When to choose negative focus for laser cutting

The “negative focus” (also known as “lower focus” or “focus inside the plate”) of the laser cutting machine is the 1 important process to adjust the laser focus position. In simple terms, negative focus refers to setting the focus of the laser below the surface of the material to be cut. The use of negative focus is mainly based on the following principles and purposes: When the focus is inside the material, the spot size of the beam at the surface of the material will become larger and the energy density will be relatively reduced. However, this is advantageous in certain situations because it creates a wider tapered light path, which is beneficial:

1. Discharge of slag: A wider incision allows auxiliary gases (such as oxygen and nitrogen) to blow away the molten metal more effectively to prevent slag hanging.

2. Get better section quality: For thicker materials, it can achieve a more vertical and smoother cutting section.

3. Protect the lens: the focus down can prevent the splash generated by cutting from directly rebounding to the focusing lens.

When should you use negative focus (main application scenario)

1. Cutting thicker metal materials (especially carbon steel)

• This is the most important application scenario of negative focus.
• Reason: When cutting thick plates, longer energy action time is required to melt the entire thickness. The use of negative coke (usually 1/3 to 1/2 of the thickness of the plate) can expand the width of the upper part of the incision, so that oxygen (when cutting carbon steel) can smoothly enter the bottom of the incision, maintain sufficient oxidation exothermic reaction, and smoothly blow away slag. If you use positive focus or zero focus (focus on the surface or above), the upper part of the incision is too narrow, which will easily lead to opaque cutting at the bottom, serious slag hanging and rough section.
• Experience reference: When cutting carbon steel plates above 6mm, it is usually necessary to start using negative coke.
• As the thickness increases, the amount of negative focus (the depth of focus into the material) needs to increase accordingly. For example, cutting 20mm carbon steel, the focal point may be set 3-5mm below the surface.

2. The pursuit of high quality cutting section

• When there are high requirements for verticality, smoothness and slag-free cutting sections, even for medium-thickness materials, a slight negative focus may be used to optimize airflow and energy distribution.

3. When performing perforation operation

• In order to prevent the high-temperature slag splash generated during perforation from damaging the lens, the focus is usually set to negative focus for perforation (low energy and deep position), and then the focus is adjusted to the setting position required for cutting after the perforation is completed.

When you should not use negative focus

1. Cut sheet material (usually <3mm)

• Reason: cutting thin plate pursuit of speed and precision. The use of zero focus or slight positive focus allows for the smallest spot and the most concentrated energy, resulting in narrower kerfs, faster speeds and finer cuts.

2. When performing high-precision precision machining

• The reason: In order to ensure the smallest spot size and the highest dimensional accuracy, the focus is usually set on the surface of the material (zero focus).

3. When using nitrogen to cut stainless steel to obtain a bright surface

• Reason: Nitrogen cutting stainless steel relies on laser energy to melt the material, and then uses high-pressure nitrogen to blow away the melt without oxygen reaction. In order to obtain a vertical, smooth, and non-oxidized “bright surface” cutting effect, zero focus or slight positive focus is usually used to ensure energy concentration and narrow and neat slits.

How to determine the best amount of negative focus?

There is no fixed value, and the best focus position needs to be determined through process testing and is affected by the following factors:

• Material type: carbon steel, stainless steel, aluminum have different focus strategies.
• Material thickness: the greater the thickness, usually the greater the amount of negative focus.
• Cutting gas: Oxygen cutting and nitrogen cutting have different focus strategies.
• Laser power and mode: Different power and beam quality (such as single mode vs multi-mode) machines have different optimal focus positions.

The common test method is:

1. Focus calibration: first find the “zero focus” position of the material surface (usually the machine has an automatic or manual calibration program).

2. Making focus test samples: under the same power, speed and gas pressure, cut a series of straight lines or patterns at different focus positions (for example, from 3mm to -3mm, one step per 0.5mm or 1mm).

3. Evaluation effect: observe the section quality, verticality, slag hanging condition, slit width and whether the bottom of each cut is cut.

4. Select the best point: comprehensive cutting quality, stability and efficiency, select the best focus position as the process parameters under the thickness of the material.

Summary

Core principle: The specific focus position is always a function of material, thickness, machine power and required quality. Optimal process parameters must be obtained through field testing and validation. When operating, be sure to refer to the process parameter manual provided by the equipment manufacturer, and fine-tune it in combination with the actual cutting effect.