The Science of Focus Position: Mastering Fiber Laser Cutting Precision

The laser beam from a fiber source is focused into a tiny spot by the cutting head's lens. The "Focus Position" refers to where this spot is located relative to the surface of the metal sheet.

When cutting Stainless Steel or Aluminum with Nitrogen (N2), we almost always use Negative Focus.

The Logic: By placing the focus inside the plate, the laser creates a wider hole at the top and a consistent path through the thickness.

The Benefit: A wider kerf allows high-pressure Nitrogen to flow more effectively, blowing away the molten metal before it can stick to the bottom. This results in the "Mirror Finish" that premium customers demand.

For Carbon Steel cutting with Oxygen (O2), Positive Focus is the standard approach.

The Logic: Placing the focus above the surface creates a larger spot diameter on the top of the plate.

The Benefit: A larger spot pre-heats a wider area, facilitating the chemical reaction between Oxygen and Iron. It creates a wide kerf at the top to help discharge the heavy slag produced during oxygen cutting, preventing the nozzle from clogging and ensuring a smooth bottom edge.

For very thin materials (under 2mm), Zero Focus is often used.

The Logic: It provides the highest energy density at the exact point of contact.

The Benefit: Maximizes cutting speed while keeping the kerf as narrow as possible, which is ideal for intricate, high-speed precision parts.

The focus position directly dictates the shape of the cut:

• Focus too high: The top of the cut becomes too wide, leading to "burning" or melting on the upper edge.
• Focus too low: The gas cannot reach the bottom of the cut, leading to "dross" or an incomplete cut.