| 摘要 |
A self-tapping screw S1 comprises a single or double helix of threads which have a flank angle of less than 45 DEG , on a core with constrictions (5) between neighbouring threads, the ratio of the diameter over the threads to the minimum diameter of the constrictions being less than 1.5 and greater than 1.2, more particularly about 1.3. The axial spacing (h/2) between neighbouring threads is preferably 0.24 to 0.30 times, more particularly 0.27 times, the outside diameter. The ratio of the diameter at the thread roots to the minimum diameter of the is preferably 1.04 to 1.10, more particularly 1.07. The forward end of the screw may have cutting edges, preferably constituted by milled recesses. The threads may have indentations, preferably V-shaped, to the root of the thread profile and preferably arranged along each of several coarse-pitch helical lines (e.g. three at 120 DEG spacing) crossing the threads with a direction of inclination, with respect to the longitudinal axis of the core, preferably contrary to that of the threads. <IMAGE> |
