Inaccurate focus positioning is one of the primary causes of degraded cutting quality. Its symptoms include widened kerfs, severe dross accumulation on edges, and—in extreme cases—the inability to fully cut through thick plates. A focus deviation of just 0.1 mm can result in an increase of over 30% in the roughness of the cut surface.

I. Three Major Causes of Focus Shift

Mechanical Looseness in the Cutting Head: Prolonged high-speed motion can cause the focusing lens barrel or nozzle connections to loosen, thereby altering the physical position of the focal point. Connections should be tightened regularly and secured with thread-locking adhesive to prevent loosening.

Sensor Feedback Misalignment: In capacitive height-sensing systems, factors such as dross accumulation on the nozzle, uneven material surfaces, or high environmental humidity can cause deviations in the feedback signal regarding the follow height, resulting in the actual focal point drifting away from the set value. The nozzle face should be cleaned with alcohol, and the sensor recalibrated using a flat metal plate.

Thermal Lens Effect on Optics: During prolonged, high-power cutting operations, the focusing lens heats up and expands; this causes the focal length to shorten and the focal point to drift deeper into the material. It is essential to ensure that the water-cooling system is functioning correctly and to utilize high-quality lenses with low absorption rates.

II. Rapid Calibration Procedures

Dot Method (Applicable to CO₂ Lasers): Cut a piece of acrylic at an angle; the narrowest point of the cut kerf corresponds to the actual focal point.

Ramp Method (Applicable to Fiber Lasers): Cut a ramp-shaped test piece; the smoothest and most lustrous area of the cut surface corresponds to the optimal focal position.

It is recommended to perform a calibration check weekly using a dedicated focus measurement tool. By recording any observed drift trends, you can proactively prevent fluctuations in cutting quality.

Incorporating mechanical fastener checks into your daily inspection checklist—and recalibrating the focus every time the nozzle or lens is replaced—are critical steps in ensuring consistent cutting performance.