Fiber laser capacity: maximum thickness is not the number to specify on
Every fiber laser carries two thickness numbers for mild steel, and most machines get specified on the wrong one. The maximum tells you what the laser can sever on a clean sample. The production-ready figure tells you what it will run on your floor, on the steel you actually buy.
The short answer. With oxygen assist, a fiber laser runs mild steel up to about 16 mm in production at 6 kW and about 80 mm at 60 kW. Its maximum rating is higher at every power level:
| Laser power | Runs in production up to | Can reach at maximum |
|---|---|---|
| 6 kW | 16 mm (5/8") | 25 mm (1") |
| 12 kW | 30 mm (1-1/8") | 40 mm (1-5/8") |
| 20 kW | 40 mm (1-5/8") | 50 mm (2") |
| 30 kW | 50 mm (2") | 60 mm (2-3/8") |
| 40 kW | 70 mm (2-3/4") | 80 mm (3-1/8") |
| 60 kW | 80 mm (3-1/8") | 120 mm (4-3/4") |
Specify a machine on the production number, not the maximum. The rest of this article explains why the two differ and when the gap matters.
Ask what thickness a fiber laser cuts in mild steel and you will get a single number back. That number is almost always the maximum: the thickest plate the machine can physically sever, under good conditions, on a sample.
It is a real number. It is just not the number that governs your shop. There is a second figure, the thickness the machine runs reliably shift after shift, and the gap between the two widens as material gets thicker. At 6 kW that gap is about nine millimetres. At 60 kW it is forty.
In production terms that runs from about 16 mm of mild steel at 6 kW to about 80 mm at 60 kW, with the maximum ratings sitting higher again. Specifying on the wrong one is how a machine ends up technically capable of the job and commercially wrong for it.
What are the two thickness numbers on a fiber laser spec?
Production-ready thickness is the zone where a shop holds stable speeds, clean edges and minimal rework. It is the thickness you can quote against and schedule with confidence, because it runs on standard, sustained parameters rather than on a setup somebody has to babysit.
Maximum possible thickness is the upper edge of what the machine can sever. It is real capability, but speeds slow, edge finish usually calls for secondary work, and the process is far less tolerant of ordinary material variation.
How thick can a fiber laser cut mild steel?
Oxygen assist throughout. Mild steel is the baseline case: with oxygen assist it is the deepest cutting of the common materials at a given power level.
| Laser power | Production-ready range | Maximum possible |
|---|---|---|
| 6 kW | 1 to 16 mm (5/8") | 16 to 25 mm (1") |
| 12 kW | 1 to 30 mm (1-1/8") | 30 to 40 mm (1-5/8") |
| 20 kW | 1 to 40 mm (1-5/8") | 40 to 50 mm (2") |
| 30 kW | 1 to 50 mm (2") | 50 to 60 mm (2-3/8") |
| 40 kW | 1 to 70 mm (2-3/4") | 70 to 80 mm (3-1/8") |
| 60 kW | 1 to 80 mm (3-1/8") | 80 to 120 mm (4-3/4") |
Reaching the maximum at any power level requires laser-grade steel with a clean surface, free of mill scale and rust, and a consistent material temperature.
Why do two ratings exist at all?
The answer is beam power density, meaning the laser power concentrated into the focused spot. Below a minimum density the cut destabilizes. Density is set by two things, the power available and how small the beam is focused, and the relationship between them drives the whole trade-off.
A smaller beam raises density but narrows the kerf. A lower-power laser can still reach thick material by focusing to a smaller spot to keep density up. That smaller beam cuts a narrower slot, and a narrow slot is harder to clear. Molten metal has less room to eject, so the cut is more prone to fusing, dross and poor edges the moment the material is less than ideal.
A larger beam is more forgiving but demands more power. Opening the beam widens the kerf, which gives assist gas room to expel melt and makes the process markedly more tolerant of real-world material. But a larger beam spreads the same power over more area, so you need more wattage to hold the minimum density.
That is the real reason higher-power lasers cut thick sections more reliably. Not simply that they have more power, but that the power lets them run a wider, more forgiving kerf while keeping density in range.
When does material quality decide the cut?
A narrow, high-density beam reaches heavier material only when conditions are near perfect: the surface clean, the steel laser-grade, the temperature consistent. Push into the maximum range and four things stop being minor variables and start determining whether the cut succeeds.
Mill scale on the surface. Scale creates hot spots that penetrate ahead of the cut and leave veins down the edge, disrupting an already tight kerf.
Metallurgy. Variation in steel chemistry and cleanliness changes how the material melts and ejects, shifting the process off its narrow window.
Material temperature. Inconsistent starting temperature changes heat balance in the cut zone and undermines repeatability.
Surface rust. Rust and other surface contamination alter beam absorption and introduce inconsistency in piercing and along the cut.
In the production-ready range, the wider kerf and larger process window absorb these deviations, and material quality matters far less. In the maximum range, the same deviations are what separate a clean cut from a scrapped part. That is the practical meaning of the two ratings.
Which rating should you specify against?
It is easy to anchor on the maximum figure. It is the bigger number and it is the one on the brochure. For a production buyer it is the production-ready rating that governs throughput, edge quality and scrap.
The question worth asking is not whether a machine can cut 40 millimetre plate. It is what that machine does on 40 millimetre plate, every shift, on the steel your supplier actually delivers.
Once a machine is on the floor, holding it inside that production range is a maintenance question rather than a specification one, and fiber laser maintenance covers what that takes.
If you need thick material with forgiveness in day-to-day production rather than on a sample, the answer is generally to step up to a higher wattage, so the thickness you run sits inside the production range on a wide, stable kerf instead of at the ragged edge of maximum capability. That is a larger purchase and it is worth being plain about that. It is also often the cheaper machine over its life, because the cost of the alternative does not appear on the quote. It appears as slow cycles, secondary operations and scrapped parts.
A note on these figures
These parameters come from the manufacturers' own factory cutting tests and are provided for reference. Actual results vary with the grade and surface condition of the material, assist gas supply and pressure, the cutting head and optics, the control system, and other conditions specific to your shop. The maximum thickness figures represent the upper edge of capability, where speeds slow and edge finish typically requires secondary work.
Because these principles hold across the fiber laser industry, the table is a useful baseline whichever manufacturer you are evaluating. AC Machinery carries fiber lasers from more than one manufacturer and quotes across them, so this comparison is not tied to one brand.
Where AC Machinery can be more precise is on a specific configuration. Tell us what you cut, how thick, and in what volume, and we will confirm the validated cutting parameters for the machine we quote you.
About Adam Bender
Adam spent roughly twenty years as a field technician servicing plasma and laser cutting machines before taking over AC Machinery. These notes come from that work.
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Common questions
Short answers to the questions we are asked most about this topic.
It depends on power, and on which of the two ratings you mean. With oxygen assist, a 6 kW laser runs 1 to 16 mm in production and can reach 25 mm at maximum. A 12 kW runs to 30 mm and reaches 40 mm. A 20 kW runs to 40 mm and reaches 50 mm. At the top of the range, a 60 kW laser runs to 80 mm in production and can reach 120 mm.
Production-ready is the thickness a machine runs shift after shift on standard parameters, with stable speeds, clean edges and minimal rework. Maximum is the thickest plate it can physically sever, at slower speeds, usually with secondary work on the edge, and with much less tolerance for ordinary variation in the material.
Not simply because it has more power. A cut needs a minimum beam power density to stay stable, and a lower-power laser can only reach that density by focusing to a smaller spot. A smaller spot cuts a narrower kerf, and molten metal has less room to eject from it. More power lets a machine hold that density while running a wider, more forgiving kerf.
In the production-ready range, far less than people expect, because the wider kerf and larger process window absorb the deviations. In the maximum range it is often decisive. Mill scale, steel chemistry, starting temperature and surface rust each shift the process off a window that is already narrow.
For production work, no. The maximum tells you what the machine can do on a sample coupon. The production-ready rating is what governs throughput, edge quality and scrap on your floor. If you need thick material with forgiveness day to day, the usual answer is to step up a wattage so that thickness sits inside the production range.
Laser-grade steel with a clean surface, free of mill scale and rust, and a consistent material temperature. Those conditions are straightforward to describe and harder to guarantee on every sheet a supplier delivers, which is the practical reason the two ratings differ.