Patent GB-541619-A (GB)
Title: Method of and means for producing relief in screw thread taps and dies
IPC: B24B3/22
Filed: 1940-03-02
Abstract: 541,619. Milling-machines; screwing-dies taps. COVENTRY GAUGE & TOOL CO., Ltd., HARLEY, S. J., and WAINWRIGHT, J. E. March 2, 1940, No. 3932. [Class 83 (iii)] [Also in Group XXIII] Screw thread taps or dies are relieved by progressively accelerating or decelerating the speed of rotation of the blank or its axial traverse during the time that the lands of the blank are passing the relieving tool thus modifying the helix to give the desired relief. In some cases the rotation of the blank is accelerated or retarded while the flutes of the blank are passing the tool so as to obtain a staggered relief. In one form the rotary motion of the blank 2 is alternately accelerated and retarded relative to its traversing movement past the cutting or grinding tool 5. For this purpose the rotary face plate 6 communicates motion to the work which is carried on dead centres 3, 4 through a lever 7 carrying a driving pin 8. A pin 13 on the face plate carries a gear 12 engaging a stationary gear 11. The pin 13 also carries a cam 14 which communicates motion to the lever 7 and the work through a roller 15. The cam shown in Fig. 3 has four alternate rising and falling parts a ... d and in the relief of a fourfluted tap the part a decelerates the tap and gives lateral relief to the leading flank of the thread on one land. The part b accelerates the tap during the passage of the second land and flute past the wheel and imparts lateral relief to the following flank. Similarly parts c and d relieve the leading and following flanks respectively of the third and fourth lands. If successive lands are to have narrow staggered threads, which combine to form the full width thread groove, the cam shown in Fig. 4 is employed. In this cam the parts a<SP>1</SP> ... d<SP>1</SP> are separated by concentric portions e ... h which operate while the tool is passing the lands. If both thread sides, Fig. 6, are to have lateral relief some of the side surfaces are relieved on the first revolution of the tap and the remainder during a second revolution. For this purpose, the rotation may be transmitted from a constant-speed driving-wheel 20 through a lever 24 connected to the driven wheel by a pin 27. The cam 23 which modifies the motion of the driven wheel has, in the case of a four-fluted tap, two accelerating portions a<SP>2</SP>, b<SP>2</SP> and a decelerating portion c<SP>2</SP>. The lever gear rotates at It times the blank speed. In operation, the cam parts a<SP>1</SP>, b<SP>2</SP> cause acceleration during the cutting of the first two lands to give relief on one side while the part c<SP>2</SP> causes relief of the opposite flanks of the third land and so on so that three complete revolutions of the blank are required before all the lands are finished. The degree of acceleration and retardation of the work may be varied by shifting the position of the roller 25 on the lever 24. In a modification, instead of obtaining the side relief by modifying the work rotation, the traversing movement of the work may be varied. The relief may be over the whole or part only of each land of the tap.