Choosing assist gas

What often goes wrong when choosing assist gas

On a busy South African shop floor, assist gas is often chosen out of habit or price, not fit for the job. This leads to rough edges, slow cutting, wasted gas or jobs that come back from powder coating with flaking paint.

Workshops running mixed material sometimes forget to check that the correct cylinder is open, or they assume compressed air will do everywhere. We see this when mild steel lands up with heavy oxide, or stainless jobs come out with a dull grey edge that needs extra buffing.

What we have learned from real cuts

Oxygen is the go-to for mild steel because it speeds up the cut by creating an exothermic reaction at the kerf, but it leaves a dark oxide on the edge. This oxide must be removed before any paint or powder goes on, or you’ll fight adhesion failures down the line.

Nitrogen keeps the edge bright on stainless and aluminium and avoids an oxide layer, but your gas flow and purity must be high. Stainless cut with oxygen can rust or pick up weld issues. For thin sheet, clean dry compressed air will work, but only if your compressor delivers oil-free air at stable pressure: any oil or moisture will leave spatter or contaminate the lens.

A job that proved the difference

Last year, a customer in Germiston was cutting brackets for marine use. They used oxygen by mistake on 316 stainless, thinking it would be faster. The job finished quickly, but every part needed rework to remove the oxide, and some still corroded at sea. We tested nitrogen on the same setup, and the next batch was ready for use straight off the bed.

On another job, a panel beater in Durban tried compressed air for mild steel covers to save gas costs. The edges came out with light brown streaks and powder coating failed. Switching to oxygen fixed the cut, but now they prep the edge before paint.

Practical steps to get assist gas right

Every metal and finish has a best fit for assist gas. You need to check your gas supply, line up the right settings, and match your edge to what the next step in production needs.

Here’s what we do before running a material on the CutLine:

• Confirm the gas type and cylinder are correct for the material
• Check for leaks, pressure and purity in the gas lines
• Set assist gas pressure based on material: lower for oxygen on mild steel, higher for nitrogen on stainless and aluminium
• Inspect the protective window under the cutting head for dust or oil
• Test cut a sample and inspect the edge for oxide, burr and finish
• Decide on post-cut cleaning if oxide is present, before paint or welding

When to ask us for help

If you find your cut edges aren’t matching what you see at training, or a job is failing paint or weld tests, send us a message through our contact form with your site, cut system and a description of the edge. We’ll look at your nest brief, gas setup and settings.

It’s always better to check with the applications team before committing to a run, especially if you’re switching between mild steel, stainless and aluminium. We’ll help you line up the right gas for the job and avoid wasted time or rework.