## 1 Introduction  
In the mass production customization stage of manufacturing products, the preparation process of process technology and equipment occupies a key position, which will directly affect the effective play of productivity [1]. Therefore, processing technology design is a key step in the machinery manufacturing industry, and its quality of processing technology design directly affects product quality and processing efficiency. Process design is limited by the manufacturing resources and abstract conditions in the product's own design; therefore, it is also a systematic project. Hence, [PERSON] et al. [2] considered computer-aided process planning (CAPP) to be a link between computer-aided design (CAD) and computer-aided manufacturing (CAM), and it is also an important part of the current computer-integrated manufacturing systems. However, the product modularization and intensive design concept of today's products lead to the diversification of the product's own processing elements and constraints, and the demand for production equipment and processing conditions also presents a trend of diversification. Thus, CAPP is a very complex project [3]. Therefore, [PERSON] et al. [4] believe that CAPP is the weakest link in today's computer-integrated manufacturing system, and it is also an area that needs further study. As mentioned by [PERSON] et al. [5], the detailed steps of the process planning process include receiving and analysing design drawings, selecting raw materials of suitable shapes,selecting the process technology, deciding the process sequence and flow, and preparing processing plans. It concludes equipment requirements, flow sequence of products in the equipment, cutters and tools, processing step design, processing parameter selection, measuring tools and tooling design, tool path planning, process cost, and time analysis. Finally, obtaining an executable CNC program, process cards, operating procedures, and other process documents. Therefore, the goal of CAPP is to transform the product from the state of design drawings into the final physical product, with reasonable efficiency and quality. Among the above-mentioned process activities, sorting the processing tasks is the core that determines the quality and efficiency of product processing. It is also the most complex and difficult task in the entire CAPP process, which meets all the constraints and enables the smooth processing of the product. This work can be divided into three steps: (1) According to the design drawing of the product, analyse all its processing tasks and obtain the required processing equipment, tools, processing orientation, processing accuracy, and other information according to the processing tasks. (2) Analyse the mutual constraints between all processing tasks of the product, sort the processing tasks to form an effective processing task chain, and optimise task sorting to make the overall processing time the shortest or the processing efficiency the best. (3) On the basis of forming an effective processing task chain, the tasks of the processing task chain are divided into workstations in order, such that the processing time between the workstations is as balanced as possible. Step three is the machining production line balancing problem, which is a secondary optimisation based on the ordering of processing tasks by step two, and the tasks are equally distributed in each workstation. This process is carried out so that the products can be produced in the minimum cycle time between the workstations, productivity can be improved, and the idle time of production equipment can be reduced, thereby improving equipment utilisation. Therefore, the design of the machining line in the mass flow production mode has to focus on improving line balance while ensuring the machining line meets diverse machining process constraints and complicated process requirements in terms of part dimensions.