Electric spindle for numerical control machines structured to lock and rotationally drive a tool about a predetermined reference axis locally coinciding with the longitudinal axis of the tool; the electric spindle comprising: an outer structural casing that is structured to be rigidly attachable to a generic numerical control machine; an electric motor that is housed within the structural casing and is provided with a drive shaft, which projects/emerges with its front end outside the structural casing while remaining locally coaxial to the reference axis of the electric spindle; a rotating thrust-bearing bushing or sleeve, which is fitted in an axially rotatable manner on the front end of the drive shaft via interposition of a first set of annular rolling bearings, and is maintained coaxial to the reference axis of the electric spindle by a second set of annular rolling bearings, which are interposed between the rotating bushing or sleeve and the structural casing; and at least one disassemblable tool-holder head that is structured to be attachable, in a rigid and stable though easily releasable manner, to the front end of the drive shaft, is structured so as to accommodate and retain, in a rigid and stable though easily releasable manner, the shank of a generic tool for numerical control machines, and is provided with a peripheral crown that is dimensioned/structured to stably abut, selectively and alternatively, on the front end of the drive shaft or on the rotating thrust-bearing bushing or sleeve.