Assist gas for fiber laser cutting: oxygen vs nitrogen vs compressed air

Which gas for what

The pressures come from one compiled 3 kW parameter table (MachineMFG); your laser maker's table wins. Bodor puts compressed air at its best up to about 6 mm and oxygen from 6 mm up in carbon steel. The IPG speed table recommends nitrogen for thin carbon steel from 3 kW up, for speed. Bystronic also mixes oxygen into nitrogen for mild steel, at 20–30 % lower gas cost than nitrogen.

Speed: what the same table shows

• Stainless, 3 mm: 8–9 m/min with nitrogen at 16 bar, 9–10 m/min with air at 20 bar.
• Stainless, 10 mm: 0.6–0.8 m/min with nitrogen and 0.7–0.9 m/min with air, both with “some burrs”.
• Mild steel with oxygen: 4–4.5 m/min at 3 mm and 1.1–1.3 m/min at 10 mm; the edge goes from bright to semi-bright at 10 mm and frosted from 12 mm.
• Aluminium: the same speeds with nitrogen and with air up to 8 mm — 7–7.5 m/min at 3 mm.

How much nitrogen

Musil and colleagues (MM Science Journal, 2018) cut 4 mm AISI 304 stainless at 2.5 m/min on a 3 kW Bystronic — a CO2 laser, not a fiber laser — and calculated 16.7–30.9 m³ of nitrogen per hour across six nozzle and pressure settings (1.5–2 mm nozzles, 1.2–1.8 MPa, that is 12–18 bar). The settings that cut without burrs used 21–31 m³/h; the 2 mm nozzle at 12 bar used 27.6 m³/h. Their calculation ran 10–12 % above the machine maker's own consumption chart. Consumption depends on the nozzle and the pressure, so read these as an order of magnitude for a fiber machine and measure your own from the gas bill and the beam-on hours.

Putting gas into a quote

• Gas cost per hour = consumption × price per m³. Example: 20 m³/h × €0.40/m³ = €8 an hour (assumed price — use your supplier's). A laser dealer quoted on Bystronic's site puts many users' nitrogen cost at about $8 an hour.
• Charge it on beam-on time, not on the clock: gas flows while the head cuts and pierces, not during loading and setup.
• Zorocut lets you pick O₂, N₂ or air for each job (O₂ by default for mild steel, N₂ for stainless and aluminium), but its starter speeds follow each material's usual gas and there is no separate gas line in the app. Put the gas into the hourly rate — a higher rate for nitrogen work. See the machine hourly rate and the price calculator.
• Air is not free: Bystronic recommends an oil-free high-pressure screw compressor with filters and a desiccant or membrane dryer, and the dealer puts the payback at about six months on two shifts when it replaces nitrogen.