A lock set for a door has a cylindrical spindle integrally coupled with a knob and containing a slide plate which can be actuated to lock and unlock the cylindrical spindle against rotation. The cylindrical wall of the spindle is provided with two diametrically opposing grooves which extend axially from one end thereof to a predetermined depth and which are opened at the one end so as to facilitate installation of the slide plate in the spindle. The slide plate has two opposing lugs for insertion into the grooves.