In laser cutting, the choice of focal point position is one of the core parameters determining cutting quality. When the focal point of the cutting head is below the material surface, it is called "negative focus" (or simply negative focus). Properly utilizing negative focus is an important means of optimizing laser cutting machine performance, especially improving the cutting quality of thick plates and handling special materials.

Core Application Scenario 1: Medium-Thick Plate Cutting

This is the most typical scenario for choosing negative focus. When cutting thicker plates (such as carbon steel or stainless steel over 12mm), the beam needs to penetrate a greater depth into the material. Setting the laser cutting machine's focal point below the plate surface (usually at 1/3 to 1/2 of the plate thickness) allows the narrowest beam waist (the point of highest energy density) to be located in the middle or even the lower part of the plate. This ensures a more uniform energy distribution along the thickness direction, resulting in a more consistent and vertical cut surface and effectively reducing bottom slag.

Core Application Scenario 2: Cutting Highly Reflective Materials

Using negative focus is a common strategy when cutting highly reflective metals such as aluminum and copper. By focusing the laser energy deep into the material, the proportion of energy directly reflected from the material surface can be reduced, improving the absorption efficiency of laser energy. This helps to more stably initiate and maintain the cutting process, which is also beneficial for protecting the optical system of the laser cutting machine from damage caused by strong reflected light.

Core Application Scenario 3: Pursuing Specific Cutting Quality

When the cutting task has extremely high requirements for the quality of the lower surface, negative focus is often a necessary choice. By shifting the highest energy area downwards, priority can be given to ensuring that the material at the bottom of the sheet is fully melted and cleanly blown away by high-pressure gas, thus obtaining a smooth lower edge. This is crucial in many precision cuts that do not require secondary processing.

Scenarios Requiring Caution or Avoidance in Using Negative Focus

Conversely, when cutting thin plates (e.g., below 3mm), positive defocus (focus above the material surface) or zero focus is usually used. This is because thin plates require less energy, and negative focus may cause excessive energy concentration at the bottom, leading to overheating at the top, excessively wide kerf, or even material deformation. Furthermore, in precision piercing, to obtain a smaller piercing diameter and a more controllable molten pool, zero focus or positive focus is often used in the initial stage.

Practical Operation Recommendations

Adjusting the focus when operating a laser cutting machine is a delicate process. It is recommended to preset a basic negative focus amount in the process parameter library based on the material thickness and type (e.g., -3 to -5mm for cutting 20mm carbon steel), and then fine-tune it through trial cuts. Observe the perpendicularity of the cut surface and the amount of slag to find the optimal value. Modern high-end laser cutting machines typically have an automatic focus adjustment function, which can dynamically adjust the focus position during the cutting process according to the preset process curve to achieve optimal results.

In summary, when processing medium-thick plates, highly reflective materials, or workpieces with high requirements for the quality of the lower surface, the negative defocus process should be given priority. Correctly understanding and applying this principle is a key skill for maximizing the potential of the equipment and achieving high-quality cuts. If the above does not solve your problem, you can consult a RITMAN Laser after-sales engineer for assistance.