CNC laser cutting: a beginner's guide

• 1
  How the beam removes material
• 2
  Which metals behave, and which fight back
• 3
  The five settings that decide edge quality
• 4
  File and feature rules that prevent rework
• 5
  When laser is the wrong process
• 6
  Assist gas and edge result by material

The machine is a thermal tool, not a mechanical one. A resonator produces a narrow, coherent beam, mirrors and a cutting head focus it to a spot around 0.1–0.3 mm across, and the power density at that spot climbs high enough to melt or vaporise metal in milliseconds. The head then travels along a path generated from a 2D vector file.

Three things happen at the cut front at the same time. The metal absorbs energy and turns liquid, a coaxial assist gas blows that liquid out of the kerf, and the remaining solid edge cools fast. If any one of the three is wrong, you get dross rather than a clean edge.

Fiber sources dominate metal work today because 1 µm wavelength couples far better into steel and aluminium than the 10.6 µm of CO₂. That is why a 3 kW fiber unit cuts 3 mm mild steel at several metres per minute, while the same thickness in 10 mm aluminium needs more power and a different gas.

The kerf is never zero. A typical 1–2 mm kerf removes material on both sides of the programmed line, so the operator offsets the path outward by half the kerf width. Beginners who ignore this find every hole 0.1–0.2 mm undersize.

Mild steel and stainless are the easy cases. Oxygen assist gives a fast, slightly oxidised cut on 1018 or A36; nitrogen gives a clean, oxide-free edge on 304 or 316L for welding or painting. Thickness drives everything: 1 mm cuts roughly ten times faster than 10 mm.

Aluminium reflects the beam at low power and conducts heat away fast. A 6061 sheet cuts well with nitrogen, but the edge can carry a recast layer and the surface needs a brush or a bead blast before anodising. Alloys with high silicon, such as ADC12, cut slower and rougher.

Copper and brass are the hard ones. High reflectivity sends energy back into the optics, so a fiber source with anti-reflection protection is required, and thickness is usually capped lower than steel. Beryllium copper cuts, but the dust needs extraction.

Titanium cuts cleanly with argon or nitrogen, though the cut edge must stay shiny. A straw or blue tint means oxygen got in and the edge is embrittled. Inconel and other nickel alloys cut at low speed and demand tight gas control.

Power, speed, focus position, gas pressure and nozzle standoff. Those five decide whether a cut is square, dross-free and repeatable. Change one and the others usually need to move with it.

Speed is the lever most beginners touch first. Run too fast and the beam does not fully penetrate, so you get a rough lower edge and dross that sticks. Run too slow and the kerf widens, the heat-affected zone grows, and thin sheets warp.

Focus position sets how the beam tapers through the thickness. Focus at the top surface gives a wider kerf; focus slightly inside the material gives a squarer edge on thick plate. Nozzle standoff of 0.5–1.0 mm keeps gas pressure stable.

Assist gas does two jobs: it blows molten metal out and it shields the cut from air. Oxygen raises cutting speed on mild steel but leaves an oxide layer. Nitrogen costs more per part but leaves an edge that can be welded without cleaning.

Design rules matter more than machine choice on thin sheet. A hole smaller than the material thickness will usually come out tapered or undersize, because the beam has nowhere to focus cleanly. Keep hole diameter at least 1× thickness for a reliable result.

Minimum feature width should be roughly 1× thickness as well. Anything thinner overheats, distorts, or falls away. Corners can be left sharp, but a small radius, around 0.5× thickness, lets the head maintain speed and avoids a burnt corner.

Nesting decides material cost. Parts spaced 3–5 mm apart cut cleanly; closer than that and the heat from one cut can distort its neighbour. Leave a 10 mm skeleton at the sheet edge so the frame stays rigid.

Every file should be a clean 2D vector at 1:1 scale, with one layer per thickness and no stray construction lines. A single open path stops the program cold, and a duplicate line cuts the same contour twice, which wastes time and burns the edge.

Thick sections are the first limit. Beyond roughly 25 mm in mild steel, cutting speed collapses compared with a waterjet or a bandsaw, and the edge needs secondary machining anyway. If the part is 40 mm thick, a mill is faster and cheaper.

Tolerance is the second limit. Sheet cutting holds roughly ±0.1 mm on position, which is fine for a bracket and useless for a bearing bore. The moment a drawing calls for ±0.005 mm or a Ra 0.2–0.8 μm finish, the part belongs on a machining center.

Depth is the third limit. The beam cuts through, not into. A pocket, a counterbore, a thread or a 3D contour cannot be produced by a flat-bed cutter, no matter how the file is drawn.

That is why the two processes sit next to each other in a real shop. Cut the blank flat and fast, then move it to a 3-axis or 5-axis mill for the bores, faces and threads. Doing both under one roof removes the tolerance stack between suppliers.