A feed screw and nut mechanism includes a nut having an internally threaded bore which threadingly engages an externally threaded feed screw. A runner bearing is attached to the nut and projects outwardly therefrom, and a dummy runner bearing is attached to the nut and projects outwardly therefrom and is angularly displaced from the runner bearing about the feed screw axis. The mechanism includes a stationary runner defining a runner surface that is engaged by the runner bearing to prevent rotation of the nut, and a dummy runner defining a dummy runner surface that engages the dummy runner bearing. The dummy runner and dummy runner bearing are biased toward each other such that the nut is rotatably biased in a direction to urge the runner bearing against the runner surface. The nut includes at least one rotatable drive bearing having a rotation axis perpendicular to the feed screw axis, and at least one rotatable driven bearing connected to the carriage and having a rotation axis perpendicular to both the feed screw axis and to the axis of the drive bearing. The drive and driven bearings make contact at their cylindrical surfaces to form a crossed bearing coupling for minimizing transmission to a carriage of force components which are not parallel to the axial direction along which the carriage travels. In a preferred embodiment, the nut mechanism includes a slave carriage, connected to the carriage, which supports a pair of crossed bearing couplings and which is rotatably connected to the nut to isolate the slave carraige from rotation of the nut about a transverse axis.