GB468679A - Improvements in tapping screws and methods of cutting threads - Google Patents

Abstract

468,679. Bolts. CLAYTON, C. E. EVERY-. (Illinois Tool Works) Oct. 9, 1935, No. 27905. [Class 89 (i)] [Also in Group XXII] In a tapping-screw having a threaded liolding portion and a hardened tapered tapping portion provided with a recess traversing at least some of the thread convolutions, the recess is of sufficient cross-section to accommodate cuttings and is so shaped that cuttings are directed towards the entering end of the screw. The cutting edge of the screw is sufficiently rigid to exert a compressive force on the work and so increase the effectiveness of the cutting edge. The screw comprises a holding portion 26, Fig. 1, with threads of full section and a hardened cutting portion 30 with truncated threads 38 having a conical exterior surface but the same root diameter as the threads 26. A cutting edge 34 is formed by a recess 32 which has a depth not greater than the radius of the screw and a width not more than a third of the circumference of the screw. The recess has a transverse arcuate curvature B, Fig. 3, designed to form the cuttings 40, Fig. 6, into continuous spirals which are ejected from the recess by its longitudinal arcuate curvature A. The side walls 36a of the recess are straight and parallel at the entering end of the screw, but merge into the arcuate surface 36 at the top, thus producing initial heavy duty cutting edges 34 having an acute cutting angle and final finishing edges 34a having an obtuse cutting angle. The flanks of the threads are not relieved so that the gradually expanding size of each convolution of the thread produces a compressive action on the work 42 as the screw advances axially, this compression causing a slight wave of material to form in the recess 32 in advance of the cutting edge, thereby providing a heavier cut and ensuring that continuous self-ejecting cuttings 50 will be produced instead of small chips which might clog the recess. In a modification, which is described and claimed in Specification 468,744, the cutting threads 30a, Fig. 14, maintain the same shape, core diameter and pitch as the holding threads 26a but are of gradually decreasing height towards the point of the screw, and in another modification the cutting threads 30b, Fig. 15, retain the same height as the holding threads 26b, but are formed on a core which decreases towards the point. In a further modification, the trailing edges 64, Fig. 3, of the cutting threads are chamfered so as to provide a more obtuse angle in order to relieve the pressure on the thread at this point and prevent chipping of the thread on the screw.