Patent GB-873976-A (GB)
Title: Improvements relating to machine tool control
Inventor: LAWSON ROBERT
IPC: B23C9/00
Filed: 1958-05-07
Abstract: 873,976. Copy-milling. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 7, 1959 [May 7, 1958], No. 14665/58. Class 83 (3). A machine tool limit position control system comprises, for a given direction of feed motion, a pulse generator capable of producing electrical impulses correspondingly with feed along that direction, a reversible pulse counter having a zero condition which can be selectively related to any one, constituting a limit position, of a number of positions along said direction, which counter is governed in respect of its direction of counting according to the sense of the feed motion (namely forwards or backwards along said direction) and is arranged to establish, on reaching its zero condition in response to pulses produced during a feeding motion towards the limit position, a condition for bringing about a modification of the overall feeding action, and means functioning consequently on the counter reaching its zero condition to relate the zero condition to a desired limit position for a subsequent feeding motion along the same direction. In a copy-milling machine, three screws SA, SC, SL, Fig. 1, rotated by motors AM, CM, LM, move the tool in axial, cross-feed and longitudinal directions. The tracer scans the model in longitudinal passes p1, p2, Fig. 1a, with a cross-feed x between passes, and the tracer controls the axial movement of the tool. The longitudinal and cross-feed motions are measured by pulse generators PGL, PGC. The pulses from the generator PGL are fed to reversible counters CL1, CL2, each of which counts in one direction only and after a predetermined count reverses the motor LM and causes the motor CM to impart a cross-feed. (Cutting may take place only on forward passes, in which case there is cross-feed only after each complete traverse and the return longitudinal movement is carried out at an increased speed.) The tracer controls the motor AM for the axial feed and may also control the longitudinal feed. The counter PGC stops the cutting operation when the selected area W has been covered. To cause the limits of longitudinal feed to follow the lines EC, AF, the counters CL1, CL2 are set to the required amounts for the first two traverses and a variable pulse ratio device V is set to feed pulses to ratio selectors RS1, RS2, to cause the limits to move as desired. The device V may be an electro-mechanical or electronic pulse divider such as a uni-selector having a fixed number of output pulses for each of two banks CB1, CB2. Sense selectors SS1, SS2 enable the operator to change the direction of the limit lines such as to produce an upper limit line EGH, Fig. 1b. In a modification, the cutter, after traverse in one direction, is withdrawn clear of the work to a point determined by an additional counter. On the return movement towards the work, further counting means initiate a reduced speed of the cutter travel as the cutter approaches the work. The points of reduced speed may lie on a straight line or may be at a constant distance from the work surface. In a further modification, the cutter follows round contours r 1 -r 4 , Fig. 3a, on a workpiece S. The cross-feed and longitudinal movement counters will both return to a zero count when a complete circuit has been made. When a simultaneous zero condition is sensed, an axial feed x is imparted to the cutter. All embodiments may be incorporated in a single machine. The pulse generator may be a commutator, a cam and switch, or of electronic or photo-electric type. The machine tool part may be set initially in a required zero starting position or a number of pulses may be fed to the counter(s) to bring the counter(s) to zero at a desired starting position. Each counter may employ multi-cathode cold-cathode counting tubes and may give a visual indication of its count at any moment.