Thread-rolling flat die for threaded fastener, and thread-rolling method

A thread-rolling flat die for a threaded fastener, comprising a thread-rolling surface including at least a bite surface to which a workpiece of the threaded fastener is supplied, a finishing surface formed adjacent to the bite surface, and a roll-off surface formed adjacent to the finishing surface,

• wherein the bite surface includes at least:
• a first partial bite surface having an inclination receding with distance from the finishing surface and an angle α of the inclination with respect to the finishing surface, the first partial bite surface being a portion for substantially forming a thread groove in a processed portion of the workpiece;
• and
• a second partial bite surface formed continuously from the first partial bite surface and having an inclination receding with distance from the finishing surface and an angle β of the inclination with respect to the finishing surface, the angle β being smaller than the angle α;
• wherein, in the first partial bite surface and the second partial bite surface, a crest angle of a processing tooth for forming a trough bottom of the thread groove in the threaded fastener is larger than a crest angle of a processing tooth in the finishing surface,
• wherein a crest shape of the processing tooth gradually changes from a root side to the crest angle of the processing tooth in the finishing surface along the thread-rolling surface from a starting point of the first partial bite surface toward the finishing surface, and
• wherein a length L1 along a thread-rolling direction in the first partial bite surface, a length L3 along the thread-rolling direction in the finishing surface, a length L4 along the thread-rolling direction in the roll-off surface, and a diameter of the processed portion of the workpiece of the threaded fastener have a relationship of L1>L3>L4> the diameter of the processed portion of the workpiece of the threaded fastener.