Plasma Cutting Operation & Safety
Run a plasma cutter by sizing amps to thickness, stacking consumables, setting pierce and cut height, reading dross and sparks, and controlling shock and fume.
Table of contents
A plasma cutter pushes a jet of ionized gas through a small nozzle, melts the metal and blows it out of the cut. It works on any conductive metal, including steel, stainless, aluminum and copper, and it is quick to learn. Getting a clean edge is a matter of matching amps, height and speed, and keeping your consumables fresh.
What You Will Be Able to Do
- Explain how a plasma arc forms and cuts.
- Choose amps for a thickness, and install the right consumables.
- Set pierce height and cut height.
- Read sparks and dross, then fix speed problems.
- Protect yourself from shock, arc light, fume and fire.
How Plasma Cutting Works
Inside the torch, a pilot arc forms between the electrode and the nozzle. Gas flows around the electrode and through the nozzle, where the arc ionizes it into plasma. When the torch nears the metal, the arc jumps from the nozzle to the work. That is the transferred arc, the main cutting arc. The plasma jet is often quoted at over 40,000 F at the cutting point. It melts the metal, and the gas blows it out of the kerf.
The gas does two jobs: it becomes the plasma, and it blows molten metal away. Compressed air is the most common gas for mild steel in shop systems. Some systems use nitrogen, oxygen or other mixes for better edges.
Because the arc has to transfer to the work, plasma only cuts conductive metal. It will not cut wood, plastic, glass or ceramic.
Consumables
The parts in the torch head wear out and are stacked in a set order.
| Part | Job |
|---|---|
| Electrode | Carries the arc, with a hafnium insert in the tip |
| Swirl ring | Spins the gas to steady the arc |
| Nozzle (tip) | Narrows the plasma jet |
| Shield | Protects the nozzle |
| Retaining cap | Holds the stack together |
Electrode. The hafnium wears away, leaving a pit. Replace the electrode when the pit reaches about 0.040 to 0.060 inch deep, or as your maker says. Running it to failure can damage the torch.
Nozzle. The hole wears oval, which widens the kerf and adds bevel and dross. Change the nozzle and electrode together, because they wear together.
Shield and swirl ring. Check them for damage, and keep them clean and dry.
Warning: Plasma systems run at dangerous voltages, often 240 to 400 volts DC at the torch. Always power down, by switching off or unplugging the machine, before you change consumables.
Let the torch cool, seat the parts fully, and do not cross-thread the cap.
Choosing Amps
Match the amps to the thickness, with lower amps for thin material and higher amps for thick material. Too many amps on thin sheet gives a wide kerf and heavy dross. Too few, and the arc will not get through.
Machines list a rated cut and a sever cut. A rated cut is one you can make well at a practical speed. A sever cut is the most it can get through, with a rough edge.
| Amps | Rated cut, mild steel | Sever cut |
|---|---|---|
| 30 | 3/8 inch | 1/2 inch |
| 45 | 1/2 inch | 3/4 inch |
| 65 | 3/4 inch | 1 inch |
| 85 | 1 inch | 1-1/4 inch |
| 125 | 1-1/2 inch | 2 inch |
These are typical values, and your machine's data comes first. Fit the consumables rated for your output amps. Running a low-amp nozzle at high output burns it out quickly.
Setting Up to Cut
- Power off and install the right consumables.
- Check your air. Set the supply to the machine's spec, and make sure it is clean and dry. Wet or oily air shortens consumable life and spoils cuts.
- Connect the work clamp to clean, bare metal, close to the cut.
- Set amps.
- Clear the area. Treat plasma as hot work, and remove combustibles before you start.
- Start extraction.
- Dress for it. Use a helmet or face shield, gauntlet gloves and flame-resistant clothing.
- Make a test cut on scrap.
Pierce Height and Cut Height
- Cut height, or standoff, is the distance from the nozzle to the plate while cutting, usually 1/16 to 1/8 inch. Too close damages the nozzle, and too far widens the kerf and adds bevel.
- Pierce height is the distance when the torch starts a hole in the plate. It is usually about 1.5 to 2 times the cut height, so that molten metal thrown back does not hit the nozzle and shield.
- Pierce delay is the short wait, after the arc transfers, while the arc breaks through. Too short, and the torch moves before the arc is through. Too long, and you blow a big hole.
On a hand torch, a drag shield lets you rest the torch on the plate at the right height. Without one, hold the height by eye.
Reading the Cut
Watch the sparks and the bottom edge. They tell you if your speed is right.
| What you see | Likely cause | Fix |
|---|---|---|
| Sparks stream out the bottom, slightly trailing | Good speed | Keep going |
| Sparks blow back toward you or spray on top | Too fast, or not cutting through | Slow down |
| Thick, globby dross, easy to chip off | Too slow, or amps too high | Speed up or lower amps |
| Thin, hard dross, hard to remove | Too fast | Slow down |
| Spatter on top of the plate | Worn nozzle, too fast, too high | Check consumables, speed and height |
| Heavy bevel | Worn nozzle, torch too high | Replace nozzle, lower torch |
Bottom dross comes in two kinds. Low-speed dross forms a thick, globular bead that chips off easily. High-speed dross is a thin, hard, rolled bead that clings to the bottom edge and often needs grinding. Adjust speed in steps of about 5 to 10 percent, and watch for a change.
If your settings are right and you still get dross, change the electrode and nozzle before you chase speed. Worn consumables can look like a speed problem.
Cutting on a CNC Table
On a CNC table, the machine controls height and speed, but the same rules apply.
- Torch height control (THC) reads arc voltage and adjusts torch height continuously, to hold the target cut height as the plate warps.
- Kerf compensation offsets the path, so parts come out to size.
- Lead-ins and lead-outs start and end the cut in scrap, so pierce marks stay off the finished edge.
- Common faults include a failed arc transfer, often from a poor work connection, and low gas pressure from a clogged filter.
Water Tables and Aluminum
Water tables capture fume and sparks. Cutting aluminum on a water table can release hydrogen, which can collect under the plate and ignite with a bang. If you cut aluminum on a water table, follow the machine maker's guidance. Keep the water aerated and don't leave plates submerged.
Staying Safe
Eyes and Skin
The plasma arc gives off intense ultraviolet and infrared light. OSHA minimum shades for plasma arc cutting are:
| Cutting current | Minimum shade |
|---|---|
| Under 300 amps | 8 |
| 300 to 400 amps | 9 |
| 400 to 800 amps | 10 |
Use a welding helmet, or a face shield with the right lens. Cover your skin with flame-resistant clothing.
Shock
Treat the torch and leads as live. Keep gloves and clothing dry, stay off wet floors, and inspect leads for damage. Do not stand in water at a water table.
Fume
Plasma vaporizes metal into fine fume. Downdraft tables, water tables and portable extractors help. Cutting galvanized steel without extraction can cause metal fume fever from zinc oxide. Stainless gives off hexavalent chromium. Old paint can contain lead. Use the respiratory protection your employer requires.
Fire and Noise
Sparks can travel far, and they fall through grates and cracks. Follow your site's hot work rules, and keep an extinguisher close. Plasma is loud, so wear hearing protection.
Electronics and Medical Devices
High-frequency start systems can interfere with computers and CNC controls. Anyone with a pacemaker should ask their doctor and the machine maker before working near plasma equipment.
Common Mistakes
- Running worn consumables and blaming the machine. Check the electrode and nozzle first.
- Changing parts with power on. The torch head can carry a lethal voltage.
- Mixing consumable sets. A low-amp nozzle fails quickly at high amps.
- Cutting on a poor work connection. The arc will not transfer reliably.
- Ignoring wet air. A filter and dryer cost less than torches.
Check Your Understanding
You are cutting 1/4 inch steel and sparks are spraying back at you off the top of the plate. What is going on?
You are moving too fast, or the arc is not getting through. Slow down until the sparks stream out the bottom.
A cut leaves a thick bead on the bottom that knocks off with one tap. What does that tell you?
That is low-speed dross. Speed up, or lower the amps if they are too high for the thickness.
Your cut quality dropped today, but nothing else changed. The electrode pit measures 0.060 inch. What do you do?
Power down and replace the electrode and nozzle.
A CNC table has started to cut uneven parts on warped plate. Which system should you check?
The torch height control. It uses arc voltage to hold cut height as the plate moves.
Key Takeaways
- Plasma cuts conductive metal only, using a transferred arc and a gas jet.
- Match amps to thickness, and fit consumables to the amps.
- Pierce at about 1.5 to 2 times your cut height.
- Sparks and dross tell you if your speed is right.
- Replace worn electrodes and nozzles before you adjust settings.
- Power down before you touch the torch head, and control fume at the source.