Guidelines for basic CNC plasma
This page explains how a basic CNC plasma cut is actually formed, what the arc does to the cut face, and where the process stops being economical. It is written for design engineers and buyers who need to judge a plasma-cut part before it goes to a machining supplier.
Key takeaways
How a basic CNC plasma cut is formed
A plasma torch strikes an arc between a tungsten electrode and the workpiece. Compressed gas crosses that arc, heats to roughly 20,000 °C, and turns into ionized plasma. The plasma jet melts a narrow band of metal, and the same high-velocity gas stream pushes the molten material out the bottom of the plate. What is left is a cut edge with a thin recast layer and a heat-affected zone, not a machined face.
The CNC part only handles motion. A controller reads the part program and drives the torch along the toolpath at a set travel speed while holding a set standoff. Torch height control adjusts that standoff continuously, because plate is never perfectly flat. If the standoff drifts, arc voltage drifts with it, and the cut width changes along the part.
That is the whole process in plain terms. Heat melts, gas removes, motion defines shape. Everything that goes wrong later comes from one of those three steps being out of range: not enough heat, not enough gas flow, or the wrong travel speed for the thickness.
For a design engineer, the practical consequence is simple. Plasma gives you a fast, low-cost profile cut in conductive plate. It does not give you a finished mating surface, a sharp internal corner, or a tight hole. Those features need a second operation.
- 1Conductive metals onlyCarbon steel, stainless, aluminum, copper and brass cut. Plastics and composites do not.
- 2Recast layer is normalA thin re-solidified skin sits on the cut face and may need grinding before welding or coating.
- 3Heat goes into the partThin plate can distort; thicker plate holds shape better but cuts slower.
Kerf, taper and hole quality in basic CNC plasma
Kerf is the width of material the arc destroys. On a 20–40 A entry-level system cutting 3 mm steel, expect roughly 1.0–1.5 mm. On a 100–200 A industrial system cutting 12–25 mm plate, kerf runs about 3–5 mm. These are allowances you build into the nesting, not errors you can tune away.
Taper is the second geometry effect. The top of the cut is wider than the bottom because the arc loses energy as it travels through the plate. A typical bevel angle is 2–5° per side on thicker material. On a 20 mm plate that can shift the bottom edge by more than 1 mm relative to the top, which matters if the part nests against another part.
Holes are the weakest feature. In basic plasma, a hole smaller than about 1.5× the plate thickness usually comes out out-of-round or tapered, and the entry side has a pronounced rounding. A common shop rule is to plasma-cut holes at 60–80% of finished size, then drill or ream to final diameter.
Internal corners behave the same way. The arc has a finite radius, so a sharp 90° internal corner becomes a small radius of roughly half the kerf width. If the drawing calls for a sharp corner, the plasma cut is a roughing step, not the finished geometry.
- 1Add kerf to your nestingPart-to-part gap should be at least one kerf width plus 1 mm for reliable separation.
- 2Pierce away from the edgeStart the pierce at least 3–5 mm inside the scrap side, never on the finished profile.
- 3Lead-in length mattersUse a lead-in roughly equal to the plate thickness so the arc is stable before it reaches the part.
Gas, amperage and speed ranges for basic CNC plasma
Amperage sets how much metal you can melt. A 20–40 A system handles up to about 10 mm steel in a single pass. A 100–200 A industrial system cuts 25–40 mm steel, and high-definition plasma pushes further on the same thickness with a tighter kerf. Always check the rated capacity for the specific material, because stainless and aluminum behave differently from carbon steel.
Gas choice depends on the metal. Compressed air is the standard for carbon steel and is the cheapest option. Stainless and aluminum are usually cut with nitrogen to limit oxidation on the cut face. Some shops use an air/nitrogen mix or oxygen for specific thickness ranges. The wrong gas does not stop the cut; it just leaves a rougher, more oxidized edge that costs more to clean.
Travel speed is the variable operators adjust most. Too slow and the arc overheats the kerf, producing heavy dross on the bottom and a wide heat-affected zone. Too fast and the arc cannot punch through, leaving a beveled edge and an incomplete cut on the bottom. The correct speed produces a slight lag angle and a clean, nearly dross-free bottom edge.
Airstrike or pilot arc starting, gas pressure and consumable condition round out the picture. Worn nozzle and electrode change the arc shape and shift kerf width, so consumables are a wear item with a real effect on part-to-part consistency.
- 1Air for carbon steelCheapest and most forgiving on mild steel from 3 to 20 mm.
- 2Nitrogen for stainlessLimits oxidation so the cut face cleans up faster before welding.
- 3Replace consumables on scheduleA worn nozzle widens kerf and increases dross long before the cut fails.
Where basic CNC plasma stops and machining starts
Plasma is a plate process. It cuts sheet and plate from roughly 1 mm up to 40 mm or more, depending on the power source. It does not cut solid bar, thick sections, or complex 3D geometry. If your part is a machined housing with bores, pockets and threaded holes, plasma is at best a blanking step before the part goes onto a mill.
Tolerance is the clearest boundary. General plasma work holds roughly ±0.5 mm on a good machine with a clean setup. A CNC mill holds ±0.005 mm. That is two orders of magnitude apart. Any feature with a tolerance band tighter than ±0.2 mm, or a surface finish requirement finer than Ra 3.2 μm, needs a machining operation after the plasma cut.
Edge finish is the second boundary. A plasma cut edge is typically Ra 6.3 μm or rougher, with a recast layer and possible dross. If the part will be welded, that is often acceptable after light grinding. If the edge is a sealing surface, a slide fit, or a visible face, it needs to be milled or finished.
Cost and speed still favor plasma for the right part. On a 10 mm A36 bracket with a simple profile and generous tolerances, plasma is far faster and cheaper than milling the same outline from plate. On a bracket with a ±0.05 mm bore pattern, plasma blanks the shape and the mill finishes the features.
- 1Use plasma for the outlineRough blanking of plate profiles, gussets, base plates and weldments.
- 2Use machining for the featuresBores, slots, threads, flat mating faces and anything with a tight tolerance.
- 3Combine both when it paysPlasma blank plus CNC finish is often cheaper than milling from solid.
Basic CNC plasma versus CNC machining
Use this to decide which process owns which feature on a drawing.
| Criterion | Basic CNC plasma | CNC machining |
|---|---|---|
| Typical tolerance | ±0.5 mm on profile | ±0.005 mm |
| Cut edge finish | Ra 6.3 μm or rougher | Ra 0.8–1.6 μm typical |
| Kerf or tool loss | 1.5–5 mm per cut path | Tool diameter, no recast layer |
| Best plate range | 1–40 mm conductive plate | Any solid, up to 4,000 mm |
| Holes under 1.5× thickness | Usually out-of-round | Drilled or bored to size |
| Internal corners | Radius of roughly half kerf | Sharp corner possible |
| Setup speed | Fast on flat profiles | Slower, more fixturing |
| Best use | Blanking and weld prep | Finished, tight-tolerance features |
The honest verdict
If the feature is a profile edge with a tolerance looser than ±0.2 mm, plasma owns it. If the feature is a bore, a thread, a flat face or anything tighter than ±0.05 mm, machining owns it. The cheapest route is usually plasma blank plus a CNC finish pass on the features that matter. Send us the drawing and we will tell you which is which.
Common questions about basic CNC plasma
How thick can a basic CNC plasma system cut?
It depends on the power source, not the CNC. Entry-level 20–40 A machines cut up to about 10 mm steel. Industrial 100–200 A machines cut 25–40 mm, and high-definition plasma pushes higher on the same thickness with a tighter kerf.
Always check the rated capacity for the specific material. Stainless and aluminum cut differently from carbon steel at the same amperage.
Why does my plasma cut have dross on the bottom edge?
Dross is molten metal that did not get blown clear. The usual causes are travel speed too slow, torch standoff too high, amperage too low for the thickness, or a worn nozzle.
Start by increasing travel speed in small steps and checking the bottom edge. If that does not clear it, check gas pressure and replace the consumables.
Can plasma cut holes to final size?
Rarely. Holes smaller than about 1.5× the plate thickness come out tapered and out-of-round, with a rounded entry edge.
The common approach is to plasma-cut the hole at 60–80% of finished diameter and then drill or ream to size. That keeps the hole round and on tolerance.
What tolerance should I expect from a plasma-cut profile?
General plasma work holds roughly ±0.5 mm on a well-maintained machine with a clean setup. Thicker plate and longer cuts widen that band.
If your drawing calls for ±0.1 mm or tighter, plan a machining operation after the plasma cut.
Does a plasma cut edge need cleaning before welding?
Usually yes. The cut face carries a recast layer, oxides and sometimes dross. Light grinding to bright metal removes most of it.
On stainless cut with air instead of nitrogen, the oxide layer is heavier and needs more attention before a quality weld.
When should I skip plasma and go straight to machining?
When the part has tight bores, threads, flat sealing faces, or a surface finish requirement finer than Ra 3.2 μm. Plasma cannot hold those features.
Also skip plasma when the part is a solid block or a complex 3D shape. Plasma is a plate process; it does not cut thick sections or contoured surfaces.
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