FIG. 4 shows a conventional bolt having a pilot portion for engagement in a predrilled aperture. The bolt 100 is provided with a head portion 102 and a shank portion 103. The shank portion 103 is formed with a thread 103-A (male thread portion) treaded into internal thread 106 (female thread portion) formed in an object member 106A to be engaged with the bolt. A pilot portion 105 is formed to the front end of the shank portion 103 so that the axis (not shown) of the pilot portion 105 accords with that of the shank portion 103. In such bolt, an incomplete thread portion 104 is formed at a boundary portion between the normal thread 103-A and the pilot portion 105.
When the bolt 100 is fastened to the object member 106A, the pilot portion 105 is first inserted into an insertion hole 107 of the object member 106A to keep a linearity of the bolt 100 relative to the object member 106A to a certain degree, and thereafter, the bolt 100 or the object member 106A is rotated by means of an automatic fastening machine or the like so that the normal thread 103-A of the bolt and the internal thread 106-A of the nut are threaded together, completing the fastening.
However, in the conventional art mentioned above, the following problems commonly occur.
In the fastening working between the bolt 100 and the object member 106, the bolt 100 is not perpendicularly inserted into the object member 106 with respect to the axis thereof but the bolt 100 is inserted in an inclined manner. This results in a cross point between the two spirals constituted by the tops of the normal thread 103-A and the internal thread 106-A. That is, a torque due to the driving operation of the automatic fastening machine and a thrusting force by a worker are concentrated at such cross point forming a point contacting portion.
With the bolt provided with the pilot portion of FIG. 4, the front end portion of the pilot portion 105 first contacts the internal thread 106-A, but in this time, less resisting force against the torque and the thrusting force is caused. The resisting force is increased at a time when the top of the internal thread 106-A contacts the incomplete thread portion 104, and the force is concentrated on the contact point of the top of the internal thread 106-A to the incomplete thread portion 104. In this time, the contact point (line) of the incomplete thread portion 104 is damaged and, therefore, the internal thread 106-A bites the same and is damaged.
In order to obviate such damage, the bolt 100 and the object member 106 are rotated manually by a hand for several turns (in usual, they are rotated till they are surely screw engaged), and thereafter, the fastening working has been performed by using the automatic fastening machine or the like, resulting in the lowering of the workability.