Introduction to the CNC Plasma Cutting Machine
This page explains how a CNC plasma cutting machine forms a cut, what the plasma arc does to the plate edge, and which jobs belong on it instead of a mill. Read it if you need to judge plate parts, kerf, taper and heat-affected zone before you quote.
How a CNC plasma cutting machine makes a cut
A plasma torch works by forcing gas through a small nozzle while a pilot arc ionizes it. The ionized gas leaves the nozzle at high velocity and carries current to the workpiece. That arc melts the metal locally, and the gas jet blows the molten material out of the kerf. Nothing touches the plate except the arc, so the torch never wears against the workpiece the way a mechanical cutter does.
The CNC side is separate from the physics. The controller reads a toolpath from CAM, then drives X, Y and usually a Z height axis so the torch tip stays at a fixed standoff. Torch height control matters more than most people expect. If the standoff drifts, cut width and bevel angle drift with it, and the part measures wrong even though the machine followed the path perfectly.
Watch the amperage and gas choice. Low-carbon steel cuts well on air or oxygen, stainless and aluminum usually run on nitrogen or an argon-hydrogen mix. Amperage sets how much material the arc can melt per unit of travel. Push travel speed too high on thick plate and the arc stops cutting through; too low and the bottom edge drosses up.
The result is a through-cut with a heat-affected zone along the edge. Grains near the cut line change, hardness rises on some alloys, and a thin oxide layer forms. That edge is not a machined surface. It is a starting point for the next operation, or an acceptable finish if the drawing only calls out a profile.
What a CNC plasma cutting machine can and cannot hold
Plasma is a plate process, not a precision finishing process. A well-tuned machine holds roughly ±0.5 mm on profile dimensions for 6 mm plate, and that window widens as thickness climbs. On 25 mm steel, expect ±1.0 mm or looser on the cut edge. Hole diameters run small because the arc has width, so a 10 mm hole is often programmed at 11 mm to land in tolerance.
Kerf is the width of material the arc removes. It typically runs 1.5 mm to 4 mm depending on amperage and nozzle size. CAM has to offset the toolpath by half the kerf, and that offset changes with consumable wear. A worn nozzle widens the kerf and tilts the cut, which is why operators track consumable life instead of running nozzles until they fail.
Taper is the other limit. A plasma cut edge is rarely square. On 12 mm plate you might see 2° to 5° of bevel per side, more on thick sections. If a part needs a square edge for a weld joint or a bearing seat, plasma is the wrong first operation or it needs a secondary pass.
The heat-affected zone is narrow compared with flame cutting, often 0.2 mm to 1.5 mm, but it is still there. For most structural work that is fine. For a part that will see fatigue loading or a hardened edge, the HAZ has to be removed or the process changed.
When plasma cutting beats milling and when it does not
Choose plasma when the part is flat, the material is plate, and the important feature is the outer profile or a large internal cutout. Brackets, gussets, base plates, flanges, weldments and machine guards are typical. Setup is fast, the cut is one pass, and a 6 mm steel plate 2,000 mm long takes minutes rather than hours.
Choose milling when the part needs a machined face, a bored hole, a thread, a tight tolerance, or a square edge. A CNC plasma cutting machine cannot hold ±0.005 mm, cannot leave Ra 0.8–1.6 μm, and cannot cut a pocket with a controlled floor. Those features belong on a 3-axis or 5-axis mill.
Hybrid jobs are common in our shop. We plasma-cut a blank to near-net profile, then finish the critical faces and bores on a machining center. That removes a lot of roughing time from the mill and still lands the tolerance where the drawing needs it.
Do not plasma-cut a part that fits in a mill vise and has three or fewer setups. The handling alone eats the savings. Plasma pays off on large, flat, thin or awkward plate where fixturing on a mill would be costly.
Getting a clean edge from a CNC plasma cutting machine
Edge quality comes from four settings: amperage, travel speed, gas and standoff. Set amperage to the material thickness chart, then tune speed until the dross on the bottom is minimal and easy to knock off. A clean cut has a narrow, near-vertical top edge and a slight bevel toward the bottom.
Consumables decide repeatability. The electrode and nozzle erode with every arc start, so a shop that tracks pierce count and replaces on schedule holds tolerance better than one that replaces only after a bad cut. On thin plate, a worn nozzle shows up as a wider kerf first.
Lead-ins and lead-outs matter on profiles with tight corners. Start the pierce outside the part outline, ramp into the cut, and let the kerf close before the torch exits. Piercing directly on the part edge blows molten metal back onto the nozzle and shortens its life.
Dross, bevel and a rough edge are normal on a plasma cut. If the drawing tolerates them, leave them. If not, plan a secondary operation: grinding, machining or a different process entirely. Deciding that at the quote stage is cheaper than deciding at inspection.
CNC plasma cutting against other plate processes
Use this to pick a process, not to rank one above another.
| Process | Best for | Typical tolerance | Edge condition |
|---|---|---|---|
| CNC plasma cutting | Flat plate, profiles, cutouts | ±0.5 to ±1.5 mm | Bevel 2°–5°, light dross |
| Laser cutting | Thin sheet, fine detail | ±0.1 to ±0.2 mm | Near square, small HAZ |
| Waterjet cutting | Thick plate, no heat | ±0.1 to ±0.3 mm | Square, no HAZ |
| CNC milling | Bores, faces, tight fits | ±0.005 mm | Machined, Ra 0.8–1.6 μm |
What to do with this
If the part is flat plate with a profile tolerance of ±0.5 mm or looser, a CNC plasma cutting machine is the fast, low-cost route. If it needs a machined bore, a square edge or ±0.005 mm, plasma is the blank and the mill is the finish.
Common questions about CNC plasma cutting
What thickness can a CNC plasma cutting machine cut?
Most shop machines cut from 0.5 mm sheet up to about 50 mm steel, with the cleanest results below 25 mm. Above that, cut quality drops, taper grows and travel speed falls.
The practical limit depends on amperage and gas. A 200 A oxygen setup handles thicker steel than a 45 A air unit, but it also needs more power and a bigger table.
Can plasma cutting hold a hole tolerance?
Not a tight one. Arc width makes holes cut undersize, and taper makes the wall slightly conical. A 10 mm hole may come out at 9.2 mm and need reaming.
If the drawing calls out H7 or a bolted joint, drill or bore after plasma. If it is a clearance hole, plasma alone is usually fine.
Is the heat-affected zone a problem?
It depends on the alloy and the load. Low-carbon steel is forgiving; the HAZ is thin and rarely matters for structural parts.
On hardened or fatigue-loaded parts, the HAZ can be a crack initiation site. Grind it back or choose a cold process such as waterjet.
How does kerf affect the part size?
The toolpath must be offset outward by half the kerf width. Get that wrong and every part is off by the same amount in the same direction.
Kerf grows as the nozzle wears, so a shop that measures kerf daily and updates the CAM offset holds size better than one that sets it once.
When should a shop switch to milling?
Switch when the part needs a machined face, a thread, a reamed bore, a pocket with a controlled floor, or a tolerance tighter than about ±0.2 mm.
A hybrid route is often best: plasma the profile, then finish the critical features on a 3-axis or 5-axis machining center.
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More process notes
We publish setup notes, tooling trials and inspection data from the factory floor.
