Patent GB-558037-A (GB)
Title: Improvements in and relating to means for cutting screw threads
IPC: B23G1/02
Filed: 1942-05-12
Abstract: 558,037. Lathes. CROWE, E. F. S. May 12, 1942, Nos. 6435 and 16557. [Class 83 (iii)] In cutting screw threads on lathes &c. the tool is given a reciprocating movement to and from the work axis by a peripheral cam while being traversed along the work at a speed different from the normal lead movement. The longitudinal movement is produced by screw-and-nut mechanism. The cam speed may be higher or lower than the spindle speed according to whether right or left hand threads are required. In one example, if a single lobe cam is used for the transverse movement in cutting a right hand screw of four threads per inch with a four threads per inch lead-screw, if the cam shaft is driven at 99 revolutions per 100 revolutions of the work and the lead screw is geared 1 to 100 this gives a feed of 0.0025 per revolution and thus a traverse of one pitch per revolution of the cam relative to the work spindle. If the cam makes 101 revolutions per 100 revolutions of the work a left hand thread of four turns per inch will be cut. The profile of the cam must be such that if there is no relative longitudinal traverse between the tool and work the tool describes a path corresponding to the form of thread to be cut. A multi-lobed cam may be used for cutting multi-start threads. The tool may either be a single point tool or a chaser. In the latter case the tool may first be fed to the desired depth without relative axial movement and then the axial movement effected through one pitch. In one form, Figs 1 and 4 for cutting internal threads on work 20, a threaded shaft 1 running in a nut 2 is mounted in bearings 4 in a bar 5 and carries a cam 9 actuating a tool-carrying slide 11. An oil-tight cover 6 is provided. The tool slide is returned by parallel springs 16. In a modification, Fig. 7, the shaft 1 is mounted in bearings 4 and carries a double cam 21 which imparts positive movement in both directions to the slide 27 of the tool 28. A dog 30 protruding from shaft 1 co-operates with a projection on a ring 32 having a pin which entrains a friction ring 37 which acts as a damper to ensure creep of the nut round its screw thread. Either of the above described forms may be used in a capstan lathe, Fig. 17, the nut and work being held in the chuck 38, and the boring-bar 5 held in the capstan-head 41. In another modification, Fig. 18, for externally threading work 42 by a chaser 60 the work, mounted between centres, is coaxial with a gear 45 connected by tumbler gearing to a gear 50 driving a nut on a splined screwed spindle 54 carrying a double cam imparting transverse reciprocation to the tool slide 59 at a speed related to that of the work. With the tumbler gearing engaged, the tool is adjusted adjacent the work, the saddle locked and the machine started to reciprocate the top slide which is fed by adjustment of slide 61 until the tool cuts to the required depth at one part of the work periphery. The slide 62 is now traversed by a screw 63 for about one pitch after which the thread will be complete. In a further modification, Fig. 23, a base 66 mountable for instance on the cross slide of a screw machine has a main slide 68 carrying a tool slide 69 actuated by a cam 70 on a screw 71 engaged by half-nuts 83. A wheel 74 on the work spindle is geared to a wheel 78 having an extended boss carrying the half-nuts which are normally separated by springs. When the cycle begins the unit is moved forward until plate 88 moves back the plunger 87 and actuates push-rods to close the half-nuts. The screwed shaft 71 and cam 70 are carried round by the half nuts thereby oscillating tool slide 69 and cutting the thread to depth at one point. When full depth is reached a projection engages bolt 97 and moves slide 68 lengthwise by a distance of slightly more than one pitch, against springs 98, to generate the entire thread. The slide 68 is returned by the springs. The cross slide and turret are also returned thus releasing the half nuts. A friction device 103 ensures slip between the half nuts and the screw to produce the longitudinal movement. In a further modification a boring bar 105, Fig. 30, which may be mounted at one end in a turret head, (as in Fig. 17) is carried in a bearing 106 and encloses a spindle 107 running on a nut 109 normally held against rotation by a peg 111 which may be withdrawn by a rod 114. The spindle 107 carries a cam 115 . which reciprocates a slide carrying two opposed tools 118, Fig. 31. In a further modification the work is secured to a chuck 141, Fig. 36. The lead screw 143 coacts with a nut 144 rotatable in bearings in an extension of saddle 147, the nut rotation being damped by spring-urged shoes 148. The nut carries a cam 149 operating a follower on a cross slide 151 to impart the transverse movement to the tool. This form may be modified by replacing the screw 143 by a splined shaft carrying the cam members and driven at a speed slightly different from that of the headstock spindle, the required axial movement of slide 147 at a speed dependent on the difference between the speeds of the spindle and cam being obtained by driving the front traversing screw 142.