CNC Laser Cutting Sheet Metal: How the Cut Actually Works

• 1
  What the beam does to the material in CNC laser cutting sheet metal
• 2
  Kerf, heat-affected zone, and what you can hold
• 3
  Which features the process handles and which it does not
• 4
  How each metal behaves under the beam
• 5
  Laser cut blanks in a full machining workflow
• 6
  Laser cutting compared with the alternatives

A fiber laser focuses roughly 1 kW to 12 kW into a spot 0.1 to 0.3 mm wide. Power density at the focus reaches 10^6 W/cm² or more, so the metal does not melt slowly. It reaches vaporization temperature in the time the beam dwells on one point, which is microseconds at typical cut speeds.

Cutting is not one mechanism. Below roughly 1 mm, thin steel often leaves the kerf as vapor and fine oxide fume. On 3 mm to 12 mm plate, most of the kerf is molten metal pushed out by the assist gas. Both happen in the same cut zone at different depths. That is why parameter tables differ for every thickness.

The assist gas has three jobs. It blows molten dross out of the kerf, shields the hot edge from air, and in some setups supplies the exothermic heat that keeps the cut going. Oxygen adds heat and speeds up mild steel, but it leaves an oxidized edge that usually needs tumbling or blasting before paint.

Nitrogen gives a clean, oxide-free edge on stainless and aluminum. It costs more per hour because flow rates run high, yet it removes a whole finishing step. The choice is rarely about cut speed alone. It is about what the next operation does to that edge.

Kerf is the material the beam removes, typically 0.1 to 0.5 mm depending on nozzle, focus and thickness. The controller offsets the tool path by half the kerf, so a nominal 50 mm hole comes out at 50 mm only if the offset is right for that material and thickness. Change the gas and the kerf shifts.

The heat-affected zone on a laser-cut edge is narrow, usually 0.05 to 0.2 mm on thin sheet. Grain structure changes there, hardness rises slightly, and a thin oxide layer forms. For brackets and covers this is irrelevant. For parts that will be bent to a tight radius or welded and then fatigue-loaded, it matters.

Tolerance is where expectations go wrong. A laser profiler holds roughly ±0.1 mm on position for thin sheet in good condition. It does not hold ±0.005 mm. That number belongs to milling, drilling and turning, not to a thermal cut. Design the laser profile as a blank and let machining set the critical features.

Edge quality is judged by dross, striation angle and roughness. A good nitrogen cut on 2 mm stainless lands near Ra 3.2 μm at the top and a little rougher at the bottom. If a sealing face or a bearing bore needs Ra 0.8–1.6 μm, plan a secondary machining pass instead of chasing it with gas pressure.

Minimum hole diameter is roughly equal to sheet thickness for a clean cut, and about 0.5× thickness at the limit where you accept taper and slower speed. A Ø3 mm hole in 3 mm steel is routine. A Ø1 mm hole in 3 mm steel will come out tapered and may need drilling after cutting.

Sharp internal corners are a design trap. The beam has a finite diameter, so every inside corner carries a radius of at least half the kerf, usually 0.2 to 0.5 mm. If a corner must be truly sharp, specify a relief notch, accept the radius, or move the feature to a milled operation.

Slots and narrow webs follow the same logic. A web thinner than 1× sheet thickness tends to distort as residual stress releases around it. Keep webs at 1.5× thickness or more when the part is flat and cosmetic, and add a stress-relief pass if flatness is called out.

Thick sections flip the economics. Above about 20 mm in steel, the cut slows sharply, taper grows, and oxygen cutting leaves heavy dross. At that point sawing plus CNC milling is often cheaper and far more accurate. Laser wins on 0.5 mm to 12 mm sheet, not on 40 mm plate.

Mild steel cuts easily and cheaply with oxygen, which is why A36 and 1018 brackets dominate the process. The oxidized edge is normal. If the part gets powder coating, that edge is fine after a light blast. If it gets anodized or plated, the oxide has to come off first.

Stainless 304, 316 and 316L cut best with nitrogen. Reflectivity is not the problem people assume at fiber wavelengths. The real issues are dross on the underside at high thickness and nitrogen consumption, which drives cost on 6 mm and above. 17-4PH behaves similarly but cuts slower.

Aluminum reflects and conducts heat away from the cut zone. 5052 and 6061 cut cleanly up to about 6 mm with nitrogen. Above that, dross and roughness climb fast. 7075 cuts but tends to crack near the heat-affected zone on tight bend radii, so keep bend lines away from the cut edge.

Copper and brass need high power and short focal length because they conduct heat so well. Thin C110 is practical. Thick copper is usually waterjet or milled. Titanium cuts well in an inert atmosphere but the kerf must be clean, since titanium oxide contamination ruins welds downstream.

The strongest use of the laser is as a blanking step. We cut the outline, holes and reliefs on sheet, then load the blank into a 3-axis or 5-axis mill for the features that need real tolerance. This splits the work where each process is strongest and keeps cost down.

Bend lines should sit at least 2× sheet thickness from any cut edge, and 3× if the edge will be visible. A hole closer than 2.5× thickness to a bend line will pull oval when the brake forms it. Move the hole, or form first and drill after.

Flatness is the hidden variable. Laser cutting puts a little heat into the sheet, and thin panels can bow a few tenths over a long part. If flatness is specified tighter than 0.2 mm over 1,000 mm, plan a stress relief or a machining pass rather than blaming the laser.

Welded assemblies built from laser blanks are common in chassis, enclosures and brackets. Fit-up is excellent because the profiles are consistent. What the laser cannot give you is a machined datum, so add one milled face or a drilled hole pattern for fixturing before you weld.

For prototypes and low volume, the pattern is simple. Cut the profile, mill the critical interfaces, finish as specified. We hold ±0.005 mm on milled features and Ra 0.8–1.6 μm on functional surfaces, and inspect 100% before shipment.