An actuator providing Z-direction movement within a positioning system extends between an input stage and an output stage. A nut is mounted to rotate on the input stage in threaded engagement with a screw extending into the nut from the output stage. The nut is driven by a motor also mounted on the input stage. A plate attached to the output stage through a flexible leaf spring slides along the input stage in the Z-direction with movement of the output stage, preventing rotation of the output stage about the axis of the screw. Lateral positioning of the screw results from the threaded engagement of the screw and nut. The leaf spring is formed to provide a lateral pre-load force between the screw and nut.