How Does a CNC Plasma Machine Work?
A plasma cutter is a controlled electrical arc. The arc turns gas into a 16,000 °C jet, and the CNC moves that jet along a toolpath. This page walks through the five steps of CNC plasma machine work, the settings that matter, and where the process stops being the right choice for a part.
Key takeaways
What happens inside the torch
A plasma torch holds three parts that matter: a hafnium or tungsten electrode, a copper nozzle with a small bore, and a swirl ring that spins the gas stream. Gas enters at 4 to 8 bar, gets swirled around the electrode, and exits through the nozzle bore, typically 0.9 to 1.8 mm across. That bore sets the jet diameter, which in turn sets the kerf width.
To start, a pilot arc jumps between electrode and nozzle. On high-frequency start machines this happens at roughly 5,000 to 10,000 V. The pilot arc ionizes the swirling gas and a conductive plasma column forms. Once the torch is close enough to the workpiece, usually 1.5 to 3 mm, the arc transfers to the plate and the pilot circuit drops out.
The transferred arc runs at 20 to 200 A depending on plate thickness. It heats the gas in the nozzle to roughly 16,000 °C. The gas cannot stay a gas at that temperature. It becomes plasma, expands hard, and leaves the nozzle at close to the speed of sound. The workpiece never gets that hot overall. The heat is concentrated in a spot a few millimetres wide.
That is the whole trick. Local melting plus a fast gas jet equals a cut. Everything else on the machine exists to place that spot accurately and move it at the right speed.
- 1Electrode wear sets consumable lifeHafnium electrodes on air plasma typically last 300 to 1,000 pierces.
- 2Nozzle bore controls kerfA worn nozzle widens the kerf and adds dross on the bottom edge.
- 3Gas choice changes cut chemistryOxygen gives the squarest edge on mild steel. Nitrogen suits stainless.
How the CNC side drives the torch
The CNC part of CNC plasma machine work is simple in principle. A controller reads G-code and commands three axes. X and Y position the torch over the plate. Z sets the standoff between nozzle and workpiece. On a typical 1,500 × 3,000 mm table, rapid moves run 10 to 25 m/min and cutting moves run 1 to 8 m/min.
The G-code comes from CAM software. You import a 2D DXF or a flat pattern, choose a lead-in, choose cut direction, and post it. The post processor handles the machine-specific details: pierce delay, arc voltage, and the height control loop that keeps the standoff constant while the plate moves under the torch.
Arc voltage height control is the part most people underestimate. As the torch cuts, the plate warps from heat. Without active height control the standoff drifts, the arc voltage changes, and the cut either loses penetration or gouges. A closed loop reads arc voltage 100 to 200 times per second and trims Z to hold it within about ±0.2 V.
Nesting matters too. Parts are arranged on the sheet with shared cut lines where possible. A good nest on 1,500 × 3,000 mm plate can push material utilization from 65 percent to over 80 percent.
- 1Pierce delay, not pierce powerOn 12 mm mild steel, hold the pierce 0.5 to 1.5 s before starting the move.
- 2Lead-ins off the finished edgeUse a 3 to 6 mm arc lead-in so the pierce crater sits in scrap.
- 3Cut direction affects the square sideOn a clockwise outside profile, the right-hand kerf wall is squarer.
What it cuts well and what it does not
Plasma needs an electrically conductive workpiece. Mild steel, stainless steel, aluminium and copper all cut. Thickness ranges from 0.5 mm sheet up to about 50 mm on a high-amperage machine, though edge quality falls off well before that. On 1 to 6 mm mild steel a well-tuned 45 A cut gives a clean top edge and minimal dross.
Above roughly 12 mm the cut face shows a visible drag angle. The top of the kerf leads the bottom because the arc cannot remove material fast enough at the bottom of a thick section. That drag is a physical limit, not a tuning problem. You can reduce it with more amperage and slower feed, but you cannot remove it.
Non-ferrous metals behave differently. Aluminium conducts heat away quickly, so it needs higher amperage for the same thickness and often leaves a rougher dross that has to be chipped or ground. Stainless with air plasma produces an oxide layer on the cut face. If the part will be welded or exposed, cut it with nitrogen or a nitrogen-hydrogen mix.
What plasma does not do: it does not hold tight tolerances, it does not produce a fine surface finish, and it does not cut non-conductive material. For those jobs, the part moves to a mill.
- 1Good fitBrackets, gussets, base plates, frame rails, guards, and weld-prep profiles.
- 2Poor fitBearing bores, sealing faces, threaded holes, and any fit under ±0.1 mm.
Reading a plasma cut edge
A plasma cut face has three zones. At the top there is a small rounded edge from the arc radius. In the middle the surface is relatively smooth and shows fine vertical striations. At the bottom the striations curve and, if the feed is too fast, the arc lags and leaves dross attached to the underside.
Feed rate is the main dial you turn. Too fast and the arc cannot keep up, so the bottom corners round off and dross sticks. Too slow and the kerf widens, the heat-affected zone grows, and the top edge washes away. The sweet spot is usually a 5 to 15 degree drag angle on the striations at the bottom of the cut.
Standoff is the second dial. Cutting at 1.0 to 1.5 mm gives a tighter kerf and squarer edge than cutting at 4 mm. But running too close risks a double arc, where the arc jumps from nozzle to plate and destroys the nozzle in seconds. Most shops run 1.5 to 3 mm as a compromise.
If the part needs a finished edge, plan a secondary operation. A 0.3 to 0.5 mm cleanup pass on a mill removes the heat-affected zone and gives you a dimension you can inspect. Plasma gets you a blank. Machining gets you a part.
- 1Dross on the bottom
- 2Top edge washed out
- 3Heavy taper on holes
Step by step: from file to finished cut
- 11. Prepare the flat patternExport a clean DXF with closed polylines and no duplicate lines. Add the kerf compensation in CAM, not in CAD, so you can change it per material. For 3 mm mild steel expect a 1.5 mm kerf, for 12 mm expect 2.5 to 3 mm.
- 22. Nest and set lead-insArrange parts to share cut lines. Place pierce points in scrap material, never on a finished edge. Use a 3 to 6 mm arc lead-in and a 1 to 2 mm lead-out on thick plate to avoid a divot at the end of the cut.
- 33. Set amperage and gasMatch amperage to thickness: about 30 to 45 A for 1 to 6 mm, 60 to 80 A for 8 to 12 mm, 100 to 130 A for 16 to 25 mm. Use oxygen on mild steel for the squarest edge, nitrogen on stainless, and air only when edge chemistry does not matter.
- 44. Set pierce height and delayPierce at 3 to 6 mm above the plate, not at cutting height. Hold the pierce 0.5 to 1.5 s on 12 mm steel, longer on thicker plate. Piercing too close blows molten metal back into the nozzle and shortens its life by half.
- 55. Set cut height and start the arcDrop to 1.5 to 3 mm cutting height after the pierce. Enable arc voltage height control and set the target voltage from the cut chart. Watch the first 200 mm of the cut. If the arc voltage wanders more than ±2 V, the plate is warping or the height control needs tuning.
- 66. Dial in feed rateStart at the chart value, then adjust. Aim for a 5 to 15 degree drag angle on the striations. If dross sticks to the bottom, increase speed by 10 percent. If the top edge washes out, decrease speed by 10 percent.
- 77. Inspect and deburrCheck kerf width, dross, and taper on the first part. Chip light dross with a chipping hammer or run a 0.3 to 0.5 mm cleanup pass on a mill for any face that has to fit. Remove the heat-affected zone if the part will be welded.
Plasma cutting versus milling for the same part
Judge the process by the feature, not by the part name.
| Feature or requirement | Plasma | CNC milling | Verdict |
|---|---|---|---|
| Plate 1 to 6 mm, flat profiles | Fast, low cost per part | Slower, more setup | Plasma wins |
| Tolerance on an outside profile | Around ±0.5 mm | Down to ±0.005 mm | Mill for tight fits |
| Holes under 1.5× thickness | Tapered, out of round | Drilled or bored clean | Mill or drill |
| Surface finish on the cut face | Ra 6.3 to 12.5 μm | Ra 0.8 to 3.2 μm | Mill for sealing faces |
| Heat-affected zone | 0.1 to 0.5 mm deep | None from cutting | Mill if welding critical |
| Cutting 50 mm plate | Possible with 200 A | Not practical | Plasma wins |
| Threaded holes | Not possible | Tapped in setup | Mill only |
| Prototype quantity of 1 | Very low setup | Higher setup, tighter result | Depends on the fit |
When to choose plasma, when to choose milling
Use plasma for flat profiles in plate up to 12 mm where the tolerance is loose and speed matters. Move the part to a mill as soon as a feature needs a fit, a thread, or a sealing face.
Questions engineers ask next
Can plasma hold ±0.1 mm if I slow the cut down?
No. The limit comes from the kerf width and the arc's natural taper, not from speed. Slowing down widens the kerf and grows the heat-affected zone, which usually makes the part worse.
If a feature needs ±0.1 mm, cut it oversize with plasma and finish it on a mill. A 0.3 to 0.5 mm cleanup pass is normally enough.
How thick can a CNC plasma machine cut?
Production machines with 200 A power supplies cut up to about 50 mm mild steel. Edge quality drops sharply past 12 to 16 mm, and the cut face shows a noticeable drag angle.
Most shop work sits between 1 mm and 12 mm. That range gives the best balance of speed, edge quality and consumable life.
Why does my cut have dross on the bottom edge?
Low feed rate is the usual cause. The arc lingers and the molten pool re-solidifies on the underside instead of being blown clear.
Raise the feed rate about 10 percent and check the standoff. If the plate is thick and cold, preheat is not the answer. Increase amperage instead.
Do I need oxygen or can I use compressed air?
Compressed air is the cheapest option and works fine for brackets and guards where the edge chemistry does not matter. It leaves a nitrided layer on the cut face.
For mild steel that will be welded or painted, oxygen gives a squarer edge and less dross. For stainless, use nitrogen to avoid an oxide layer that has to be ground off.
How often do consumables need replacing?
On air plasma a hafnium electrode lasts roughly 300 to 1,000 pierces depending on thickness. Nozzles last longer if you pierce at the correct height.
Piercing too close is the fastest way to kill a nozzle. Keep the pierce height at 3 to 6 mm and move to cutting height only after the arc transfers.
Can plasma cut holes I can tap later?
You can cut a pilot hole with plasma, but not a tapping-size hole. The taper means the hole is conical, and the pitch diameter will not be consistent.
Cut the hole undersize by 1 to 2 mm, then drill or interpolate it on a mill before tapping.
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