Self-penetrating screw
A screw includes a shaft, a trumpet-shaped head provided to a base end of the shaft, and a narrow-tip portion formed at a tip end of the shaft. The shaft has different diameters at a small-diameter portion contiguous to the narrow-tip portion and at a large-diameter portion contiguous to the head. Two screw threads are formed on the small-diameter portion and one screw thread is formed on the large-diameter portion. Therefore, a thread portion can be divided into a double-thread portion and a single-thread portion. It is preferable that the screw threads forming the double-thread portion have different heights. The screw is mainly used for fastening a plasterboard to a frame member.
1. A self-advancing screw, comprising:
a shaft having a circular cross section;
a head contiguously provided to a base end of the shaft, the head having a trapezoidal cone portion with an outside diameter thereof increasing from a portion connected to the shaft toward a top face, and further defining a fitting portion in which a driver bit is to be fitted and which is formed in the top face of the head, a bearing surface of the trapezoidal cone portion forming the head having a profile including an outline curved to be recessed from outside in a vertical cross-sectional front view taken along a plane including an axis, which forms the head as an outwardly flared structure;
a tip end portion of the shaft on a side opposite to the head being formed as a self-advancing portion for advancing into a member without a prepared hole, the self-advancing portion comprising a narrow-tip portion having a pointed extreme tip end;
the shaft further including a small-diameter portion contiguous to the self-advancing portion and a large-diameter portion contiguous to the head, a tapered portion being interposed between the small-diameter portion and the large-diameter portion contiguous therewith;
the small-diameter portion of the shaft being provided with two screw threads including a first screw thread and a second screw thread, only the first screw thread out of the two screw threads extending proximate to the head, such that the small-diameter portion of the shaft is formed as a double-thread portion and the large-diameter portion is formed as a single-thread portion;
an outside diameter of the single-thread portion being greater than or substantially the same as a maximum outside diameter of the double-thread portion, an outside diameter of the second thread being less than a corresponding outside diameter of the first thread formed in the double-thread portion;
the head including a flat flange portion overhanging outward from an outer periphery of the trapezoidal cone portion, an outside diameter of the curved trapezoidal cone portion being approximately twice a maximum outside diameter of the screw threads, an outside diameter of the flange portion being 2.5 to 3.5 times the maximum outside diameter of the screw threads; and
spaces being left between bases of the adjacent screw threads on the shaft.
2. A self-advancing screw according to claim 1 ,
wherein an outside diameter of the first screw thread is greater than an outside diameter of the second screw thread in the small-diameter portion of the shaft and an outside diameter of the first screw thread in the large-diameter portion is greater than the outside diameter of the first screw thread in the small-diameter portion.
3. A self-advancing screw according to claim 1 ,
wherein an outside diameter of the first screw thread and an outside diameter of the second screw thread are the same as each other in the small-diameter portion of the shaft, and an outside diameter of the first screw thread in the large-diameter portion is greater than outside diameters of both the screw threads in the small-diameter portion.