This study examined the effect of changes in center distance on a circular-arc gear pump that operates at high pressure and high speed. In principal these types of gear pumps have no trapped-oil and flow ripples. The effect of changes in center distance caused by assembly, machining, radial force and oil film force on the performance of circular-arc gear pumps was studied. The results show that the flow rate, axial force and torque increased linearly with an increase in variables x. Computer aided manufacturing for ball end milling has been developed to simulate the process of numerical control of the rotors of circular-arc gear pumps. Two methods for assessing interference are provided. The trajectories of the centers of ball end milling for rough and finish machining are simulated.