GB848464A - Improvements in or relating to servo-controlled drives, for instance for machine tools - Google Patents

Abstract

848,464. Machine tool details; milling. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 28, 1958 [May 29,1957; July 20, 1957; Dec. 2, 1957; March 26, 1958], Nos. 17048/57, 23032/57, 37419/57 and 9752/58. Class 83 (3). [Also in Group XXXVIII] Apparatus for driving a first object 4, Fig. 1, along a predetermined path in relation to a second object 3, comprises two driving mechnisms coupled together and to the objects, so that movements of the first object relative to the second object. are dependent upon the algebraic sum of the individual driving actions of the two driving mechanisms, the first driving mechanism Q being capable of a relatively high driving acceleration over only a relatively short range of driving action, and the second driving mechanism M being capable of a relatively long range of driving action but being arranged to provide only a relatively low driving acceleration, means A, responsive to a signal representing the motion required of the first object in relation to the second object along the path, for actuating the first driving mechanism, and means for controlling the second driving mechanism to prevent the first driving mechanism from reaching the end of its range of driving action. In a machine tool, the first driving mechanism or quick drive may be a short stroke hydraulic ram Q, controlled by a valve V operated by an actuator A responsive to command signals. The second driving mechanism or slow drive may be a bar, such as a leadscrew, working in combination with a rotary-to-linear converter RLC driven by a motor M. The cylinder of the ram Q is carried by the table, and the motor M and converter RLC are fixed. In modifications, the motor M is also carried on the table, or the converter RLC is carried on the cylinder of the ram Q and the ram Q attached to the table, or the motor M is carried by the table and the ram cylinder is fixed, or the converter RLC is carried by the table, and the motor and cylinder fixed, or the converter RLC only is fixed. In a further modification, the cylinder of the ram Q, Fig. 6, is fixed and the ram is attached to a slide carrying the motor M and the converter RLC. In another modification, the cylinder of the ram Q, Fig. 8, is fixed and the ram attached to an angularly adjustable table. The table carries a concentric work-table rotated by the motor M. In a milling machine having a laterally adjustable spindle 8, Fig. 9, the ram cylinder 17 is attached to the table 4, and the ram is attached to a rack 11 engaged by a worm 10 rotated by the motor 9. The worm 10, Fig. 10, is carried in a cradle 21 pivotable about the axis of a driving shaft 13. The members 10, 13, 21, may be loaded endwise hydraulically to prevent axial movement. The worm 10 is urged upwardly by an hydraulic piston engaging a lug 23 on the cradle 21 to prevent backlash. When the ram is actuated by a solenoid operated valve controlled by signals from a magnetic tape 51, Fig. 14, the table 4 is moved. The rack 11 also moves, and dogs thereon actuate switches to energize the motor 9 to move the rack. The switches operate to ensure that the ram does not move beyond certain limits in its cylinder. The switches may be replaced by a rheostat 60, Fig. 16, co-operating with a slider 62 attached to the rack 11 so that the motor 9 is operated at the required speed and in the required direction as the table 4 and rack 11 move relatively to each other. In a further modification, the switches are carried by an extension of the ram cylinder. The ram then carries a rod having a dog which operates limit switches in each direction of movement to cause the motor 9 to run in one or other direction until the dog returns to operate a switch at its central position. The signals from the tape are fed to a comparator 54, which also receives signals corresponding to the actual movement of the table. The latter signals may be produced by a Moire fringe effect using gratings 55, 56, and a photo-cell 57. Instead of using an electric motor 9, an hydraulic motor, controlled by a valve moved by the ram, may be used. In yet another modification, the motor 9 is controlled by a velocity signal from the tape. Should the ram be displaced too far, as when the command and velocity signals are not synchronized, safety switches are operated to apply a motor control over that of the tape. When the command signals are in the form of pulses, the velocity signal may be obtained by a reading head and integrator, which determine the frequency at which the pulses occur. The latter reading head is preferably ahead of the command head, so that changes in velocity are anticipated. The quick drive may be a magneto-strictive transducer, and the slow drive an hydraulic piston. The drives may each receive the same command signal. Three drives instead of two, may be provided, the two slower ensuring that the two faster drives do not exceed their limits of movement. Alternatively, the slow drive may be a lead screw and nut, to one of which the quick drive gives additional rotary movements under the control of the command signals in the manner of a corrector bar.