Expert Tips for Choosing Laser Cutting Focus

Before cutting any sheet, the distance between the laser focal point and the material to be cut must be properly adjusted.

Different focal point positions can result in varying levels of precision in the cross-section of the cut material, as well as differences in bottom slagging and even failure to cut the material.

The choice of focal point position for the laser cutting machine will depend on the workpiece and the material to be cut.

To make the right choice, it is important to inspect the laser cutting machine prior to use to ensure high-quality cuts and proper machine operation.

Positive Focus Cutting

Positive focus cutting is a technique where the focal point of the laser beam is positioned above the material surface, with the focal point located on top of the workpiece. This method offers several advantages in various cutting applications, particularly for carbon steel oxygen cutting and high-power laser cutting of thick plates.

In carbon steel oxygen cutting, a positive focus is generally preferred due to its beneficial effects on cut quality and process efficiency. The resulting cutting profile exhibits a wider kerf at the bottom of the workpiece compared to the top. This tapered shape facilitates slag removal and allows for improved oxygen penetration to the bottom of the cut, promoting complete oxidation throughout the material thickness.

The efficacy of positive focus cutting can be further optimized by adjusting the focal range. Within certain limits, increasing the positive focal distance leads to a larger spot size on the plate surface. This expanded beam diameter enhances preheating and heat distribution around the cut zone, resulting in smoother and brighter cutting surfaces on carbon steel. The improved heat distribution also contributes to more consistent cut quality and reduced risk of defects such as dross formation.

For high-power laser cutting applications, such as 10,000-watt laser pulse cutting of thick stainless steel plates, positive focus cutting offers significant advantages. It provides stable cutting quality by maintaining a consistent beam profile throughout the cut depth. This stability is particularly beneficial for slag removal in thick plate cutting, as it helps prevent molten material from adhering to the bottom of the cut. Additionally, the positive focus technique reduces the risk of reverse blue light phenomena, which can occur when the laser beam reflects off the bottom of the cut and potentially damages the cutting head optics.

Figure 1 illustrates the positive focus cutting setup and its effects on the cutting process, demonstrating the improved cut quality and reduced risk of optical damage.

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

Negative Focus Cutting

Negative focus cutting is an advanced laser cutting technique where the focal point is positioned within the workpiece, below the material surface. This strategic placement of the focal point offers distinct advantages in certain applications.

In this mode, the focal point’s deeper position results in a wider beam at the cutting surface, creating a relatively larger kerf width within the workpiece compared to positive focus cutting. While this approach requires increased cutting gas flow and higher energy input to maintain sufficient temperature throughout the cut, it offers several benefits:

1. Enhanced cut quality: Negative focus cutting is particularly effective for processing stainless steel, producing a uniform cutting surface pattern and superior cross-sectional finish. The wider beam at the surface helps to stabilize the cutting process, reducing the likelihood of dross formation and minimizing heat-affected zones.
2. Improved piercing performance: When initiating cuts through piercing, a negative focus is preferred. This technique ensures a minimum perforation spot size while maximizing energy density at the focal point. The concentrated energy facilitates rapid and clean penetration, especially in thicker materials.
3. Depth-dependent focus adjustment: As the perforation depth increases, the negative focus point should be adjusted accordingly. Generally, deeper perforations require a larger negative focus offset to maintain optimal energy concentration at the cutting front, as illustrated in Figure 2.
4. Reduced sensitivity to focal position: The wider beam at the surface makes the process less sensitive to small variations in focal position, which can be beneficial when cutting materials with surface irregularities or when working with less precise positioning systems.

However, operators must carefully balance the benefits of negative focus cutting against its increased power and gas consumption. Proper optimization of cutting parameters, including laser power, assist gas pressure, and cutting speed, is crucial to fully leverage the advantages of this technique while maintaining process efficiency.

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

Zero Focus Cutting

Zero-focus cutting is a precision laser cutting technique where the focal point of the laser beam is positioned precisely at the surface of the workpiece. This method creates a distinctive cutting profile: the upper surface near the focal point exhibits a relatively smooth finish, while the lower surface, further from the focal point, presents a comparatively rougher texture.

This technique is primarily employed in two key applications:

1. Thin Sheet Cutting: When using continuous-wave lasers to process thin metal sheets, typically under 3mm thickness. The concentrated energy at the surface allows for clean, precise cuts with minimal heat-affected zone (HAZ).
2. Foil Layer Processing: In the cutting of ultra-thin metal foils (often less than 0.1mm) using pulsed lasers. Here, the high peak power of each pulse enables rapid vaporization of the material at the focal point, resulting in exceptionally clean edges with negligible thermal distortion.

Zero-focus cutting offers several advantages, including reduced kerf width, improved edge quality on the incident side, and the ability to cut intricate patterns in thin materials. However, it requires precise focal length control and is generally less suitable for thicker materials where through-thickness consistency is crucial.

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.

The lens of the 10,000-watt cutting head is crucial to the 10,000-watt laser cutting machine. Its cleanliness directly impacts the machine’s processing performance and cutting quality.

If the lens is dirty, it can not only negatively impact the cutting results but also cause damage to the internal components of the cutting head and the laser output head.

Performing a pre-check 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;
	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 co-axiality of the nozzle outlet hole and the laser beam is a crucial factor that impacts the cutting quality. Inconsistencies caused by misalignment of the nozzle and the laser beam can result in an irregular cutting surface. In severe cases, the laser beam may hit the nozzle, leading to overheating and burning of the nozzle.

The coaxial debugging method is shown in Table 2.

Table 2 Coaxial debugging method

Nozzle	Nozzle 1.2mm	Normal coaxial	Abnormal coaxial
Tools	Sticky tapes	Three coaxial
Methods	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.

The shape of the laser spot directly reflects the distribution of laser energy in the lateral direction. The roundness of the spot indicates the uniformity of energy distribution in different directions.

A highly rounded spot with uniform energy distribution in all directions can guarantee a smooth cutting surface and consistent cutting speed in all directions, ensuring the quality of the cutting products.

Table 3 is the spot distribution check method.

Table 3 Coaxial debugging method

Nozzle	Single nozzle above 2mm	Normal laser spot	Abnormal laser spot
Tools	Black laser spot paper
Methods	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.

One of the major issues with using 10,000 watt lasers for cutting thick carbon steel plates is the overheating of the nozzle. To address this, it is important to test the nozzle temperature for various nozzle sizes and focal points at full power output before beginning the cutting process.

Before conducting the tests, it is necessary to align the co-axiality so that the light spot is centered at the nozzle outlet. Additionally, a cooling gas system for cutting should be installed for the nozzle.

Test method:

Iron buckets filled with a suitable amount of water should be placed beneath the cutting head.

The auxiliary gas (the type of gas and air pressure should be adjusted based on the specific requirements for cutting) should be blown first, and the laser should be operated at full power for 30 seconds to monitor any changes in the nozzle temperature.

The nozzle size should then be changed and the testing process repeated.

The heat generation of different nozzles should be recorded and used as a reference for the subsequent cutting process setup.

If the nozzle temperature does not remain within a narrow range, it may indicate a problem with the cutting head, nozzle, lens, or laser, and further troubleshooting should be performed before proceeding with the next step in the cutting process setup.

Conclusion

The choice between using a positive or negative focal point for laser cutting does not depend on the metal material being cut (such as stainless steel or carbon steel), but rather on the cutting method being used (such as oxidation cutting or fusion cutting).

Different workpieces processed by the laser cutting machine may require different focal patterns.

DBy considering the different effects of positive and negative focus on cutting stainless steel and carbon steel, and taking into account their individual processing needs, users can select the appropriate focus cutting method to fully utilize the performance benefits of the laser cutting machine.

When it comes to 15kW lasers from different brands, they generally have similar cutting results and processing efficiency for both carbon steel and stainless steel.