What Does a CNC Plasma Cutting Machine Do?
A CNC plasma cutting machine uses a constricted arc to melt metal and a gas jet to blow the molten pool out of the kerf. This page explains the arc, the cut edge, thickness limits, and where the process stops making sense for your part. Written for engineers and buyers who need to pick a cutting method, not a brochure.
How a CNC plasma cutting machine separates metal
Plasma is the fourth state of matter. A gas is heated until electrons break free of their atoms, so the gas conducts current. Inside the torch, a pilot arc forms between the electrode and the nozzle. Once the torch is close enough to the workpiece, the arc transfers to the plate and the pilot circuit drops out. The nozzle bore constricts the arc into a narrow column roughly 1–2 mm across.
That column reaches 15,000–30,000 °C at its core. Steel melts near 1,500 °C, so the arc cuts through plate by melting, not by abrasion. A secondary gas jet, flowing at high velocity around the arc, pushes the molten metal down and out of the kerf. The cut is complete before heat has time to soak far into the plate.
The CNC part controls motion, not the arc itself. G-code moves the torch along the programmed path at a set feed rate, holds the standoff height, and switches the arc on and off between contours. On a 4,000 mm × 400 mm table, that path can be 6 m long with 40 pierces, and the controller keeps torch height within ±0.2 mm of the set value.
What this means for a part: plasma cuts in two dimensions. It separates flat plate into profiles. It cannot produce a square vertical wall, a sharp internal corner, or a counterbore. Those features come later, from milling or drilling, or from a different process altogether.
What the cut edge looks like and why it matters
A plasma edge is not machined. It carries a slight bevel, typically 2–5° per side on carbon steel, with more bevel on thick plate and on the lead-out side of a contour. The top edge is sharper than the bottom edge because the arc widens as it travels through the material.
Dross is the main quality variable. On carbon steel, low dross comes from matching amperage, feed rate, and gas. Cut too slow and molten metal piles up on the bottom edge. Cut too fast and the arc lags, leaving a ragged kerf and heavy dross. On stainless and aluminum, dross is stickier and usually needs mechanical removal.
Heat-affected zone (HAZ) sits at 0.2–1.0 mm on steel under 12 mm thick. That zone is harder and more brittle than the base metal. For a bracket or a gusset, it does not matter. For a part that will be bent through a tight radius right at the cut edge, it does.
Tolerance on a plasma cut is process tolerance, not machine tolerance. A well-tuned 6 mm carbon steel cut holds ±0.5 mm on a good day, ±1.0 mm on a busy one. That is not the same as a milled ±0.005 mm feature. Design for it: leave 1–2 mm of stock where the edge will be machined later.
- 1Kerf width1.0–1.5 mm at 6 mm steel; 2–3 mm at 25 mm steel
- 2Bevel2–5° per side; reduce with a fine-cut nozzle and higher gas flow
- 3DrossLow on carbon steel with matched settings; heavy on stainless without a shield gas
- 4HAZ0.2–1.0 mm on steel under 12 mm; plan for it before bending
Which metals a CNC plasma cutter can handle
If the metal conducts electricity, plasma can cut it. That is the rule. Carbon steel is the sweet spot: fast, cheap, low dross, and consistent from 0.5 mm sheet up to 50 mm plate with a high-definition power supply. At 6 mm, a 100 A cut runs at 2,500–3,500 mm/min on a standard machine.
Stainless steel cuts cleanly but needs a nitrogen or H35 gas mix to avoid oxidation on the cut face. Without it, the edge discolors and dross adheres hard. Stainless also distorts more than steel because it conducts heat away slower. On 3 mm stainless, expect 0.5–1.5 mm of warp on a thin, long part unless you clamp it flat.
Aluminum cuts fast and cleanly, but its oxide layer is refractory. The arc has to break through that skin before it can melt the metal underneath. Thicker aluminum needs higher amperage and a slower feed than steel of the same thickness. Above 20 mm, most shops switch to waterjet for aluminum.
Copper and brass conduct heat so well that plasma struggles on anything over 6 mm. The arc heat bleeds into the plate faster than it can cut. Titanium cuts cleanly but requires a shielded gas to keep oxygen away from the hot edge. For any of these, ask whether the part really needs plasma or whether a milled profile is cheaper.
When plasma is the wrong choice
Plasma loses to laser below about 3–6 mm on steel if the part needs a fine edge, a tight kerf, or a small hole. A fiber laser cuts 1 mm steel with a 0.1–0.2 mm kerf and a near-vertical wall. Plasma at that thickness leaves a 1 mm kerf and a visible bevel. If the part is a cosmetic panel or a thin bracket with 2 mm holes, laser wins.
Plasma loses to waterjet on thick stainless, aluminum, and any part where HAZ is unacceptable. Waterjet cuts with no heat, holds ±0.1 mm, and handles 100 mm plate. It is slower and costs more per part. The trade is simple: if the metallurgy of the cut edge matters, pay for waterjet.
Plasma loses to milling when the part has 3D features. A plasma cut is a 2D profile. If the drawing shows a pocket, a step, or a chamfer, you need a mill after the cut, or you skip plasma and start from bar stock. Cutting a profile and then machining it can still be cheaper than milling from solid, but only when the profile removes most of the material.
Plasma is also not a finishing process. The cut edge will not hold a cosmetic finish without grinding, and grinding a plasma edge is slower than grinding a laser edge because there is more dross and more bevel to remove. Plan for that labor or choose a different cut method.
- 1Choose plasmaCarbon steel 6–50 mm, 2D profiles, structural parts, no fine holes
- 2Choose laserSteel under 6 mm, tight kerf, small holes, cosmetic edges
- 3Choose waterjetThick stainless or aluminum, no HAZ, ±0.1 mm on the cut edge
- 4Choose millingAny 3D feature, pocket, step, or hole tighter than ±0.2 mm
Plasma vs laser vs waterjet vs milling
Cut quality, thickness range, and cost profile for each process on steel plate.
| Process | Typical thickness | Kerf / tolerance | Best for |
|---|---|---|---|
| Plasma | 0.5–50 mm steel | 1–3 mm / ±0.5 mm | 2D profiles, structural parts, medium plate |
| Fiber laser | 0.5–20 mm steel | 0.1–0.3 mm / ±0.1 mm | Thin sheet, small holes, cosmetic edges |
| Waterjet | 1–150 mm any metal | 0.8–1.2 mm / ±0.1 mm | Thick plate, no HAZ, stainless, aluminum |
| CNC milling | Any solid | Tool-dependent / ±0.005 mm | 3D features, pockets, tight holes |
Which process to pick
Pick plasma for 2D carbon steel profiles from 6 to 50 mm where ±0.5 mm and a 3° bevel are acceptable. Switch to laser below 6 mm for fine holes and clean edges, and to waterjet when the cut edge cannot see heat or the plate is thick stainless or aluminum. If the part has any 3D feature, cut the profile with plasma and finish it on a mill, or start from solid.
Common questions
How thick can a CNC plasma cutter cut?
On carbon steel, a 100 A air plasma cuts up to about 25 mm cleanly and severs up to 40 mm. A high-definition 200–400 A system reaches 50 mm and beyond, with slower feed and thicker kerf.
On stainless and aluminum, practical thickness drops. Above 20 mm, most shops move to waterjet because plasma dross and HAZ become hard to control.
Why does my plasma cut have dross on the bottom edge?
Dross usually means the feed rate is too slow for the amperage, or the torch standoff is off. Slow travel lets molten metal pool at the bottom instead of being blown clear.
Raise the feed rate in 10% steps and check the edge. If dross persists, check the nozzle for wear, then check the pierce height. On stainless, add a shield gas or switch to a nitrogen mix.
Can a plasma cutter cut holes smaller than the kerf?
No. The arc needs a kerf width of clearance, so the smallest circle is roughly 1.5× the kerf. On a 1.5 mm kerf, that means a 2.5 mm minimum hole.
Holes that small also come out tapered and out of round. If the drawing calls for a 2 mm hole, cut a 4 mm pilot with plasma and drill to size on a mill.
Does plasma cutting harden the edge?
It creates a heat-affected zone 0.2–1.0 mm deep on steel under 12 mm. The metal there is harder and less ductile than the base plate.
For most structural parts this is harmless. If the part will be bent through a tight radius at the cut edge, remove 1 mm of stock from that edge before forming, or switch to waterjet.
How accurate is CNC plasma cutting compared to milling?
Plasma holds about ±0.5 mm on a well-tuned 6 mm steel cut, and it applies to the profile, not to features. Milling holds ±0.005 mm on the same feature.
That gap is why plasma is a first-op process. Cut the blank or profile, then machine the critical faces and holes on a mill. Trying to hold ±0.05 mm on a plasma cut is not realistic.
Can plasma cut aluminum and stainless steel?
Yes, if the metal conducts. Aluminum cuts fast but needs higher amperage per millimeter of thickness. Stainless cuts cleanly with a nitrogen or H35 gas mix to keep the edge from oxidizing.
Copper and brass are difficult above 6 mm because they draw heat away from the cut zone too fast. For those, waterjet is usually the better call.
Need a cut part checked against the process?
Send the drawing and we will tell you whether plasma, laser, waterjet, or milling is the right first operation. Quote and DFM feedback within 12 hours.
12-hour quote100% inspectionNo minimum order
More process notes from the shop
We publish setup notes, tooling trials and inspection data from the factory floor.
