Abstract: A center chuck assembly for rotatably supporting the end of a cylindrical workpiece comprises a shaft rotatable in a mounting pipe to be mounted on a receiving member and having an operating flange and a disc formed on the outer end thereof. The disc has a spiral groove on the end surface thereof. A chuck holding member having chuck members slidably fitted within holes formed radially in the chuck holding member, is rotatably fitted on the disc of the rotating shaft. The chuck members have teeth on one side thereof, and these teeth engage with the spiral groove on end surface of the disc. By rotating the operation flange relative to the chuck holding member, the chuck members are caused to project outwardly by means of engagement of said teeth and spiral groove and to abut the inside surface of the workpiece at a position spaced from the end of the workpiece, thereby supporting the workpiece securely. By using this center chuck assembly, end surface of the workpiece can be machined easily in one process.