CNC laser tube cutting: a guide to how the beam behaves

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  What CNC laser tube cutting actually does to the metal
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  How wall thickness and material set the cutting window
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  Which tube shapes and features suit the process
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  CAD details that decide whether the cut is clean
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  Where the process sits next to sawing, plasma and milling
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  What the cut edge means for the next operation
• 7
  CNC laser tube cutting against three common alternatives

A fiber laser focuses a beam onto the tube wall. Absorbed energy melts and partly vaporizes the metal, and an assist gas blows the melt out through the kerf. The cut kerf is narrow, typically 0.1 to 0.4 mm on thin walls, so the heat-affected zone stays small. On a 2 mm stainless wall the HAZ usually runs 0.05 to 0.15 mm deep.

Because the head can rotate and the chuck can spin the tube at the same time, the beam never has to reach around a corner. That simultaneous motion is what separates tube cutting from flat sheet cutting. A 5-axis head lets the nozzle stay perpendicular to the surface on a curved wall or a corner radius, and it can tilt to pierce a hole without striking the far wall.

The cut itself removes material. Nothing is chipped away by a tool edge, so there is no cutter mark and no built-up edge. What you get instead is a narrow kerf, a light oxide or nitride layer on the cut face, and a small taper that grows with wall thickness.

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  Kerf0.1–0.4 mm on thin walls; grows with wall thickness
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  HAZ depthAbout 0.05–0.15 mm on a 2 mm stainless wall
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  Edge finishRa 1.6–3.2 μm as cut, before any finishing

Laser power, cutting speed, focus position and assist gas pressure are tied together. Change one and the others move. On mild steel a 3 kW fiber source cuts 2 mm wall at roughly 4 to 6 m/min; 6 mm wall drops to about 1 to 1.5 m/min and needs oxygen assist for a clean edge. Push the speed too high and the beam no longer fully penetrates. The cut face shows dross on the bottom edge.

Stainless and aluminium behave differently. They conduct heat away from the kerf faster, so they need higher peak power and nitrogen assist to avoid an oxidized edge. A 4 mm 304 wall typically runs at 2 to 3 m/min with nitrogen at 12 to 16 bar. Cut it with oxygen instead and the edge turns dark and rough.

Reflective and highly conductive metals are the hard cases. Copper and brass absorb little of a 1 μm beam at room temperature, so piercing needs a higher power spike and a longer dwell. Titanium cuts well but must be shielded, since a hot titanium edge reacts with air. For thin titanium tube, nitrogen assist keeps the cut face clean.

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  Mild steelOxygen assist, 3 kW cuts 2 mm at 4–6 m/min
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  StainlessNitrogen assist at 12–16 bar, 2–3 m/min on 4 mm wall
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  AluminiumHigher power, nitrogen assist, watch for dross
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  Copper and brassLonger pierce, higher peak power, tight focus

Round tube is the easiest case. The chuck spins it continuously and the head follows the surface, so a straight cut, a mitre or a saddle notch all come out in one pass. Square and rectangular tube need the head to track four flat faces and four corner radii. Corner quality depends on how well the controller blends speed through the radius.

Holes, slots and cut-outs are where the process earns its place. A 5-axis head can pierce a hole on a curved wall without hitting the opposite side, and it can cut a slot on the inside of a bend. Hole diameter below about 1.2 times the wall thickness gets tricky, because the pierce spatter can stick to the opposite wall.

Long parts are routine. On a machine with 4,000 mm travel, a single tube can carry several features along its length without repositioning. That matters for frames, rails and handles where the datum shifts if you cut in two setups. Deep saddle joints on thick tube are the limit. When the wall passes roughly 8 to 10 mm, plasma or a mill often wins on cost.

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  Good fitMitres, saddle notches, holes, slots, cut-outs
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  Watch outHoles smaller than 1.2× wall thickness
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  Poor fitVery thick wall, deep 3D contours along the axis

Model the tube as a solid and cut the features as through-cuts. Leave a small land at the end of a slot so the drop-out does not fall into the chuck. A tab of 0.5 to 1 mm is enough on thin wall, and it is snapped off after the part leaves the machine.

Keep the cut path away from the weld seam on welded tube. The seam is harder than the parent metal and can cause a small step in the kerf. Rotate the tube so the seam lands on a face that will be machined later, or specify seamless tube when the edge matters.

Design for the kerf. A nominal 0.2 mm kerf means a 10 mm slot comes out at about 10.2 mm. If the slot is a locating feature, call out the finished size and let the shop compensate, or plan a light machining pass. Do not stack a laser-cut edge against a tight tolerance unless the drawing allows for it.

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  Add a tab0.5–1 mm land keeps the drop-out in place
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  Mind the seamKeep cut paths off the weld seam on welded tube
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  Allow for kerf0.2 mm kerf adds to slot and hole size

Sawing gives a square end and nothing else. It is fast and cheap on straight cuts, but a mitre or a notch needs a second operation. Plasma cuts thick wall and handles large diameter, yet the kerf runs 1 to 3 mm and the edge needs grinding. Waterjet avoids heat entirely and holds a tight edge, but it is slow on tube and the abrasive cost adds up.

A mill can cut a tube feature and hold ±0.005 mm, but it removes material with a tool. On a thin wall the cutting force can deflect the tube, and a long part may need a fixture to stay rigid. The laser does not push on the part, so thin wall stays straight.

The practical split is simple. Straight cuts and heavy wall go to the saw or plasma. Tight features on thin to medium wall go to the laser. Features that need a machined fit, a thread or a bore go to the mill, often after a laser cut brings the tube to near-net shape.

A laser-cut edge is not a finished edge. It carries a thin oxide or nitride layer and a small taper. On a 3 mm wall the taper is usually under 0.1 mm per side, but it is there. If the edge is a sealing face or a bearing seat, plan a light machining pass to remove 0.1 to 0.2 mm.

For welded assemblies the laser edge is usually fine as-is. Weld prep angles are easy to cut, and the narrow kerf means less filler metal. Clean the oxide off stainless before welding, since the layer can cause porosity.

If the part will be anodized, note that the cut face anodizes differently from a machined face. The oxide layer from cutting can leave a colour shift. Bead blasting or a light etch before anodizing evens it out. For a uniform look across a visible part, machine or blast the cut edges first.

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  As cutGood for weld prep and non-critical edges
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  Light machine passRemoves 0.1–0.2 mm for sealing or bearing faces
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  Before anodizingBlast or etch to even out the colour