Patent GB-769572-A (GB)
Title: Improvements in or relating to method and apparatus for reproducing a pattern outline
IPC: B23Q35/128
Filed: 1954-06-02
Abstract: 769,572. Electric control systems. CINCINNATI MILLING CO. June 2, 1954 [Aug. 7, 1953], No. 16308/54. Class 40 (1). [Also in Groups XXV, XL (b) and XL (c)] In a copying machine controlled by a pattern line, a beam of light is oscillated, e.g. by a cathode-ray tube, across the pattern line, electric impulses produced during the forward and backward sweeps of the beam as it crosses the pattern being compared as to their phase to produce an electric signal of polarity and magnitude corresponding to the sense and magnitude of the deviation of the centre of oscillation of the beam from the pattern line. This signal re-centres the beam relative to the pattern, maintains the beam oscillations perpendicular to the pattern at the crossing point, advances the pattern relative to the beam and at the same time controls the cutting of the work. The beam is biased laterally, i.e. in the direction of oscillation, to compensate for the diameter of the cutting tool, and biased longitudinally to introduce " anticipation ". The pattern line 154, Fig. 7, is provided with loops 450 at sharp corners such as 460, 464 thereof, the beam following the loops so that the corners may be negotiated without slowing down of the driving mechanism. Other corners such as 462 when negotiated automatically cause slowing down of the cutter advance. An arcuate beam scan 198 is used to effect additional anticipation. General. Figs. 1, 2, 3, 4. As applied to a copy-milling machine the work 33 and patterns 34 are fixed to a longitudinally movable table 32. The cutter 39 is carried by a cross-slide 35 which also carries the scanning head 44 adjustably mounted thereon. Movements of the table and cross slide are effected by hydraulically-operated pistons 51, 54, Fig. 3, controlled through sine and cosine valves 58, 59 by an eccentric 57, the co-ordinate movements of table and slide producing a resultant movement of the cutter and work in the direction of maximum eccentricity indicated by an arrow 122. The cutter and pattern feed is thus controlled by adjusting the orientation of the eccentric. The eccentric is formed by ball-races 100, 101, Fig. 2, mounted eccentrically on a sleeve 106 in the scanner head 44, and co-operating with the respective valves 58, 59. The eccentricity of the ball-races 100, 101 may be varied by adjustment of a graduated dial 120 to vary correspondingly the sensitivity of the control system, and the orientation of the eccentric may be adjusted by a hand wheel 108 for manual feed of the cutter or through a gear-wheel 109 during automatic control. The scanning head 44 includes a vertically disposed cathode-ray tube 130, Fig. 4, the usual quadrature deflecting coils 188, 189 therefor being mounted on a yoke 137, angularly adjusted around the C.R.T. neck in synchronism with the angular setting of the eccentric through a gear-wheel 227 which also drives the gear 109. The yoke also carries two adjustably mounted permanent magnets, Figs. 5, 6 (not shown), arranged to produce quadrature magnetic fields aligned respectively with those produced by the deflecting coils, the permanent magnetic fields being adjustable by rotating the magnets through thumbscrews as 216. The quadrature deflecting coils are fed respectively with a pyramidal waveform 179, Fig. 8, and a rectified sine waveform 197, derived from a reference waveform 178 by a circuit arrangement, Fig. 18 (not shown), so that the trace 198, Fig. 7, formed on the C.R.T. screen is arcuate in shape. This trace is biased laterally by one of the permanent magnets to compensate for cutter diameter and is biased longitudinally by the other permanent magnet to effect " anticipation. " The C.R.T. trace 198 is projected by lenses 148 on to the pattern and reflected thereby on to a photo-electric multiplier tube 153. Operation. Figs. 20, 21. The photo-tube, Fig. 19 (not shown), generates negative pulses as the scanning beam 198 crosses the pattern line. These pulses are fed to an inverting valve 256, Fig. 20, which feeds corresponding positive pulses to a clamping and gating circuit 264-267 where the A pulses generated during the forward beam sweeps are separated from the B pulses generated during the backward beam sweeps. The A and B pulses are separately amplified in stages 268, Fig. 20, and 283, Fig. 21, and fed as positive pulses through respective pulse widener stages 288, 289 to stages 314, 321, where the pulses are converted to sinusoidal form and added vectorially at the secondaries of transformers 317, 322. The resultant output from line 325 is an alternating signal voltage varying in magnitude and phase sense according to the magnitude and sense of the lateral deviation of the beam trace 198 from its centred " on line " position relative to the pattern line 154. This A.C. signal is fed through a phase-sensitive rectifier 337-338, Fig. 22, the D.C. output from which is fed to power tubes 351, 353, Fig. 23, forming with tubes 350, 352 a normally balanced bridge circuit in which the coil 354 is normally de-energized. Thus, immediately the beam trace deviates from " on line " the resultant signal upsets the balance of the bridge and energizes the coil 354 with a current proportional to the deviation and of a polarity dependent on the sense of the deviation. The coil 354, Fig. 2, is movable within the field of a pot-magnet 368, and is axially displaced according to its energization to move the plunger 373 of the control valve 365, Fig. 3, causing the hydraulic motor 400 to rotate the eccentric 57 through the gear 227, in the correct sense to steer the beam trace back " on line. " The scanning yoke 137 is also rotated to keep the trace normal to the pattern line at the crossing point. At the same time the armature 377, Figs. 2, 22 of a variable differential transformer 336 is decentred by the off-normal movement of the plunger 373 to introduce a bucking voltage in opposition to the signal voltage at the input of the rectifier 337-338. The piston 373 moves the armature 377 until the bucking voltage cancels out the signal, when the bridge, Fig. 23, is again balanced to de-energize the coil 354. As the trace is steered back towards the " on line " position, the deviation signal is reduced so that the bucking voltage predominates and upsets the bridge balance in the reverse sense to return the plunger 373 of valve 365 towards its centred position where the bucking voltage is zero. " Stiction " is prevented by oscillation of the plunger 373 about its central position by higher harmonics present in the circuit of coil 354. In negotiating a corner of the pattern such as 460, Fig. 7, in the direction 456, the additional A, B pulses produced by the trace as its tips cross the perpendicular line 461, cancel one another so that the trace follows the loop 450 and negotiates the corner without any sharp change of direction. When sharp changes such as the corner 462 are negotiated, the rapid rotation of the motor 400, Fig. 3, causes a blocker valve 61, Fig. 3, to throttle the return flow of fluid from the cylinders 50, 53 associated with the table and cross-feed drives and so reduce the feed rate of the cutter. Fail safe arrangements, Fig. 3. Should the beam trace leave the pattern a magnet 424 is de-energized whereby a spring-biased trip lever 420 is retracted to permit a plunger 401 of a cut-out valve 88 to be moved to the left by its biasing spring 421. This stops operation of table, cross-slide and motor 400. The magnet 424 is normally energized by thyratrons, Fig. 24 (not shown), whose energization is dependent on the A, B pulses. Modifications. The pattern normally consists of a white line 154 on a dark, non-reflecting background. These conditions may be reversed in which case the photo-tube will produce positive output pulses.