Basic Knowledge of CNC Plasma Cutting

A plasma torch holds a tungsten or hafnium electrode inside a copper nozzle. Gas flows through the gap between them. A pilot arc strikes between electrode and nozzle, then transfers to the workpiece once the torch is close enough. This arc heats the gas past 20,000 °C, stripping electrons and turning it into plasma. The ionized stream exits the nozzle at near-sonic speed and melts the metal in its path.

Molten material is not vaporized. Most of it is blown out of the kerf by the gas flow, which is why cut quality depends as much on pressure and standoff as on current. The nozzle constricts the arc, so the cut width stays narrow. Cut speed, amperage, gas type and torch height are the four settings an operator tunes for a given plate.

Conductive metals only. Aluminum, stainless, carbon steel, copper and brass all cut. Plastics, wood, glass and most composites do not, because there is no current path for the arc to follow.

The plasma process existed before CNC. What changed was motion control. A gantry or cantilever table carries the torch on X and Y axes, and a Z axis sets pierce height and cut height. The controller reads G-code, so the torch path comes from a CAD file rather than a template.

That matters for repeatability. A hand-held torch wanders. A gantry repeats the same path to within a fraction of a millimeter across a full sheet, which is what makes nested parts and production runs practical. Kerf compensation is handled in the CAM software: the toolpath is offset by half the kerf width so the finished part lands on nominal dimension.

Standard gantry tables at our shop handle plate up to 4,000 mm on the long axis. Thicker plate cuts slower, and the controller de-rates feed rate automatically as amperage saturates.

• 1
  Pierce delayHolds the torch still until the arc punches through, typically 0.3–1.5 s on 6–25 mm plate.
• 2
  Initial heightSet from the ohmic or floating head sensor before each cut.
• 3
  Kerf compensationApplied in CAM, not at the machine.
• 4
  Lead-inKeeps the pierce mark off the part edge.

A plasma cut edge is never machined-flat. The top edge is close to square, the bottom edge shows a slight bevel, and the kerf is wider at the bottom than at the top. This is normal. Taper grows with plate thickness and with cutting speed. On 6 mm mild steel, taper is often under 3°. On 25 mm, expect more.

The cut face has a drag line where the arc exits the bottom of the plate. Dross forms as a bead or a hard film depending on gas mix. Air plasma tends to leave more dross on stainless than oxygen plasma, but oxygen plasma needs a clean, dry gas supply and is more sensitive to pierce height.

For a hole, plasma is the wrong tool below roughly 1.5× plate thickness in diameter. Smaller holes come out tapered and out of round. If you need a Ø6 mm hole in 12 mm plate, drill it or mill it after cutting. If you need a Ø20 mm hole in the same plate, plasma will hold it within about ±0.5 mm and you can ream it if the tolerance is tighter.

Keep part outlines simple and avoid sharp internal corners. A plasma arc has a finite kerf radius, so a 90° internal corner becomes a small radius whether you want it or not. Design it as R1.5 mm or larger and the part comes off the table ready to use.

Watch the heat. Plasma puts a heat-affected zone into the edge, typically 0.5–2 mm deep depending on thickness. That zone is harder than the base metal and can crack if the part is bent sharply along a cut edge. If a part will be formed, either cut it oversize and machine the bend line, or cut with the correct gas mix and accept a larger bend radius.

Long thin parts warp. A 3 mm strip cut from 6 mm plate will bow from residual stress released by the cut. If flatness matters, cut from stress-relieved plate or plan a machining pass after cutting.

Lettering and decorative panels are a good fit. Minimum stroke width should be about 1.2× the kerf, so on a 1.5 mm kerf keep strokes at 2 mm or wider. For laser marking on a plasma-cut panel, the marked character height should be at least 1.5 mm.

Plasma gives you a blank, not a finished part. Tolerances are loose by machining standards, and the cut face carries oxide and a heat-affected layer. Anything that must seal, bear load, or fit a bearing needs a secondary operation.

The usual sequence is plasma for the outline, then CNC milling for critical features. On our 5-axis and 3-axis centers we hold ±0.005 mm on milled features, so a plasma-cut bracket can be brought to drawing tolerance on the holes and mating faces without re-cutting the whole profile.

This combination is often cheaper than cutting the entire part from solid. A 12 mm steel plate bracket with a complex outline wastes most of the material if it is milled from a block. Plasma removes the bulk in seconds, then the mill does the precise work.

If the part will be coated, plasma dross and oxide must come off first. Bead blasting and tumbling handle most of it. Anodizing on plasma-cut aluminum can show a color difference at the cut edge because of the oxide layer, so we usually machine or blast the edge before anodizing.