CNC Laser Metal Cutting: How the Beam Actually Cuts Metal

• 1
  What the Beam Does to the Metal
• 2
  Thickness, Tolerance, and Kerf Limits
• 3
  Assist Gas and Material Pairing
• 4
  Where Laser Cutting Fits in a Machining Job
• 5
  Reading a Bad Cut Before You Call It a Defect
• 6
  Process Choice by Part Requirement

Cutting sheet metal with a laser is a thermal separation process, not a mechanical one. A focused beam raises a spot on the sheet above melting point in milliseconds. The molten metal is then pushed out of the kerf by a coaxial gas jet, and the CNC moves the head along the programmed path. There is no cutter to wear, so the same program runs the same way on part one and part four thousand.

The beam itself is only half the story. Cut width, dross, and heat-affected zone are decided by the balance between laser power, cutting speed, focus position, and assist gas pressure. Push the speed up and the kerf narrows, but the beam starts to lag and you get striations. Drop the speed and the kerf widens, the heat-affected zone grows, and thin sections can bow.

A fiber source at 1 kW to 6 kW covers most sheet work. CO2 still holds a niche on thicker plate and on some non-metals, but the maintenance bill is higher. Disk and direct-diode heads sit between the two on beam quality. For mild steel up to 20 mm and stainless up to 12 mm, a 6 kW fiber machine with nitrogen assist is the usual production setup.

The cut edge is a record of what happened in the kerf. A clean edge shows fine, even striations angled slightly backward. Dross on the bottom means the gas did not clear the melt, usually from too much power at low speed or a focus point set too deep. A brown oxide edge on stainless means oxygen got into the assist gas.

Thickness capability is a gas and power question, not a single number. Mild steel cuts cleanly to about 20 mm with oxygen assist on a 6 kW fiber machine; stainless and aluminum top out nearer 12 mm with high-pressure nitrogen. Above those ranges the kerf turns rough and the taper grows, so the parts usually move to a machining center instead.

Tolerance on a laser is asymmetric. The profile can hold roughly ±0.1 mm on thin sheet, but the kerf taper and the heat-affected zone do not disappear. If your drawing calls for ±0.005 mm, or for a bore that must round to a few microns, laser cutting is the wrong first operation. Cut a blank with stock and finish it on a mill.

Kerf width matters for the flat pattern you send us. A typical fiber kerf runs 0.1 mm to 0.3 mm depending on material and thickness, and the CAM offset must account for it. Ignore the kerf and every hole comes out undersized by roughly half the kerf width.

Feature size sets a second limit. Holes smaller than the material thickness are hard to keep round, and a hole under 1 mm in 3 mm stainless will show taper. Slots narrower than the kerf cannot be cut at all. Design minimum hole diameter at least equal to thickness, and keep webs at 0.8 × thickness or thicker.

Assist gas does two jobs: it clears molten metal from the kerf and it shields the cut zone from air. Oxygen is the cheap option on mild steel. It adds exothermic heat, so it cuts faster and thicker, but it leaves an oxide edge that needs removal before painting or welding.

Nitrogen is inert and gives a bright, oxide-free edge on stainless, aluminum, and copper alloys. It costs more per part because flow rates are high, typically 15 bar to 20 bar on stainless. If the part will be welded, anodized, or visible, pay for nitrogen. If it is a bracket that gets powder coated, oxygen is fine.

Compressed air sits in the middle. It is inexpensive and works on thin mild steel and some aluminum, but the edge quality is inconsistent and the nitrogen content varies with the compressor. We do not recommend air for cosmetic or welded parts.

Material grade changes the recipe. Aluminum 6061 cuts well but reflects the beam, so it needs higher power and a careful focus. Copper and brass reflect even more and are usually cut on higher-wattage machines with nitrogen. Titanium cuts cleanly with nitrogen or argon, but the cut zone must stay shielded. Galvanized and coated steels release zinc fumes, so extraction has to be sized for it.

Most parts we quote are not purely cut or purely machined. A typical bracket starts as a laser blank, then goes to a 3-axis mill for the mounting face and the tapped holes. The blank cuts in minutes and the mill only touches the features that need tolerance. That split keeps cost down without giving away accuracy.

Nesting is the other lever. Flat parts nest tightly, so material utilization on a 4,000 mm sheet is high and the per-part cost drops with quantity. Curved or deep-formed parts do not nest well, which is one reason a stamped or cast part can beat laser on high volume.

Edge condition matters downstream. A laser edge is a heat-affected zone, not a machined surface. If the part will be anodized, the cut edge takes dye slightly differently than the machined face. If it will be welded, oxide on an oxygen-cut edge has to be ground back first. Both are solvable; both are cheaper to plan for at the drawing stage.

We run laser blanks alongside 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and a Ø400 mm rotary table. That means a laser-cut blank can move straight into milling, turning, or finishing without a second supplier. One setup stream, one inspection record.

Dross on the underside is the most common complaint. On mild steel it usually means the focus is too deep or the gas pressure is too low for the speed. On stainless it often means the nitrogen pressure dropped. Raise pressure first, then re-check focus.

Striations that lean forward instead of backward mean the beam is running ahead of the melt. Slow the cut or raise power. Vertical, rough striations with a wide kerf point the other way: the beam is dwelling too long, so speed up and reduce power.

Burn marks and a brown edge on stainless come from oxygen contamination. Check the nozzle for damage, check the gas purity, and check that the pierce point is not sitting on the part edge. A worn nozzle also widens the kerf and softens the focus.

Taper shows up as a hole that is wider on top than on the bottom. Some taper is normal. If it exceeds 0.1 mm on thin sheet, the focus is off center or the nozzle is misaligned. For holes that must be true, drill or mill them after cutting.