Thermal cutting §
Thermal cutting removes material by heating it to its melting or boiling point with a concentrated energy source (laser, plasma arc, oxy-fuel flame, or electron beam) and ejecting the molten or vaporised material. It is the dominant method for 2-D contour cutting of sheet and plate, and the only practical way to cut very thick sections (> 50 mm) without mechanical force.
Authoritative sources
Common principles §
All four thermal cutting processes share the same underlying physics:
- Energy-density threshold. The material must be heated above its melting point (or boiling point, for keyhole-mode laser and EB) at the cut front. The required power density is where is the delivered power and is the effective interaction area. Below this threshold the material merely heats without cutting.
- Heat transfer. Once the cut front is established, heat conducts laterally into the workpiece, creating a heat-affected zone (HAZ). Convection (gas flow) and radiation also carry energy away. The HAZ width is governed by the thermal diffusivity of the material and the dwell time (thickness / cut speed).
- Assist / cutting gas. A gas jet (N₂, O₂, Ar, air) serves two roles: it ejects the molten or vaporised material from the kerf, and (for O₂ on steel) it supplies additional exothermic energy that sustains the cut. Without the gas, the melt re-solidifies and the cut stalls.
- Penetration–kerf trade-off. Greater penetration depth requires more energy, which widens the kerf and enlarges the HAZ. This fundamental trade-off is what differentiates the four processes: laser and EB concentrate energy into a tiny spot (narrow kerf, limited thickness), while plasma and oxy-fuel spread energy over a wider zone (wider kerf, greater thickness).
The material properties that govern cuttability (thermal conductivity, melting point, oxidation behaviour, electrical conductivity) are covered in Materials processing.
Process comparison §
| Process | Kerf width | HAZ | Max thickness (steel) | Edge quality (Ra) | Material restrictions | Relative cost | Typical applications |
|---|---|---|---|---|---|---|---|
| Laser (fibre) | 0.1–0.5 mm | 0.1–0.5 mm | ~25 mm (O₂), ~15 mm (N₂) | 10–20 µm | Most metals; non-metals with CO₂ | Medium–High | Sheet metal, automotive, aerospace |
| Plasma (HD/UHD) | 1–3 mm | 0.5–2 mm | 50–100 mm | 25–50 µm | Electrically conductive only | Low–Medium | Structural steel, shipbuilding, fabrication |
| Oxy-fuel | 5–15 mm | 2–5 mm | 300 mm | 50–100 µm | Ferrous metals only | Low | Thick plate, structural, shipbuilding |
| Electron beam | 0.1–0.5 mm | < 0.1 mm | 50 mm | 5–15 µm | Most metals (vacuum-compatible) | Very high | Aerospace, nuclear, superalloys |
Laser cutting §
A focused laser beam (CO₂ at 10.6 µm, or fibre at 1.06 µm) heats the material to its boiling point; an assist gas (N₂, O₂, or air) ejects the molten metal.
Key parameters: laser power (W), spot diameter (µm), cut speed (m/min), assist gas pressure (bar), focal position (mm above/below surface).
Details
- Conduction mode (low power, thin sheet): the beam heats the surface; conduction spreads the heat. The gas blows away the melt. Used for < 2 mm steel with N₂.
- Keyhole mode (high power, thick section): the beam vaporises the metal, creating a plasma-filled keyhole. The recoil pressure and gas flow eject the melt. Used for 2–25 mm steel with O₂ (exothermic) or N₂ (inert).
- Kerf: 0.1–0.5 mm for fibre laser on steel. The kerf tapers with thickness.
- Heat-affected zone (HAZ): 0.1–0.5 mm for steel; negligible for aluminium with N₂.
- Materials: carbon steel, stainless, aluminium, copper (fibre laser), titanium. Non-metals (wood, acrylic, fabric) are cut with CO₂ laser.
Keyhole-mode cross-section §
The laser and electron-beam equipment used here is the same technology family as in Joining (laser welding, EB welding); the difference is that the beam is scanned along a contour rather than held at a stationary joint.
Plasma cutting §
An electric arc ionises a gas (argon, hydrogen, nitrogen, or air) to a plasma jet at 20 000–30 000 °C. The jet melts and the gas flow blows away the molten metal.
Key parameters: arc current (A), gas flow (L/min), electrode-to-workpiece distance (standoff, mm), cut speed (m/min).
Details
- Conventional plasma: cuts up to 20 mm steel, kerf 2–5 mm, Ra 25–50 µm. Used for structural steel, shipbuilding, and fabrication shops.
- High-definition (HD) plasma: a constricted arc and higher gas pressure reduce the kerf to 1–2 mm and improve edge quality. Cuts up to 50 mm.
- Ultra-high-definition (UHD) plasma: approaches laser quality on 10–25 mm steel at a fraction of the cost.
- Materials: any electrically conductive metal (steel, aluminium, copper, titanium). Non-conductive materials cannot be cut.
Plasma arc constriction cross-section §
Oxy-fuel (flame) cutting §
A preheating flame (oxygen + acetylene or propane) brings the steel to its ignition temperature (~1 300 °C); a high-pressure oxygen jet then oxidises (burns) the metal, and the reaction heat sustains the cut.
Key parameters: preheat flame type, oxygen cutting pressure (bar), cut speed (m/min), kerf width (mm).
Details
- Only works on ferrous metals (carbon steel, low-alloy steel) that oxidise exothermically. Stainless steel, aluminium, and copper cannot be oxy-fuel cut.
- Thickness range: 5 mm to 300 mm. The only practical method for very thick plates (> 100 mm).
- Kerf: 5–15 mm (wide). HAZ: 2–5 mm with a decarburised layer.
- Edge quality: rough (Ra 50–100 µm); usually followed by machining or grinding for critical dimensions.
Oxy-fuel flame geometry §
Electron-beam (EB) cutting §
A focused beam of high-energy electrons (150–200 kV) in a vacuum melts and vaporises the material. The beam is deflected by electromagnetic lenses.
Key parameters: beam current (mA), accelerating voltage (kV), scan speed (m/s), vacuum level (Pa).
Details
- Kerf: 0.1–0.5 mm, very straight edges, minimal HAZ (< 0.1 mm).
- Thickness: up to 50 mm steel in a single pass.
- Limitation: requires a vacuum chamber, so the part must fit inside. Used for aerospace (titanium, superalloys) and nuclear components where HAZ must be minimised.
- Cost: very high capital and operating cost; used only when laser cannot meet the HAZ or kerf requirement.
EB column in vacuum §
The EB source is the same technology used in Joining for electron-beam welding; cutting simply scans the beam along a contour.
Decision heuristic §
When selecting among the four thermal cutting processes, apply these rules in order:
- Material check. If the material is non-ferrous (stainless, Al, Cu, Ti) or non-metallic, oxy-fuel is excluded. If the material is non-conductive, plasma is excluded. If the part must fit in a vacuum chamber and HAZ must be < 0.1 mm, EB is the only option.
- Thickness. > 100 mm → oxy-fuel (only practical option). 25–100 mm → plasma (HD/UHD). < 25 mm → laser or plasma; < 15 mm → laser preferred for quality.
- Tolerance and edge quality. ± 0.2 mm or Ra < 20 µm → laser or EB. ± 0.5 mm → HD/UHD plasma acceptable. ± 1 mm or looser → conventional plasma or oxy-fuel.
- Cost and volume. Low volume / one-off → plasma or oxy-fuel (low capital). High volume with tight tolerances → laser (amortised over many parts). Aerospace / nuclear with HAZ-critical requirements → EB despite cost.
Example problem §
A fabrication shop needs to cut 12 mm carbon steel plate (S355) into 2 m × 1 m panels with a tolerance of ±0.5 mm and no further machining. The production rate is 200 panels per week. Which thermal process is most appropriate, and why?
Solution
- Oxy-fuel: kerf 5–10 mm, HAZ 2–5 mm, edge Ra 50–100 µm. The ±0.5 mm tolerance is not achievable without secondary machining. Rejected.
- Plasma (conventional): kerf 2–4 mm, edge quality moderate. Tolerance ±0.5 mm is marginal. Possible but not optimal.
- Laser (fibre, 6 kW, O₂ assist): kerf 0.3–0.5 mm, HAZ 0.2 mm, edge Ra 10–20 µm. Tolerance ±0.2 mm easily achieved. Cut speed ~10 m/min for 12 mm S355. Selected.
- Electron beam: overkill; requires vacuum, very high cost. Rejected.
The fibre laser at 6 kW with O₂ assist is the correct choice: it meets the tolerance, produces a clean edge requiring no secondary operation, and the throughput (200 panels × ~2 min cutting each ≈ 400 min ≈ 7 h of cutting per week) is well within a single machine’s capacity.
Parent §
This page is a class in the Subtractive manufacturing topic. 4. Electron beam: overkill; requires vacuum, very high cost. Rejected.
The fibre laser at 6 kW with O₂ assist is the correct choice: it meets the tolerance, produces a clean edge requiring no secondary operation, and the throughput (200 panels × ~2 min cutting each ≈ 400 min ≈ 7 h of cutting per week) is well within a single machine’s capacity.