Patent GB-1223218-A (GB)
Title: Machine tool control system
IPC: G05B19/18
Filed: 1969-06-19
Abstract: 1,223,218. Programmed electric control. INTERNATIONAL BUSINESS MACHINES CORP. June 19, 1969 [July 5, 1968], No.30979/69. Heading G3N. In a system for a machine tool controlled by a programme e.g. punched tape, a material removing element is moved in a succession of linear scans over a workpiece, the material removal being controlled by a means sensing the flow of energy to the element. Each scan removes an incremental layer of material until a determined depth is removed which may be perfomed by an electric discharge, a laser beam, or a stream of abrasive fluid. The system is particularly suited to the forming of cavities in a plane surface. The element may travel in a series of spaced parallel lines e.g. 43 Fig.4A in one, say X major direction for a first scan, after which the scanning sequence is reversed so that the element scans in a Y major direction, see Fig. 4B (not shown), for a second scan when the sequence reverts to the first scanning pattern et seq. Traverse may be commenced from any corner and either an X major or Y major scan pattern. In the embodiment described an E.D.M. machine has a continuous wire electrode which is advanced at intervals during which the element is moved periodically to a calibration position, the period between calibrations being decreased as the element cavity approaches a depth limit, in order to improve accuracy and uniformity of depth. The energy flow as measured by a current transformer 12 Fig. 1 is used to control the depth of cut and frequency of calibration. A current signal I is supplied through an integrator 13 to a level sensor 15, which at a determined value releases a signal to a sequence controller illustrated in Fig. 3 (not shown) which temporarily disables operation of the machine and moves the element to a calibration position, advances the wire and returns the element to its former position and reinitiates operation. The signal I is also supplied to an amplitude sensor Fig. 2A (not shown) where gates are set dependent on whether the value lies between certain limits e. g. > 3, > 5, > 7 amps. If the current level is too high the electrode wire erodes more quickly hence more frequent calibrations are necessary. If I is > 7 a signal is released to a sequence controller, Fig. 3, and a reset signal is supplied to level sensor 15 to initiate recalibration. The determined value in the level sensor is reduced by the programme arrangement in order that recalibration will be performed more frequently as the required depth is approached. The I > 7 signal also prevents the wire from fusing to the cavity bottom when a high point is encountered, and several calibrate sequences may be performed before the wire has been burned sufficiently to clear an obstruction. A current of less than 3 amps causes a signal to stop the X-Y traverse, electro wire being advanced until I is > 3 when the X-Y traverse is continued. When I is >5 the X-Y traverse is reduced to one quarter of normal running speed by arrangements illustrated in Figs. 2A, 21 (not shown). At the calibration position the wire is advanced until electric contact is established between the wire and a fixed electrode which contact develops a signal to reset gates which return the electrode to its previous position. The element may be alternatively fixed and relative movement performed by traverse of the work table. Servo-drives for all motions may be stepping motors rotatable in increments according to digital input data, and positional sensors likewise generate digital data in the form of 14 bit characters. A punched tape reader 42 distributes numerical data stored in registers. Before a rectangular cavity is to be eroded, four sets of X, Y co-ordinates defining limits in a horizontal plane and the depth co-ordinate Z are fed into position controller and sequencer 26 which controls the machine until the required cavity is cut, or there is an unscheduled event. In either of these two instances control is returned to the tape reader and programmer 42. For irregular or non linear cavities e.g. ellipses, a series of lines of variable length are scanned in one direction say X each line being traversed under the control of the sequencer, after which the next set of line data is drawn from the position controller. A series of spaced lines could be cut and a similar direction scan could follow with the second group of lines interlaced with a first group. This could be followed by further scans in the Y direction. For each new line a new set of data is read from the tape, but generation of non-rectangular contour data by digital computer is envisaged. In addition to cavity machining, parts of a workpiece could be eroded to leave one or more features in relief. A logic system is described in the Specification to control the machine.