PURPOSE: To prevent a threaded bar from damaging or crushing the thread ridge as well as to cut any burr, becoming a hindrance to the screwing of a nut, perpendicular to an axis, not projecting it outwards.

CONSTITUTION: Two recess parts 22 and 34, having two internal threads 27 and 36 being screwed in each external thread of a threaded bar to be right angles to end faces 26 and 35 being opposed to each other, are installed in a rectilinearly reciprocating male blade 21 and a stationary blade 31 at one side in the front. These internal threads 27 and 36 situate their ridge parts 27a and 36a at the end faces 26 and 35, and they are installed after making a protuberant part 28, having a short circular root part 29 and a crest part 30 forming a part of the internal thread 27 at a spot nearer to a terminal part at the thick wall side of a crest part 27a of the movable blade 21, project toward a clearance, whereby any burr to be produced out of the thread ridge is made so as to be formed into a part of the external thread.