Patent GB-396246-A (GB)
Title: Improvements in and relating to the control of the feeding mechanism of machine tools
IPC: B24B49/04
Filed: 1932-10-24
Abstract: 396,246. Copying-lathes. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Mershon, A. V.; 823, State Street, Schenectady, New York, U.S.A.) Oct. 24, 1932, No. 29715. Convention date, Oct. 23, 1931. [Class 83 (iii).] In machines for turning work to exact size or shape, the feed of the tool is electrically controlled by a coil arranged in the arm of a Wheatstone bridge, an armature in the field of the coil being moved by a feeler bearing on the work so as to produce an " out-ofbalance " current to operate electromagnet means to vary, stop, or reverse the feed when a predetermined size is reached. The invention is shown in Fig. 1, applied to a machine for grinding a rotary spindle 19 supported on a slide 12 which can be reciprocated on a bed 10 by a rack and pinion operated by bevel gear 52, 53, 54, which is reversed at each end of the stroke by tappets 48, 49 engaging a lever 62 to slide the shaft 55 carrying the bevel wheels 53, 54. The rotary grinding wheel 20 is mounted on a screw-operated transverse slide 24 which is fed towards the work by a ratchet wheel 26 and moved away from the work by a weighted cord 29 which rotates the ratchet wheel clockwise only when a retaining pawl 41 is released. The anti - clockwise slow feed of the ratchet wheel is effected at two different speeds governed by arms 32, 33 which carry feedpawls 30, 31 and are moved by branches 35, 36 of a lever 40 actuated by a projection 65 on the lever 62 at the end of each reciprocation of the table 12. The arrangement is such that (1) a coarse feed is given by the pawl 30 until this pawl is released by an electromagnet 109, (2) a fine feed is given by the pawl 31 until this pawl is released by an electromagnet 110, and (3) the grinding wheel ceases to be fed and remains in contact with the work to give a smooth finish until an electromagnet 111 releases the retaining pawl 41 when the grinding wheel is withdrawn from the work by the weight 28. The desired feeds are controlled by energizing the electromagnets 109, 110, 111 at times determined by the movement of a feeder 75 which presses upon the work and operates an armature 72 moving between coils 69, 70 inserted in the arms of a Wheatstone bridge which is supplied with current from A.C. mains 87. Coils 82, 83 in the other arms of the bridge are connected through a transformer 86 and rectifier 90 to leads 91 which operate the needles 96 of micro-ammeters which are adjusted to engage in succession with mercury cups 98, 99, 100 as the " out-of-balance " current in the bridge increases due to movement of the armature 72 in the field of the coils 69, 70 as the work reaches a predetermined size for which the feeler has previously been set by engagement with a sample shaft. The circuits established through the cups 98, 100 energize switches 103, 104, 105 which close circuits in succession through the electromagnets 109, 110, 111. An arm 123 moving with the table 12 engages contact blocks 116, 117 at each end of the stroke and ensures that the electromagnets 109, 110, 111 are only operated at the ends of the stroke of the table. Modifications are described, (1) in which there is no reciprocation of the table, (2) in which the pawls 31, 41 are released by a single electromagnet when a smooth finish is not required, (3) in which one feeding speed only is provided ; a single pawl 30 being controlled by an electromagnet simultaneously with the retaining pawl 41, (4) in which a second feeler carried by a box 67 slidable on the pillar 68 engages the sample bar during the grinding operation. Fig. 8 shows the invention applied to turning a surface 143 to a shape determined by a templet 144 over which moves a feeler 141 mounted on a box 67 slidable vertically on guide 181 and carrying a second feeler 75 which bears against the tool box 151. This tool box and the box 67 carrying the coils 69, 70 are mounted for simultaneous movement on a screwed shaft 178 rotated by a pulley 175 under the control of a clutch 177 operated by a hand-lever 171 which also carries a switch plate 170 for closing the circuit through a clutch 160 for engaging the tool-feed motor 159 with the feed pinion 162. The arrangement is such that the speed of the motor 159 is controlled through a resistance 166 by the " outof-balance " current of the Wheatstone bridge 69, 70, 82, 83 through the micro-ammeter 96, mercury cup 97, and switch 105 acting as previously described.