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A parallel manipulator of hexaglide structure has been proposed to develop a knee surgical robot. The surgical robot provides enhanced safety by its mechanical and software constraint. The working envelope of a parallel manipulator is mechanically limited by its mechanical parameters, such as link lengths, sliders' strokes etc; and is easily to be designed only to moving and orientating within the small local area of operation. This feature protects both the patients and medical staffs from unexpected harms when the manipulator malfunctions. The software constraint provides a surgical boundary for bone cutting so that a surgeon can easily assign a desired cutting shape of bone and rely on it for bone cutting. Software constraint provides different impedances which is determined by the position and direction of the cutting movement and implemented by impedance control strategy. Therefore the manipulator with software constraint possesses both the skill of the robot in accurate cutting and the skill of reflecting the designated impedance to the surgeon in being able to judge kinesthetically. The software constraint can be used to substitute a sequence of aligning and positioning templates in conventional surgical procedures; and to achieve better outcomes. The advantage of using the hexaglide structure was also demonstrated in this paper for its simple inverse kinematics and high rigidity.