Patent GB-670143-A (GB)
Title: Improvements in and relating to curve followers
IPC: B23Q35/128
Filed: 1949-11-14
Abstract: 670,143. Copy milling machines. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 14, 1949. [Nov. 18, 1948] No. 29081/49. Class 40 (i). [Also in Group XL (b)] In a copy-milling machine for profiling a turbine blade in accordance with a templet curve, the latter is mounted on a table traversible in mutually perpendicular co-ordinate directions by lead screws driven by electric motors, which drive other lead screws through Selsyn linkages to correspondingly traverse the worktable of a milling machine which positions the workpiece to be profiled relatively to a stationary rotary cutter. The templet is examined by a radially mounted photoelectric scanner having prismatic optical means for rotating the image in synchronism with an A.C. supply, to develop phase variable pulses in the photocell output which are combined electronically with the supply voltage in thyratron circuits embodying saturable reactors to produce signal voltages electronically controlling the co-ordinate traversing motors, and a further motor which turns the scanning device about a radial axis, so that the optical axis of the scanner is maintained in correspondence with the templet curve, and the radial axis is maintained perpendicular to the curve, so that the workpiece is traversed relatively to the cutter so as to be profiled in correspondence with the templet. In Fig. 1, the templet curve 2 is mounted on a table 4 and illuminated by a lamp 22. The scanner 13 having a lens system focused on a portion of the curve, is mounted on a slotted radial arm 15 pivoted on a spindle 16 by a motor 43 and contains an axially rotatable tube 18 (Fig. 2) carrying internally a Dove prism 19, and externally the rotor core of a synchronous motor, whose stator windings 18a are internally supported by the scanner housing, whose upper end is closed by a disc 20 having a small circular aperture 21 slightly displaced from the optical axis, through which light falls on a photocell 25. The synchronous motor is driven at e.g. 1,800 r.p.m. from the A.C. supply 23, and the Dove prism rotates the image of that portion of the curve in the lens field at 3,600 r.p.m., i.e. once per cycle, so that an end of the image traverses the photocell once per half cycle, and the pulsating cell output, is amplified by valves 32, 35 (Fig. 4) and transformer coupled in phase to the grids of thyratrons 38, 39 having anodes fed in antiphase from the A.C. supply 23 through the transformer 40, and the primaries of the saturable reactors 47, 48, whose secondaries are connected in bridge with the balanced secondary of the supply transformer 46. When the optical axis of the lens system is normally positioned to intersect the templet curve, the voltage pulses from the photocell due to the traversals of the curve image respectively coincide with the maxima of the positive half cycles of the thyratron anode voltages which strike at the respective 90 degrees phase points, so that the mean anode currents are the same. Deviations of the optical axis from the curve cause one end of the traversing image to pulse the photocell early, and the other end to pulse it late, in respective dependence on the sense of the deviation so that the thyratrons respectively strike early and late in the half cycles of positive anode voltage, and the mean anode currents are unbalanced in accordance with the sense of the deviation. The saturable reactor bridge is correspondingly unbalanced to produce an alternating signal dependent in magnitude and phase sense on the deviation of the optical axis. The signal energizes the stator winding 45a of a two-phase Selsyn whose quadrature stator winding 45b is energized from a potentiometer across the A.C. supply, and which has a rotor driven from the spindle 16 on which the scanner arm is pivoted. The rotor winding 45c develops an alternating signal which is applied in antiphase by a balanced transformer 50 to the grids of valves 51, 52, whose anodes are supplied with antiphase alternating voltages from the supply through a balanced transformer 53, and which operate as a phase sensitive rectifier developing a direct voltage EY controlling the speed and direction of rotation of the Y motor through a known balanced thyratron control circuit energizing the split field windings of the Y motor so that when EY is maximum the motor runs full speed in one direction, while when EY is minimum it runs full speed in the opposite direction. For the mid value of EY the motor is at rest. The quadrature winding 45d of the Selsyn rotor supplies'a similar phase sensitive rectifier 55 which develops a direct voltage EX controlling the X motor' through a similar power amplifier. The X and Y motors directly drive lead screws to traverse the templet table in co-ordinate directions, and are connected through Selsyn linkages 9, 10, 11, 12 to drive similar lead screws traversing the milling worktable 5. If the radial arm 15 supporting the scanner deviates from a position normal to the templet curve, the phase of both the photocell pulses is advanced or retarded relative to the positive half cycles of the anode voltages of thyratrons 38, 39 in dependence on the sense of the deviation, so that the mean current of both thyratrons increases or decreases relatively to the normal value. A corresponding direct voltage is developed across the cathode resistor 70, and controls the direction and speed of rotation of the motor 43 through a balanced thyratron centrol circuit similar to the above, to maintain the radial arm supporting the scanner in perpendicularity to the templet curve. When the scanner axis intersects the curve and its radial arm is perpendicular thereto, the control voltage EX is at maximum and EY is at zero so that the X motor runs at full speed and the Y motor is at rest. The movements of the templet and milling tables are thus controlled so that the optical axis of the scanner follows the curve, and the milling cutter reproduces the curve profile on the workpiece. If the latter is to be cut the same size as the templet, the distance of the scanner optical axis from the spindle is made the same as the cutter radius, but the scanner is adjustable along the slot in the arm 15 to vary the scale of the reproduction.