Steel laser cutting accuracy: what sets the real tolerance floor

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  How the cut actually removes steel
• 2
  What moves steel laser cutting accuracy off target
• 3
  Taper, dross, and the edge you cannot inspect with a caliper
• 4
  Where steel laser cutting accuracy stops being enough
• 5
  Turning laser-cut blanks into parts that assemble
• 6
  Laser cutting vs. CNC milling for steel features

A fiber laser focuses 1,070 nm light into a spot roughly 0.1–0.3 mm wide. Power density at that spot reaches 10^6 W/cm² or more, so the steel does not melt politely along a line. It forms a narrow keyhole of vaporized metal, and the assist gas drives the molten film down through the kerf.

That means the kerf is a dynamic channel, not a saw blade of fixed width. Cut width depends on spot size, focus position, gas pressure, and how fast the head travels. Change the feed rate and the kerf changes with it.

The practical consequence: steel laser cutting accuracy is partly a machine specification and partly a process outcome. Two shops with the same 6 kW source can hold very different numbers on the same 6 mm plate.

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  Kerf widthTypically 0.2–0.5 mm on thin steel, wider as thickness climbs.
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  Heat affected zoneUsually under 0.1 mm on carbon steel at normal feed rates.
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  Focus positionOften set slightly below the surface on thicker plate.

Thickness is the first lever. On 1–2 mm mild steel a good fiber machine holds profile position within about ±0.05 mm. By 10 mm the same machine may drift to ±0.15 mm or worse, because the kerf widens and the beam loses coherence as it travels through the cut.

Assist gas is the second. Oxygen gives a hot, reactive cut on carbon steel and runs fast, but it leaves a rougher oxide edge and more taper. Nitrogen gives a clean, cool cut with better edge quality, at the cost of speed and gas consumption.

Then come the boring ones: nozzle condition, lens contamination, and sheet flatness. A chipped nozzle changes gas flow and pulls the kerf sideways. A warped sheet changes standoff and the focus walks off the surface. Neither shows up in the machine spec sheet.

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  PowerHigher power cuts faster but can widen the kerf on thin stock.
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  Feed rateToo fast leaves dross and rounded corners; too slow over-burns.
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  Material gradeA36 and 1018 cut predictably; 4140 and 4340 need tuning.

A laser cut edge is rarely square. The top of the kerf is narrower than the bottom on oxygen cuts, and the striations you can see on the cut face show the direction of melt flow. On a 6 mm plate the taper can be 0.05–0.1 mm per side, which matters if the part drops into a mating slot.

Dross is the other give-away. Hard, adherent dross on the bottom edge means the melt is not being cleared, usually from low gas pressure, wrong focus, or a feed rate pushed too high. It also means the dimensional result is unreliable, because the same conditions that trap dross tend to widen the kerf.

For a bracket that bolts to a frame with 1 mm clearance, none of this matters. For a shim that sits between two machined faces, it does.

Laser cutting is a 2D thermal process. It gives you a profile. It does not give you a bore with a controlled diameter, a flat face, or a thread. If the drawing has an H7 hole, a perpendicular face, or a surface finish callout, the laser is only the first operation.

Hole quality is the usual flashpoint. A laser-cut hole under about 1× material thickness tends to come out tapered and slightly undersized, so shops often cut it small and ream it, or drill it after cutting. A 10 mm hole in 10 mm steel is near the practical edge of what a laser will hold cleanly.

Sharp internal corners are another limit. The beam has a finite radius, so a true 90° internal corner is not possible on a laser. You get a radius roughly equal to half the kerf. If the design needs a sharp corner, that is a milling job.

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  Hole-to-thickness ratioBelow 1:1, expect taper and rework.
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  Internal corner radiusAt least half the kerf width; design for it.
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  Datum featuresLaser edges are poor datums; machine them if they matter.

A common workflow is laser, then CNC. The laser produces the flat outline fast, then a 3-axis or 5-axis mill finishes the bores, faces, and slots that carry the tolerance. This keeps the cutting time low and puts the machine time where the accuracy is actually needed.

The reason to separate the two is thermal history. A laser edge has a thin recast layer and a heat affected zone. If you later weld or anodize the part, that edge behaves differently from a machined face. Machining 0.2–0.3 mm off the cut edge removes the recast layer and gives you a clean surface.

For steel parts that must hit ±0.005 mm, the laser cut geometry is not the finished geometry. It is a near-net blank. Plan the stock allowance that way and the whole route becomes predictable.

GreatLight runs 127 high-precision CNC machines alongside cutting operations, with 16 simultaneous 5-axis centers and a maximum processing size of 4,000 mm. That lets us take a laser-cut blank and finish it in the same shop rather than bouncing the part between vendors.