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Focus Concept

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Focal length

In short, the focal length is the distance from the lens to the focal plane. In our case the focal plane is the workpiece — the metal sheet.

The position of the focused spot (focal point) above the focal plane is called positive focus, and the position below the workpiece is called negative focus.

Why focus position matters

The focal-length value affects the spot size on the surface or inside the metal workpiece: as the focal length increases, the spot diameter grows and so does the kerf width.

Positive focus

This is the focus position in which the laser beam is focused above the workpiece surface.

As a rule, this focus setting is used for oxygen (flame) cutting of carbon steel.

Such a cut promotes slag removal and helps oxygen reach the lower surface of the workpiece to participate in the full oxidation reaction.

The higher the positive-focus value, the larger:

  • the laser spot diameter on the workpiece surface;
  • the heating and heat input around the kerf;
  • the smoothness and brightness of the carbon-steel cut face.

Negative focus

The cutting focus is located inside the workpiece. In this mode, because the focus is far from the cutting surface, the kerf is relatively wider than the spot size on the workpiece surface. At the same time the assist-gas flow is strong and the temperature is sufficient.

  • When cutting stainless steel, negative focus is preferred: the cut face has a uniform texture and a clean cross-section.
  • Piercing the plate before cutting: because piercing has a certain height, negative focus is used to guarantee the smallest spot size and the highest energy density at the pierce point.

Zero focus

The cutting focus is on the workpiece surface. As a rule, the cut face near the focus is relatively smooth, while the lower face — far from the cutting focus — is rougher.

This mode is mainly used for:

  • continuous-wave laser cutting of thin sheets;
  • pulsed laser vaporization at high peak power when cutting layers of metal foil.

The choice of focus position in laser cutting is determined not by the material of the workpiece (stainless or carbon steel) but by the cutting method (oxidation cutting or fusion cutting).