Laser Cutting

The physics of the cut

Laser cutting works because a lens or mirror can concentrate optical power into an absurdly small area. Take a 2 kW beam and focus it to a 0.2 mm spot and you deliver power over a circle of about 3.1 × 10⁻⁴ cm². The resulting power density is:

Power density  q = P / A

  P = 2,000 W
  A = π·(0.01 cm)² = 3.14 × 10⁻⁴ cm²

  q = 2,000 / 3.14 × 10⁻⁴
    ≈ 6.4 × 10⁶ W/cm²

That is millions of watts per square centimetre — far above the roughly 10⁵–10⁶ W/cm² threshold where a metal surface heats fast enough to melt before the heat conducts away. Above ~10⁸ W/cm² the metal vaporizes outright. The whole craft of laser cutting is delivering enough density to melt or vaporize a column of metal through the sheet, then clearing that melt before it freezes back into the kerf.

The energy needed to remove material per unit volume sets a floor on how fast you can cut. To melt and eject steel you must supply both the sensible heat to reach the melting point and the latent heat of fusion:

Energy to melt unit volume:
  E_v = ρ·[ c_p·(T_m − T_0) + L_f ]

  ρ   = 7,850 kg/m³           (steel density)
  c_p = 490 J/(kg·K)          (specific heat)
  T_m = 1,510 °C, T_0 = 20 °C
  L_f = 270 kJ/kg             (latent heat of fusion)

  E_v ≈ 7,850 × (490·1490 + 270,000)
      ≈ 7.8 × 10⁹ J/m³  (≈ 7.8 J/mm³)

So roughly 8 joules melts a cubic millimetre of steel. Cut a 0.3 mm kerf through 5 mm plate and you must melt about 1.5 mm³ per millimetre of travel — about 12 J/mm. A 2 kW beam delivering, say, 60% useful coupling supplies ~1200 J/s, so the speed limit is around 100 mm/s, or 6 m/min, before the melt front lags the beam. Real machines run somewhat slower to leave margin for clean ejection.

Spot size, focus and depth of field

You cannot focus a real beam to a point. Diffraction sets a minimum focused spot diameter that grows with wavelength λ and focal length F, and shrinks with the raw beam diameter D entering the lens:

Focused spot diameter (diffraction limited):
  d ≈ 4·M²·λ·F / (π·D)

  Rayleigh range (depth of focus):
  z_R ≈ π·(d/2)² / (M²·λ)

This is the central trade-off in cutting optics. A short focal length gives a tiny spot and very high power density — great for thin sheet and fine features — but a shallow depth of focus, often well under a millimetre. A long focal length gives a fatter spot but a focal column deep enough to span thick plate. M² is the beam quality factor; a single-mode fiber laser (M² ≈ 1.1) can be focused far tighter than a multimode beam, which is why fiber lasers cut thin sheet so cleanly and quickly.

Where you place that focus along the cut depth is a process parameter in its own right. Put the focus at or slightly above the top surface for thin material to get the narrowest kerf. Drop it a third of the way into the plate for thick sections so the high-density region covers more of the cut. A focus error of half a millimetre on 15 mm stainless is enough to stop the bottom of the kerf clearing — the melt freezes and welds the cut shut. This is why modern heads measure standoff capacitively and autofocus to within tens of microns.

The assist gas does the dirty work

The beam melts; the gas evacuates. A coaxial nozzle surrounds the beam and blows high-pressure gas straight down the kerf, accelerating the molten pool and ejecting it out the bottom before it resolidifies. Without it, the melt would refill the channel and you would get a glowing weld bead instead of a cut. The choice of gas defines the cutting regime:

	Fusion (Nitrogen)	Reactive (Oxygen)	Air	Vaporization
Mechanism	Melt + inert blow-out	Melt + exothermic burn of iron	Melt + mild oxidation	Solid/liquid → vapour
Typical pressure	15–25 bar	0.5–6 bar	6–15 bar	varies
Edge quality	Bright, oxide-free, weldable	Oxidised (black) skin	Light oxide	Minimal recast
Speed (mild steel)	Slower, power-limited	Fastest (burn adds ~40–60% of heat)	Moderate	Thin only
Best for	Stainless, aluminium	Thick mild / carbon steel	Cost-sensitive mild steel, thin	Foils, non-metals, micro-cuts
Running cost	High (gas volume)	Low	Lowest	varies
Catch	Gas consumption dominates cost	Edge needs cleaning before welding/paint	Compressor moisture/oil risk	Low throughput on thick stock

Oxygen cutting is a quiet trick: the iron itself burns. 2Fe + O₂ → 2FeO releases roughly 4–5 MJ per kg of iron oxidised, which on thick mild steel contributes more energy to the cut than the laser does. That is why an oxygen cut on 20 mm carbon steel runs faster, and at lower laser power, than a nitrogen cut — but it leaves a dark oxide edge that must be ground before welding or painting. Nitrogen keeps oxygen out entirely, giving a bright, paint-ready edge, but you pay for it in gas: 20 bar nitrogen at a 2 mm nozzle moves a lot of cubic metres per hour.

Worked example: nitrogen cut on 6 mm stainless

Estimate a feasible cutting speed for 6 mm 304 stainless with a 4 kW fiber laser, 0.25 mm kerf, nitrogen fusion cut. The melt energy for stainless is close to steel, ~8.5 J/mm³ including latent heat:

Volume removed per mm of travel:
  V' = kerf × thickness = 0.25 × 6 = 1.5 mm³/mm

Energy needed per mm of travel:
  E' = V' × E_v = 1.5 × 8.5 ≈ 12.75 J/mm

Useful power (assume η ≈ 0.5 coupling/ejection efficiency):
  P_useful = 4,000 × 0.5 = 2,000 W = 2,000 J/s

Max speed:
  v = P_useful / E' = 2,000 / 12.75
    ≈ 157 mm/s ≈ 9.4 m/min

Real machine tables for this exact case list around 7–8 m/min — slower than the energy ceiling because part of the budget goes to the heat-affected zone, kerf walls, and the need to fully clear the melt for an oxide-free edge. The calculation gets you the right order of magnitude and shows why doubling power roughly doubles speed until ejection, not energy, becomes the limit.

What controls cut quality

• Striations. The cut edge shows fine vertical lines — frozen ridges left by the oscillating melt front. Lower striation amplitude means a smoother edge; it improves with the right speed, focus, and gas pressure, and degrades sharply if you cut too fast.
• Dross (burr). Resolidified melt clinging to the bottom edge. Caused by too little gas pressure, wrong focus, or excessive speed so the melt is not fully ejected before it freezes. The single most common quality complaint.
• Kerf taper. The kerf is slightly wider at top than bottom because the beam converges and diverges. Minimised by correct focus placement and adequate depth of focus.
• Heat-affected zone (HAZ). The thin band beside the kerf whose microstructure changed from the thermal cycle — usually 0.1–0.5 mm. Matters for fatigue-critical parts and for hardenable steels that may form brittle martensite at the edge.
• Perpendicularity. Edge squareness, graded by standards such as ISO 9013. Degrades on thick plate and at high speed.

Failure modes and trade-offs

• Burn-through and dross on thick plate. If focus or gas is wrong, the bottom of the kerf stops clearing; the melt bridges across and the cut fails to separate. Cure: lower focus into the plate, raise gas pressure, slow down.
• Back-reflection on shiny metals. Copper, brass and aluminium reflect a large fraction of the incident beam — especially at CO₂ wavelengths — and that reflected power can travel back up the optical chain and damage the laser. Fiber lasers with back-reflection isolators and the shorter 1.06 µm wavelength (better absorbed) made these metals routinely cuttable.
• Piercing spatter and lens damage. Starting a cut requires piercing a hole, which throws molten spatter upward toward the protective cover lens. Ramped or pulsed piercing and a clean nozzle reduce contamination; a fouled lens absorbs power, overheats and shatters.
• Plasma shielding. At very high power density the vaporized metal can ionise into a plasma plume that absorbs and defocuses the incoming beam, choking the cut. Cross-jet gas and tuned parameters keep the plume from blocking the beam.
• Thermal lensing in optics. Absorbed power in dirty or aging lenses shifts their focal length during a long cut, drifting the focus and quietly ruining edge quality. Watched via power and edge monitoring.
• Speed-vs-quality trade. Pushing feed rate to cut cycle time widens striations and grows dross; the economic optimum is usually well below the maximum speed the machine can sustain.

Where laser cutting fits among processes

Laser cutting is fast, accurate, and contact-free, with a tiny kerf and no tool wear, which makes it the default for medium-gauge sheet metal in flat blanks. But it competes with several alternatives, each with a niche. Plasma cutting is cheaper and faster on very thick conductive plate but leaves a wider, rougher kerf. Waterjet cutting handles any material, including stone, glass and composites, with zero heat-affected zone, but runs slower and at higher consumable cost. Wire EDM gives micron accuracy on hardened conductive metal but is glacially slow. Mechanical punching and shearing beat the laser on cost-per-part for very high volumes of simple shapes. The laser wins when you need tight tolerances, complex contours, fast changeover between parts, and a clean edge — and the part is thin-to-medium metal rather than 50 mm plate.