Key Takeaways
- A single CNC plasma machine can cut both mild steel and aluminium to a high standard, but each metal needs its own amperage, speed and gas settings rather than one universal setup.
- Aluminium conducts heat far faster than steel, so cuts generally need to run quicker and with a tighter, more consistent torch standoff to stop molten metal clinging to the edge.
- Removing the natural oxide layer and any oil or grease from aluminium immediately before cutting improves arc contact and reduces the need for post-cut finishing.
- Shielding gas choice, whether compressed air, nitrogen or an argon-hydrogen blend, has a direct effect on dross, edge oxidation and overall finish.
- Treating steel and aluminium as two related but separate cutting disciplines, rather than one set of settings for everything, is the fastest route to consistent results on both.
Most fabrication shops cut more than one type of metal, which is why the practical answer is a single capable CNC plasma cutting machine rather than a separate table for every material. Plasma cutting works by forcing a gas through a constricted nozzle and ionising it into plasma, an intensely hot, high-velocity jet that melts a narrow path through metal and blows the molten material clear. A machine such as the Plazmax CutAce is built to handle mild steel, stainless steel and aluminium with equal confidence, provided the settings are adjusted to suit whichever material is on the table. Not every material qualifies for plasma cutting: metals such as lead and tin melt too easily to be cut cleanly.
Steel and aluminium are the most common pairing on an Australian fabrication shop floor, and both are well suited to plasma cutting, but they do not behave the same way under the arc. Aluminium is relatively soft and an extremely fast thermal conductor, and while that makes it entirely manageable with the right approach, it also means the settings that produce a clean cut on steel will not automatically work on aluminium.
Steel vs Aluminium: Key Cutting Differences
Before adjusting parameters, it helps to see where the two materials diverge in practice. The table below summarises the main differences an operator needs to account for when moving between mild steel and aluminium on the same machine.
| Factor | Mild Steel | Aluminium |
|---|---|---|
| Thermal conductivity | Moderate; heat stays close to the cut line | High; heat spreads quickly through the plate |
| Recommended standoff | Standard torch height settings | Tighter, more consistent standoff, roughly 1/8 inch (3mm) |
| Typical shielding gas | Compressed air or oxygen-based mixes | Compressed air, nitrogen, or argon-hydrogen for thicker plate |
| Surface preparation | Minimal surface oxide to manage | Oxide layer forms almost immediately and should be removed before cutting |
| Common quality issue if mishandled | Dross and taper on thicker sections | Molten metal clinging to the edge if the cut is too slow |
Why Aluminium Dissipates Heat Differently and Affects Arc Control
Because aluminium conducts heat so efficiently, energy from the arc spreads through the plate instead of staying concentrated at the cut line, which can make the melt zone harder to control. This calls for a different approach than steel, not just a different setting on the same approach.
Speed matters more with aluminium than it does with steel. Cutting too slowly lets heat build up, and molten metal can cling to the edge of the cut rather than being blown clear by the gas stream. This often looks like a consumables problem, when in reality the operator is simply cutting aluminium as though it were steel.
Amperage needs to be set carefully for the thickness on the table, and the standoff, meaning the distance between the torch and the plate, needs to stay close to 1/8 inch (3mm) unless a specially designed drag shield is fitted. Because that standoff has to remain consistent cut after cut, even across plate that is not perfectly flat, accurate and repeatable plate detection matters more here than it does on more forgiving materials. Plazmax's laser height sensor technology detects plate position using a laser rather than the traditional method of the torch physically touching down, which means faster, more accurate positioning with less wear on the torch itself, regardless of whether the plate is new, aged or coated.
Air quality plays a bigger role than many operators expect. Any oil or moisture in the compressed air line can destabilise the arc and produce an imperfect edge. Before running a full batch of aluminium, it is worth sacrificing a small test piece to dial in the settings, rather than assuming steel parameters will transfer across.
Managing Aluminium Oxide and Minimising Post-Cut Finishing
Post-cut finishing costs time and money that a correctly set up cut should not need. Getting the job right in one pass starts with understanding what is sitting on the surface of the plate before the arc ever touches it.
Aluminium oxide is a hard, non-conductive layer that forms on bare aluminium almost as soon as it is exposed to air. Because it does not conduct electricity, it interferes with the arc's ability to make clean contact with the base metal, which can throw off precision and leave imperfections that need to be cleaned up afterwards.
The fix is straightforward:
- Remove the oxide layer with a wire brush or sandpaper immediately before cutting, so the arc strikes bare metal rather than the oxide skin.
- Degrease the surface with something like acetone to remove oil and other contaminants.
- Carry out both steps right before the cut, not the night before, since the oxide layer and any surface film can reform quickly.
Choosing the Right Shielding Gas for the Job
Gas selection has a direct effect on cut quality, influencing dross levels, edge oxidation and cutting speed. There is no single correct answer, because the best choice depends on the material, the thickness and how the finished edge needs to look.
Compressed air is the most common default. It is versatile and cost-effective, and for many general fabrication jobs on steel and aluminium it is entirely adequate.
Nitrogen is a step up for jobs where an oxide-free, high-quality edge matters more than running cost, since it produces a cleaner cut with less surface oxidation than air.
For thicker plate or high-precision work, argon-hydrogen mixtures are often used, particularly where cut quality tolerances are tight enough to justify the extra cost of a speciality gas.
Well-maintained consumables make just as much difference as gas choice. A worn nozzle or electrode will undermine even a correctly selected gas and standoff setting, so keeping genuine consumables in good condition, and knowing when it is time to replace them, is worth building into the same routine as gas and parameter selection.
Steel and aluminium reward different habits, not different machines. Once an operator stops assuming that settings transfer directly between the two, and instead treats them as two related but separate disciplines, cut quality on both materials becomes far more predictable. Consistency in standoff, gas choice and pre-cut preparation is what separates a clean single-pass cut from a job that needs grinding and rework afterwards.
Frequently Asked Questions
Can one CNC plasma machine cut both steel and aluminium well?
Yes. A capable CNC plasma system can cut mild steel, stainless steel and aluminium to a high standard, provided the amperage, speed, standoff and gas are adjusted for whichever material is being cut.
Why does aluminium need a faster cutting speed than steel?
Aluminium conducts heat much faster than steel, so cutting too slowly allows heat to build up and molten metal to cling to the cut edge instead of being blown clear.
Do I need to clean aluminium before plasma cutting it?
Yes. Aluminium develops an oxide layer almost immediately on exposure to air, and removing it with a wire brush or sandpaper, along with degreasing the surface, immediately before cutting improves arc contact and cut quality.
What gas is best for cutting aluminium?
Compressed air works well for general aluminium cutting, nitrogen produces a cleaner, oxide-free edge, and argon-hydrogen mixtures suit thicker plate or high-precision applications.