## 1 Introduction  
As the main component of the propulsion device, propellers importantly affect the propulsion ability of ships. Machining defects such as scratches, protrusions, and burrs on the blades will increase the frictional resistance between blades and water flow during the propulsion process which reduces fuel conversion and ship speed. Consequently, ameliorating the machining quality of blades is of great significance to improve the propulsion ability of ships. Large propeller blades are mostly manufactured by heavy-duty CNC machine tools, such as the CNC seven-axis five-link propeller machining heavy-duty turning and milling machine tool CKX5680 led by Wuhan Heavy Duty Machine Tool Group Corporation [1], the CNC seven-axis six-link propeller machining heavy-duty machine tool VTM11000 led by Zhongchuan Heavy Duty Machine Tool Corporation Limited, and the CNC machine tool HPMC-110 of HNK Machine Tool [2]. However, there are two shortcomings with heavy-duty CNC machine tools: (1) After completing the single-sided machining of the propeller, it is necessary to turn the propeller over through a special flipping mechanism, and the opposite side of the propeller can only be machined after the secondary clamping. The flipping process will cause a certain degree of damage to the machined surface, and secondary clamping will result in positioning deviation on both sides. (2) The blade is in a cantilever state during machining, and the current machining auxiliary shoring cannot completely neutralize the concomitant vibration and deformation caused by the cutting force; consequently, the improvement of surface machining accuracy is limited. This investigation solves the shortcomings in traditional single-sided machining of propellers by means of double-sided machining. In recent years, academia has produced a profound study on the theory of double-sided machining [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. [PERSON], [PERSON], et al. have studied the simultaneous machining of thin-walled surfaces with double-end cutter milling, which greatly improves the flatness and straightness of the workpiece surface [3]. A novel double-sided milling strategy of thin-walled parts based on dual collaborative parallel kinematic machines was designed by [PERSON], [PERSON], et al. The double-sided milling strategy doubles the material removal rate while eliminating the cumbersome in-process steps used in conventional machining [4]; [PERSON], [PERSON], [PERSON], et al. presented a new method to finish flexible plates with high accuracy and high productivity, in which both surfaces are finished simultaneously with synchronization so that the thrust forces are canceled out on both sides [5; 6; 7]. [PERSON], [PERSON], et al. introduced a newly designed mirror machining device and analyzed the stiffness of the support mechanism and the workpiece. The distribution of the synthetical stiffness is obtained from the stiffness model and improved by adding redundant actuation. The results show that the stiffness of the support mechanism and the minimum synthetical stiffness are significantly improved [8]. [PERSON], [PERSON], et al. proposed a new path smoothing algorithm based on the geometric constraints to guarantee the smoothness of twin-tool paths and the synchronization of the two cutters for the nine-axis twin-tool milling of blade profiles. The results of simulation and experiments show that the two surfaces of the blade can be simultaneously and entirely machined, and the contour errors do not exceed the tolerance range [9]. [PERSON], [PERSON], et al. used a newly developed ISF machine and investigated the process of double-side incremental forming [10]. The interaction method has a certain reference significance for double-sided machining. [PERSON] proposed a thin-walled parts mirror milling stability monitoring technology and the developed mirror milling system and successfully realized the efficient and stable mirror milling of large thin-walled parts [11]. [PERSON], [PERSON], et al.