Thick plate laser cutting: process, gas and edge quality

Thick plate laser cutting starts where sheet cutting stops being fast. On a 12 kW fibre laser, 12 mm mild steel runs at roughly 1.6–2.2 m/min in oxygen, while the same machine takes 3.5–4.5 m/min through 6 mm. Double the thickness and the time per metre more than doubles, because the cut is no longer a narrow kerf travelling through thin material; it is a gas-assisted thermal process where the assist gas does a large share of the work.

That shift changes which decisions matter. Below 8 mm, the gas is mostly a setting. Above 10 mm, gas choice, pressure, focus position and pierce strategy decide whether the part comes off clean or comes off with a taper, dross and a burnt corner at every lead-in.

This guide works through the numbers a design engineer needs to specify a thick plate part, and the limits that say when the plate should leave the laser for another process.

What counts as thick plate on a fibre laser?

Above about 8 mm, and in production terms 10–20 mm. A 12 kW source with a good optical chain will cut mild steel to 20 mm all day; 22–25 mm is possible on some machines with oxygen but the speed and edge quality fall away quickly, and the pierce time starts to dominate the cycle.

Wavelength matters here. A fibre laser delivers about 1,070 nm, roughly a tenth of the wavelength of the CO2 laser it replaced, and steel absorbs far more of that energy. The practical effect is a tighter focus spot and a higher power density, which is why a 12 kW fibre machine cuts 12 mm plate at a speed that took a 4 kW CO2 machine twice the time. It is also why the cut edge on thick plate is narrower and the heat-affected zone smaller than the older process.

The thickness itself changes the physics of the kerf. Molten metal has further to travel out of the cut, the gas jet loses pressure down the kerf, and the bottom of the cut lags the top. That lag is what produces taper, dross and the occasional failure to sever at the end of a long profile.

Oxygen or nitrogen for thick plate?

Oxygen for mild steel and CRCA. Nitrogen for stainless, aluminium and anything that will be welded without pickling. That is the decision rule, and the numbers behind it are speed, edge chemistry and cost.

Oxygen does not just blow the melt out; it burns it. The exothermic reaction between iron and oxygen adds energy to the cut, which is why 12 mm mild steel runs two to three times faster in oxygen than in nitrogen at the same power. The trade is an oxide layer on the cut face, typically 5–15 µm thick, a wider kerf and a rougher surface. For a bracket that will be shot blasted and powder coated, that edge is fine. For a stainless part that will be welded and left visible, it is not.

Nitrogen is inert. It ejects the melt without reacting, so the edge stays bright and weldable, but the process needs 12–16 bar at the nozzle and runs 30–50% slower. On 12 mm stainless that is 0.6–0.9 m/min against the oxygen speed on the same thickness of mild steel. The gas consumption at those pressures is the largest single cost line in the cut, which is why nitrogen cutting on thick plate is quoted differently from oxygen cutting.

Gas purity is part of the same decision and is usually invisible on the drawing. Oxygen at 99.5% cuts, but the residual nitrogen and argon in a lower-purity supply show up as intermittent dross on 16 mm and above, so thick work wants 99.9% or better. Nitrogen for stainless wants 99.999%: at 99.9% the few parts per million of oxygen are enough to tint the cut face yellow-brown, which then has to be pickled or brushed out before welding. A supplier buying gas on price alone will pass that cost to you as a finishing operation.

Focus, nozzle and standoff: the three settings that decide the edge

Focus position is the setting most often left at a default. On thick mild steel in oxygen, the focus sits inside the material, often 2–4 mm below the surface, so the beam stays wide and hot at the bottom of the kerf and the cut severs cleanly. Raise it toward the surface and the top edge sharpens while the bottom starts to dross. On stainless in nitrogen the focus is usually closer to the surface or slightly above, because the jet has to drive molten metal down and out without the extra energy of an exothermic reaction.

Nozzle choice follows the gas. Oxygen on thick mild steel runs a 1.6–2.0 mm double nozzle at 0.8–1.2 mm standoff; a larger nozzle costs pressure and a smaller one is easily blocked by spatter. Nitrogen at 12–16 bar usually takes a 2.0–2.5 mm single nozzle, because the flow has to be high enough to clear a deep kerf.

Standoff is a maintenance item disguised as a setting. A nozzle that has drifted 1 mm high loses cut quality on 16 mm plate before it shows any problem on 3 mm sheet. That is why the first-part check on a thick job includes the nozzle condition, not only the dimensions.

What edge quality can you actually call out?

Call the standard and the class, then measure. ISO 9013:2017 covers thermal cuts from oxyfuel, plasma and laser, and sets out geometrical product specifications and quality tolerances with classes for perpendicularity and for surface roughness. A drawing that says "cut edge to ISO 9013" without a class is a drawing that will be argued about at inspection.

The perpendicularity part is where thick plate earns its reputation. A taper of 0.25° per side sounds trivial until it is measured: over 12 mm of thickness it is 0.052 mm per side, and over 20 mm it is 0.087 mm. At 0.5° the same 12 mm gives 0.105 mm per side. So a 12 mm hole in 12 mm plate that measures 0.2–0.4 mm oversize is normal behaviour, not a machine fault, and the fix is a drilled hole rather than a tighter laser setting.

Surface roughness on a laser cut in mild steel typically falls in the 10–40 µm Rz band depending on thickness and gas, against 25–60 µm for a comparable oxyfuel cut. Measure it with a roughness gauge along the cut direction, not across it, and record where on the profile the reading was taken: the bottom third of a thick cut is always rougher than the top.

Then there is the heat-affected zone. Laser cutting leaves 0.1–0.3 mm on 12 mm mild steel, a fraction of the 1–3 mm that oxyfuel leaves. It still matters where the cut edge becomes a bend line or a weld prep. Bending directly on a laser-cut edge puts the harder zone in the tension fibre of the bend, and on 12 mm plate at a tight radius that is where a crack starts.

Dross is the other edge feature to write into the callout. Hard dross, the brittle, welded-on slag that needs a chisel, comes from too little gas pressure or a focus that is too deep; soft dross, which wipes off, comes from excess pressure or too much power at the bottom of the kerf. The two look similar on a photo and have opposite fixes, so inspection should record which one is present rather than the word "dross" alone. On a blasted and powder-coated part, a 0.3 mm soft dross bead will still show as a ridge after coating.

Nesting 12 mm plate without turning it into scrap

Thick plate nesting is a weight problem before it is a layout problem. A 2,500 mm × 1,250 mm sheet in 12 mm steel weighs 294 kg at 7,850 kg/m³, and a 2,000 mm × 1,000 mm offcut still weighs 188 kg. Nothing about that sheet is moved by hand, and the clamp zone on the bed has to stay clear of the nest.

Work the arithmetic before the layout. Take a bracket 600 mm × 300 mm in 12 mm with six 30 mm holes. Flat area is 0.180 m², minus 0.0042 m² of holes, so 0.1758 m² and 16.6 kg per part. On a full sheet, a 4 × 4 nest with 5 mm gaps leaves 2,415 mm × 1,215 mm used, which fits inside 2,500 mm × 1,250 mm. Sixteen parts weigh 265 kg, leaving about 29 kg of scrap and a yield near 90% before the clamp zone is taken out.

Cut length drives the cycle time. Each part runs 1,800 mm of perimeter plus 565 mm around the six holes, so 2,365 mm or 2.37 m of cut. At 1.8 m/min that is 1.32 minutes, and seven pierces at 4 seconds add 28 seconds, giving roughly 1.8 minutes per part and 29 minutes of cutting for the sheet. Handling 294 kg in and 265 kg out is a separate operation with a separate risk.

That is also why thick plate nesting rewards a part revision that removes one hole. Six holes become five, one pierce and 94 mm of cut disappear, and the saving multiplies across every sheet in the order.

Where thick plate laser cutting stops and the iron worker starts

Above 20 mm, on our floor, the laser is out. Oxyfuel cutting on the iron worker handles 30–40 mm plate for structural parts, gussets and base plates, at a wider kerf and with a heat-affected zone that has to be allowed for in the weld procedure. Plasma sits between the two for some shops; we route the work to the process that gives a controllable edge rather than the one that is nearest.

There are also parts that fit in the thickness range but not the process. A hole under 12 mm in 12 mm plate, a slot narrower than the kerf plus 0.5 mm, a cut edge that doubles as a bearing surface, a sharp internal corner that needs a radius below 0.5 mm: all of those want a drill, a mill or a different blank. Thick plate laser cutting is a blanking and profiling process, and it is a poor substitute for machining where the feature carries the function.

Finally, distortion. Cutting releases residual stress in hot-rolled plate, and a long slender part can bow after it is cut free even though nothing touched it. The fix is sequencing, leaving tags until the end, and not nesting a 60 mm wide strip between two large parts where the released stress has nowhere to go.

The 12 kW route at Ganesh

Plate work runs on the 12 kW fibre laser with the 40 ft × 16 ft bed, backed by two 6 kW and two 3 kW machines for thinner work. The bed length means a 3,000 mm part is cut in one set-up with no splice and no re-registration, which matters on canopy rails, coach panels and long brackets where a joint would show after coating. Mild steel, CRCA, stainless, aluminium, brass and copper are all cut in-house, to 20 mm on the laser, with plate from 30 to 40 mm handled on the iron worker.

Downstream, the same parts go to the 200 T, 5-axis brake with the 4,450 mm bed for forming to 10 mm, and to the seven-tank pretreatment and 18 ft × 14 ft powder booth when the drawing calls for a finish. Because the cut, the form and the coat sit in one flow, a thick plate part does not lose its datum between operations.

The laser cutting capability page lists the bed sizes and materials. The 12 kW fibre laser article covers what the source changed for thin sheet, and the mild steel thickness and edge quality guide covers the 1–8 mm band this article deliberately leaves out.

What to put on the DXF and the drawing

1. Grade and thickness, not just "MS 12 mm". IS 2062 E250 or E350 changes the cut parameters and the bend behaviour.
2. Gas expectation, where the edge matters: oxygen, nitrogen, or "supplier's choice, edge to be blasted and coated".
3. Edge class to ISO 9013, plus any functional face that must be dross-free.
4. Minimum hole diameter, and whether holes under 1:1 with thickness may be drilled instead.
5. Pierce and lead-in allowance, especially on parts with a tight corner at the start of the profile.
6. General tolerance to ISO 2768-1 class where the drawing has no individual callouts, so the profile has a default.
7. Lifting and packing, because a 16-part nest of 12 mm brackets is a 265 kg bundle that has to arrive undamaged.

Send the DXF with the revision and the gas requirement through the RFQ route, and the nest and the edge callout come back with the price. If the plate is above 20 mm, say so on the enquiry: the answer will be a different process and a different lead time, and it is better to know that before the drawing is released. More on the wider process sits under the laser cutting and fabrication hub.