How Does the Focal Length of a Laser Cutting Machine Affect the Cutting Process?

In laser cutting technology, focal length is one of the key parameters determining cutting quality and efficiency. The focal length directly affects the focusing degree and energy density of the laser beam. When a laser beam is focused by a focusing lens to a very small focal point, changes in focal length significantly alter the spot diameter, energy concentration, and the beam’s effective range within the material. A shorter focal length results in more concentrated light energy, suitable for high-speed, high-precision cutting of thin plates; while a longer focal length provides a more stable energy distribution, better suited for thicker plates or materials with uneven surfaces.

Different focal length settings significantly affect the laser cutting depth, kerf width, edge quality, and cutting speed. For example, a shorter focal length helps achieve smoother edges and finer kerfs, while a longer focal length performs better when cutting thicker or highly reflective materials. Appropriate focal length selection can improve production efficiency while ensuring processing quality and effectively extending the lifespan of the laser lens and cutting head. Therefore, understanding and mastering the adjustment principles of focal length is a crucial step in optimizing laser cutting processes and improving overall cutting performance.

What is Focal Length?

In laserové řezací stroje, focal length refers to the distance between the laser beam and the focal point after passing through the focusing lens. The core of laser cutting lies in the concentration of energy, and the focal length is the key parameter determining the degree of light energy focusing. Different focal lengths directly affect the spot diameter and energy density, thus significantly impacting the cutting effect.

When the laser beam passes through the focusing lens, it is compressed into a high-energy-density focal area. This focal point is the point of highest energy concentration during the entire cutting process, where the material is rapidly heated, melted, or even vaporized, achieving the cut. A shorter focal length results in more concentrated laser energy, higher temperatures at the focal point, and a sharper cut; while a longer focal length results in a larger spot size and more uniform energy distribution, but relatively lower cutting precision.

Generally, laser cutting machines are equipped with lenses of different focal lengths depending on the application. Short focal length lenses are more suitable for processing thin plates, high-precision parts, and complex contours; long focal length lenses are more suitable for thick plates, high-power cutting, or materials with significant surface undulations. Understanding the meaning and function of focal length helps operators adjust the optimal parameters under different materials and working conditions to achieve ideal cutting results.

The Impact of Focal Length on Cutting Characteristics

The core of laser cutting lies in the focusing and control of light energy. Lenses with different focal lengths alter the energy distribution of the laser beam on the material surface, thus affecting the cutting depth, kerf width, edge quality, and overall cutting speed. Understanding the role of focal length in these key characteristics helps users find optimal matching solutions in actual processing, improving cutting efficiency and processing quality.

Focal Length and Cutting Depth

Changes in focal length directly determine the laser beam’s penetration ability within the material. Short focal length lenses can generate higher energy density, resulting in stronger beam focusing on the material surface. Therefore, in thin plate cutting, they can quickly penetrate the material, achieving deeper and sharper cuts. However, when cutting thicker materials, short focal lengths have a shallower depth of focus, and the laser energy is prone to attenuation during penetration, leading to an uneven cut surface.

In contrast, long focal length lenses have a wider depth of focus range, and the laser energy is more evenly distributed in thick plates, enabling stable depth cutting. Short focal lengths are suitable for high-speed cutting of thin plates, while long focal lengths are more suitable for deep cutting of thick plates. Choosing the right focal length allows you to maintain ideal cutting depth and penetration for different material thicknesses.

Focal Length and Kerf Width

Kerf width reflects cutting precision. Short focal length lenses, due to their strong focusing ability and smaller spot diameter, can create extremely fine kerfs, making them particularly suitable for machining parts requiring high precision and a high surface finish. However, because of their shorter depth of focus, even slight deviations in the focal position can easily lead to uneven kerfs or edge ablation.

Long focal length lenses have a larger spot size and a slightly wider kerf, but their greater depth of focus makes them less demanding on material flatness, making them more suitable for machining thick plates or irregular surfaces. When precision and detail are the primary concerns, a short focal length should be used; if cutting stability and material adaptability are emphasized, a long focal length is more advantageous.

Focal Length and Edge Quality

Edge quality is an important indicator for evaluating cutting results. Short focal length cutting produces a small spot size and concentrated energy, resulting in smooth edges and minimal burrs. However, if the laser power is too high or the focus is improperly adjusted, melting marks can easily occur.

While longer focal lengths offer slightly lower cutting precision, their greater depth of focus results in more uniform light energy distribution and a smaller heat-affected zone, thus maintaining smooth edges and minimizing deformation when cutting thick plates. Shorter focal lengths achieve precise and delicate edge quality, while longer focal lengths offer a better balance of stability and aesthetics in thick plate cutting.

Focal Length and Cutting Speed

Laser cutting speed is closely related to light energy density. Shorter focal length lenses concentrate energy more effectively, allowing for higher cutting speeds in thin plate processing; while longer focal lengths, due to lower energy density, have relatively slower cutting speeds, but offer stable performance in high-power thick plate cutting, effectively preventing thermal deformation and scorching. Shorter focal lengths improve production efficiency, while longer focal lengths ensure processing stability. Optimizing cutting speed should consider power, material thickness, and focal length parameters comprehensively.

In summary, focal length is one of the key factors affecting the performance of laser cutting machines. Short focal length lenses offer advantages such as high precision and high speed, but have a shorter depth of focus and require more precise focusing. Long focal length lenses are suitable for thick plates and complex materials, providing more stable cutting but with slightly lower precision. In actual production, the focal length should be flexibly selected based on the type and thickness of the material and processing requirements to achieve the best cutting results and economic benefits.

Advantages and Disadvantages of Short Focal Length Lenses

Short focal length lenses are widely used in laser cutting technology, especially in thin sheet metal processing and high-precision parts manufacturing. The shorter the focal length, the stronger the laser beam focus and the higher the energy concentration, resulting in faster cutting speeds and finer cutting effects. However, while improving cutting performance, short focal lengths also place higher demands on the focusing accuracy of the equipment, lens cleanliness, and operational stability.

Main Advantages of Short Focal Length Lenses

Higher Energy Density and Cutting Speed: Short focal length lenses can compress the laser beam into a smaller spot size, generating extremely high energy density. This means that at the same power, the light can melt and vaporize the material in a shorter time, thus significantly increasing the cutting speed. This characteristic is particularly suitable for high-efficiency mass production scenarios.

Fineer Kerfs and Smoother Edges: Due to the smaller spot diameter, short focal length cutting can achieve narrower kerfs and smoother cutting edges, with a relatively smaller heat-affected zone. This is a significant advantage for industries that require high precision, such as electronic components, mechanical parts, and decorative metal sheets.

Suitable for complex graphics and micro-machining: Short focal lengths allow for more precise energy control, making lasers more flexible in cutting complex curves, small holes, or patterns. Whether for engraving, micro-hole machining, or cutting thin sheet structures, excellent detail reproduction can be achieved.

Short focal length cutting, with its high energy density, high precision, and high speed, is particularly suitable for laser processing of thin plates and precision parts, making it an ideal choice for manufacturing companies seeking high efficiency and high quality.

Limitations and Disadvantages of Short Focal Lengths

Limited Depth of Focal Length and Lower Error Tolerance: While short focal lengths concentrate energy, the shallow depth of focal length makes them extremely sensitive to the focal point. Even slight surface irregularities or focus misalignment can rapidly degrade cutting quality, potentially leading to uneven kerf, slag buildup, or incomplete cuts.

Higher Risk of Material Damage: Due to the highly concentrated energy, cutting highly reflective or low-melting-point materials can easily cause ablation, discoloration, or even excessive penetration. If this high-energy-density thermal effect is not properly controlled, it can affect the appearance and dimensional accuracy of the finished product.

Higher maintenance frequency and cost: Short focal length lenses have a shorter optical path distance and more concentrated energy in the focal area, making them more prone to dust accumulation or thermal contamination, thus affecting focusing performance. To ensure cutting stability, more frequent lens cleaning and calibration are required, increasing maintenance costs.

While short focal lengths offer superior speed and precision, they demand more from operators and require more complex maintenance. To achieve high-quality processing, it is essential to ensure equipment cleanliness, precise focusing, and stable optical path control.

Overall, short focal length laser cutting is suitable for thin sheet materials, high-speed production, and precision parts machining, significantly improving cutting quality and production efficiency. However, it is not a universal solution. For thick plates or irregular surface materials, short focal lengths may lead to unstable cutting due to insufficient depth of focus. Only by combining material characteristics, equipment configuration, and production requirements, and rationally matching focal length parameters, can a balance between performance and quality be achieved.

Advantages and Disadvantages of Long Focal Length Lenses

Long focal length lenses play a crucial role in thick plate laser cutting and high-power laser processing. Compared to short focal length lenses, long focal length lenses offer greater depth of focus, a wider energy distribution range, and stronger material adaptability. They maintain stable spot quality during cutting, achieving good cutting consistency even with slightly uneven material surfaces. However, these lenses also have limitations, often performing worse than short focal length systems in terms of precision and speed.

Key Advantages of Long Focal Length Lenses

Greater Depth of Focus and Higher Processing Tolerance: The greater depth of focus of long focal length lenses means a wider range of high energy density maintained within the material. This makes them more stable when cutting thick plates or materials with some surface undulations, less prone to affecting cutting quality due to focus shift. This characteristic is particularly important for industrial applications with tolerance errors or slight vibrations.

Reduced Risk of Material Damage and Smoother Cutting: Due to the relatively uniform energy distribution and smaller heat-affected zone of long focal length lenses, heat accumulation during cutting is easier to control, effectively reducing the risk of melting, scorching, or deformation. This makes it safer and more stable when processing uhlíková ocel, nerezová ocel, hliník plates, and reflective materials.

Low maintenance frequency and longer lens life: The longer optical path of a long focal length results in a lower heat load on the lens surface due to laser energy, reducing the risk of optical contamination or thermal damage from high temperatures. Compared to short focal length systems, it is less dependent on lens cleanliness and focusing accuracy, allowing for longer maintenance cycles and thus lower operating costs in the long run.

With their large depth of focus, low heat-affected zone, and high stability, long focal length lenses are ideal for thick plate cutting and high-power processing, particularly suitable for industrial applications requiring high stability and consistency.

Limitations and disadvantages of long focal length lenses

Slightly lower cutting accuracy and detail: Due to the larger spot diameter and relatively lower energy concentration of long focal length lenses, the kerf width is wider, resulting in lower accuracy compared to short focal length systems. In the processing of microstructures, curved contours, or complex engravings, long focal lengths struggle to achieve high-precision detail.

Relatively Slow Cutting Speed: Increased focal length means a wider laser energy distribution range and a lower energy density per unit area, resulting in a slightly slower cutting speed. For mass production of thin plates at high efficiency, long focal lengths cannot match the processing speed of short focal length solutions.

Higher power requirements: To ensure cutting depth and penetration, long focal length systems typically require higher-power laser generators. This increases energy consumption and equipment investment, potentially leading to higher operating costs for small and medium-sized manufacturing enterprises.

While long focal lengths offer excellent depth of focus and stability, they have inherent limitations in cutting speed and precision. In thin plate or high-precision processing applications, their performance may be inferior to short focal length systems.

Overall, the advantages of long focal length laser cutting lie in its stable, reliable, and highly tolerant processing characteristics, making it ideal for thick plate cutting, high-power processing, and materials with irregular surfaces. Despite slight shortcomings in cutting speed and precision, with proper power configuration and cutting parameter optimization, long focal lengths can still achieve excellent cutting results. In practical applications, companies should comprehensively select the focal length based on factors such as material thickness, production type, and equipment power to achieve the best balance between efficiency, quality, and cost.

Factors to Consider When Choosing a Focal Length

In laser cutting, focal length is not a fixed parameter; it requires comprehensive judgment based on material characteristics, thickness, cutting requirements, and equipment configuration. Different focal lengths correspond to different optical performances and cutting characteristics. Only by scientifically selecting the focal length can the optimal balance between speed, precision, and stability be achieved.

Typ materiálu

Different materials have different absorption rates, reflectivities, and thermal conductivity characteristics for laser beams, thus requiring different focal lengths. For example, carbon steel and stainless steel have high laser absorption rates, and short focal length lenses can easily achieve high-quality cutting; while highly reflective materials such as aluminum and copper easily reflect laser energy, using long focal length lenses can reduce the risk of lens damage and achieve a more stable cutting process.

Furthermore, for brittle materials (such as ceramics or glass), choosing a lens with a larger depth of focus helps control thermal stress and prevent edge cracking. Different materials have significantly different adaptability to focal lengths; the focal length should be matched according to the material’s absorption characteristics to ensure stable and safe cutting.

Tloušťka materiálu

Material thickness is one of the core factors determining focal length selection. In thin sheet machining, short focal length lenses concentrate energy for high-speed cutting and extremely fine kerfs, making them ideal for materials such as stainless steel and carbon steel (0.5–6 mm thick). However, when the material thickness exceeds 10 mm, the insufficient depth of focus of short-focal-length lenses causes energy attenuation, leading to slag buildup or uneven melting on the cut surface. In such cases, long focal length lenses, with their greater depth of focus and more uniform energy distribution, better ensure the cutting penetration and kerf stability of thicker plates.

Using short focal lengths for thin sheets and long focal lengths for thick sheets is the most common configuration in laser cutting. Correctly matching the focal length significantly improves cutting quality and efficiency.

Cutting Characteristics and Process Requirements

Different cutting targets correspond to different process priorities. When the processing task emphasizes detail, precision, and smooth edges, such as the manufacturing of electronic components, logo engraving, or high-end decorative parts, short focal lengths are more suitable. If the cutting target is a structural component, equipment frame, or thick plate welded parts, the focus is on the consistency of cutting depth and strength. In this case, the beam of a long focal length is more stable, achieving better overall quality.

Furthermore, for scenarios requiring mixed processing of varying thicknesses (such as multi-functional laser cutting production lines), adjustable focal length lenses or automatic focusing systems can be selected for flexible switching. Focal length selection should be based on process objectives—shorter focal lengths are preferred for precision, while longer focal lengths are more suitable for stability.

Machine Specifications and Laser Power

Focal length is closely related to laser power and the optical path system. Different models of laser cutting machines differ in beam divergence angle, lens structure, and spot shape, so the same focal length may perform differently on different devices. For example, high-power laser generators (above 6kW) are often paired with long focal length lenses to achieve thick plate cutting, while medium- and low-power devices may not be able to penetrate materials due to insufficient energy density when using long focal lengths.

In addition, the machine’s cooling system and lens dustproof design also affect the lifespan and stability of the focal length. Focal length selection should match the machine specifications, power level, and lens system to ensure maximum optical performance.

Overall, focal length selection is a key aspect of laser cutting system optimization. It not only determines the cutting depth and speed but also affects the kerf quality, material adaptability, and equipment lifespan. When developing processing techniques, companies should comprehensively consider material type, thickness, cutting requirements, and equipment performance, optimizing parameters based on actual test data. Only with a scientifically matched focal length can a laser cutting machine achieve a comprehensive performance of high efficiency, high stability, and high quality.

Shrnout

In the field of laser cutting, focal length is not merely an optical parameter, but a crucial factor affecting cutting efficiency and processing quality. A suitable focal length enables higher energy density, more precise cutting edges, and more stable cutting speeds, resulting in ideal performance in processing various materials and thicknesses. Conversely, an inappropriate focal length, even with a high-power laser generator, can lead to problems such as excessively wide kerfs, edge ablation, or incomplete cutting, impacting production efficiency and finished product quality.

From a practical application perspective, short focal length lenses perform better in thin-plate and high-precision cutting, suitable for processing parts with high detail requirements; while long focal length lenses are suitable for cutting thick plates, complex structures, or materials with uneven surfaces. When selecting equipment, companies should comprehensively consider multiple factors such as material type, processing depth, cutting speed, and maintenance costs to achieve a balance between cost and performance.

AccTek Laser is committed to providing high-performance, highly stable laser cutting solutions to customers worldwide. Whether you are seeking fine processing or require high-power thick plate cutting, our fiber laser cutting machines can achieve optimal cutting results based on different focal length options. Precise optical configuration, intelligent focusing system, and stable equipment performance make your production process more efficient and reliable. Choosing AccTek Laser means choosing a higher-quality laser cutting experience.

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