What Plasma Cutting Actually Does

Plasma cutting is controlled lightning. Your machine forces compressed air through a very narrow nozzle, strikes an electric arc through that gas stream, and superheats it until it becomes plasma — the fourth state of matter, hot enough to melt steel almost instantly. The arc does the melting; the blast of gas does the clearing, shoving molten metal out of the kerf and leaving a narrow, clean slot behind.

The practical upshot is that you can cut mild steel, stainless, aluminium, brass and copper with the same machine, using nothing more exotic than a decent compressor. That is what makes plasma so appealing to the small workshop. An angle grinder will cut metal, but it will also wander, burnish the edges and take an age on anything thicker than 3mm. Oxy-fuel gear cuts well on thick steel but struggles with stainless and aluminium, and it wants two cylinders. Plasma sits neatly in the middle: fast, versatile, and now genuinely affordable.

The Kit You Actually Need

Modern inverter plasma cutters are light, run happily off a standard 13A supply in the smaller sizes, and have transformed what home workshops can do. As a rough guide to amperage:

• 20–30A — ideal for sheet metal up to about 4mm, bodywork, brackets and fabrication jobs.
• 40A — a sweet spot for most UK workshops. Cuts 6–8mm cleanly and will sever 10–12mm if you take your time.
• 50A and above — needs a dedicated supply and a serious compressor, but will slice through 20mm plate.

Whatever the machine, the real limiting factor is usually air. Plasma cutters need a steady flow of clean, dry, oil-free compressed air, typically somewhere between 60 and 80 psi. A small 24-litre hobby compressor will run out of puff on long cuts, so fit a moisture trap and an inline filter, and drain your tank regularly. Water vapour in the airline is the single most common cause of rough edges and short-lived consumables.

Budget for consumables too: electrodes, nozzles, swirl rings and shield cups are all wear items. Buy spares in packs, because a worn nozzle will ruin an otherwise perfect cut.

Safety Gear: What You Shouldn't Skip

The arc is intensely bright and produces ultraviolet and infrared radiation that will burn your eyes and skin faster than you might expect.

• Face shield or welding helmet with a shade 8 to 11 lens. Flip-up goggles are not enough for cutting.
• Leather gauntlets, long enough to cover your wrists. Spatter finds every gap.
• Flame-retardant overalls or a leather apron. Avoid synthetic clothing — it melts onto skin.
• Ear defenders. Plasma cutting is louder than it looks, especially indoors.
• Good ventilation. Fumes from galvanised, painted or coated metal are genuinely nasty. Cut outdoors or use extraction, and never cut sealed containers, tanks or drums.

Keep a fire extinguisher within reach and clear the area of anything flammable. Sparks travel further than you think, and hot slag drops straight down — so mind what's underneath your bench.

Setting Up: Air, Amps and Travel Speed

Set your air pressure to the figure in your machine's manual, then check it while the torch is flowing, not just at rest. Fit a clean nozzle and tighten it properly — a loose shield cup lets air escape sideways and distorts the arc.

Set the amperage to match your material. Turning the dial to maximum on 2mm sheet is a recipe for a ragged, blown-out cut. Most machines have a recommended chart printed inside the lid; trust it as a starting point, then adjust.

For cut height, a drag shield lets you rest the torch directly on the workpiece, which is the easiest way to learn. If you're using a standoff tip, keep it around 1.5–3mm above the metal. Consistency here matters more than anything else — bouncing the torch up and down changes the cut width and leaves you with a wavy edge.

Technique: Getting Clean Edges

Start your cut at the edge of the material wherever possible. Piercing mid-plate is harder on consumables and tends to splash molten metal back at the torch, particularly on thicker stock. If you must pierce, tilt the torch slightly, fire up, then straighten as the arc punches through.

Move at a steady, even pace. The right speed is usually faster than beginners expect — if you're seeing a shower of sparks spraying out the bottom of the cut, you're roughly on the money. If sparks are arcing upwards, you're too slow. Clamp a straight edge or a piece of flat bar as a guide for long cuts, or fit a circle-cutting attachment for tidy holes and flanges. Let the torch do the work and don't force it.

Reading the Cut: Dross, Bevel and Wear

Your finished cut tells you everything you need to know:

• Heavy dross (slag) on the bottom — too slow, too much amperage, or a worn nozzle.
• Excessive bevel, top edge rounded — too fast, or cut height too high.
• Ragged, gouged kerf — moisture in the air line, or a damaged shield cup.
• Arc won't start or cuts out — check your earth clamp is on clean, bare metal, and that the electrode isn't spent.

Aluminium cuts quickly and wants a faster travel speed than steel; stainless cuts hot and benefits from a slightly lower amperage. Thin sheet will warp if you linger, so clamp it down and keep moving.

Like any shop skill, plasma cutting rewards a bit of practice on scrap. Spend an afternoon with some offcuts, get a feel for speed and height, and you'll be producing clean, weld-ready edges in no time.