An expandable mandrel including a threaded spindle with a drive for rotating the spindle, wedge pieces supported on the spindle with core chuck lugs facing outwardly for engaging the inner surface of a core, the wedge pieces having outwardly facing inclined cam surfaces, matching cam surfaces on the core chuck lugs to expand the core chuck lugs when the wedge pieces are moved relatively axially by the spindle, the wedge pieces and core chuck lugs being free to rotate with the spindle until locked by an annular gear and a stopping rack so that the wedge pieces and core chuck lugs are held against rotation to allow the spindle to rotate relatively thereby moving the wedge pieces axially and the core chuck lugs outwardly.