The small spot size obtained by focusing the laser beam provides high intensities for material treatment. Above and below the focus the intensity in the beam drops. The depth of focus expresses how quickly the beam becomes wider and the intensity drops. A shorter focal length results in a smaller spot size and a smaller depth of focus.

Generally, we must position accurately the focal point with reference to the surface of the workpiece. And the position must be kept constant during processing.

The beam waist or focus (the point where the beam diameter is smallest) should either be located on the surface of the workpiece (when using oxygen as a cutting gas, Fig. 6) or up to 75% of the material thickness of the workpiece. The sensitivity of focusing is less in high-power lasers than in lower-power ones. The sensitivity to focal positioning is also greater in some materials than in others.

However, focal position is a parameter that must be controlled in order to ensure optimum cutting performance. Also:

• Variations in material and thickness may require alterations of focus
• Variations in the laser beam shape or the mode and changes in the temperature of the cooling water and contamination on the lens may change the focal position

Nozzle size and standoff distance

Gas assistance is essential in laser cutting. Therefore, nozzle geometry and standoff distance are important. Nozzle design and fl ow dynamics through the nozzle differ substantially from other thermal cutting processes (Fig. 7). This is mainly due to the compactness and diameter of the nozzle, which is always larger than the kerf produced below it. As a result, only a portion of the gas jet formed by the nozzle penetrates the kerf.

Nozzle standoff distances depend on the design of the nozzle. The standoff distance for standard nozzles used in laser cutting applications should be smaller than the diameter of the nozzle, as turbulence and pressure variations may appear in the cutting gas jet with larger distances (Fig. 8). At short nozzle standoff distances, the kerf itself acts as a nozzle and the geometry of the nozzle tip is not so critical. If large nozzle standoff distances are required, great care must be taken in designing the nozzle tip, especially when nozzle pressure exceeds 2–3 bar (30–45 psi).

Typical nozzle diameters are in the range of 0.8–3mm (0.03–0.12 in). So that the nozzle standoff distance should be in the range of 0.5–1.5mm (0.02–0.06 in) for best cutting results.

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