Sheet screw

#1. Field of the Invention

#The present invention relates to a sheet screw, and more particularly to an improvement in a thread-forming or self-tapping screw which is especially designed for screwing into a metallic sheet, a thin metallic plate coated with a synthetic resin film or the like sheet member.

#2. Prior Art

#The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

#FIG. 1 is a front elevation of a sheet screw embodying the present invention;

#FIG. 2 is an enlarged fragmentary elevation showing in detail the screw end portion formed with a blank according to the present invention;

#Referring now to the drawings, a sheet screw 10 according to the present invention has a screw head 11, a threaded shank 12 of substantially cylindrical shape connected at its top to said screw head 11, and a non-threaded substantially pyramidal end 13 which extends from the bottom of said threaded shank 12 to terminate in a point 14. Said pyramidal end 13 is divided into two parts, viz. a tip portion 15 with at least five edges 16 extending divergently from the point 14, and a base or frustum portion 17 with at least three rounded ridges 18. In other words, the tip portion 15 has at least five flat sides 19, and therefore, an angle .theta..sub.1 in FIG. 3 must be at least greater than 108.degree.. The base portion 17 as illustrated in the embodiments of FIGS. 1 to 13 has a substantially square cross section, however, it may be replaced by a modified base portion (not shown) having a triangular, pentagonal or other regular polygonal cross section, but the number of sides 20 of a base portion must be less than the number of sides of a tip portion. As a matter of fact, providing an even number of flat sides 19, 20 is preferable from the technical view point. For example, if the base portion has four sides, the tip portion may preferably be a regular hexagon or octagon. Further, it should be noted that each of said sides 20 of the base portion 17 must be smoothly continuous with a pair of adjacent rounded ridges 18 so as to provide a smoothly continuous contour line as shown in FIG. 6.

#In the above-described construction of the sheet screw 10, the pyramidal angle, i.e., the vertically opposite angle of the base portion 17 as indicated by .theta..sub.2 in FIG. 7 may vary preferably within the range of 25.degree. to 35.degree., while, the pyramidal angle, i.e., the vertically opposite angle .theta..sub.3 (FIG. 7) should be equal to or larger than said angle .theta..sub.2. The most preferable value of the angles .theta..sub.2 and .theta..sub.3 should be determined in accordance with thehardness and thickness of the sheet material to which the screw 10 is to be applied as well as with the size of the screw to be employed. Generally, in the case of the sheet 21 being made of iron or similar hard material of a considerable thickness, the angles .theta..sub.2, .theta..sub.3 should be increased to about 35.degree., while, if the sheet 21 is of aluminum or similar softer material of relatively smaller thickness, the angles .theta..sub.2, .theta..sub.3 should be decreased to about 25.degree. .

#Further, the number of polygonal sides 19 and 20 of each of the base portion and the tip portion may also vary in accordance with the hardness and thickness of the sheet 21. Generally, it is preferable to increase the number of the sides 19, 20 for harder and thicker material. However, as far as the base portion 17 is concerned, eight or more polygonal sides 20 should be avoided because a base portion having that many sides will approach the shape of a circular cone, and as a result, the stress concentration on each of the many rounded ridges 18 will be dispersed, so that the initial aperture 22' (FIG. 9) cannot be effectively enlarged enough to form a protrusion 23 of a desired height without development of undesirable cracks around the aperture 22.