| 摘要 |
A screw (3, 5, 6) includes a shank portion (31, 51) and a head portion (311, 511) on one end of the shank portion (31, 51). The shank portion (31, 51) has a continuous spiral screw thread (32, 52, 62) with a plurality of turns formed thereon. The screw thread (32, 52, 62) has an upper thread surface (32a, 52a), a lower thread surface (32b, 52b), and a cutting line (32c, 52c) that interconnects the upper and lower thread surfaces at a radial edge of the upper and lower thread surfaces distal to the shank portion (31, 51). Each of at least some of the turns of the screw thread (32, 52, 62) is formed with from two to six adjacent concave facets (321, 521, 621) along the cutting line (32c, 52c). Each of the concave facets (321, 521, 621) has a radial width ranging from about 60 to 120 degrees. Each pair of the adjacent concave facets (321, 521, 621) forms a pointed corner (322, 522, 622) at the cutting line (32c, 52c). <IMAGE> |
