Patent GB-498972-A (GB)
Abstract: 498,972. Taps. FAIRWEATHER, H. G. C. (National Acme Co.) April 4, 1938, No. 10338. [Class 83 (iii)] In a screw-threading tap for forming double tapered threads in work Such as pipe couplings, the chasers are moved radially by an axially displaceable wedge-core which is moved during tapping by two oppositely disposed cam surfaces on a rotary sleeve, rotation of the sleeve being produced by relative axial movement between the body of the tap and an outer ring which engages the end of the work. Circular chasers 12 are carried by holders 11 radially slidable in the body 5 attached to the shank 4, and having hooked ends in positive engagement with slots in the 'wedge-core 7. The core is mounted on a plunger 6 slidable within the body 4 and normally pressed rearwardly by a spring 38. A rod 25 passing transversely through the plunger carries at each end a roller 24 'engaging cam grooves 23 in members 23<1> attached to a sleeve 3 rotatable on the body. An outer ring 2 carries a roller 20 running in a helical groove 22 in the sleeve and also carries a key 17 sliding in a similar helical slot 22<1> in the sleeve and a roller 15 guided in an axial groove 16 in the body to prevent relative rotation of the ring and body. In operation the work A is rotated and the tap fed vertically downwards until an adjustable ring 13 on the ring 2 meets the end of work and prevents further feed of the rings. Axial travel of the roller 20 along the helical groove 22 rotates the sleeve 3 and the cam surfaces 23 thereon move the core 7 axially, first in a direction to reduce the diameter of cut for tapping one half of the coupling and then in a direction to increase the diameter for the other half. The point of transition from one taper to the other is adjusted, in order to preserve the same diameter at each end of the work, by adjusting the ring 13 relatively to the ring 2 to vary the point at which it meets the work in relation to the point at which the chasers commence to cut. At the end of the tapping operation a wedge 34, Fig. 5, on the ring 2 engages a roller 36 on a plunger 27 to move it radially outwards, against a spring 37 and disengage it from an annular groove 28 in a ring 29 rotatably supported on the sleeve 3. The spring 38 may then draw back the core 7 and sleeve 3 to collapse the tap. The plunger 27 is carried by a bracket 271 attached to the shank and is located axially in the bracket by means of opposed wedges 41, 42 and screws 43, 44 in order to adjust the axial position of the plunger and so vary the initial diameter of cut. The plunger and the sides of the groove 28 are bevelled as shown so that when the last few threads are being cut on the work and the chasers tend to move outwards due to decreased load, the partial withdrawal of the plunger by the wedge 34 will allow the core to recede and counteract the tendency. The extent of taper produced on the work may be varied by changing the cams 23<1>, or by tilting the latter through adjustment of the wedges 45 which clamp them to the sleeve 3. The cams may be formed in two independently adjustable parts to allow one taper to be varied without affecting the other. The plunger 6 is formed of two parts relatively adjustable by a screwed stud 6<111> so that the position of the core and the compression of the spring 38 may be varied. The core 7 may be detachable from the plunger 6 and interchangeable to suit the size and number of chasers used. To re-set the tap, as it is raised from the work the ring 2 falls until the roller 20 engages the end of the groove 22 and a yoke coacting with a groove 21 in the ring then meets a stop on the machine to draw' the sleeve 3 downwards until the plunger 27 snaps into the groove 28. Chasers.-The front part of each chaser is shaped to correspond to the taper on the first half of the work and the last few threads are shaped to the taper of the second half, the taper on the chaser in each case being slightly more than that required on the work to compensate for distortion of the chaser during operation.