I. Machining accuracy
The machining accuracy of laser cutting is the overall accuracy determined by the performance of the machine, beam quality, and processing.
Even if the cutting is performed according to the program, there are cases where the processed product cannot meet the accuracy requirement.
Therefore, it is necessary to take countermeasures according to different situations.
- The overall size of the processed product has changed
This is due to the diameter of the laser focus on the slit and the width of the slit formed by the surrounding combustion zone.
Although under the same conditions, the same offset compensation value can be used to ensure the accuracy of the same workpiece.
However, the setting of the focus position is determined by the feeling of the processing machine operator.
And the thermal lens action will also cause changes in the focus position.
Therefore, it is necessary to check the optimal offset compensation value regularly.
- There is a difference in the dimensional error in the machining direction (partial)
The dimensional accuracy and dimensional accuracy of the upper part of the sheet are different.
This phenomenon should consider two reasons.
First, the roundness and intensity distribution of the beam are not uniform, causing the slit width to vary along the machine direction.
The solution is to perform optical axis adjustment or cleaning of the optical components.
Secondly, when the workpiece is thermally expanded, the length of the processed shape is shortened.
- Changes caused by warpage
Although the dimensional accuracy is within the required range, warpage may occur due to thermal deformation.
When processing aluminum, copper, stainless steel, etc., it is very significant, and it is affected by physical properties such as linear expansion coefficient and heat capacity.
As far as the shape is processed, the larger the aspect ratio, the larger the amount of warpage.
The use of low-calorie processing conditions and processing lines to work on the processing program, but has not completely solved the problem.
The residual stresses of the machined parts also have an effect on the warpage and dimensional errors, so we need to maintain a certain configuration direction for the machining program.
- Pitch accuracy change
When machining many holes, the accuracy of the spacing between the holes may vary.
Since the holes are opened in the case of thermal expansion, the pitch becomes small after the cooling shrinks.
We can correct the precision of the shrinking part in the program or use the shape scaling function flexibly.
In any case, after the initial processing, the processing size is measured and the error is compensated.
When the interval accuracy does not vary with the machining position, but deteriorates throughout the machining zone, the cause is the deterioration of the mechanical precision.
- Roundness change
It is unavoidable to process the cutting surface of the hole in laser processing to produce slope.
The lower diameter is larger than the diameter of the back surface, and the roundness on the slightly smaller side of the back is generally evaluated.
- The difficulty of perforation
The holes required for processing at the beginning of the cutting process are referred to as perforations.
The thicker the plate, the more unstable the perforation.
It can be said that in the thick plate cutting with a plate thickness greater than 1.0mm, 70% of the processing failure occurs due to poor perforation.
In order to carry out a stable perforation, the processing characteristics of the perforations are described here.
- The principle of perforation
During the perforation, the molten metal generated in the previous processing is deposited around the pores on the surface of the workpiece.
After the illuminating, the surface of the workpiece is heated, and the heating is performed by slow heating until the final penetration is continuously performed.
In this method, if the thickness of the plate is greater than 9.0mm, the piercing time will increase sharply.
However, the pore size is about 0.5 mm, which is narrower than the slit and has a small thermal influence.
Therefore, if the processing capacity is increased and the output energy is increased, it is difficult for the molten metal to be completely discharged from the upper portion of the aperture, and excessive combustion occurs.
The CW condition is a method of setting a focus position slightly above the surface of the workpiece, increasing the processing aperture, and rapidly heating.
Although a large amount of molten metal is scattered on the surface of the workpiece, the processing time is greatly shortened.
The absorption of laser energy also occurs on the walls of the perforated holes.
During the perforation process, the irradiated laser light is multi-reflected in the perforations and propagates downward while being absorbed.
In order to shorten the piercing time, it is necessary to supplement the absorption by the hole wall.
The weakened energy, that is, the need to increase the output power during the perforation process.
Moreover, in order to reduce the thermal influence on the wall of the hole, it is necessary to increase the output power while shortening the piercing time as much as possible, and reducing the illumination of the laser around the hole wall.
- Four principles for dealing with defects in perforations
When defects occur during the perforation process, it is necessary to analyze the causes of various phenomena and find out the treatment methods.
(1) The moment when the defect occurs
Be sure to identify defects that occur during the perforation process or when the cutting begins after the end of the perforation.
If it occurs during the perforation process, the output power and air pressure conditions in which the problem occurs are corrected according to the specific conditions at the time of the start of the perforation or the conditional switching during the perforation.
If the defect occurs before the end of the perforation, it is because it is necessary to extend the perforation time by switching to the cutting condition before the penetration.
If a machining defect occurs at the start of cutting, it is because the molten metal portion accumulated around the surface of the hole is difficult to pass, so it is necessary to set a pulse condition or a low speed condition at the starting position.
(2) The location of the defect
If a perforation defect occurs in a specific position at the processing platform, it is because the laser optical axis is offset from the center of the nozzle.
This requires adjusting the optical path deviation.
If the perforation position is too concentrated or the perforation is made in the vicinity of the cutting line, the perforation defect may also be caused due to the excessively high processing position temperature.
The SS400 plate having a thickness of 12.0 mm was used as a workpiece, and the material temperature was changed from normal temperature to 200 ° C to investigate the relationship between the defects and the processing defects.
The data is the ratio of the occurrence of perforation defects and cutting defects by 50 perforations under various temperature conditions.
The higher the temperature, the greater the incidence of defects.
It is therefore necessary to study the processing sequence and to improve the procedure to perforate and cut along the lines that are not overheated.
(3) Time when poor perforation occurs
As the processing time goes on, the number of occurrences of poor processing is only increased when there is no decrease.
The reason may be the output power variation caused by the vibration of the vibrator.
If the cooling time is increased, the reason may be caused by the action of the optical lens of the optical component.
In this case, you will need to repair the optical components and contact your supplier.
(4) Materials with poor perforation
For materials that have poor perforation, it is important to confirm that the record has been processed well in the past.
If there is a record of past processing, there is no need to adjust the processing conditions.
It can be identified as a defect in the processing machine and optical components, and inspection is performed to find out the cause.
- Appropriate perforation conditions
The thicker the workpiece, the greater the proportion of the perforation time in the overall processing time, and the requirement for shortening the perforation time is increased.
The processing condition parameters effective for shortening the piercing time are the pulse peak output power and the pulse waveform and the average output power.
- Prevent the appearance of whiskers when perforating stainless steel
When the stainless steel is cut, there is a slag-like metal slag around the surface of the hole, and scratches may occur on the surface of the mirror surface and the stripe surface.
Further, when the whisker-like metal slag comes into contact with the nozzle of the electrostatic induction machining head, an alarm of abnormal focus occurs.
Depending on the thickness of the panel, the corresponding processing method is also different.
The larger the piercing pulse frequency, the less metal slag.
When the set auxiliary gas pressure is high, the metal slag can be reduced.
- Precautions when perforating highly reflective materials
When cutting high-reflection materials such as copper and pure aluminum, it is necessary to apply a beam absorber to the surface of the workpiece.
The beam absorber not only has the effect of improving the processing ability, but also has a function of resisting reflection from a safety point of view.
The processing conditions need to reduce the pulse frequency and increase the energy per pulse of the pulse peak.
Moreover, by increasing the gas pressure, the molten metal is extruded into the interior of the panel to improve the processing ability.