How Fiber Laser Cutting Works — From Beam to Edge

What Is Fiber Laser Cutting?

Fiber laser cutting uses a concentrated beam of light to melt, burn, or vaporize sheet metal along a programmed path. The “fiber” part refers to how the beam is generated and delivered — through optical fibers doped with rare-earth elements like ytterbium, rather than through gas-filled tubes or mirror assemblies.

This technology has largely replaced CO2 lasers in sheet metal fabrication over the past decade. The reasons are practical: fiber lasers cut thin metal 3-5x faster, consume 50-70% less electricity, and require almost no optical maintenance. For shops processing steel, stainless steel, and aluminum under 20mm thick, fiber laser is the clear standard.

At Laser Tuan Thinh, our high-power fiber laser systems run daily across mild steel, stainless, aluminum, copper, and brass — from 0.5mm decorative panels to 20mm structural plates.

How the Beam Works

A fiber laser generates light at a wavelength of 1.06 micrometers (1,064 nm) — roughly ten times shorter than the 10.6 μm wavelength of a CO2 laser. This shorter wavelength is absorbed more efficiently by metals, which is why fiber lasers cut reflective materials like aluminum and copper that would damage older CO2 systems.

Beam generation starts with semiconductor diodes that pump energy into a fiber core doped with ytterbium ions. The fiber itself acts as both the gain medium and the delivery mechanism — there are no mirrors to align, no gas to replenish, and no beam path exposed to contamination.

Beam delivery is equally straightforward. The light travels through a flexible fiber optic cable directly to the cutting head. CO2 lasers require rigid mirror assemblies that must be precisely aligned and regularly maintained. Fiber delivery eliminates this entirely.

Focus and spot size determine cut quality. The beam passes through a collimating lens and then a focusing lens inside the cutting head. For typical sheet metal work, the focal length produces a spot diameter of 0.02-0.1mm at the material surface. The focus position — where the beam’s narrowest point sits relative to the material — is critical:

• Focus on surface: Best for thin material, clean top edge
• Focus inside material: Better for thicker cuts, more energy at depth
• Focus below material: Used for certain nitrogen cuts on stainless

The focus window is roughly ±0.5-1mm. Outside this range, cut quality degrades rapidly — wider kerf, rougher edges, increased dross.

Assist Gas: Nitrogen vs Oxygen vs Air

The laser beam does the melting, but assist gas does the actual material removal. Gas flows coaxially through the nozzle at high pressure, blowing molten metal out of the cut. The choice of gas fundamentally changes the cut result.

Oxygen cutting

Oxygen creates an exothermic reaction with carbon steel. The metal doesn’t just melt — it combusts. This reaction adds energy to the cut, allowing the laser to cut thicker material or cut faster than the beam power alone would permit. A 4kW laser cutting 10mm mild steel with oxygen can match a 6kW laser cutting with nitrogen.

The tradeoff: oxygen leaves an oxide layer on the cut edge. This dark, slightly rough surface is acceptable for most structural work but may need grinding before welding or painting. For powder coating, oxide edges are typically fine — the coating adheres well.

Nitrogen cutting

Nitrogen is inert — it simply blows molten metal out without reacting. The result is a clean, bright, oxide-free edge straight off the machine. This matters for stainless steel (where oxidation ruins corrosion resistance), aluminum (where oxide interferes with anodizing), and any visible edge that won’t be finished.

Nitrogen cutting requires significantly higher pressure (12-20 bar vs 1-5 bar for oxygen) and consumes much more gas. For a typical shop, nitrogen costs 3-5x more per meter of cut than oxygen.

Compressed air

Shop air (filtered and dried) is the budget option. It’s roughly 80% nitrogen and 20% oxygen, so the result is a compromise — slight oxidation but much cheaper than pure nitrogen. For internal brackets, hidden components, and prototype work, air cutting is often sufficient.

Kerf, HAZ, and Edge Quality

Three characteristics define a laser cut’s quality: kerf width, heat-affected zone (HAZ), and edge roughness.

Kerf width

The kerf is the width of material removed by the cut. Fiber lasers produce kerf widths of 0.1-0.3mm — roughly half that of CO2 lasers and a fraction of plasma or waterjet cuts. This narrow kerf means:

• Less material waste
• Tighter nesting (parts packed closer together on the sheet)
• Smaller features and finer details are possible
• Dimensional accuracy within ±0.05-0.1mm

The NC program compensates for kerf automatically — the beam path offsets by half the kerf width so the finished part matches the drawing dimensions.

Heat-affected zone

Every thermal cutting process heats the material adjacent to the cut. This heat-affected zone undergoes microstructural changes — the steel may harden, become brittle, or develop residual stresses. Fiber laser cutting produces the smallest HAZ of any thermal process, typically 0.05-0.2mm wide.

Why this matters for downstream processes:

• Bending near a laser edge: If the bend line runs through the HAZ, the hardened material may crack. Keep bend lines at least 2x material thickness away from cut edges.
• Welding: The narrow HAZ means less pre-existing distortion, but the hardened zone can affect weld penetration. Grinding the edge before welding eliminates this.
• Fatigue life: For cyclically loaded parts, the HAZ can be the initiation point for fatigue cracks. For critical applications, consider specifying edge grinding.

Edge quality factors

Three machine parameters control edge quality:

1. Focus position — Must be precisely set for each material and thickness
2. Cutting speed — Too fast creates striations and dross; too slow creates excessive HAZ
3. Gas pressure — Insufficient pressure leaves dross (resolidified metal) on the bottom edge

A well-tuned fiber laser produces edge roughness of Ra 1.6-3.2 μm on thin material — smooth enough for most applications without secondary finishing.

Material Capabilities

Modern fiber lasers in the 6-12kW range handle a broad spectrum of materials and thicknesses.

Speeds vary significantly with laser power. The values above are representative for a 6kW system. A 12kW machine cuts 2mm mild steel at 25+ m/min.

Design Tips for Laser Cutting

Good design reduces cost, improves quality, and avoids production delays. These rules apply broadly to fiber laser cutting.

Minimum hole diameter

Hole diameter should be greater than or equal to material thickness. A 2mm thick sheet can reliably have 2mm diameter holes. Below this ratio, the beam dwells too long in a small area, causing excessive heat buildup and distorted holes.

For holes smaller than material thickness, consider drilling or punching as a secondary operation.

Avoid sharp inside corners

Sharp inside corners (90° with zero radius) are problematic. The laser must decelerate to zero, change direction, and accelerate again — creating a larger HAZ and a slightly rounded corner regardless. Specify a minimum inside radius of 0.5mm (or material thickness, whichever is larger) for clean corners and reduced stress concentration.

Minimum feature spacing

Keep the distance between features (holes, slots, edges) at least equal to material thickness, and preferably 1.5-2x thickness. Closer spacing concentrates heat and can cause warping or incomplete cuts.

Tab and microjoint placement

Small parts (under ~30mm) need tabs (microjoints) to prevent them from tipping into the cut gap and being damaged by the beam. The NC programmer places these automatically, but be aware:

• Tab marks require grinding or snapping off
• Place non-critical edges where tabs will be positioned
• Indicate on the drawing which edges are cosmetic

Minimum slot width

Slots should be at least 1.5x material thickness wide. A 2mm sheet should have slots no narrower than 3mm. Narrower slots risk the laser re-cutting the opposite wall.

Text and engraving

Laser-engraved text should use a minimum font height of 3mm and stroke width of 0.3mm. Smaller text may be illegible or inconsistent. For serialization or logos, consider marking (surface etching) rather than through-cutting.

When Laser Cutting Reaches Its Limits

Fiber laser cutting excels at flat sheet parts, but certain geometries require different approaches:

• 3D contours: Laser cuts a flat profile; forming happens at the press brake
• Threads: Laser-cut holes are round; tapping or hardware insertion adds threads
• Tight tolerances (under ±0.05mm): Possible but requires slower speeds and post-measurement; consider CNC machining for critical features
• Very thick material (above 25mm): Plasma or waterjet becomes more economical

Understanding these boundaries helps you design parts that play to the laser’s strengths — speed, precision, and flexibility — while planning secondary operations where needed.

Get a Quote

If you’re designing sheet metal parts and want to understand how laser cutting fits your project, Laser Tuan Thinh can help. Upload your DXF files for an instant quote, or contact us to discuss material selection, tolerances, and finishing options.