Laser Cut Basics Every Engineer Should Know

• 1
  How a laser cut actually removes metal
• 2
  Thickness, kerf, and material limits
• 3
  Heat-affected zone and what it does to the part
• 4
  What tolerance a laser cut can hold
• 5
  Using a laser cut blank before CNC machining
• 6
  Laser cut vs CNC milling: pick by feature

A focused beam hits the sheet and the irradiated spot heats in microseconds. The metal either melts or vaporizes. Assist gas then pushes the molten pool out of the kerf, leaving a narrow slot behind. The beam travels along the contour and the cut appears as a continuous line, not as a series of drilled holes.

Absorption depends on wavelength and on surface condition. A 1,064 nm fiber beam couples well into steel, stainless, and aluminium once the surface is clean. Reflective and highly conductive metals, such as copper and brass, need more power and a tighter focus to reach the same penetration depth.

The kerf is not zero. A fiber source leaves a slot roughly 0.1–0.5 mm wide, and that width changes with nozzle diameter, focus position, and gas pressure. If your part has a 0.3 mm tab between two cutouts, the kerf can consume most of it.

Cut quality comes from three settings: power, speed, and focal position. Too much power at low speed burns the edge and widens the heat-affected zone. Too little power leaves dross attached to the bottom face. There is a narrow window where the edge is clean and the taper stays small.

Thickness sets the first hard boundary. A fiber laser cutting mild steel handles roughly 20 mm, stainless around 12 mm, and aluminium around 10 mm. Beyond that, the beam scatters, the kerf widens, and the edge turns rough. Those numbers move with power, so a 6 kW source and a 12 kW source do not share the same ceiling.

Thin sheet has its own problem. Below about 0.5 mm, heat input warps the part and the edge distorts. You get a cut, but the flatness is gone. Nesting tight parts on thin stock usually makes this worse.

Reflective metals demand attention. Copper, brass, and bare aluminium reflect a large share of the beam back toward the optics. Modern heads handle this better than older CO2 machines, but the cut is still slower and the edge is less predictable.

Material thickness also decides whether a laser cut is the right first operation at all. If the part needs pockets, threads, or a bored bore, the laser only gives you the outline. Everything else comes from a mill or a lathe after the blank is cut.

Every laser cut leaves a heat-affected zone, usually 0.05–0.3 mm deep. Inside that band the grain structure changes. Hardness can rise, and for some alloys the material becomes more brittle than the parent plate.

For mild steel and most aluminium grades, the zone is harmless. You deburr the edge and move on. For 17-4PH stainless, tool steel, or any part that will be hardened later, the zone matters. It can cause a hardness gradient that shows up after heat treatment.

Titanium and its alloys need more care. The molten edge can pick up oxygen and nitrogen from the atmosphere, forming a hard, brittle layer. A proper assist gas shield keeps that layer thin. A poor shield leaves a discolored edge that will crack under load.

If the edge will be a sealing face or a fatigue-critical surface, plan a secondary operation. A light face mill or a grinding pass removes 0.1–0.2 mm and takes the zone with it. That step is cheap insurance on parts that see cyclic load.

A laser cut holds roughly ±0.1 mm on thin sheet over short spans, and it drifts as the part grows. Over a 1,000 mm length, expect ±0.2 mm or worse. Thermal expansion during the cut and table positioning both add error.

The kerf taper adds a second error. The top of the cut is wider than the bottom on thicker plate, so a hole measures differently at each face. On a 6 mm plate the difference can reach 0.1 mm. If a dowel pin must fit, that gap is already too large.

Positional tolerance is separate from dimensional tolerance. Hole-to-hole distance depends on how the machine moves between features, not on the cut width. A good nesting plan keeps related features close so the error does not stack.

Because of these limits, a laser cut suits brackets, plates, covers, and frames. It does not suit bearing bores, press fits, or anything with a ±0.005 mm callout. Those features belong on a CNC machine with a rigid setup.

Many parts start as a laser cut blank and finish on a mill. The blank gives you an accurate outline and holes that locate the part in the fixture. The mill then cuts pockets, bores, and faces that the beam cannot reach.

This sequence saves material and cycle time. A 4,000 mm plate can be nested with several parts, cut in one pass, and then loaded on a machine with a 4,000 × 400 × 150 mm travel envelope. The outline is already done.

Watch the stock allowance. Leave 0.3–0.5 mm on any edge that will be machined, because the as-cut face is rough and slightly tapered. Clamping on a rough cut edge also shifts the part, so locate on a hole, not on the outer profile.

If the blank was cut from stainless or titanium, check the heat-affected zone before the finishing pass. A 0.2 mm cleanup cut is usually enough. On hardened alloys, verify hardness after the cut, not after the finish.

For thin sheet parts that need flatness, plan the cut last. Cutting before a forming or welding step reintroduces distortion that you already removed.