UK CNC Plasma Cutter Guide

• 1
  What the arc actually does to the plate
• 2
  What changes between a hobby table and an industrial machine
• 3
  Speed, thickness and where the edge goes wrong
• 4
  Where the plasma edge meets a machining operation
• 5
  What to measure and what to put on the drawing
• 6
  Setting up a job that holds tolerance
• 7
  Which cutting route fits which part

A plasma torch forces gas through a narrow nozzle around a tungsten electrode. A pilot arc ionises that gas, and the constricted jet leaves the nozzle at roughly 20,000–30,000 °C. The jet melts a narrow channel through the plate, and the same gas flow blows the molten metal out of the bottom of the kerf. The plate never touches the electrode. Cutting depends on the workpiece closing the circuit.

Because the arc carries current through the metal, the part must be electrically conductive. Steel, stainless, aluminium, copper and brass all cut. Plastics, composites and ceramics do not, since they cannot sustain the transferred arc. If a job mixes steel plate with GFRP panels, those two families need different processes on the shop floor.

The cut edge is not a machined surface. It is a re-solidified zone with a heat-affected band behind it. On 6 mm mild steel the HAZ typically runs 0.2–0.8 mm deep, and it widens as thickness climbs. On 20 mm plate it can reach 1.0–1.5 mm. That band is harder than the parent metal and can chip a fine boring tool if you machine straight into it.

Cut quality splits into three things you can measure: kerf width, edge squareness and dross. Kerf on a 1.5 mm nozzle runs about 1.0–1.5 mm; a 3.0 mm nozzle opens it to 2.5–3.5 mm. Squareness drifts on thick plate because the jet loses energy as it travels down the kerf. Dross is the re-solidified metal clinging to the bottom edge. All three respond to speed, gas and current.

A UK CNC plasma cutter is usually described by its cutting current and its table class. Entry tables run 30–45 A on a 1,250 × 2,500 mm bed and cut up to about 6 mm. A 65–85 A air unit holds 12–16 mm. Heavy industrial machines run 130–200 A and cut 25–50 mm with a water table and a high-definition torch. The machine class decides more about your part than the brand does.

The gas package matters as much as the amperage. Compressed air is cheap and works for mild steel, but it leaves a nitrided edge that is hard and prone to rust. Nitrogen on stainless keeps the cut edge free of oxides but raises cost. Oxygen on mild steel gives the squarest edge above 12 mm, though it needs a clean nozzle and a strict pierce routine. Choose the gas for the material, not for the machine.

A water table or downdraft bench controls fume and stops thin plate from warping. Thin sheet under 3 mm lifts and distorts when heat builds in one area. Water also traps the smoke that air plasma produces, which matters for shop ventilation. On a 2,000 × 4,000 mm bed with a water table, a 6 mm sheet stays flat through a full nest.

Torch height control is the quiet differentiator. Arc voltage sensing holds the standoff at 1.0–1.5 mm through a cut. If the torch drifts up, the kerf widens and dross forms. If it dives, the nozzle touches the plate and the consumable dies. A machine with good height control outcuts a higher-amperage machine without it on anything under 20 mm.

Cut speed is the single setting that decides edge quality, and it has a real window. Run too fast and the jet cannot melt through the full section, so the arc trails backwards and leaves a heavy bevel on one side. Run too slow and the extra heat widens the kerf, rounds the top edge and drops a hard dross bead on the bottom. The good window is often only 15–20 percent of the nominal speed.

Thickness sets the ceiling. Above roughly 25 mm on air plasma, the arc loses coherence and the bottom edge starts to lag behind the top. You can still cut 40 mm with a 200 A oxygen setup, but the taper on a 20 mm hole becomes a problem. Holes under 1.5 × the plate thickness rarely hold size on a plasma table. Drill or mill those instead.

The pierce is where most consumables die. Piercing 12 mm plate at full current blows a crater and can shatter the nozzle. Shops pierce at reduced current, hold a 3–6 mm standoff and delay the move for 0.3–1.0 s. On stainless above 10 mm, a pilot pierce away from the edge and a lead-in arc of 5–10 mm protects the part and the consumable.

Consumables wear on a schedule, not on a hunch. On mild steel, a 1.5 mm nozzle typically holds tolerance for 400–800 pierces before the kerf opens past 0.2 mm. Track pierce counts and replace the nozzle and electrode as a set. A worn electrode drifts the arc, and a drifted arc is what most cut-quality complaints turn out to be.

Plasma cuts the outline. It does not produce a bearing bore, a seal face or a dowel fit. When a drawing calls out ±0.005 mm, the plasma blank becomes raw stock for a mill or a lathe. That combination is common and it works well, provided the shop leaves enough material for the cutter to get under the HAZ.

The rule we use is a 0.8–1.2 mm allowance on any plasma edge that will be machined. That is enough to remove the hardened band on 6–12 mm plate and still leave a clean surface for a finish pass. Skip the allowance and the first pass runs through a hard, abrasive skin that shortens insert life and can pull the part out of the vise.

Plasma also hardens the top edge on air-cut mild steel. A nitrided layer a few hundredths of a millimetre deep is enough to dull a HSS drill. If a hole sits close to a plasma edge, cut the hole undersize on the table and drill it after. That sequence is faster than fighting a work-hardened rim.

For welded assemblies, plasma edges are usually fine. A cut edge at Ra 12–25 μm takes MIG and TIG weld without trouble, and a light grind on the bevel removes dross before welding. If the part is an architectural face, plan a finishing step: bead blasting, brushing or powder coating will hide the kerf lines that a bare cut leaves visible.

Measure kerf on a finished part, not on the machine display. Take the difference between the programmed contour and the measured edge on three sides and average it. A consistent offset means the kerf compensation is wrong. A varying offset means speed, height or consumable wear is drifting during the cut.

Squareness is the check most drawings omit. On 12 mm mild steel, expect the top edge to be about 0.2–0.4 mm wider than the bottom on a good cut. If the drawing tolerances the side face, state it. Otherwise the shop will assume the cut face is non-critical and may ship a part you reject later.

Dross comes in two forms and they point in opposite directions. A hard, bubbly bead that needs a chisel means speed is too low or the torch is too high. A thin, easy-to-flick spatter means speed is slightly high. Both are fixable inside the same setup, so send the sample back with a photo rather than a written description.

For anything that will be inspected, ask for the cut parameters with the parts: current, gas, speed, nozzle size and pierce count at the time of the cut. A part that passes today can fail after 600 more pierces on the same nozzle. Parameter records turn a quality argument into a simple comparison.