Laser cutting is one of the important uses of lasers, that is, focusing the laser beam on the material surface with a focusing lens to melt the material.
At the same time, the melted material is blown away by the compressed gas coaxial with the laser beam, the laser beam and the material move relative along a certain track, so as to form a certain shape of slit.
The process of laser cutting is completed by the cooperation of motion mechanism, control system, laser source and laser head.
Therefore, the factors affecting the cutting effect mainly come from the above four parts.
The main influencing factors are: beam energy distribution, laser power, focus diameter, focus position, cutting speed, nozzle diameter and so on.
Among all the factors affecting the cutting effect, the most influential is the focus position.
Changing the focus position is equivalent to changing the beam diameter incident on the surface of the plate and the incident angle to the inside of the plate, which will affect the formation of the slit and the reflection of the beam inside the slit, and then affect the width of the slit.
The width of the cutting seam will affect almost all cutting effects, such as the roughness of the cutting surface and the adhesion state of the slag at the bottom.
What is the focus position?
The focus position (Z) refers to the distance between the focus and the upper surface of the material to be cut.
Generally, the focus on the plate surface is called zero focus, the focus above it is called positive focus, and the focus below it is called negative focus. The schematic diagram is as follows:
Influence of focus position
The following figure shows the variation relationship between the focus position (Z) and the width (W) of the upper part of the cutting seam of the processing material.
When the focus is on the plate surface, the slit width is the smallest.
Changing the focus position, whether it is positive defocus or negative defocus, the slit width will become wider.
With different focal lengths of the cutting heads used, the extent of slit widening is also different. Generally, the shorter the focal length of the cutting head lens and the smaller the focal depth, the greater the variation of the slit with the focal position.
Before cutting any plate, the distance between the focus and the plate must be adjusted.
Usually, the choice of focus position will be different with different cutting materials, so how to choose it correctly?
How to choose the appropriate focus position?
Positive focus cutting
The focus is above the cutting material.
After the light beam reaches the material surface, the irradiation range becomes wider and diffuses in the cutting seam, making the lower part of the cutting seam larger than the upper part. It is suitable for oxidative cutting, such as oxygen cutting of carbon steel, which helps oxygen reach the bottom of the workpiece and participate in sufficient oxidation reaction;
At the same time, the larger slit at the bottom also helps to remove the slag.
Generally, for oxygen cutting of carbon steel, within a certain range, the larger the positive defocus, the larger the spot size on the material surface, and the brighter and smoother the cutting surface;
Beyond a certain range, the energy of the lower part may not be enough, which will also cause impervious cutting or slag hanging at the bottom.
2KW laser – positive focus cutting 15mm carbon steel with oxygen
Negative focus cutting
During negative focus cutting, the focus is inside the plate, and the laser is focused inside the material, which ensures that the lower part of the slit has sufficient energy density.
The slit is wide at the top and narrow at the bottom, and the cutting amplitude of the upper part is large, which improves the fluidity of the melt;
However, the lower part has a small cutting width and needs more air flow. Generally, negative defocus is used when cutting with air or nitrogen.
6kW laser – negative focus cutting 10mm stainless steel with nitrogen
Zero focus cutting
During zero focus cutting, the minimum spot size can be obtained on the plate surface, so the melting range is relatively narrow and the cutting seam is relatively small, which is suitable for high-precision cutting of thin materials.
2KW laser – zero focus cutting 1mm brass with nitrogen
Cutting characteristics and application scope of different focus positions
Scope of application
|The slit at the bottom of the plate is larger than that at the top, which helps the bottom participate in full oxidation reaction and slag discharge.||Oxygen cutting of carbon steel|
|The focus is inside the plate, and the cutting seam of the upper part of the plate is larger, which ensures that the lower part has sufficient energy density.||Nitrogen/air cutting of stainless steel, carbon steel, galvanized steel, aluminum and copper|
|The slit is the narrowest and the machining precision is high||Sheet/foil cutting
In recent years, the power range of fiber laser is increasing year by year.
For laser cutting applications, it has also changed from KW to 10000 watts.
Our laser factory has continuously explored the application of high-power lasers and developed a unique HBF high brightness flat mode laser output.
While ensuring excellent thick plate cutting quality, it also takes into account the needs of thin plate cutting efficiency.
Common focus position of high power laser cutting
Positive focus cutting mild steel with oxygen
Negative focus cutting stainless steel with oxygen
In the actual cutting application, it is also necessary to select the appropriate focus position according to the specific cutting requirements.