Method for molding screw thread of metal pipe

A method for forming a screw thread on a metal pipe, in which a screw thread extending about a cylinder center line of the metal pipe is formed on the metal pipe, the method comprising the steps of:

• holding the metal pipe using a clamping die having a clamping surface including a preforming surface portion corresponding to the screw thread, and thereafter pressing an end portion of the metal pipe with a pressing die in a direction of the cylinder center line to thereby form a primary formed portion on the metal pipe, the primary formed portion having a shape corresponding to the preforming surface portion and extending helically about the cylinder center line;
• and
• subsequently, aligning either one of a rib portion or a recessed groove portion provided on an outer forming surface of an outer forming die with the primary formed portion, the outer forming surface curved to correspond to an outer circumferential surface of the metal pipe, the rib portion or the recessed groove portion having a shape corresponding to the screw thread, and thereafter pressing the outer circumferential surface of the metal pipe with the outer forming surface by moving the outer forming die toward the cylinder center line, while inserting an inner forming die into the metal pipe and aligning another of the rib portion or the recessed groove portion provided on an inner forming surface of the inner forming die with the primary formed portion, the inner forming surface curved to correspond to an inner circumferential surface of the metal pipe, and thereafter pressing the inner circumferential surface of the metal pipe with the inner forming surface by moving the inner forming die away from the cylinder center line, to thereby form a final formed portion on the metal pipe between the rib portion and the recessed groove portion, the final formed portion serving as the screw thread.