Patent ID: 8245553

Claim:
A method of producing an ultrathin-wall seamless metal tube having 4.0% or less in terms of t/D, the ratio of wall thickness t to diameter D, by diameter-expanding drawing that is a cold drawing method of an ultrathin-wall seamless metal tube using a floating plug, comprising: arranging an entry solid die and a delivery solid die in series and in an abutting relation on the same pass line, the entry solid die increasing, remaining constant or decreasing in diameter from an engaging entry side toward a finishing exit side, the delivery solid die decreasing in diameter at least in the vicinity of the engaging entry; and inserting a hollow shell into the entry solid die and the delivery solid die, causing a tapered plug, the diameter of which increases gradually from the engaging entry side toward the finishing exit side, to float along the pass line, gripping an end of the hollow shell by use of a chuck, and drawing the hollow shell in the direction in which the hollow shell moves from the entry side toward the exit side, whereby in the entry solid die region, the wall thickness is reduced while a mid-wall diameter of a hollow shell, which is an average value of outside diameter and inside diameter of the hollow shell, is being expanded between the solid die and the plug, and in the delivery solid die region, the tapered plug is caused to float by performing reduction working of the outside diameter.