Patent GB-849211-A (GB)
Title: Improvements relating to the automatic control of machine tools
Inventor: VOLES ROGER
IPC: G05B19/41
Filed: 1955-12-13
Abstract: 849,211. Milling; special processes. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 13, 1956 [Dec. 13, 1955], No. 35695/55. Class 83 (3). [Also in Group XXXVIII] A mechanism for the automatic control of machine-tools comprises means for causing a cutting-tool to make spaced traverses on a workpiece in response to instructions derived from a record, and in such a way that edges may be formed on the workpiece, and means whereby the distance between successive tra-- verses of the tool is variable in response to variations in curvature of the workpiece represented by the instructions, so as to reduce the dependence of the height of the ridges on the curvature. In a turbine-blade milling-machine of the type described in Specification 848,411, a convex cutter produces a surface having a series of ridges or cusps. (Alternatively, a cylindrical cutter or end-mill may produce a surface having a sawtooth form.) It can be shown that the height h of a cusp is given by where h 1  and h 2  are the distances shown in Fig. 1, the point B being parametrically halfway between one cut and the next, and that where R is the radius of the cutter, and T is the common parameter of the co-ordinates x, y, defining the section on the surface, which may be convex or concave. A.C. voltage analogue signals representative of the quantities shown in Fig. 2, which may be obtained from a differentiating-device as described in Specification 849,091, are fed to resolvers 6, 7 as well as a signal representative of # from a common inputshaft 5. The output of the resolver 6 is fed to a detector 8, and the D.C. output signal is fed to a squaring-device 13, through a potentiometer 9 loaded by a spring 10 and coupled by a clutch 11 to the T-shaft 12. The output of the device 13 is fed to an analogue adder 15 through another D.C. potentiometer 14, which is preset to divide the input signal by R. Similarly, the output of the resolver 7 is fed into a phasesensitive detector 16, the D.C. output of which is fed to the analogue adder 15 through a double potentiometer arrangement 17, 18, which is loaded and coupled as is the potentiometer 9. A preset signal voltage from a source 19 is applied to the adder 15 via a potentiometer 20. The output of the adder 15 is fed via a switch 21 to a second adder 22 connected between the output of the tacho-generator 23 and the input of the servo-amplifier 24 in the servo feed-back path controlling the rotation of the T-shaft. At the end of a longitudinal traverse of the tool, a signal from the sequence-controller (of the machine in Specification 848,411), engages the clutch 11 and moves the switch 21 to position b. The T-shaft rotates together with the potentiometers 9, 17, 18. The signal from the potentiometer 14 to the adder 15 represents 8h 1  as the output of the detector 8 is and thus is multiplied by dT by potentiometer 9 and squared in tne squarer 13. Similarly a signal representing 8h 2  is fed to the adder 15. The adder 15 compares the sum 8(h 1 +h 2 ) with the signal 8hmax set in on the potentiometer 20. If hmax>h 1 +h 2 , the adder 15 maintains a positive output signal to open the adder 22 and allow the T servomotor to rotate the shaft 12, until h 1 +h 2  tends to hmax. A brake 25 then clamps the T-shaft, the switch 21 is moved to position a, the spring 10 returns the potentiometers to zero and the tool commences its next traverse. In a modification, the T-shaft may be driven by a stepping-switch and the potentiometers may comprise uni-selectors. The mechanism may also be simplified if the surface is such that h 2  may be neglected; a constant-height of cusp may then be obtained when a tool has been reground by adjusting the potentiometer 14. If a surface is tapered, two potentiometers 20 may be provided, there being a throw-switch to select the appropriate potentiometer according to the direction of traverse. An inhibitor-gate in the output of the adder 22 may be provided to arrest the T-shaft in response to a signal from the record if a discontinuity in the surface is approached.