| 摘要 |
The shank portion of a screw has a surface formed with a screw thread throughout the length thereof and a lower section formed with a flute at the surface thereof. The flute extends from a tip of the shank portion and has a longitudinal length which is inclined with respect to a longitudinal axis of the shank portion. The flute is defined by a planar side wall, which is inclined relative to the longitudinal axis, and a curved bottom. The planar side wall has a surface with screw thread portions that are disposed in a staggered arrangement in a direction receding from a direction of screw advancing rotation. The flute has a depth which decreases gradually from the tip of the shank portion, and which has a maximum value equal to radius of the shank portion. The longitudinal length of the flute forms an angle ranging from 12.degree. to 32.5.degree. with the longitudinal axis. The screw further has a head portion with an annular skirt adjacent to the shank portion. The annular skirt has one side which is formed with an annular recess around the shank portion and a plurality of radial teeth in the annular recess.
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