The Relationship Between Laser Cutting Focus and Burrs

The focal position in laser cutting has a direct and deterministic relationship with burr formation. In short, the focal position directly affects the energy distribution of the beam within the material, thereby determining the kerf quality, the efficiency of molten-material expulsion, and whether burrs ultimately form. The relationships are detailed below.

Basic Principle: How the focal position affects the cutting process

The principle of laser cutting is to irradiate the workpiece with a high‑energy‑density laser beam, causing it to rapidly melt or vaporize, while a辅助 gas (such as oxygen or nitrogen) blows away the molten material to form the cut.

Focus: The point where the laser beam’s energy is most concentrated and its spot size is smallest.

Focus position: refers to the location of the focal point relative to the workpiece surface. It is divided into three cases:

1. Positive defocus: The focal point is located above the workpiece surface.

2. Negative defocus: The focal point is located below the workpiece surface.

3. Zero defocus: The focal point is precisely on the workpiece surface.

Specific relationship between focus position and burr

The nature of the burr is that the residual melt, which has not been completely blown away, re-solidifies at the bottom of the cut. The focus position affects the burr mainly by affecting the cut shape and air flow force.

1. is out of focus (focus on the workpiece)

• Energy distribution: After the beam enters the material, it is divergent, and the energy density gradually decreases from top to bottom. The energy is strongest on the upper surface and weakest on the lower surface.
• Incision shape: easy to form a V-shaped incision with a wide upper and a narrow lower.

Effect on burr:

• Due to insufficient energy at the bottom, the bottom of the material cannot be completely melted or gasified.
• The flow rate of the auxiliary gas is accelerated at the narrow bottom, but the blowing force may not be sufficient to completely strip and blow away the viscous melt.

Results: It is easy to produce a large number of stubborn burrs on the lower surface. This is one of the most common causes of burrs.

2. Negative defocus (focus below workpiece)

• Energy distribution; the beam converges inside the material, and the energy density reaches the highest in the middle or lower part of the material.
• Incision shape: easy to form a waist drum-shaped incision with narrow upper and lower and wide middle.

Effect on burr:

• The advantage is that the bottom energy is insufficient and the material can be melted more thoroughly.
• However, the middle of the incision is the widest, which may cause the airflow to spread here and weaken the blowing force when reaching the bottom.
• If the parameters are not properly matched, the melt may still adhere to the bottom due to the unclean row.

Results: Compared with positive defocus, the burr condition will be improved, but a small amount of soft burr may still be produced. For thick plate cutting, negative defocus is sometimes used to ensure that the bottom is cut through.

3. Zero defocus or best focus (focus on the surface of the workpiece or a specific depth)

• Energy distribution: The most concentrated area of energy is located on the surface of the workpiece, which is the best for thin plate cutting. For thick plates, a “best focus” position is usually required, that is, the focus goes deep into a certain 1 depth inside the material (for example, 1/3 of the thickness of the plate) to balance the upper and lower energy.
• Incision shape: the most ideal vertical, parallel incision.

Effect on burr:

• The energy distribution throughout the incision area is relatively uniform, and the material can be continuously and stably melted.
• The parallel cuts provide a smooth passage for the auxiliary gas, which can carry enough kinetic energy to smoothly discharge the melt from the bottom.

Result: This is the best state to obtain no burr or very few burr incisions. The melt was blown away “cleanly” with no residue.

Summary and Analogy

You can understand it like this:

• Is out of focus: like a “blunt chisel”, the top is very hard, but at the bottom is boring, can only “rub” off the material, leaving a rough.
• Negative defocus: Like “blasting” inside the material, although it can explode, the exit may not be neat and will bring out some debris.
• Best focus: Like a “sharp scalpel”, neatly cut from top to bottom, clean and neat.

Practice Guide: How to Adjust Focus Based on Glitches

In actual operation, if burrs are found, the following steps should be followed to check and adjust:

1. Observe the burr shape:

• There are a lot of hard, granular burrs on the bottom: it is likely that the bottom is not cut through due to being out of focus (focus is too high) or insufficient power/too fast speed. The focus should be adjusted downward.
• There is a small amount of soft, easy-to-fall beard-like burr at the bottom: it may be negative defocus (focus is too low) or insufficient gas pressure/impure gas that causes the melt to not be blown out. You can try to fine-tune the focus upwards and check the gas parameters.

2. Focus calibration and testing:

• Use a focus pointing instrument to accurately determine the zero focus position.
• In the cutting parameter software, the adjustment of the ± value is usually performed on the basis of the 0 point. Do 1 sets of focus tests (e. g., from +1 to -3 in 0.5 steps), cut the same figure, and then observe which position has the smoothest cut and the least burr. This position is the best focus for current materials, thicknesses and equipment.

Other key factors that affect glitches (to be considered in coordination)

Focus is not the only factor and must be optimized in conjunction with other process parameters:

Auxiliary gas:

Pressure: Insufficient pressure cannot effectively blow away the slag; too high pressure may cause turbulence, which will affect stability.

Purity: Especially when cutting stainless steel, it is important to use high-purity nitrogen (99.99% or more). Residual oxygen will form oxidized slag and aggravate burrs.

Cutting speed: The speed is too fast, the energy input is insufficient, and the material is not completely cut through, forming bottom burrs; the speed is too slow, excessive ablation, rough cuts, and burrs may also be produced.

Laser power: power and speed to match. Power is too low to cut through.

Nozzle: The aperture, height and concentricity of the nozzle directly affect the airflow state. The non-concentricity is a common cause of uneven cut quality and burrs.

The material itself: the composition of the material, the surface state (such as whether there is rust, oil) also has an impact.

Focus position is the primary adjustment parameter to control laser cutting burr. The ideal focal position creates a vertical, smooth cut and allows the assist gas to efficiently discharge the melt. When you encounter a burr problem, you should first systematically conduct a focus test to find the best focus, and then coordinate to adjust other parameters such as gas and speed to obtain a perfect burr-free cutting effect.