## 1 Introduction  
In order to meet the lightweight requirements of aerospace, a large number of thin-walled parts with complex form features are widely designed and applied, which poses great challenges to existing manufacturing technologies [1, 2]. At present, the thin-walled parts of aerospace are produced via chemical milling after traditional metal cutting, in which the excess metal on the surface of the parts is removed by chemical milling to form features such as reinforcing ribs and structural bosses [3, 4, 5]. Chemical milling is a non-traditional machining process where a chemical solution is used to selectively remove material from a workpiece using a strong chemical reagent known as etchant [6, 7]. Before chemical milling, the surface of parts should be coated with maskant. Then, some maskant is trimmed away, and the uncovered area is subject to etching. The maskant could be cut by mechanical cutting or by laser ablation, which is called the scribing process in chemical milling [8, 9]. The scribing process in chemical milling has become one of the key processes for manufacturing and chemical milling aviation equipment parts [10]. Laser scribing in chemical milling is developed from traditional manual scribing, which is an important process in the chemical milling for aerospace parts. In essence, it is a technological process of laser ablation for a chemical milling maskant on the surface of aluminum or titanium, which is to scribe form patterns on the surface of parts with the metal substrate material not damaged. Laser scribing combines a laser with multi-axis CNCmachining to ensure scribing is carried out efficiently and precisely; it is programmed based on the geometric pattern information and process parameters of chemical milling. Laser scribing can be used for the first and second scribing on complex surfaces and provides a new solution to solve the bottleneck problem of scribing in the chemical milling process of aerospace complex thin-walled structural parts [11]. When laser scribing is combined with five-axis numerical control, the processing of complex patterns of three-dimensional parts can be realized. In general laser processing, laser scribing parameters such as laser power, pulse repetition frequency, feed speed, incident angle and other parameters are usually set before the start of processing and do not change during the process. However, due to the restriction of laser scribing trajectory and dynamic performance of machine tools, when the feed speed is set at a higher level, the actual feed speed is often difficult to reach the programmed value. The actual feed speed is subject to a variety of factors, including the structure of machine tools, loads, and curvature changes of machining characteristics, so the specific control will become more difficult. Therefore, it is very important to study the accurate control method of removal amount in NC laser scribing and realize the equal removal of materials to ensure the accurate manufacturing of protective adhesive surface patterns in chemical milling. The scribing quality strictly depends on the process parameters, and it is important to choose optimized process parameters according to the productivity and the required cut properties. As early as the 1970s, laser scribing was proposed and applied to semiconductor manufacturing [12]. In 1987, [PERSON] from Boeing North American Inc Dallas, TX, USA [13] disclosed a laser method for scribing chemical milling, in which the Nd:YAG (wavelength = 1.06 pump), the Nd:Glass (wavelength = 1.06 m) and CO\({}_{2}\) (wavelength = 10.6 um) laser beams controlled by NC or CNC were used to scribe the maskant. In 1992, [PERSON] [14] proposed a laser-assisted masking process, which is a method of accurately applying maskant to a workpiece using one or more scanned laser beams and a maskant delivery device.