Basic Knowledge of Laser CNC Cutting
This page covers how a laser CNC cutting head actually removes material, which metals and thicknesses suit the process, and where it stops being the right choice. It is written for design engineers and buyers who need to pick a cutting method and write a usable drawing. After reading, you should be able to judge when laser cutting wins and when milling, waterjet, or EDM does the job better.
What Laser CNC Cutting Actually Does
A focused beam, a moving gantry, and a gas jet. Everything else is tuning.
Beam Source, Motion, and Assist Gas
A focused beam melts, burns, or vaporizes metal along a programmed path. Three subsystems do the work: the resonator that generates the beam, the motion system that moves the cutting head, and the nozzle that delivers assist gas to the cut zone. Get any one of them wrong and edge quality drops fast.
Most industrial machines today use a fiber resonator, which carries the beam through a flexible optic cable to the head. CO2 resonators still run in shops that cut thick mild steel or non-metals, because a 10.6 μm wavelength couples well to those materials. Nd:YAG units survive in legacy drilling and welding cells. For sheet and plate under 25 mm, fiber has largely replaced the rest.
Motion comes from a gantry or a flying-optic setup. The controller reads the nesting file, then drives X and Y servos while the Z axis holds the focus offset. Acceleration matters more than top speed on thin sheet: a machine that hits 120 m/min but takes 0.5 s to reverse will lose to a slower head with sharper dynamics on a dense nest.
- 1Fiber resonator1 μm wavelength, wall-plug efficiency above 30%, low maintenance.
- 2CO2 resonator10.6 μm wavelength, best for thick mild steel and organics.
- 3Assist gasOxygen for speed on carbon steel, nitrogen for clean edges on stainless.
- 4Focus offsetSet above, at, or below the surface depending on thickness and gas.
Kerf, Heat-Affected Zone, and Edge Finish
Kerf is the width of material the beam removes. A typical fiber cut on 3 mm stainless leaves a kerf of 0.15–0.25 mm. The kerf sets the minimum inside corner radius you can draw: a 1 mm slot in 6 mm plate is not a laser feature, it is a milling feature. Designers who ignore this end up with rounded corners they did not want.
The heat-affected zone on a laser cut edge is small, usually under 0.1 mm on thin sheet, but it is not zero. On 4140 or 4340 steel the cut edge hardens slightly and may need a stress relief pass before finishing. On 6061-T6 aluminum the zone softens a little, which matters if the part sees fatigue loading near the cut edge.
Dross is the re-solidified metal that sticks to the bottom edge. It comes from too much heat or too little assist gas pressure. A clean nitrogen cut on stainless produces almost no dross; an oxygen cut on mild steel leaves a light oxide that usually comes off in a tumble or a bead blast. If your drawing calls for Ra 0.8–1.6 μm on the cut face, plan a secondary operation.
- 1Kerf width0.1–0.3 mm on thin sheet, wider as thickness climbs.
- 2Minimum holeRoughly equal to material thickness on a good fiber machine.
- 3Taper0.02–0.1 mm per side depending on thickness and focus.
- 4Edge finishAs-cut is typically Ra 3.2–6.3 μm, not a finished surface.
Laser Cutting vs Other CNC Processes
Pick the process that matches the geometry, not the one that is already busy.
| Process | Best for | Typical tolerance | Watch out for |
|---|---|---|---|
| Laser cutting | Flat sheet and plate, 0.5–25 mm | ±0.1 mm on thin sheet | Taper, dross, HAZ on hardened steel |
| CNC milling | 3D pockets, threads, tight bores | ±0.005 mm | Setup count, tool access, cycle time |
| Waterjet | Thick plate, no HAZ, mixed stacks | ±0.1–0.2 mm | Slow on thin sheet, abrasive cost |
| Wire EDM | Hardened tool steel, sharp corners | ±0.005 mm | Conductive material only, slow |
| Plasma | Mild steel over 25 mm | ±0.5 mm | Wide kerf, rough edge, big HAZ |
Which Metals Cut Well and Which Fight Back
Mild steel and stainless are the easy cases. A 6 mm 304 sheet cuts clean with nitrogen assist and holds ±0.1 mm on most profiles. Aluminum cuts fast but reflects the beam, so the machine needs the right lens and a pierce routine that does not blow back into the optics. Copper and brass reflect even more, and thin copper may need a longer-wavelength source or a different process entirely.
Titanium cuts well but the edge picks up oxygen if the assist gas is not pure enough. Grade 5 Ti-6Al-4V is cut routinely in aerospace work, provided the shop blankets the cut with argon and controls the pierce. Thick titanium above 10 mm is usually better done on a waterjet, because the laser cut face needs more cleanup than the waterjet edge does.
Some materials should not be laser cut at all. PVC releases hydrogen chloride and attacks the machine; polyurethane foam and certain composites leave a charred edge that fails a cleanliness spec. For those, route the part to milling, waterjet, or die cutting. Sending them to the laser is a rework waiting to happen.
- 1Mild steelOxygen assist for speed, nitrogen for a cleaner edge.
- 2Stainless 303 / 304 / 316LNitrogen assist, minimal dross, good edge color.
- 3Aluminum 6061 / 5052Cuts fast, watch reflectivity and pierce spatter.
- 4Copper, brass, PVCReflective or toxic. Confirm with the shop before quoting.
Drawing Rules That Save Rework
Give the shop a flat DXF plus a PDF with the drawing views. The DXF defines the cut path; the PDF tells the operator which edges are cosmetic and which are functional. A single note like "cut edge Ra 3.2 μm acceptable" removes a phone call and a possible remake.
Keep inside corners at a radius of at least half the kerf, and ideally equal to the material thickness. Put the part number on a tab or a slug, not in the middle of a cosmetic face, unless you want a laser mark there. Laser marking works down to a character height of 1.5 mm, which is small enough for most serial numbers.
If the part needs a tight bore, a thread, or a counterbore, plan a second operation on a mill. Laser gets you the blank in minutes; milling brings the feature to ±0.005 mm. That split is usually cheaper than trying to hold a precision bore on a laser, and it keeps the tolerance where it belongs.
- 1File formatDXF for the cut path, PDF for the drawing notes.
- 2Corner radiusAt least half the kerf, ideally one material thickness.
- 3Mixed processLaser the blank, mill the bores and threads.
- 4MarkingLaser marks down to 1.5 mm character height.
Common Questions
How thick can laser CNC cutting go?
Fiber machines cut mild steel up to about 25 mm and stainless to roughly 20 mm, though edge quality drops as thickness climbs. Above 20 mm, oxygen assist on mild steel gives a usable edge but the kerf widens and taper grows.
If your part is over 25 mm, waterjet or plasma is usually the better route. Send us the drawing and we will say which process holds the tolerance you need.
Can laser cutting hold ±0.005 mm?
Not on the cut edge. A laser cut profile typically holds ±0.1 mm on thin sheet, and that number widens with thickness and taper. The ±0.005 mm figure applies to our CNC milling and turning operations.
A common workflow is to laser the blank and mill the critical features. That keeps cost down and puts the tight tolerance on the surfaces that need it.
What is the difference between oxygen and nitrogen assist gas?
Oxygen feeds an exothermic reaction that adds heat and raises cutting speed on carbon steel. It leaves a light oxide on the cut face, which is fine if you plan to paint or powder coat the part.
Nitrogen is inert. It blows molten metal out without adding heat, giving a clean, oxide-free edge on stainless and aluminum. It costs more per part and runs slower on thick mild steel.
Does laser cutting leave a heat-affected zone?
Yes, though it is small. On thin sheet the HAZ is often under 0.1 mm. On 4140 or 4340 steel the cut edge hardens and may need stress relief before finishing or assembly.
If the part sees fatigue loading, tell us at quote time. We can adjust the cut parameters or route the part to a process with no thermal input.
When should I choose milling instead of laser cutting?
Choose milling when the part has 3D geometry, threads, tight bores, counterbores, or a surface finish tighter than Ra 1.6 μm. Laser only cuts through the sheet; it cannot create a pocket or a shoulder.
For flat parts with a few critical features, we often run both: laser for the outline, milling for the bores. That is usually the lowest-cost path to a finished part.
What files do you need for a laser cutting quote?
A flat DXF or DWG of the cut path, plus a PDF drawing with material, thickness, tolerance, and finish notes. STEP files help if the part also needs milling or turning.
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