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 (9) to which an electric stator (8) also built-in in the supporting assembly is operatively coupled, the stator (8) having a plurality of circumferential channels (6) for circulating therethrough a refrigerating and lubricating fluid.