PROBLEM TO BE SOLVED: To secure enough strength of locking a lower side plate W1 of work W with a piercing nut 1, driven into the work W comprising double-layered plates W1 and W2; in the bottom of the piercing nut 1, an inner cylinder 1b concentric with the screw hole 1a, and an outer cylinder 1d, shorter than the inner cylinder 1b, are provided to form an annular groove 1c in between; the inner cylinder 1b has a tapered periphery for larger outer diameter downward and the outer cylinder 1d has detent space 1e formed on its inner surface.;SOLUTION: An inner volume of the annular groove 1c, located above the bottom of the outer cylinder 1d, is set to be larger than the volume of the work W pushed into the groove 1c, so that a pushed-in portion W1a of the lower plate W1, is completely taken into the annular groove 1c. In addition, the space 1e are formed as to become gradually deeper upward. In the phase when the outer corner of the pushed-in portion W1a of the lower plate W1 reaches the space 1e, over-hung swellings are formed in the material portion W1a to prevent slip-off and idle turn of the pierce nut.;COPYRIGHT: (C)2002,JPO
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