The laser is created by mixing high purity helium, high purity CO2 and nitrogen in the gas mixing unit. The laser generator generates the laser and a cutting gas such as N2 or O2 is added to process the object. The laser energy is highly concentrated and can instantly melt and gasify material.
This method effectively solves the difficulties of processing hard, brittle and refractory materials, offering high speed, precision and minimal deformation. It is ideal for processing precision parts and micro components.
Various factors can affect the quality of laser cutting, including cutting speed, focus position, auxiliary gas pressure, laser output power and other process parameters. Other factors such as external light path, part characteristics (reflectivity and material surface condition), cutting torch, nozzle and plate attachment can also affect cut quality.
These factors are particularly significant in the processing of stainless steel sheets, resulting in problems such as large nodules and burrs on the back of the part, poor roundness when the hole diameter is 1-1.5 times the plate thickness, and straight lines that are not straight into the corners. These issues continue to challenge the sheet metal industry in laser processing.
Figure 1 Laser cutting melting principle
- Auxiliary Gas
- Nozzle
- Nozzle height
- Cutting speed
- Melted
- Slag
- Cutting roughness
- Heat affected zone
- Slit width
Tumor Accumulation Problem
We have carried out repeated tests and found that the cutting stand equipped with the laser cutting machine is not suitable for processing sheet metal.
The main reasons are:
(1) If the angle R is large, there will be a large contact area between the top of the bracket and the processed plate, leading to a greater probability of splash reflection if the laser beam is only cut at the top of the bracket. On the other hand, if the angle R is small, the probability of splash reflection will be low when processing thin sheets.
(2) If the distance is small and the slope is low, the space available for the laser beam to penetrate further will be limited. The smaller the reflection space, the greater the reflection force, causing the cutting tumor to adhere more firmly to the opposite side of the plate.
Fig. 2 Improved support
Based on the above considerations, we have improved the cutting stand equipped with the machine:
(1) Reduce the R angle and increase the distance from the top of the support to the base surface, as well as increase the slope. This significantly reduces spatter and tumor on the back of the workpiece, allowing the tumor to fall off easily with a gentle touch of a tool.
(2) During the cutting process, it was found that applying oil to the surface of the plate reduces the adhesion of cutting spatter. The oil forms a protective film on the surface of the plate, making it difficult for the splash to adhere to the plate.
Furthermore, the oil film is more effective in guiding the laser beam, especially when machining plates with extremely smooth surfaces, such as mirror stainless steel. This is because the oil film is easier to absorb the laser beam compared to a smooth plate surface, which makes the beam easier to penetrate and position.
Therefore, we begin to evenly coat the front and back of the board with metal rolling oil, which has a high flash point. This has led to a significant reduction in spatter and tumor build-up on the processed part, especially on the back, which is much better than before.
(3) After repeated adjustments to the focus position in the cutting parameters, technicians discovered that the best laser focus position for cutting the sheet is a little less than 1/2 of the sheet thickness.
However, with plate deformation or excessive cutting air pressure, the cutting quality becomes unstable when the thin plate vibrates slightly or is locally affected by high gas pressure.
But when the focus is adjusted to about 2/3 of the plate thickness (correcting the focus deviation caused by deformation or vibration), it effectively prevents the formation of fine burrs under the same plate and air pressure conditions.
As a result, the cutting quality of the part was greatly improved.
Small hole roundness problem
When using a laser cutting machine, it can be challenging to produce high-quality holes about 1 to 1.5 times the thickness of the board, especially for round holes.
The laser cutting process involves drilling, insertion and cutting, which requires changing intermediate parameters. This results in a delay during the transition, causing the round holes in the finished product to become distorted.
To overcome this issue, we optimize drilling and insertion time to better align with the cutting process. This eliminates the noticeable change in parameters and results in a higher quality result.
Corner straightness problem
In laser processing, several important parameters (such as acceleration factor, acceleration, deceleration factor, deceleration and corner residence time) play a crucial role in processing thin sheet parts, which are outside the conventional adjustment range.
During the machining process of thin sheet metal parts with complex shapes, frequent corners often occur. The laser beam should slow down at each corner and then speed up again. These parameters determine the pause time of the laser beam at each point.
(1) If the acceleration value is too high and the deceleration value is too low, the laser beam may not penetrate the board well at the corners, resulting in low permeation and increased scrap rate.
(2) If the acceleration value is too low and the deceleration value is too high, the laser beam will penetrate the board at the corners, but the low acceleration value causes the laser beam to remain at the acceleration switching point and deceleration for a long time, causing the plate to melt and vaporize continuously under the influence of the continuous laser beam, leading to a non-straight line at the corners. (Other conventional factors that affect cutting quality, such as laser power, gas pressure and workpiece clamping, are not considered here).
(3) When processing thin sheet parts, it is recommended to reduce the cutting power as much as possible without compromising the cutting quality, so that there is no obvious color difference or burning on the surface of the workpiece.
(4) The cutting gas pressure should be minimized, which can greatly reduce the local micro tremors of the plate under strong air pressure.
Based on the above analysis, what values should be set for proper acceleration and deceleration? Is there a proportional relationship between the two that should be followed?
To determine ideal values, technicians continually adjust acceleration and deceleration, mark each cut piece, and record the adjustment parameters. Through repeated sample comparisons and careful examination of parameter changes, it is found that when cutting stainless steel in the range of 0.5 to 1.5 mm, the appropriate acceleration value is between 0.7 to 1.4 g, and the deceleration value is between 0.3 to 0.3 to 1.4g. 0.6g. There is a general rule that the value of acceleration is approximately twice the value of deceleration.
This rule also applies to cold-rolled sheets of similar thickness, but for aluminum sheets of similar thickness the values must be adjusted accordingly.
Conclusion
By successfully addressing the factors affecting cutting quality, the quality of stainless steel sheet products processed by us has significantly improved in terms of reducing cutting tumors and improving cutting subtlety.
As modern technical workers, it is important to make a commitment to learning, have the courage to explore new solutions and adhere to the principle of “production excellence”. This approach guarantees the production of high quality products and success in intense economic competition.
