In metal processing, laser cutting machines use high-energy-density laser beams to achieve material melting, vaporization, and separation. The focal position is one of the key process parameters in laser cutting. It directly affects energy distribution, molten pool morphology, kerf quality, and cutting stability. Proper control of the focal position is a critical prerequisite for obtaining high-quality cutting results.
I. Basic Definition of Laser Cutting Focal Position
The focal position in laser cutting refers to the point where the laser beam is focused by the cutting head to achieve the smallest spot diameter and the highest energy density. Taking the workpiece surface as the reference zero point, the focal position can be divided into three states:
Positive focus (focus located above the workpiece surface)
Zero focus (focus located on the workpiece surface)
Negative focus (focus located inside the workpiece material)
Different focal positions change the way laser energy couples with the material, thereby affecting the cutting process and cutting quality.
II. Influence of Focal Position on Laser Energy Distribution
The laser beam reaches its maximum energy density at the focal point. As the beam moves away from the focus, the spot diameter increases and the energy per unit area decreases. Changes in focal position lead to the following effects:
Variation in energy concentration
The closer the focus is to the material surface, the more concentrated the laser energy, resulting in higher initial melting efficiency.
Variation in energy distribution along material thickness
In negative focus conditions, laser energy forms a longer effective interaction zone inside the material, which is beneficial for thick plate cutting.
Changes in molten pool morphology
Different focal positions affect the depth, width, and stability of the molten pool, thereby influencing kerf geometry and cut surface quality.
III. Effects of Different Focal Positions on Cutting Quality
1. Effect of Positive Focus on Cutting Quality
When the focus is positioned above the workpiece surface, the laser spot on the material surface is relatively larger:
More uniform energy distribution at the cutting entry surface
Wider kerf at the top surface
Relatively weaker downward expulsion of molten metal
This condition is suitable for thin materials and applications with higher requirements for surface appearance quality.
2. Effect of Zero Focus on Cutting Quality
When the focus is located on the workpiece surface:
Laser energy density reaches its maximum
Cutting speed is relatively high
Kerf width is smaller
Kerf perpendicularity is improved
Zero focus is commonly used for high-speed cutting of thin and medium-thickness plates, providing a good balance of overall cutting quality.
3. Effect of Negative Focus on Cutting Quality
When the focus is positioned inside the workpiece:
Laser energy extends deeper into the material
Molten pool depth increases
Assist gas can more effectively expel molten material from the bottom of the kerf
Negative focus is widely applied in thick plate cutting and helps reduce dross formation while improving full penetration capability.
IV. Influence of Focal Position on Kerf Shape and Cut Cross-Section
Improper focal positioning may lead to the following issues:
Kerf taper with wider top and narrower bottom, or vice versa
Uneven striation patterns on the cut surface
Increased dross at the bottom edge
Incomplete cutting or cut interruption
Proper focal adjustment helps improve kerf perpendicularity, cross-section smoothness, and overall cutting consistency.
V. Synergistic Effect of Focal Position and Assist Gas
During laser cutting, assist gases such as oxygen, nitrogen, and air interact closely with the focal position:
When the focus is positioned higher, the gas mainly acts on the surface molten zone
When the focus is positioned lower, the gas more effectively assists molten pool ejection
The focal position and nozzle height jointly influence gas flow stability
Focal adjustment must be coordinated with gas pressure and nozzle diameter.
VI. Common Effects of Improper Focal Position Settings
Incorrect focal positioning may result in:
Reduced cutting speed
Lower laser power utilization efficiency
Unstable cutting quality
Increased thermal load on optical components
Therefore, in practical applications, the focal position should be dynamically optimized based on material thickness, laser power, and cutting process parameters.
VII. General Principles for Focal Position Adjustment in Practical Applications
The following guidelines are commonly applied:
Zero focus or slight positive focus for thin sheet cutting
Negative focus for thick plate cutting
Increased negative focus for high-power laser cutting
Fine focal adjustment for high-reflectivity materials to improve energy coupling efficiency
The laser cutting focal position is one of the most important process parameters affecting cutting quality. By properly controlling the focal position, laser energy distribution can be optimized, melting efficiency can be improved, and kerf geometry and cutting stability can be enhanced. In actual production, precise focal adjustment should be carried out according to material properties, equipment parameters, and process requirements to achieve stable and reliable cutting performance.