CNC Waterjet Laser Cutting: How Each Process Removes Metal

• 1
  CNC waterjet laser cutting: two ways to separate metal
• 2
  What a waterjet kerf and edge actually look like
• 3
  Laser cutting: fast, tight, and sensitive to heat
• 4
  When the part should go to a mill instead
• 5
  How tolerance and finish change the process choice
• 6
  What we check before a cut job is released
• 7
  Waterjet vs laser vs milling: pick by feature

A waterjet does not cut with a blade or a beam. It pressurizes water to roughly 60,000 psi, pushes it through a sapphire orifice around 0.3 mm across, and mixes in garnet abrasive. The jet leaves the nozzle near 900 m/s. Metal leaves the kerf as eroded particles, not as chips and not as melt.

Because the tool is cold, there is no heat-affected zone. A 25 mm stainless plate stays at room temperature a millimeter from the cut. That matters for parts that will be finish-machined later, and for materials that crack or harden when they see a hot beam.

A laser works the other way. The beam melts and vaporizes metal along the cut line, and the assist gas blows the molten material out of the kerf. The heat is concentrated, so the cut moves fast, but the edge absorbs energy and the material around it changes.

Neither process is a milling operation. Both cut a profile through a plate. They produce a 2D shape, not a pocket, not a thread, not a flat face held to a tight tolerance. Keep that boundary in mind before you send a print to either machine.

The jet spreads as it travels down through the material. At the top of the cut the kerf is close to the nozzle diameter. At the bottom it is wider, and the edge carries a taper. On thin sheet the taper is hard to measure. On 50 mm plate it can reach a fraction of a millimeter per side.

Speed controls edge quality. Run the head fast and the jet lags. The cut line curves backward near the bottom, and the surface shows striations. Slow the head down and the edge straightens out and the striation angle drops. Most shops quote a quality number, from Q1 for a rough separation cut up to Q5 for the finest edge.

Abrasive waterjet cuts almost anything. Steel, stainless, titanium, Inconel, copper, brass, aluminum, stone, glass, composites. Thickness is not a hard limit in the way it is for a laser. A 100 mm block of aluminum or a 200 mm slab of steel can be cut on the right machine.

The trade is speed and edge finish. A waterjet edge is matte and slightly rounded at the top. If the print calls for Ra 0.8–1.6 μm on the cut face, the waterjet part still goes to a mill or a grinder afterward. Waterjet is the roughing step, not the finished surface.

A fiber laser focuses a beam to a spot a few tenths of a millimeter wide and moves it at high speed. On 1–3 mm mild steel the cut runs an order of magnitude faster than a waterjet on the same sheet. Positioning accuracy is excellent, and the kerf is narrow, often under 0.2 mm.

The heat-affected zone is the cost. On mild steel it is small and usually harmless. On 4140 or 4340 the laser edge can harden, and a hardened edge is a crack starter under load. On titanium and some aluminum alloys the zone can pick up oxygen or change microstructure. That edge may need to be machined away.

Dross is the other issue. Molten metal that does not clear the kerf re-solidifies on the bottom edge. It shows up on thicker material, on high-carbon steel, and when the assist gas pressure drifts. Dross has to be ground off, which adds a manual step and a second setup.

Laser cutting is a thermal process, so it will always leave a visible edge. For brackets, panels, and enclosures in thin sheet, that edge is fine. For a fatigue-critical part in hardened steel, it is not. The drawing should say which one you have.

If the feature has depth, it is not a cutting job. Pockets, counterbores, slots with a floor, threads, chamfers, and bosses all need a rotating tool that removes material in three dimensions. A waterjet or a laser can only trace a contour through the plate.

If the tolerance is tighter than ±0.1 mm, think carefully. Both processes hold position well, but the kerf taper and the edge condition eat into the budget. On a 6 mm aluminum bracket a waterjet can land inside ±0.1 mm. On a 40 mm steel plate the same cut will not.

If the edge is a functional surface, plan a secondary operation. A seal face, a bearing seat, or a mating surface that must sit flat needs milling, turning, or grinding after the profile is cut. The cutting process gets you the shape; the machining process gets you the surface.

A common route is cut-then-machine. Cut the profile on a waterjet, then put the blank on a 3-axis or 5-axis mill for the pockets, holes, and faces. That sequence saves spindle time and plate material, and it keeps the tight tolerances on the machine that can hold them.

Ask what the drawing actually needs. A profile held to ±0.2 mm with an as-cut edge is a cutting job. A profile held to ±0.05 mm with a smooth face is a machining job. The gap between those two statements is where most process arguments start.

For CNC work we hold ±0.005 mm (±0.0002 in) on critical dimensions, with surface finish from Ra 0.2–0.8 μm on fine work up to Ra 1.6–3.2 μm as-machined. Those numbers come from a spindle, not from a jet or a beam. A cutting process cannot reach them on the cut face.

Material thickness pushes the same decision. Under 6 mm, laser is usually the fastest route for sheet metal profiles. Between 6 mm and 25 mm, the choice depends on edge quality and material. Above 25 mm, waterjet is often the only practical option, especially on stainless, titanium, and Inconel.

Then there is the secondary question: what happens after the cut. Deburring, tumbling, anodizing, plating, and powder coating all sit downstream of the cut. The edge condition from the cut affects how much hand work those steps need.

The first check is the drawing itself. Which dimensions are functional, and which are reference? A cut profile usually has two or three critical dimensions and a dozen that only describe the shape. Knowing which is which decides the process and the inspection plan.

The second check is the material. Stainless 316, 17-4PH, TC4 titanium, Inconel, and magnesium all behave differently at the cut edge. We check thickness, condition, and any prior heat treatment. A pre-hardened plate and an annealed plate do not get the same laser settings.

The third check is downstream. Is the part going to anodize, plate, or powder coat? Does the edge need to be free of burrs and dross? A cut edge that looks acceptable in the shop can show up as a coating defect later, so the finishing step gets planned before the cut, not after.

We run 127 high-precision CNC machines across three plants, including 16 simultaneous 5-axis machining centers. For a cut-then-machine route, the profile comes off the cutter and goes straight to a mill for the tight features, with inspection before the part ships.