Abstract: A tool joint for drill pipes having a female thread 4 and a male thread 3 on both ends and having shoulders 5 and 8 on the external side of the male threaded portion and on the internal side of the female threaded portion respectively to cause both shoulders to be in flat contact with the corresponding tips. Also, the length from the shoulder on the tubular body side of each threaded section is made longer than the connection length of the male thread side of the general-purpose type tool joint and is made shorter than the connection length of the female thread side of the same. Also, the clearance ranges from 0 to 0.5 mm when hand-tightened at the internal shoulder. As compared with the conventional API tool joint with an external shoulder, the tool joint of the present invention also features an internal shoulder and can thus withstand higher torque, being interchangeable with conventional API tool joints.

Abstract: In a plug mill, the plug is contained in a container supported by a carriage which is moved from the plug recovering position to the inlet guide provided in front of the mill along a straight transfer rail positioned at an angle with respect to the pass line of the mill and at a speed sufficient to impart a predetermined moment of inertia to the plug. When the carriage is advanced to a position near the inlet guide, the carriage is rapidly stopped to eject and supply the plug to the front end of the mandrel bar by utilizing the moment of inertia.