What Is CNC Plasma Cutting?
What is CNC plasma cutting? It is a thermal process where a constricted arc melts conductive plate while a CNC table moves the torch along programmed paths. This page covers the arc physics, the gas and height settings that decide edge quality, and the part types where milling beats cutting.
How the arc forms and why standoff decides quality
A plasma torch starts with a pilot arc between the electrode and the nozzle. Once the torch is close enough to the workpiece, the arc transfers to the plate and the nozzle constricts it into a narrow column. Compressed gas blows through that column, gets ionized, and reaches roughly 15,000–30,000 °C. The metal does not burn in the usual sense; it melts, and the gas jet pushes the molten material out of the kerf.
The CNC side is motion. A controller reads G-code from a CAM nesting file and drives X, Y and often Z axes, holding torch standoff inside a narrow window. A change of 1 mm in standoff changes the kerf width and the bevel noticeably, which is why arc voltage feedback on the Z axis is not optional on plate thicker than about 6 mm.
Cut direction adds a second variable. The plasma arc swirls as it leaves the nozzle, so one side of the kerf ends up squarer than the other. CAM software assigns cut direction per contour: holes run clockwise, outside profiles run counter-clockwise on most mild steel setups. Get this wrong and a 12 mm hole comes out tapered.
Consumables set the practical limit. Electrode and nozzle wear widens the kerf, raises dross, and eventually causes a hard misfire. On a production table, tracking pierce count and replacing consumables on a schedule beats waiting for a bad cut.
Gas choice and amperage for common plate
The gas does two jobs: it shields the molten pool and it drives the ejection. For mild steel, compressed air is the usual choice below about 12 mm because it is cheap and cuts fast. Oxygen gives a sharper top edge and less dross on thicker mild steel, but it costs more per part and the cut face oxidizes harder.
Stainless and aluminum behave differently. Air cutting leaves an oxide layer on stainless that has to be ground off before welding, so shops running stainless usually switch to nitrogen, sometimes with a nitrogen or water shield. Aluminum cuts well on air or nitrogen; the edge is rough but clean up is easy.
Amperage tracks thickness in a roughly linear way. A 45 A setup handles 6–10 mm mild steel, 105 A covers around 20–25 mm, and 200 A and above is where 40–50 mm plate becomes practical. Cutting speed is not a free variable: too slow overheats the top edge and dumps dross on the bottom, too fast leaves a heavy bevel and a ragged drag line.
Piercing thick plate is its own problem. A pierce on 25 mm steel can blow a crater 10 mm across, so CAM places pierces off the part outline where possible, or ramps in from the scrap side. Holes under roughly 1.5 times the plate thickness are usually better drilled than cut.
Where plasma fits and where it does not
Plasma earns its place on plate and sheet that will be welded or bolted afterward. Brackets, gussets, base plates, chassis rails, flanges, ducting, and gaskets all come off a plasma table fast and with a cut face that a welder can work with directly. A 1,500 × 3,000 mm nest with thirty parts runs in a few minutes once the lead-ins are set.
The limits are dimensional. A well-tuned plasma cut holds roughly ±0.5 mm on thin sheet and worse as thickness climbs; the kerf tapers a few tenths of a millimeter per side. Holes come out slightly conical. If a drawing calls for ±0.05 mm, or a hole that must accept a dowel pin, plasma is the wrong process and the part belongs on a mill.
Heat is the other boundary. Plasma leaves a heat-affected zone a few tenths of a millimeter deep, plus a recast layer at the cut edge. On mild steel this rarely matters. On hardened or highly stressed parts it can, and on some aerospace alloys the edge has to be machined back before service.
Thin sheet reverses the picture. Below about 1 mm, heat input distorts the sheet faster than the table can move, and laser or waterjet gives a flatter part with a tighter kerf. Plasma stays economical from roughly 3 mm upward.
Edge condition, dross and secondary operations
A good plasma cut has a square top edge, a slight bevel on the lower third, and a light, granular drag line at the bottom. That is normal and does not need fixing on most welded assemblies. What you do want to remove is hard dross, the blobs that stick to the bottom edge and break off under a chipping hammer.
Dross forms from too much heat, too slow a travel speed, or a standoff that drifted high. Raising speed in small steps and checking the bottom edge after each pass is the fastest way to dial it in. On stainless, dross often means the gas mix is wrong rather than the speed.
Secondary operations add time but not much cost. A quick pass with a grinder or a stroke sander removes dross and the recast layer on a weld prep. Tumbling or bead blasting is common for brackets that will be painted, and it also hides the drag line if the part is visible.
If the part needs a functional surface, a flat face, or a bore, plan the sequence so plasma does the rough outline and a mill does the critical features. Cutting the profile first and machining afterward keeps the setup simple, because the plasma edge is already close to net shape. A machine shop with both processes under one roof avoids the double-shipping problem.
Plasma against the other cutting and machining options
Tolerance and edge figures are typical for well-tuned equipment, not guarantees.
| Process | Typical thickness | Edge and tolerance | Best for |
|---|---|---|---|
| CNC plasma cutting | 3–50 mm plate | ±0.5 mm, kerf 1.5–4 mm | Brackets, base plates, weld prep |
| Laser cutting | 0.5–20 mm sheet | ±0.1 mm, kerf under 0.5 mm | Thin sheet, tight outlines |
| Waterjet | 1–150 mm, any metal | ±0.1 mm, no heat | Heat-sensitive, thick plate |
| Oxy-fuel cutting | 20–300 mm steel | ±1.5 mm, wide kerf | Very thick mild steel only |
| CNC milling | Any solid stock | ±0.005 mm | Bores, faces, tight holes |
Pick the process before you pick the machine
If the part is plate that gets welded and ±0.5 mm is enough, cut it on plasma and move on. If the drawing carries a bore, a flat face, or anything tighter than ±0.05 mm, cut the profile on plasma and machine the critical features on a mill in the same shop.
Questions engineers ask next
Can plasma cut non-ferrous metal?
Yes, if the material conducts electricity. Aluminum and stainless cut cleanly on air or nitrogen, and copper and brass cut too, though they need higher amperage and the edges oxidize fast.
The catch is the cut face. On stainless the air-cut edge carries an oxide that has to be removed before welding, so specify the gas mix in the quote if the part is a weldment.
Why does my hole come out tapered?
Kerf taper is built into the process. The arc is wider at the bottom of the cut than at the top, so every hole is slightly conical. Cut direction and the correct lead-in radius reduce it, they do not remove it.
If the hole must be cylindrical, cut it undersize and ream or drill it. Holes below about 1.5 times the plate thickness are usually cheaper to drill than to cut.
How thick can a CNC plasma table cut?
On mild steel, 200 A and above handles roughly 40–50 mm, and thicker plate is possible with a larger power supply and slower travel. Stainless and aluminum top out lower, typically around 30–40 mm.
Above roughly 20 mm the cut face bevels more and the pierce crater grows, so shops often switch to oxy-fuel or waterjet for that range. The right answer depends on the edge quality the drawing actually needs.
Does the heat-affected zone matter?
For welded structures, usually not. The HAZ on mild steel is a few tenths of a millimeter deep and the weld procedure covers it.
For hardened or high-stress parts it does. The recast layer at the cut edge can initiate cracks, so the edge gets machined back. Tell the shop if the part sees fatigue loading.
What file does a plasma table need?
A 2D DXF or a nested CAM file with cut paths, lead-ins, and pierce points. The CAM step is where cut direction, kerf compensation, and pierce placement get decided.
Sending a 3D model alone usually means the shop rebuilds the flat pattern. A flat DXF with the material and thickness noted gets a quote back fastest.
Can plasma replace milling for a prototype?
For flat plate parts, often yes. A bracket that would take an hour of setup on a mill can be cut in minutes, then finished with a light grind.
It stops being a replacement as soon as the part needs a bore with a tolerance, a flat mating face, or a thread. Those features belong on a machining center, and mixing both processes in one shop keeps the part moving.
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