Building on a recently proposed contact-mechanical theory of friction control by external vibration, the case of large-amplitude normal oscillation is revisited. It is shown that the coefficient of friction can be expressed in particularly simple form if the waveform of the displacement oscillation is triangular or rectangular, and the contact stiffness is constant. The latter requirement limits the scope of the exact solutions to contacts between a plane and a flat-ended cylinder or a curved shape with a wear flat, but the adopted methodology also enables efficient numerical solution in more general cases.