CNC Plasma Cleavage Explains: How a Constricted Arc Cuts Plate

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  What CNC plasma cleavage explains about the arc itself
• 2
  Gas, amperage and speed: the three knobs that decide the edge
• 3
  Where plasma holds tolerance and where it does not
• 4
  Which metals take a plasma cut cleanly
• 5
  Reading the cut edge as a process record
• 6
  When a plasma-cut part should go to the mill instead
• 7
  Plasma cutting versus milling: which route fits the part

Plasma cleavage starts with a gas pushed through a small nozzle and heated until it ionizes. The result is a fourth state of matter that conducts electricity far better than the cold gas around it. In a transferred-arc torch, the workpiece becomes part of the circuit: current leaves the electrode, passes through the constricted arc, and returns through the plate.

The constriction matters. A narrow nozzle forces the arc into a tight column, which raises current density and temperature at the same time. That column melts a narrow band of metal instead of heating a broad area. The molten metal is then blown out of the kerf by the same gas stream that formed the plasma.

Cut quality is mostly a balance of three settings. Amperage sets how much metal the arc can melt per second. Gas flow sets how cleanly the molten pool is expelled. Travel speed sets how long the arc stays in one spot. Push any one of them too far and the cut edge tells you immediately.

A CNC control turns those settings into G-code motion. The torch follows a programmed path, holds standoff height with a voltage feedback loop, and keeps travel speed constant through corners where a human hand would slow down and dump heat into the part.

Nothing here is exotic. The arc is just a controlled electrical discharge, and the cut edge is the visible record of how well that discharge was controlled.

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  Transferred arcCurrent flows through the plate, not just the torch.
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  Constricted columnA small nozzle raises current density and narrows the kerf.
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  Gas does two jobsIt forms the plasma and blows molten metal clear.

Amperage is the first knob to set. Low-current cuts on thin sheet run clean at 20–40 A, while 1 inch plate may need 200–400 A to keep travel speed reasonable. More amperage means more heat, a wider kerf, and a larger heat-affected zone. That zone is where hardness climbs and where a subsequent machining pass has to remove material anyway.

Gas choice changes the cut chemistry. Compressed air is cheap and works well on carbon steel, but it leaves an oxidized edge. Nitrogen gives a cleaner cut on stainless and aluminum. Oxygen raises cutting speed on mild steel but produces a heavily oxidized face. Argon-hydrogen mixes suit thick stainless where edge chemistry matters.

Travel speed is the setting most operators get wrong. Too slow and the arc washes back over the top edge, rounding it and building dross on the bottom. Too fast and the arc lags, leaving a jagged edge and an incomplete cut. The sweet spot is where the sparks exit the kerf at a slight forward angle, not straight down.

Standoff height is the fourth variable, and the CNC handles it. Torch height control reads arc voltage and adjusts Z to keep the nozzle a fixed distance above the plate. When that loop is mistuned, the kerf widens on one side of the part and narrows on the other.

For a given material and thickness, the torch manufacturer publishes a cut chart. Treat it as a starting point, then tune speed first, gas second, and amperage last.

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  Air or nitrogenCheap cuts on carbon steel; cleaner edges on stainless.
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  OxygenFast on mild steel, oxidized face, needs cleaning after.
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  Argon-hydrogenThick stainless where edge chemistry matters.
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  Voltage feedbackKeeps standoff constant so kerf width stays even.

Standard plasma cutting is a roughing process by machining standards. A well-tuned torch on 6 mm mild steel holds roughly ±0.5 mm on profile position, with kerf width varying by 0.2–0.4 mm along the cut. That is fine for brackets, gussets, and weld-prepped plates. It is not fine for a bearing bore.

High-definition plasma narrows the gap. A constricted nozzle with a smaller orifice and a more stable arc can hold ±0.2 mm on thin plate and produce an edge angle under 3°. Even so, the cut face carries a recast layer of 0.05–0.15 mm that is harder than the base metal and may need to be machined or ground away.

The heat-affected zone is the second boundary. On 10 mm carbon steel, HAZ depth is typically 0.5–1.5 mm depending on speed and amperage. For structural work that zone is harmless. For a part that will be cyclically loaded, it is a fatigue initiation site unless the edge is dressed back.

Taper is the third boundary. Every plasma cut has some edge angle because the arc widens as it travels through the plate. On thin material the taper is nearly invisible. On 25 mm plate it can be 5–8° per side, which changes the effective size of a hole or a slot by several tenths of a millimeter.

If the drawing calls for ±0.005 mm on a bore, plasma is the wrong first operation. If the drawing calls for a 300 mm by 200 mm plate outline with a 0.5 mm profile tolerance, plasma is the right one.

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  Kerf widthTypically 1.5–4 mm depending on amperage and nozzle.
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  Recast layer0.05–0.15 mm of hardened metal on the cut face.
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  Edge taperGrows with plate thickness; check holes after cutting.

Any metal that conducts electricity can be plasma cut. Carbon steel is the easiest and most common: it cuts fast, holds a stable arc, and tolerates air as the plasma gas. Grades like A36 and 1045 are routine, and 4130 or 4140 cut well as long as the plate is clean and dry.

Stainless steel cuts well but needs the right gas. Nitrogen or an argon-hydrogen mix keeps the edge from oxidizing, though the cut face still carries a heat tint that has to be removed before welding. Grades 304, 316, and 17-4PH all cut, but thick sections of 17-4PH can crack at the edge if the plate is not preheated or the cut is not followed by a stress relief.

Aluminum cuts fast because of its high thermal conductivity, but the oxide layer has a melting point far above the metal underneath. That mismatch produces a rougher edge and more dross than steel. Thicknesses up to about 50 mm are practical; beyond that the arc struggles to clear the kerf.

Copper and brass conduct heat away so quickly that plasma cutting becomes inefficient. Thin sheet is workable. Thick copper is usually waterjet or milled instead, because the arc cannot put heat into the cut zone faster than the material pulls it away.

Titanium cuts with plasma but demands an inert shield gas. Any oxygen at the cut face embrittles the edge, so the shielding must be complete and the cut face is typically machined back afterward.

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  Carbon steelEasiest and fastest; air works as the plasma gas.
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  StainlessNitrogen or argon-hydrogen; remove heat tint before welding.
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  AluminumFast but drossy; practical up to about 50 mm.
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  Copper and brassHeat sink effect makes thick sections impractical.

A plasma cut edge carries three zones. The top edge shows a small rounded radius where the arc entered. The middle is a near-vertical drag face. The bottom is where the arc exits, and that is where dross forms if the settings are off. The angle of the drag lines tells you whether travel speed was correct.

Dross is the most common defect. It comes in two forms. Low-speed dross is a soft, easily chipped bead that forms when the arc lingers and the molten pool is too large to blow clear. High-speed dross is a hard, tenacious bead that forms when the arc cannot keep up and the molten metal freezes before it exits.

The fix differs for each. Low-speed dross means raise travel speed or lower amperage. High-speed dross means lower travel speed or raise amperage. Adjusting gas flow without changing speed rarely solves either one.

Edge squareness follows the same logic. An excessive standoff lets the arc flare before it reaches the plate, widening the top of the kerf. A standoff that is too small risks nozzle contact and a double arc that damages both the nozzle and the part.

For parts that will be welded, a slight bevel is often acceptable because the weld fills it. For parts that will be assembled dry, the bevel has to be accounted for in the drawing tolerance or removed by a secondary machining pass.

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  Top radiusWhere the arc enters; grows with excessive standoff.
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  Drag linesTheir angle shows whether travel speed was right.
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  DrossLow-speed is soft and chippy; high-speed is hard and tenacious.

Plasma and milling solve different problems. Plasma removes metal with heat and leaves a rough, heat-affected face. Milling removes metal with a cutting edge and leaves a dimensionally accurate face. The decision is not about which process is better; it is about which one matches the drawing.

Choose plasma when the part is flat, the profile is 2D, the material is conductive, and the tolerance is looser than about ±0.2 mm. Typical work includes base plates, mounting brackets, gussets, flanges, weld-prepped outlines, and rough blanks that will be machined later.

Choose milling when the part has pockets, bores, threads, tight tolerances, or a fine surface finish. A plasma-cut blank that then goes to a 3-axis or 5-axis mill is a common and efficient combination: plasma removes the bulk of the material quickly, and the mill brings the critical features into tolerance.

There is a middle ground. For plate up to about 6 mm with profile tolerances around ±0.1 mm and no secondary features, high-definition plasma can finish the part in one operation. The trade-off is that the cut face carries a recast layer, so any part that will see fatigue loading or a sealing surface still needs a machining pass.

The rule we use on the floor is simple. If a feature has a tolerance tighter than ±0.2 mm, a surface finish requirement, or a mating function, it gets machined. If it is a flat outline that will be welded or bolted against something forgiving, plasma is faster and cheaper.

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  Plasma winsFlat 2D profiles, conductive metal, tolerance looser than ±0.2 mm.
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  Mill winsPockets, bores, threads, tight tolerances, fine finishes.
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  Hybrid routePlasma cuts the blank, the mill finishes the critical features.