut from about 1 mm up to 12–20 mm. High-definition and high-amperage systems push that to 50 mm and beyond on mild steel, with 100 mm possible on specialty setups. The practical ceiling is set by your power supply, not by the material.
Tolerance is where plasma and milling part ways. A well-tuned plasma table holds roughly ±0.5 mm on thin plate and drifts to ±1.0 mm or worse as thickness climbs, mostly because of taper and thermal movement. Compare that with a CNC mill at ±0.005 mm. The two processes are not competing for the same feature.
Material choice is broad but not unlimited. Mild steel, stainless steel, aluminum, copper and brass all cut well. Aluminum conducts heat away quickly, so it needs higher current and faster travel. Stainless forms a tough oxide that needs a nitrogen or argon-hydrogen mix to keep the edge clean.
Thin sheet under about 1 mm is a poor fit. Heat input distorts it before the cut finishes, and the kerf is a large fraction of the part width. Laser or waterjet handles that range better. Plasma wins once the plate is thick enough that thermal distortion is small relative to the section.
- 1Typical toleranceAbout ±0.5 mm thin plate, ±1.0 mm thick plate.
- 2Best thickness range3–25 mm mild steel for a balance of speed and edge quality.
- 3Poor fitFoil-thin sheet and tight-tolerance finished features.
- 4Fine featuresHoles under about 1.5× plate thickness need a drill instead.
Where CNC Plasma Cutting Fits in a Shop Workflow
Plasma cutting is usually the first operation on a plate part, not the last. It blanks the outline, opens large holes and cuts the weld-prep bevels. Anything that needs a bearing fit, a sealing face or a thread moves to a CNC mill or lathe afterward.
That split is deliberate. Plasma removes metal at meters per minute across a full plate. Milling removes it at millimeters per minute along one contour. Letting plasma rough the shape means the mill only has to skim the edges, which shortens cycle time and tool wear on every part in the run.
For weldments, plasma has a second advantage: the beveled edge is already there. A 30° or 45° bevel cut straight from the nesting software replaces a separate edge-prep pass. On thick structural parts that can be the difference between one setup and three.
Nesting also matters. Because the torch path is just coordinates, parts can be packed tightly on a standard plate size with a shared cut line between them. Scrap drops, and the number of pierces drops with it, which extends consumable life.
- 1Roughing firstPlasma blanks the outline; milling finishes the fits.
- 2Integrated bevelsWeld prep comes off the table with the part.
- 3Shared cut linesCommon-edge nesting cuts pierce count and scrap.
CNC Plasma Cutting vs Laser vs Waterjet
Ranges are typical shop values for mild steel; exact numbers depend on the machine and setup.
| Criterion | CNC plasma cutting | Laser cutting | Waterjet |
|---|---|---|---|
| Mechanism | Conducted arc melts metal | Focused beam melts or vaporizes | Abrasive erodes metal |
| Comfortable thickness | 3–25 mm | 0.5–12 mm | 1–100 mm |
| Typical tolerance | ±0.5 to ±1.0 mm | ±0.1 to ±0.2 mm | ±0.1 to ±0.3 mm |
| Edge taper | 1–3° on thick plate | Small, under 1° | Essentially none |
| Heat-affected zone | 0.2–1.0 mm | 0.05–0.3 mm | None |
| Capital cost | Lowest of the three | Higher | Highest |
| Running cost | Consumables, gas | Power, assist gas | Abrasive, water, nozzle |
| Best for | Plate blanks and weld prep | Thin sheet, fine detail | Thick stock, no HAZ |
Pick the Process Before You Pick the Tolerance
If the part is 3–25 mm plate that will be welded and the edge only needs to be straight, CNC plasma cutting is the fastest and cheapest way to get there. If the edge is a finished surface or the tolerance is tighter than ±0.5 mm, cut oversize on plasma and finish on a CNC mill. Do not try to hold a bearing fit with a torch.
Common Questions About CNC Plasma Cutting
Can CNC plasma cutting hold a tight tolerance?
Not to machining tolerance. A well-tuned table holds roughly ±0.5 mm on thin plate and ±1.0 mm or worse as thickness grows, mainly because of taper and thermal expansion during the cut.
When a drawing calls for ±0.05 mm or a bearing fit, the practical route is to plasma-cut the blank oversize and finish the critical edge on a CNC mill or lathe. The plasma pass removes most of the material cheaply; the mill only has to skim the last 0.3–0.5 mm.
What gases do I need for different materials?
Compressed air is the usual choice for mild steel and gives an acceptable edge at low cost. Oxygen produces a cleaner, squarer edge on carbon steel but needs dedicated consumables.
Nitrogen suits stainless steel and aluminum because it limits oxidation. Argon-hydrogen mixes are used where edge chemistry matters, for example on stainless parts that will be welded without post-cleaning. Each gas change means a consumable change and a new cut chart.
Why is the edge beveled rather than square?
The arc diverges as it passes through the plate, so it removes more material at the top than at the bottom. On 6 mm plate the lean may be under 1°; on 20 mm it can reach 1–3°.
You can reduce taper by cutting at lower current with a smaller nozzle, or by using a high-definition system with a narrower arc. You cannot remove it entirely. If the edge must be square, plan a finishing pass.
How do I know if the cut speed is right?
Read the dross. Heavy, hard-to-remove dross on the bottom edge usually means the travel is too slow and the arc is dwelling. Light, brittle dross that flakes off with a scraper means the speed is close to correct.
Too fast gives a rounded top edge, a narrow kerf and an incomplete cut at the bottom. That is the failure mode to avoid, because the part is often scrap by the time you see it.
Can plasma cut stainless and aluminum as well as steel?
Yes, as long as the material conducts. Stainless steel, aluminum, copper and brass all cut with the right gas and current settings. Aluminum needs higher current and faster travel because it conducts heat away from the cut zone quickly.
Stainless forms a refractory oxide that resists the jet, so nitrogen or an argon-hydrogen mix is normally used instead of air. Non-conductive materials such as plastics, glass and most composites cannot be plasma cut at all.
When should I choose laser or waterjet instead?
Choose laser for sheet under about 3 mm with fine detail and tight tolerance, where the heat-affected zone is small and the edge comes off the machine nearly finished.
Choose waterjet when the material cannot take heat at all, when it is very thick, or when you need a square edge with no HAZ. Choose plasma when the plate is thick, the edge will be welded, and cost per part matters more than edge finish.
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