How to Choose Laser Cutting Focus?

The distance between the laser focal point and the material to be cut must be adjusted before any sheet is cut off.

Different focal point positions often result in different levels of fineness in the cross-section of the cut material, different bottom slagging and even failure to cut the material.

Depending on the workpiece and the material to be cut, the choice of focal point position for the laser cutting machine will vary.

So how do you make the right choice?

In order to cut high-quality sheets and ensure the normal operation of the laser cutting machine, it is also necessary to do a proper inspection of the machine before it starts working.

Positive Focus Cutting

Positive focus cutting means cutting with the focal point on top of the workpiece, positioning the focal point above the material being cut.

For carbon steel oxygen cutting, it is suitable for a positive focus.

The bottom of the workpiece has a larger cutting width than the upper surface, which is conducive to slag removal and facilitates the oxygen reaching the bottom of the workpiece to participate in the full oxidation reaction.

Within a certain focus range, the larger the positive focus point, the larger the spot size on the surface of the plate, the better the preheating and heat supplement around the slit, the smoother and brighter the cutting surface of carbon steel.

For the 10,000-watt laser pulse method to cut stainless steel thick plates, the positive focus cutting quality is stable, which is beneficial to slag removal and is not easy to reverse blue light, as shown in Figure 1.

The effect of a 12000W laser cutting carbon steel with a positive focal point

Fig. 1 The effect of a 12000W laser cutting carbon steel with a positive focal point

Negative Focus Cutting

Negative focus cutting means the cutting focus is inside the workpiece.

This mode, due to the focal point away from the cutting surface, the cutting width is relatively larger than the cutting point in the workpiece surface, while the need for cutting airflow needs to be large and temperature to be sufficient.

Cutting stainless steel is suitable for negative focal point cutting, with uniform cutting surface pattern and good cross-section.

Perforation of the plate before cutting, due to a certain height of perforation, perforation uses a negative focal point, which can ensure that the location of the perforation spot size is minimum, energy density is maximum.

The deeper the perforation position, the greater the negative focus point, as shown in Figure 2.

The effect of a 6000W laser with a negative focus on stainless steel

Fig. 2 The effect of a 6000W laser with a negative focus on stainless steel

Zero Focus Cutting

Zero-focus cutting means that the cutting focal point is on the workpiece surface, in this case, the cutting surface close to the focal point is relatively smooth, while the lower surface away from the cutting focal point appears rough.

This is mainly used when cutting thin sheets with continuous lasers and when cutting foil layers with pulsed lasers using peak power vaporization.

2000W laser cutting of thin carbon steel sheets at zero focus

Fig. 3 2000W laser cutting of thin carbon steel sheets at zero focus

A Thorough Inspection Makes For Better Cutting

1) Cleanliness of the lens.

10,000-watt cutting head lens is very important to 10,000-watt laser cutting machine, its cleanliness directly affects the processing performance and quality of the laser cutting machine.

If the lens is dirty, it will not only affect the cutting effect but also further cause the cutting head inside and the laser output head burned.

Pre-checking before cutting can prevent serious damage.

The red light detection method is shown in Table 1.

Table 1 Red light detection method

Nozzle Single nozzle above 2mm Clean lens Dirty lens
Methods 1. Red light is indicated when the laser is turned on; Red light detection method Red light detection method dirty lens
2. The white paper is about 300mm away from the cutting head nozzle to view the red light;
3. If there are black spots or irregular black objects in the red light, the lens is dirty and needs to be cleaned before cutting and debugging.

2) Coaxial debugging.

The coaxiality of the nozzle outlet hole and the laser beam is one of the important factors affecting the quality of cutting.

If the nozzle is not on the same axis as the laser, slight inconsistencies can affect the cutting surface.

Serious cases of the laser will hit the nozzle, causing the nozzle heat and burn.

The coaxial debugging method is shown in Table 2.

Table 2 Coaxial debugging method

Nozzle Nozzle 1.2mm Normal coaxial Abnormal coaxial


Sticky tapes Three coaxial

Normal coaxial

Abnormal coaxial


1. Coaxial adjustment at focal point 0, so that the laser is in the center of the nozzle;
2. Light at focal point ±6mm;
3. If focus 0 and ±6mm hitting light point are in the center of the nozzle, it is normal.

If not, it needs to replace the cutting head or laser light path offset.

(3) Check the spot distribution.

Spot shape directly feedback the lateral distribution of laser energy.

Spot roundness indicates the uniformity of energy distribution in different directions of the spot.

The high roundness of the spot with consistent energy distribution in all directions can ensure the smoothness of the cutting surface and the consistency of cutting speed in all directions to ensure the quality of cutting products.

Table 3 is the spot distribution check method.

Table 3 coaxial debugging method


Single nozzle above 2mm Normal laser spot Abnormal laser spot


Black laser spot paper

Normal laser spot

Abnormal laser spot


1. The black laser spot paper is about 300mm away from the cutting head nozzle to make a spot;
2. If there are black spots in the spot, the lens is dirty;
3. If the spot is not round and the center is shifted, then the laser spot is poorly distributed

(4) Nozzle heat range test.

The biggest problem with 10,000 watt lasers for cutting thick carbon steel plates is the nozzle heat, so before cutting it is necessary to test the nozzle heat for different nozzle sizes at different focal points for full power output.

Before testing, it needs to adjust the coaxial so that the light spot is in the center of the nozzle output; install nozzle cooling gas for cut.

Test method:

Iron buckets (containing an appropriate amount of water) placed underneath the cutting head.

It blows the auxiliary gas first (gas type and air pressure should be set according to the actual gas required for cutting), with full power light out for 30 seconds to measure the nozzle temperature change.

Change the nozzle size and continue testing as above.

Summarize the heat generation of different nozzles and provide a reference for the subsequent commissioning of the cutting process.

If the nozzle does not generate heat in a narrow range, check the cutting head, the nozzle, the lens and the laser problem, and then proceed to the next cutting process debugging.


The choice of a positive or negative focal point for laser cutting is not determined by the material of the metals (stainless steel, carbon steel), but by the cutting method (oxidation cutting, fusion cutting).

The different workpieces processed by the laser cutting machine require the use of different focal patterns.

Due to different types of cutting head focus position and different ways of adjustment, users can choose the appropriate focus cutting method according to the different effects of positive and negative focus in cutting stainless steel and carbon steel, combined with their own processing needs to give greater play to the performance advantages of the laser cutting machine.

The 15kW lasers with different laser brands have basically the same cutting effect and processing efficiency in carbon steel and stainless steel processing.

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