Plasma Cutting Operation & Safety

Plasma cutting is the go-to process in most modern fabrication shops for cutting conductive metals up to around 1 to 2 inches thick. A plasma cutter ionizes a gas (air, nitrogen, or oxygen) with a high-voltage electrical arc, then forces the superheated, electrically conductive gas jet through a small nozzle at near-sonic velocity. Temperatures at the cutting point exceed 40,000 degrees F - hot enough to instantly melt and blow any conductive metal out of the kerf. Plasma cuts mild steel, stainless steel, aluminum, copper, brass, and galvanized stock with equal ease, which is a significant advantage over oxy-fuel (which only cuts carbon steel).

This guide covers how plasma works, consumable selection, amp-to-thickness sizing, cut technique, dross troubleshooting, and the safety hazards unique to plasma.

How Plasma Cutting Works

Inside the torch head, high voltage creates an arc between the electrode (in the back of the torch) and a copper nozzle at the front. A compressed gas (air, for most shop plasma systems) flows around the electrode and exits through the nozzle orifice. The arc ionizes the gas stream, creating plasma. A pilot arc - the initial arc between electrode and nozzle - ignites the plasma jet. When the torch approaches the workpiece, the arc transfers from the nozzle to the work, becoming the main cutting arc. The nozzle constricts the plasma jet into a tight, high-energy column that melts the metal while the gas velocity physically blows molten material out the underside of the cut.

Because plasma cutting requires an electrically conductive workpiece to establish the transferred arc, plasma will not cut non-conductive material - no wood, glass, plastic, or ceramics.

Consumables - The Parts That Wear Out

Every plasma torch contains a stack of consumables arranged in a specific order. Learn the stack:

| Consumable      | Function                                               |
|-----------------|--------------------------------------------------------|
| Electrode       | Emits the arc; tipped with hafnium on air-plasma systems|
| Swirl ring      | Spins the gas stream for a stable plasma column        |
| Nozzle (tip)    | Constricts and focuses the plasma jet                  |
| Shield cap      | Protects the nozzle; directs secondary gas on some systems|
| Retaining cap   | Holds the stack together                               |

• Electrode - The hafnium insert in the electrode face erodes during cutting, forming a small pit. When the pit reaches about 0.040 to 0.060 inch deep (check your manufacturer's spec), replace the electrode. A worn electrode causes arc wander, poor cut quality, and can lead to torch head damage if run to failure.
• Nozzle (tip) - The orifice elongates or becomes oval as it wears. Worn nozzles give wide, tapered kerfs and heavy top dross. Replace in pairs with the electrode - they wear together.
• Swirl ring - Long-lived but can get gouged if metal splatters back into the torch. Inspect every few electrode changes.
• Shield cap - Sacrificial. Blast marks and pitting on the inside of the cap are normal; crack it or drop it and it is trash.

Always let the torch cool before changing consumables. Never cross-thread the retaining cap - plasma torch threads are fine and easy to strip.

Amp and Thickness Selection

Plasma machines are rated in output amps. More amps = thicker steel, but also wider kerf and slower electrode life. Most machines spec a rated cut (continuous production cut) and a sever cut (maximum thickness the machine can break through, not necessarily cleanly).

A rough rule of thumb:

| Output Amps | Rated Cut (mild steel) | Sever Cut |
|-------------|------------------------|-----------|
| 30 A        | 3/8 inch               | 1/2 inch  |
| 45 A        | 1/2 inch               | 3/4 inch  |
| 65 A        | 3/4 inch               | 1 inch    |
| 85 A        | 1 inch                 | 1-1/4 inch|
| 105 A       | 1-1/4 inch             | 1-1/2 inch|
| 125 A       | 1-1/2 inch             | 2 inch    |

Always install the amp-rated consumable set for your output setting. Running a 65-amp torch tip on 85-amp output will burn the tip in minutes.

Hand Torch vs. Machine (CNC) Torch

• Hand torch - Built for handheld use with a pistol grip, trigger, and a drag shield that allows the nozzle to rest directly on the workpiece. Ideal for field repair, demolition, and one-off cuts.
• Machine torch - Straight-barrel torch designed to mount on a CNC gantry or track. No trigger - the CNC controller fires the arc. Uses higher-precision consumables and typically longer electrode life because cut parameters are controlled precisely.

The fundamentals of cut quality (pierce height, cut height, cut speed) apply to both. CNC just automates them.

Pilot Arc and Starting

Most modern plasma machines use a pilot arc to start the cut. Squeeze the trigger and a small arc forms inside the torch head. When the torch approaches the workpiece (within about 1/4 inch), the arc transfers to the work and the cut begins.

Older "high-frequency start" plasmas used a high-voltage spark to ionize the gas and can interfere with computers and pacemakers - know what you are standing near before striking an arc.

Blowback start plasmas (common on smaller handheld units) move the electrode mechanically to create the pilot arc without HF interference. These are ideal around CNC controllers and electronic equipment.

Pierce Height, Cut Height, and Standoff

• Pierce height - The distance from the nozzle to the workpiece during initial piercing. Typically 1.5 to 2 times the normal cut height, so the molten blowback does not splash back onto and ruin the nozzle and shield. A 1/4 inch pierce height is typical for 1 inch steel.
• Cut height (standoff) - The working distance from the nozzle to the plate during a cut. Usually 1/16 to 1/8 inch for most amperages. Too close: damaged consumables, arc shorting to the nozzle. Too far: wandering arc, wide kerf, heavy bevel.
• Dwell delay (pierce delay) - The time the torch hovers at pierce height after the arc transfers, before motion starts. Long enough for the arc to punch fully through the plate. Too short and the arc blows sideways before breaking through.

Hand-torch users can use the drag shield to rest the torch on the plate - this automatically sets the correct cut height. For unshielded tips, maintain 1/16 to 1/8 inch standoff by eye.

Cut Speed Tuning

Every plasma cut has an ideal speed. Too slow and the arc melts a wide, beveled kerf with heavy bottom dross. Too fast and the arc lags, leaves an unfinished cut, or produces top dross and a negative bevel.

How to tune:

• Start near the manufacturer's recommended speed for your amperage and thickness.
• Watch the sparks. On a good cut, sparks stream down and slightly ahead of the torch, and the cut sounds like a steady hiss with a slight roar.
• Too slow = sparks blow back toward the operator, excessive bottom dross, wide kerf.
• Too fast = sparks trail behind the torch, arc struggles, incomplete cut, top dross.

Adjust in 5 to 10 percent increments and observe the kerf.

Dross Interpretation

Dross is re-solidified metal clinging to the kerf edge. It comes in two flavors:

• Top dross - Usually a thin line of bead-like spatter on top of the cut. Cause: cut speed too fast, or cut height too high.
• Bottom dross - Heavy oxide deposits stuck to the bottom edge. Two kinds:
  • Hard/high-speed dross - A thin, hard line. Cut speed is slightly too fast. Slow down.
  • Soft/low-speed dross - A thick, easily chipped bead. Cut speed too slow, or amperage too high. Speed up or drop amps.
• No dross - Sweet spot. The cut edge is clean and the bottom is smooth.

Worn consumables also cause dross that looks like a speed problem but is actually a hardware problem. If you are running at recommended parameters and still getting dross, change the electrode and nozzle before chasing speed.

Eye, Lung, and Electrical Hazards

Eye Protection

Plasma arcs produce intense visible and UV radiation. Shade requirements by amperage:

• Up to 40 A - Shade 5
• 40 to 60 A - Shade 6
• 60 to 80 A - Shade 8
• 80 to 300 A - Shade 9 to 12 depending on current

Most hand-plasma operators wear a shade 8 or 9 flip-front face shield as a good all-around setting. Auto-darkening welding hoods with plasma settings are also common - confirm the shade switches correctly before cutting.

Fume Extraction

Plasma cutting vaporizes metal and generates fine particulate fume. Cutting galvanized, painted, or stainless stock produces particularly nasty compounds (zinc oxide, chromium VI, lead, nickel) that cause acute and chronic respiratory disease. Required controls:

• Downdraft table or water table for CNC plasma - vents fume downward and away from the breathing zone
• Portable fume extractor for hand cutting - positioned within 12 inches of the cut
• PAPR respirator for heavy cutting on coated or stainless material
• Never cut galvanized in a closed space without extraction. Zinc fumes cause metal-fume fever.

Electrical

Plasma open-circuit voltages run 240 to 400 V DC - more than enough to kill. Never change consumables with the machine powered on. Unplug or switch off the unit at the machine before servicing the torch. Damaged torch leads, worn lead connections at the machine, or wet work surfaces can create ground fault paths through the operator.

Noise

Plasma cutting is loud - often 95 to 105 dB at arm's length. Wear earplugs or earmuffs on any extended cutting session.

CNC Plasma Basics

On a CNC plasma table:

• The torch height control (THC) reads the arc voltage continuously and automatically adjusts torch height to maintain the target cut height as the plate warps. If the THC is off or failing, expect inconsistent cut quality.
• Pierce programming lifts the torch to pierce height, fires the arc, dwells for the programmed pierce delay, drops to cut height, then motion begins.
• Cut kerf compensation - The kerf width varies with amperage and consumable wear. Programming software adjusts tool path to compensate so parts come out to the correct dimension.
• Lead-ins and lead-outs are short entry/exit tangents to the cut path, placed in scrap material, so pierce spatter and end-of-cut marks do not damage the finished edge.
• Common fault conditions: transfer fault (arc did not transfer - check workpiece ground), torch collision (THC failed or plate moved), low gas pressure (filter clogged or line undersized).

Day 1 Checklist

• Plasma machine unplugged/off while installing consumables
• Correct amp-rated consumable set installed
• Shield cap, nozzle, electrode, swirl ring all seated; retaining cap snug, not cross-threaded
• Air supply set to the machine spec (typically 90-120 psi, clean and dry)
• Work clamp on clean bare metal, close to the cut
• Shade 8 minimum eye protection, gauntlet gloves, leather sleeves or jacket
• Fume extraction in position
• Combustibles cleared 10+ feet; fire extinguisher within reach
• Test cut on scrap to confirm settings and consumable condition

Plasma is a forgiving process once the consumables and parameters are right. The most common new-operator mistake is running with worn consumables and blaming the machine for poor cut quality. The second most common is ignoring the fume. Treat both with respect and the cutter will earn its keep for years.