a piece - threading device on a numerically controlled lathe comprises a supporting assembly having an inner rotatively driven body which , on a side thereof facing a workpiece to be threaded , comprises a plurality of threading tools , the rotatively driven body having a built - in electrical rotor to which an electric stator also built - in in the supporting assembly is operatively coupled , the stator having a plurality of circumferential channels for circulating therethrough a refrigerating and lubricating fluid .