This paper describes a strategy of assembly operations by robotic systems under various uncertainties, and makes clear conditions that robotic programs need to satisfy to deal with the uncertainties. A sphere in a six dimensional space is used to model the uncertainties of the position and orientation of a tip of a manipulator. The uncertainties of an operation force and torque are also modeled by a sphere in a six dimensional space. The authors' strategy keeps a manipulating object in contact with another object at the final stage of assembly operations, and controls the manipulator in a way that the force between the objects stays in the sphere of forces. The authors describe conditions on robotic programs that keep the force between the objects within the sphere.>