Laser cutting mild steel sheet: thickness, tolerance, cost

Two panels, same outline, same 2 mm mild steel, quoted a third apart. The difference is rarely the machine rate. It is the pierce count, the nest and whether the part needs a secondary operation, and all three are visible in the DXF before anyone cuts metal.

Laser cutting mild steel sheet is a process with a wide working range and a narrow economic one. The 12 kW fibre laser will cut 20 mm plate, and it will also cut the same 20 mm at a speed that makes a 4 mm part look cheap by comparison. Understanding where the time goes is how a design engineer avoids specifying a part that is technically possible and commercially silly.

What thickness of mild steel can a fibre laser cut?

From 0.8 mm to 20 mm on the 12 kW machine, with the useful band for production work between 1 mm and 12 mm. Below 1 mm the sheet distorts from heat input and the handling dominates; above 12 mm the cut speed falls away and the kerf widens enough to change the nest.

The support machines matter as much as the flagship. On this floor the 12 kW laser carries the 40 ft × 16 ft bed and the thick end, while 6 kW and 3 kW machines take thinner gauges and smaller nests. A job that needs 1 mm sheet at high volume is better placed on a 3 kW machine running flat out than on the 12 kW machine between heavy plates, because the setup and the sheet handling are the same and the hourly cost is not.

Thickness also decides the assist gas. Nitrogen at 12–16 bar gives a clean, oxide-free edge on 1–4 mm, which suits parts that go straight to powder coating or to a weld without pickling. Oxygen is faster and cheaper from about 4 mm up, at the cost of a thin oxide film on the cut face that has to come off before coating. The choice belongs on the drawing as an edge requirement, not as a machine setting left to the operator.

How do speed and kerf change with thickness?

Cut speed falls faster than thickness rises, which is the whole economics of the process. Doubling the thickness roughly triples the cut time, and the pierce time grows with it. The kerf widens at the same time, so each part takes more metal out of the nest.

Treat the speeds as a planning figure and not a guarantee. They move with the gas, the nozzle, the focus position, the material's surface condition and the machine's condition. The kerf column is more stable, and it is the one that changes the part: a 0.25 mm kerf at 2 mm is 12% of the thickness, so the CAM programmer compensates for it and the drawing never sees it.

ISO 9013 class is the term that gets misused in drawings. The standard classifies thermal cuts by the perpendicularity and the drag of the cut face, and it applies to the cut edge rather than to the part dimensions. Class 2 at 6 mm is a normal laser result. Class 1 at 12 mm means a second operation, because the drag line at that thickness is inherent to the process rather than to the machine.

What does laser cutting mild steel sheet cost in machine time?

Build it in machine seconds, not in currency, and the comparison survives a change of shop. Take a 400 × 300 mm panel in 2 mm mild steel with a 1,400 mm outline and twelve 8 mm holes, so 302 mm of hole perimeter. The total cut length is 1,702 mm.

At 24 m/min the cut takes 1,702 ÷ 24,000 = 0.071 minutes, or 4.3 seconds. The thirteen pierces at 0.4 seconds each take 5.2 seconds, which is more than the cutting. Add 3 seconds of rapid traverse between features and the part needs about 12.5 seconds of beam time.

Now change the thickness to 6 mm and keep the geometry. Laser cutting mild steel sheet at that gauge runs at 6.5 m/min, so 1,702 mm takes 15.7 seconds, the pierces take 1.2 seconds each for 15.6 seconds, and the rapids are the same 3 seconds. The part needs about 34 seconds, which is 2.7 times the 2 mm part for three times the thickness. That ratio, not the material price, is why a 6 mm bracket costs what it does.

The last number is the one buyers miss: pierce count is not proportional to part size. A small bracket with forty holes can take longer than a large panel with four. Chain cutting and common-line cutting remove pierces by starting the next contour where the last one ended, and the nesting decision that does it is made in CAM. The nesting guide covers the patterns that remove the most pierces.

What tolerance does laser cutting hold on mild steel?

±0.10 mm on hole position is the working band up to 3 mm, and it holds because the machine positions the head to about 0.05 mm and the kerf is compensated in CAM. The band widens with thickness, and it widens for reasons the positioning system cannot fix.

Hole diameter is the first to move. At 2 mm the kerf is 0.25 mm and the cut face has a slight taper, so an 8 mm hole measures within ±0.10 mm on the entry side and about 0.05 mm smaller on the exit. At 12 mm the kerf is 0.60 mm and the taper is larger, so the diameter band opens to ±0.25 mm and the hole is measurably barrel-shaped. A drawing that calls ±0.10 mm on a 12 mm hole needs a boring operation, not a better laser.

Position is more stable than size because it depends on the machine rather than the cut. A hole pattern cut in one nesting holds ±0.10 mm across 1,000 mm, and the same pattern cut in two separate setups holds ±0.25 mm, because the second setup re-datums the part. That is the argument for keeping a critical hole pattern in one nest even when it costs yield.

What does the cut edge look like at 2 mm and at 12 mm?

At 2 mm the edge is smooth, with a narrow heat-affected zone and a drag line you can feel but not see. At 12 mm the same edge carries a visible drag line, a wider heat-affected zone and a roughness that a paint film will not cover. That difference is what the ISO 9013 class is describing, and it is why a 12 mm part that will be painted usually needs the oxide removed.

Measured on mild steel, the heat-affected zone runs about 0.1–0.2 mm at 2 mm and up to 0.4 mm at 12–16 mm. It is hard, and on a part that will be bent near the cut edge it is a crack initiation site: bend a 12 mm plate with the cut edge on the outside of the bend and the hard zone can open. The fix is to keep the bend line at least 1.5 times the thickness from the cut edge, or to normalise the edge, which is not a laser operation.

The oxide layer is the other practical difference. An oxygen-cut edge carries a thin grey-black oxide that interferes with powder adhesion and shows through a thin paint film as a dark line. A seven-tank pretreatment removes it, which is why the coating route matters when the drawing chooses oxygen over nitrogen. A part that leaves the laser and goes straight to a weld without pretreatment has to have the oxide removed at the joint, or the weld porosity follows.

Dross is the third symptom and the easiest to diagnose. Dross on the underside means the focus is low or the gas pressure is short; dross only at corners means the cut speed dropped without the power following. Neither is a material fault, and both are corrected at the machine rather than by changing the drawing. What the drawing controls is whether dross is acceptable: a part that will be coated needs the underside clean, because powder does not hide a hanging droplet. The mild steel edge quality guide covers how roughness and drag are measured against the class.

How much does the nest change the material bill?

It changes the scrap and the machine time together, which is why nesting is the first lever in a laser quote. The 400 × 300 mm panel is 120,000 mm². On a 1,000 × 2,000 mm sheet, two across at a 4 mm web and six down gives twelve parts and 72% utilisation, because 808 mm of the 1,000 mm width and 1,824 mm of the 2,000 mm length are used.

On a 1,250 × 2,500 mm sheet the same part goes three across and eight down: 24 parts, 2,880,000 mm² of the 3,125,000 mm² sheet, or 92%. The material per part falls from 166,667 mm² to 130,208 mm², which at 2 mm and 7.85 g/cm³ is 2.04 kg against 2.62 kg. Against a finished part of 1.884 kg, the scrap falls from 0.73 kg to 0.16 kg, a saving of 0.57 kg of steel per part, and 24 parts per sheet instead of 12 halves the number of sheet loads.

The gain is not automatic. Three across needs the sheet to be 1,250 mm wide, and not every supplier stocks that width. It also needs the part rotated so the grain runs the same way in every position, which matters if the panel is bent after cutting. And it needs the pierce points shared, which is a CAM decision rather than a layout accident. The thickness and price guide works the same arithmetic across gauges.

How is this run at Ganesh Multitech?

Mild steel and CRCA are cut on the 12 kW fibre laser with a 40 ft × 16 ft bed, supported by two 6 kW and two 3 kW machines, across 0.8–40 mm for custom parts. The bed size matters for long parts: a 12 m rail or a full canopy panel is cut in one setup rather than in sections that have to be joined.

Cut parts feed the 200 T, 5-axis press brake with a 4,450 mm bed for bending to 10 mm, the three CLOOS robotic MIG cells and the manual welding stations, and then seven-tank pretreatment and powder coating. Cutting, bending and coating under one roof is what makes a ±0.5 mm formed tolerance defensible, because the datum does not change hands three times.

The company has manufactured since 1992 across five units in Mumbai (Vasai) and Nagpur, under ISO 9001:2015 and ISO 14001:2015. Customers include Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways. The laser cutting capability page lists the machine range and the materials behind these numbers.

What belongs in the DXF and on the drawing?

• Material and grade, for example IS 513 D at 2 mm, with the thickness tolerance class.
• The cut edge requirement: ISO 9013 class, and whether the oxide layer is acceptable.
• Holes below one times the thickness, flagged so the route can add a drill operation.
• The critical hole pattern, marked to stay in one nest.
• Bend lines and the bend allowance basis, so the flat pattern is not re-derived by the supplier.
• The visible face, so pierce points can be placed away from it.
• Thread and tap callouts, which are a secondary operation and not a laser feature.
• The finish and its thickness, because a 0.25 mm kerf and a 70 µm coat both move a tight edge dimension.
• The DXF RFQ checklist covers the layer naming and the closed-contour checks that stop a file being re-quoted.

When laser cutting is the wrong route

Laser cutting is the wrong route for a part above about 50,000 pieces a year that fits a press, where stamping removes the cut time from the piece price entirely. It is also wrong for a hole smaller than 0.8 times the thickness, for a cut edge that needs a ground finish, and for a part that needs a counterbore or a thread, because those are separate operations whatever the cutting method.

We do not cut above 20 mm on the laser, we do not mirror-polish a cut edge, and we do not hold a class 1 cut on 12 mm plate without a second operation. Where a drawing needs any of those, the route changes and the quote should say so at the top. Send the DXF, the material and the volume through the RFQ form, and the laser cutting and fabrication hub carries the edge-quality and tolerance guidance that sits behind the table above.