Method of cold drawing seamless metal tubes each having an upset portion on each end

A method of cold drawing a seamless metal tube having an upset portion on each end, comprising the steps of:

• defining a reducing area between first and second relatively movable members, the first member being a closed periphery reducing die and the second member being a plug secured to a front end portion of a main push-pull rod, the reducing die having an internal surface including a rearward inlet portion which tapers rearwardly, a forward outlet portion which tapers forwardly, and a small diameter bearing portion between the inlet and outlet portions, the plug having a large diameter bearing portion between a rearward rearwardly tapered portion and a forward small diameter bearing portion;
• moving the reducing die and plug longitudinally with respect to each other at least twice to vary the size of the reducing area and fixing the reducing die and the plug in three different positions selected from first, second and third relative positions, the forward small diameter bearing portion of the plug being radially aligned with the small diameter bearing portion of the die in the first relative position so that the inner and outer peripheries of the reducing area therebetween respectively have a first inner diameter and a first outer diameter, the large diameter bearing portion of the plug being radially aligned with the small diameter bearing portion of the die in the second relative position so that the inner and outer peripheries of the reducing area therebetween respectively have a second inner diameter greater than the first inner diameter and a second outer diameter equal to said first outer diameter, and a large diameter bearing portion of the plug being radially aligned with the outlet portion of the die in the third relative position so that the inner and outer peripheries of the reducing area therebetween respectively have a third inner diameter equal to the second inner diameter and a third outer diameter greater than the first and second outer diameters;
• and
• successively drawing a mother tube through the die and around the plug through the reducing area in three steps, each of said steps being performed when the plug and die are fixed in a different one of the first, second and third positions, including the steps of relatively fixing the plug and die in one of the first and third relative positions during the first step, then relatively moving the plug and die to the second relative position and relatively fixing the plug and die in the second relative position during the second step, and then relatively moving the plug and die to the other of the first and third relative positions and relatively fixing the plug and die in said other of the first and third relative positions during the third step, to thereby successively form first, second and third tube portions having inner and outer diameters corresponding to the reducing area during each of the three steps, the first and third tube portions forming upset portions with the second tube portion therebetween, whereby there is formed a tube having an outer upset portion and an inner upset portion at the first and third tube portions at the respective opposite ends of the second tube portion.