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This paper presents a gauge-based registration method, a femur-mountable robot for hip arthroplasty named ARTHROBOT, and the surgery procedure using this system. In the gauge-based registration, a reamer-shaped gauge is inserted into the femoral medulla for relative positional information of the femur to the robot. A mounting frame and a minirobot are then mounted on the patient's femur for accurate machining. This gauge-based registration method drastically reduces the processes in preoperative planning by eliminating the need of computer tomography scanning or other image processing methods, compared to other robotic systems that are used for hip surgery. Also, this surgical system reduces damage to the femur because only the metaphyseal region of the femoral canal needs to be machined, while leaving the diaphyseal hard bone untouched. Experiments were performed on 18 composite bones and 12 pig bones. In the composite bone group, orientation (anterversion, varus/valgus and flexion/extension) errors were made at 0.13°±0.77°, 0.14°±0.38°, and -0.27°±0.33°, and the maximum position error was at 1.00 mm. In the pig bone group, orientation errors were made at -0.03°±0.65°, 0.31±0.27°, and -0.36°±0.36°, and the maximum position error was at 1.12 mm. Also, 93% of the gaps between the bone and the implant surface were under 0.25 mm. The developed system shows sufficient machining accuracy and precision for clinical application.