GB335578A - Improvements appertaining to screw cutting tools - Google Patents

Abstract

335,578. Standard Telephones & Cables, Ltd., and Clarke, G. W. June 26, 1929. Screwing - dies; taps. - In a screwcutting die or tap the cutting angle varies over the length of the tool, and a single cutting- edge only may be provided. A tap is formed by cutting a full screw-thread on a tapered bar 1 and then removing a portion of the tapered part so as to form an oblique face 3 whose edges 4 - - 16, Figs. 1 and 3, constitute the cutting- edges of regularly increasing angle. The small diameter of the tap corresponds to the size of hole to be tapped and the large diameter to the finished thread. The first teeth cut a deep narrow groove which the succeeding teeth deepen slightly and widen whilst the teeth 12 - - 16 have a reaming or burnishing action. Alternatively, the screw-thread may be cut on a parallel bar and the tops of the first threads tapered as shown in Fig. 6, or the bar and the tops of the threads may each be slightly tapered. In another modification in which the depth of cut does not increase uniformly, the bar consists of alternate cylindrical and conical portions l, k, Fig. 11, and in this form the cutting angle may also be made to vary irregularly by cutting along a curved surface as shown in this Figure. The variation in cutting angle may also be produced by cutting a tapered flute in the tapered and threaded bar. A screwing die is formed from a block 73, Fig. 17, by providing a screwed bore tapered from n to p and then cutting away in an oblique plane n, q. The larger diameter of the bore is sufficient to allow the unthreaded work to penetrate as far as the first cutting tooth 76, and the variations of cutting angle and depth of cut are similar to those of the tap shown in Figs. 1 and 3. Alternatively, the bore may be parallel and the tops of the first cutting teeth tapered, or both the bore and the teeth may be tapered as shown in Fig. 21. In this Figure the block is cut away on a curved surface 74 similarly to the tap shown in Fig. 11. The other features of the tap may be incorporated in the die and a single curved or tapered flute may be provided. The tap or die may be modified in other ways, such as by omitting alternate threads over the whole or a portion of its length or several rows of cutting-edges, each having teeth of varying angles, may be provided. Specifications 8345/87, [Class 61, Hand tools &c.], and 7109/01 are referred to.