## 1 Introduction  
The tunnel boring machine (TBM) is the main equipment for tunnel construction at present. The disc cutters installed on the cutter head of TBM are the direct rock-breaking tool, which needs to be replaced frequently. At present, the disc cutters are mainly replaced manually, which is dangerous, inefficient and difficult. The rise of the robot industry provides a new way of replacing TBM disc cutters. Since Bouygues company [1] of France first proposed using robots to replace disc cutters for TBM in 2007, researchers in Germany, Japan, China, and other countries have successively developed robots to replace disc cutters for TBM in the following ten years [2; 3; 4]. However, most of them are experimental products, and the working effect of a few products applied in practical engineering cannot meet the actual needs [4]. The application of integrated disc cutters is the premise for the robot to replace the disc cutters for TBM. To this end, the research team designed a new type of integrated disc cutter. The purpose of this paper is to design the end-effector of the cutter-changing robot for an integrated disc cutter developed by our research team. The main function of the robot is to dock and twist the fastening bolts of the integrated disc cutter. Many researchers have carried out relevant research on the use of robots to assemble and disassemble bolts. [PERSON] et al. [5] designed a bolt connection device for steel beam assembly on the construction site. The robotic bolting device consists of a bolting end-effector and a gantry-type robotic manipulator, which places the bolts to each bolting position. [PERSON] et al. [6] designed a bolt-screwing tool based on a pneumatic slip ring structure, which can realize two-DOF motion of clamping-releasing and rotating. The tool consists of a pneumatic slip ring with a sealed structure and a cylinder-driven gripper. [PERSON] et al. [7] present a novel spiral search technique developed to improve the rate of successful engagement between the robot end-effector and the screw heads despite uncertainties in the location of the screws. [PERSON] and [PERSON] [8] introduced a robotic system that combines force and visual sensors to help remove screws from laptops. Their work focuses on building and testing computer vision modules that automatically find screws. [PERSON] et al. [9] proposed a novel approach for flexible manipulator conducting screwing task based on robot-environment contact classification. They used the logistic regression method to classify the contact state according to the force signal to judge whether the bolt is tightened. Most of the above studies are aimed at the design of the robot end-effector or the development of the related control strategies and computer vision algorithms, and there is no deviation analysis of the robot's working object. However, the applicability of the robot end-effector is an important factor affecting the performance of the robot. The targeted design of the robot end-effector according to the deviation analysis results of the robot's working object can greatly improve the applicability of the robot. To compensate for the positioning deviation of the end of the robot and ensure the function of the designed end fault-tolerant mechanism, it is necessary to first analyze the tolerance modeling of the integrated hob. Tolerance modeling is the accurate representation of tolerance, tolerance zone and geometric elements after change by the mathematical model. The 3D tolerance analysis technology is very suitable for the deviation analysis of complex assembly [10; 11]. Common 3D tolerance analysis models include the Small Displacement Torsor model [12; 13], T-Map model [14; 15], Matrix model [16], Unified Jacobian-Torsor model, SDT [17; 18] and DLM (Direct Linearization Method) [10; 19]. The Jacobian-Torsor model combines the small displacement spinor theory and the Jacobian matrix in robotics.