Process Notes: Cutting 6 mm Carbon Steel Tube Without Slag, Taper or Burrs

Six millimetre carbon steel tube is the workhorse of structural and mechanical fabrication, and it is also the size at which gas choice, focus position and pierce strategy start to matter. This note summarises the settings we use on LP-T Series machines for Φ50 to Φ200 mm mild steel tube, along with the failure modes that appear when one of the three is wrong.

For oxygen-assisted cutting of 6 mm mild steel, use compressed air or a 1:1 nitrogen and oxygen blend only when downstream weld quality allows it; pure oxygen gives the fastest cut and the cleanest edge, but it leaves an oxidised layer that has to be ground before welding. Nitrogen produces the brightest, least oxidised edge at roughly 20 to 30% lower speed, and is the better choice when parts go straight to MAG welding or to galvanising.

Focus is where most shops lose time. With a 150 mm focal length and a 0.4 mm spot, the focal position on a 6 mm wall should sit roughly 0.2 to 0.3 mm below the surface for nitrogen cutting and 0.6 to 0.8 mm below for oxygen. Too shallow a focus and the kerf widens while slag builds on the lower edge; too deep and the beam loses focus before breaking through, which shows up as a pronounced taper.

Piercing 6 mm tube deserves its own routine. A single-shot pierce at full power produces a wide oxide ring, while a short multi-shot pierce of three to five pulses between 0.3 and 0.6 seconds keeps the pierce hole round and the entry-side dross small. On larger diameters it pays to back the pierce point off the tube axis so the slug cannot fall back into the melt pool.

The three defects worth naming are slag on the lower edge, which points to too little oxygen or a dirty nozzle; taper, where the cut narrows towards the exit, caused by gas pressure dropping along the cut or a worn protective lens; and burrs, which appear when the focus is off or the tube is clamped eccentrically. Check nozzle condition and gas pressure before you touch the focus position — in our experience that order resolves most reported problems.