## 2 Theoretical Study on Laser Scribing Process  
### Laser Scribing Process Flow and Requirements  
As an important process in chemical milling, the laser scribing process has an important effect on the quality of milling parts. In aerospace chemical milling parts, a large number of reinforcing ribs are designed as \"I-shape\" or \"T-shape\" structures to maximize weight reduction while ensuring the strength and rigidity of the casing. The chemical milling process of parts includes: chemical degreasing, sand blowing, coating maskant, scribing, uncovering maskant and chemical milling [29; 30]. The laser scribing process of the engine casing is as follows [31]: (1) The blank part is coated with maskant after chemical degreasing and sand blowing; (2) First scribing according to the engraved pattern; (3) First chemical milling; (4) The part is secondly coated with maskant; (5) Second scribing according to the engraved pattern; (6) Second chemical milling; (7) Obtaining the final part. The chemical milling process of aeroengine casing parts is shown in Figure 1.  
In the scribing process of chemical milling parts, there are obvious differences in the actual patterns between the first and second scribing. The laser beam is generally perpendicular to the surface of the cylinder in first scribing. The second scribing is on the sidewall of the stiffener, which formed the approximate arc surface after the first chemical milling. In second scribing, the laser beam must be inclined to etch with the sidewall of the stiffener section, as shown in Figure 2. Compared with first laser scribing on two-dimensional plane parts, the biggest characteristic of second scribing on three-dimensional parts is the change of \"laser beam direction\". During first laser scribing on two-dimensional plane parts, the direction of the laser beam is always perpendicular to the plane, which is always in a vertical position and consistent with the direction of gravity. In second scribing on three-dimensional parts, the scribing surface is no longer on a horizontal plane, and the direction of the laser beam and gravity is inconsistent. In order to ensure the scribing quality and laser absorption rate, it is usually required that the laser beam is perpendicular to the workpiece surface. The angle between the laser incident direction and the vertical direction is defined as \"\(\theta\)\", which is changed with the surface shape of the workpiece. Therefore, it is of great significance to study the influence of the laser scribing angle on scribing depth to ensure the quality and accuracy of first/second scribing.  
The requirements of scribing are as follows:  
1. The maskant scribing with the substrate should not be damaged;
2. The scribing lines are generally complex curves;
3. First laser scribing needs to be perpendicular to the part surface according to the pattern;  
Figure 1: Manufacturing process of chemical milling parts.  
Figure 2: Section diagram of laser scribing. (**a**) Schematic diagram of stiffener for laser scribing; (**b**) Schematic diagram of twice laser scribing.  
4. Second laser scribing needs to be inclined to the sidewall, which is generated by first chemical milling;
5. The upper and lower edges of the scribing lines are continuous and smooth, with high accuracy;
6. The accuracy of second scribing is \(\pm\)0.05 mm.