Selection of Positive/Negative Focus During Laser Cutting

When laser cutting, the choice between positive defocus (positive focus) and negative defocus (negative focus) primarily depends on the material type, plate thickness, and process requirements. Below is a clear, straightforward decision-making guide.

Core Principles

In-focus (zero defocus): Energy is most concentrated, resulting in the narrowest cut kerf. Suitable for cutting thin sheets with high precision and at high speeds, it prioritizes “refinement.”

Negative focus (negative defocus): The energy is more dispersed, resulting in a wider cut seam. Suitable for thick plates, it enhances slag removal and protects the lens, with a focus on “penetration power and quality.”

When should you use in-focus (zero defocus) cutting?

Applicable scenarios: Cutting thin plates (typically less than 3 mm), especially in applications with high requirements for both precision and speed.

1. Precision cutting of thin plates:

• Material examples: thin carbon steel sheet, stainless steel sheet, brass, aluminum sheet, etc.
• Reason: Thin plates produce less molten slag, so a wide kerf is not required to remove it. The use of positive focus can obtain the smallest spot and the highest energy density, so as to achieve extremely high cutting speed and smooth, vertical cross section.

2. High precision and narrow slit:

• Application examples: precision parts, electronic components, jewelry, hollow patterns, etc.
• Reason: The kerf produced by the positive focus is the narrowest, with less material loss, and can cut very fine contours and sharp corners to avoid excessive melting.

3. The pursuit of cutting speed:

• When cutting thin plates, the high energy density in the positive focus mode means that faster feed speeds can be used, thereby improving production efficiency.

Summary: When cutting thin materials, and the primary goal is speed, accuracy and narrow kerf, the priority is to choose a positive focus.

When to cut with negative focus (negative defocus)?

Application scenario: cutting medium and thick plate (usually ≥ 3mm), especially when good slag discharge and section quality are required.

1. Plate cutting:

• Examples of materials: carbon steel, stainless steel, aluminum plate, etc. above 3mm.
• Reason: Thick plates produce a large amount of molten metal (slag). The negative coke provides a wider slit, allowing the auxiliary gas (such as oxygen or nitrogen) to more effectively blow the slag away from the bottom, preventing slag hanging and cutting through.

2. Need high quality section:

• Reason: For thick plates, if positive focus is used, energy is concentrated on the upper surface, which will cause the upper part to be smooth and the lower part to be rough. The negative focus makes the energy distribution more uniform in the thickness direction of the plate, so as to obtain a more consistent and smoother cutting surface from top to bottom.

3. Protect the focusing mirror:

• Reason: When cutting thick plates, especially steel plates with rust coating or materials that are prone to a large amount of splash, the negative focus setting makes the focus located inside the workpiece, which increases the distance from the upper surface of the workpiece to the focusing mirror and reduces the metal splash. The risk of hitting and damaging the expensive focusing mirror.

4. Specific process (such as aluminum alloy nitrogen cutting):

• Reason: Nitrogen is usually used as an auxiliary gas when cutting aluminum alloys. High pressure air flow is required to blow away the molten aluminum. The wide kerf provided by the negative coke is very important for the flow and slag discharge of high-pressure nitrogen, and is the key to ensure that the cutting is free of burrs and the effect of bright surface.

Summary: When cutting thicker materials and the primary goal is to ensure cutting through, reduce slag, obtain uniform cross-section and protect equipment, negative coke is preferred.

Quick Decision Flow Chart

Tips in practice

1. The optimal amount of defocus is tested: the above rules are general guidelines. The optimum amount of defocus for a particular machine tool and material needs to be determined by process testing. The operator will do a “slit test” to find the optimal parameters by observing the quality of the section and the hanging slag.

2. setting of negative focus: the amount of negative focus is usually not a fixed value, but a range related to the thickness of the plate. A common starting reference point is 1/4 to 1/3 of the sheet thickness. For example, cutting 12mm of carbon steel, the test can be started from -2mm to -3mm.

3. Material is the key: different materials have different sensitivity to focus. For example, stainless steel is more sensitive to focus than carbon steel.