ELECTRO-DISCHARGE MACHINING APPARATUS

Abstract: An electro-discharge texture machining apparatus includes an electrode carrier (10) movable towards and away from a workpiece surface and arranged to "scan" such surface. An electrode (12) on the carrier is supplied with short fixed duration high-voltage initiating pulses and longer fixed (but adjustable) duration low-voltage maching pulses each commencing with an initiating pulse.

Description: This invention relates to an electro-discharge machining apparatus and also to a method of performing such machining. In the steel rolling industry, there is a requirement to provide steel rolls with a rough or textured surface. During rolling, such rolls form a textured surface on steel strips and such a surface plays an important role during subsequent processing operations. For example, when making panels for the automotive industry, the texture of the steel strip influences the behaviour of the strip during pressing and also the appearance of a final paint finish. The conventional shot-blast texturing method for steel rolls is difficult to control and so it is difficult to meet the stringent requirement of steel users with this method. There has been proposed an apparatus for texturing steel rolls by electro-discharge machining. This apparatus comprises a line of electrodes mounted at spaced positions along the roll and an individual actuator for each electrode to move the electrode towards and away from the roll so as to achieve the correct gap between the electrode and the roll. The provision of an individual actuator for each electrode ensures that the electrodes adapt to the camber of the roll and satisfactory results are obtained with this apparatus. However, this apparatus suffers from the disadvantage that it requires a large number of actuators which result in the apparatus being complicated and expensive. It has also been proposed in GB-B-2005582 to utilize a single electrode which is traversed along the whole length of roll. The radial position of the electrode relative to the roll is varied in accordance with the machining current detected. Two current sources of high and low voltage are employed and these are impulse operated. In operation a sequence of pulses consisting of a single low voltage pulse followed by a plurality of high voltage pulses is repeatedly applied to the electrode. During the low voltage pulse radial feeding of the electrode towards the workpiece is carried out until current starts to flow. The low voltage pulse is then terminated. No radial movement of the electrode occurs during the remainder of each sequence. It will be noted that the duration of the low voltage pulse (and hence the duration of suspension of high voltage pulses) is indeterminate. Such an arrangement can give rise to difficulties in obtaining a readily repeatable, consistent texture on the roll and it is an object of the present invention to provide a method and an apparatus in which this difficulty is avoided. In accordance with the invention there is provided a method of electro-discharge texture machining a workpiece which comprises scanning an electrode over the surface to be textured whilst servo-controlling the proximity of the electrode to the surface, whilst repeatedly applying electrical pulses to the electrode from high and low voltage sources characterised in that the electrical pulses comprise low voltage machining pulses and high voltage initiation pulses, said low voltage machining pulses being applied at a fixed (but adjustable) frequency and for fixed (but adjustable) duration and at a fixed (but adjustable) current level, and said high voltage initiating pulses being of relatively short fixed duration coinciding with the commencement of said low voltage pulses, and in that servo-control of the proximity of the electrode to the surface is carried out continuously. The invention also resides in an apparatus comprising a workpiece support, on electrode support supporting at least one electrode for movement towards and away from the surface the workpiece and also in a direction generally parallel to the surface of the workpiece, servo-control means for controlling movement of the electrode support towards and away from the workpiece in accordance with machining conditions detected, scanning means for scanning the electrode over the surface of the workpiece to be textured, and high and low voltage sources from which electrical pulses are repeatedly applied under the control of a pulse generator circuit to the electrode, characterised in that the generator circuit is arranged to provide low voltage machining pulses at a fixed (but adjustable) frequency and for a fixed (but adjustable) on duration and also to provide high voltage discharge initiation pulses of relatively short fixed duration coinciding with the commencement of the low voltage pulses, in that there is provided a current controller for controlling the machining pulse current to a fixed (but adjustable) level and in that said servo-control means is arranged to operate on a continuous basis to maintain a desired distance between the electrode and the workpiece. In the accompanying drawings: Figure 1 is a section view of an electrode holder for use in one example of an electrodischarge machining apparatus in accordance with the invention; Figure 2 is a plan view of the electrode holder of Figure 1; Figure 3 is a diagrammatic sectional view of an actuator which carries the electrode holder; Figure 4 is a diagram showing the complete apparatus; Figure 5 is a graph showing waveforms of machine and initiation pulses used in the apparatus. Figure 6 is a diagram of a pulse generator included in the circuits shown in Figure 4; Figure 7 is a diagram of a current selector circuit, and Figure 8 is a fragmentary view showing another example of the invention. Referring firstly to Figures 1 and 2 the electrode holder includes a body 10 formed of a suitable electrical insulating material (such as "Tufnol" (RTM)). This body is attached to the end of a tubular stem 11. A plurality (in this case five) of separate electrodes 12 are secured to the body 10. These electrodes are formed of a suitable metal (chosen to suit the material of the rolls to be treated) and are secured to the body in such a manner that they do not touch one another. Individual metal mounting plates 13 are used for this purpose and each such plate 13 has a screw terminal 13 a thereon for making an individual electrical circuit connection to that electrode. Each electrode 12 has a longitudinally extending passageway 12 a for dielectric liquid supply. This passageway opens on to the end face of the electrode and communicates with a manifold 10 a in the body 10. This manifold 10 a communicates in turn with the interior 11 a of the stem 11. Referring now to Figure 3, a diagrammatic representation of a servo-actuator which is used to displace the electrode holder is shown. The electrode holder stem 11 is fitted into a socket in member 15 which is attached to a block 16 movable by means of a screw 17 and a ball nut 18. The block 16 has a connector 19 through which the dielectric liquid is supplied. The block includes passageways (not shown in detail) which connect this connector to the passage in stem 11 and to four dielectric liquid nozzles 20 which are adjustably mounted on the block 16 to enable the nozzles to be positioned close to the workpiece and provide an adequate flow of dielectric liquid to the working area. The screw 17 is driven, through a belt drive 21 by a motor 22, the axis of the shaft of which is spaced from and parallel to the axis of the screw 17. Figure 4 is an overall diagram of the apparatus. The servo-actuator is mounted on a compound slide 29 on a lathe which has supports (not shown) supporting and rotating a roll 30 to be treated. A stepper motor 31 drives the slide 29 longitudinally of the roll, i.e. in a first direction parallel to the roll axis, and another stepper motor 32 drives the slide 29 towards and away from the roll axis in a second direction perpendicular to the roll axis. The servo-actuator is so mounted that the electrode holder is movable in a direction parallel to said second direction. Control of the apparatus is effected by means of a computer which receives inputs from a front panel 34 including switches and a keypad and which drives a VDU display 35 if required. The computer provides control signals for the stepper motors 31, 32, via a stepper motor controller 36. It also provides digital signals to control the on and off durations for the electrical discharges, the current level to be applied and the gain of the servo-loop controlling the motor 22. The digital signals controlling on and off durations are supplied to a digitally controlled pulse generator circuit 37. This circuit, which is shown in more detail in Figure 6, includes four 8 bit latches 40, 41, 42, 43 which are loaded with the computer output word to store 32 bits of data in total. Latches 40 and 42 are loaded respectively with the lowest and highest 8 bits of a 16 bit data word representing the number of micro-seconds for which the discharge current is required to be on and latches 41 and 43 are likewise loaded with a 16 bit data word representing the off time duration in each cycle. A 16 bit counter 44 is loaded periodically with the on or off time duration word and is counted down to zero by clock pulses from a 1MHz oscillator 45. At zero count a flip-flop circuit 46 is caused to change state. The Q and Q outputs of this flip-flop circuit are connected to terminals of the latches so that the appropriate pair of latches provide outputs to the counter and a monostable circuit 47 provides a signal to a preset terminal of the counter to cause these outputs to be loaded. The Q output of the flip-flop circuit 46 provides an M/C pulse and another monostable circuit 48 connected to be triggered by the rising edge of the M/C pulse provides a 3µS INIT pulse coinciding with the start of each M/C pulse. A logic circuit 49 receives these M/C and INIT pulses and routes these (under appropriate constraints which are not relevant to this description) to a bank of current control circuits 50 (one for each electrode). Thus, in the present example, everything shown within the box 50 is repeated for each electrode. Each current control circuit 50 contains a current switch selector circuit 51 which receives an 8 bit data input from a current code circuit 53 (common to all the current control circuits 50) which itself receives an 8 bit current code input from the computer. Each circuit 50 includes a plurality of low voltage current supply switch circuits 54 and a high voltage switch supply circuit 55. The circuits 54 control the connection of a 60V supply 56 to an output rail 57 and circuit 55 controls the connection of a 300V supply to the rail 57. The circuits 54 are connected to the rail 57 by resistors 59 of different values so that, by selecting which ones of the circuits 54 are to be energised, the current level can be controlled. For example, the resistors 59 may have values (reading from left to right) of 40 ohms, 20 ohms, 20 ohms, 8 ohms, 8 ohms, and 8 ohms. These are chosen so that, in use, the circuits 54 can pass 1 amp, 2 amps, 2 amps, 5 amps, 5 amps and 5 amps respectively, so that by selecting appropriate combinations every current level from 1 amp to 20 amps in one amp steps can be obtained. The circuit 51 consists basically of an 8 bit input buffer 60 for the digital input and an opto-isolator 60 a for each buffer output. The M/C and INIT signals are also received by the circuit 51. These also drive opto-isolators 61 and 62. The opto-isolator 61 output is connected to the inputs of eight NOR gates 63 which have inputs from respective ones of the individual opto-isolators 60 a and the outputs of these gates are used to drive the individual switch circuits 54. The opto-isolator 62 drives the high voltage switch circuit 55. The opto-isolators are used to isolate the computer and the other control circuits not included in circuit 50 from the switching circuits. This enables the switching circuits to be powered by floating power supplies which are used because there are some conditions in which it is desired to reverse the polarity of the electrodes. The roll 30 is always at earth potential and a reverse polarity switch 65 is used to connect either the rail 57 or a return conductor 65 associated with the supplies 56, 58 to the roll and the other to the electrode. This switch 65 is controlled by the computer 33. The current code circuit 53 consists basically of an 8 bit latch and buffer. It may include a circuit for disabling the latch output if an RF detector 67 outputs a d.c. signal representing the r.f. picked up near the electrodes and indicating that arcing is occurring instead of the required spark discharge. For controlling the servo-actuator, a gap voltage detector 67 is provided in each circuit 50. This detector is basically a sample and hole circuit controlled by the logic circuit 49 and providing an opto-coupled output representing the gap voltage for the corresponding electrode at the sampling time. The outputs from the five detectors 67 are supplied to a low wins gate 68 so that the lowest voltage received from the detectors 67 is supplied as a feedback signal to the servo-control for the motor 32. The low wins output is supplied to a comparator 69 which provides a "short-circuit" signal to the logic circuit 49 if the voltage is below a set minimum. The low-wins output is also supplied to a variable gain amplifier 70 which is digitally controlled by an 8 bit signal from the computer 33 and amplifies the error between the low wins output and an adjustable reference voltage. In the event of a short circuit, the logic circuit overrides the variable gain amplifier in the servo-control to cause the electrode holder to move a short distance away from the roll. The M/C pulses are also suppressed for a time. In the absence of a short circuit indication, however, the servo-control acts to move the electrode holder away from the roll if the detected voltage is below the reference level and towards the roll if it is above the reference level. When the voltage is actually at the reference level and there is no error, the spacing of the electrodes from the roll is correct, although in practice the electrode holder will be repeatedly moved in and out, the amplitude of the movement and hence the texture applied to the roll depending on the gain of the variable gain amplifier. It will be appreciated by the skilled reader that the use of the "low wins" gate arrangement in the servo-feedback loop has the effect of controlling the electrode position such that all the electrodes save the one which is actually in control, are further from the roll than that electrode. The use of the high voltage INIT pulses at the start of each M/C pulse ensures that a discharge is established between each of the electrodes and the roll. In use, the roll is positioned on the lathe and the slide 29 is then moved to a starting position. The servo-mechanism is initially set up to drive the electrode holder towards the roll until the discharge commences when the normal closed loop operation is initiated. The computer controls the on and off duration for the discharge and the current level during each pulse, as well as determining the gain of the variable gain amplifier in the servo-loop. All these variables can be adjusted by the operator to obtain the surface finish required. The electrode holder is moved along the rotating roll so that the whole surface of the roll is treated. Figure 1 shows an electrode holder with a row of rectangular section electrodes arranged in a straight line. Many variations of electrode shape and arrangement are, however, possible. Electrodes could, for example, be of circular or flat section, and they could be arranged in any desired pattern. The use of several electrodes ensures that a good rate of material removal is possible. As shown in Figure 8, there may be several electrode holders 110 on independently controlled actuators 110 a each with its own servo-motor 122 and each having its own distinct servo-control loop as described above. Other arrangements are possible in which the roll and the electrodes are positioned in a dielectric bath instead of using the nozzles described above. The control circuits described above are also applicable to more general types of electro-discharge machining as distinct from roll surface treatment. In such cases the circuit of Figure 6 may be changed to provide a plurality of the INIT pulses during the period of each M/C pulse. In addition the control may include further high voltage switch devices which are energised for the whole duration of each M/C pulse in the event of the discharge failing to be established by the low voltage M/C pulses alone.

Claims: A method of electro-discharge texture machining a workpiece which comprises scanning an electrode (12) over the surface to be textured whilst servo-controlling the proximity of the electrode to the surface, whilst repeatedly applying electrical pulses to the electrode from high and low voltage sources (55 and 54) characterised in that the electrical pulses comprise low voltage machining pulses and high voltage initiation pulses, said low voltage machining pulses being applied at a fixed (but adjustable) frequency and for fixed (but adjustable) duration and at a fixed (but adjustable) current level, and said high voltage initiating pulses being of relatively short fixed duration coinciding with the commencement of said low voltage pulses, and in that servo-control of the proximity of the electrode to the surface is carried out continuously. An electro-discharge machining apparatus for carrying out the method claimed in Claim 1 comprising a workpiece support, on electrode support (10) supporting at least one electrode (12) for movement towards and away from the surface the workpiece (30) and also in a direction generally parallel to the surface of the workpiece (30), servo-control means (68, 70, 22) for controlling movement of the electrode support (10) towards and away from the workpiece (30) in accordance with machining conditions detected, scanning means (31) for scanning the electrode over the surface of the workpiece (30) to be textured, and high and low voltage sources (56, 58 respectively) from which electrical pulses are repeatedly applied under the control of a pulse generator circuit (37) to the electrode (12), characterised in that the generator circuit (37) is arranged to provide low voltage machining pulses at a fixed (but adjustable) frequency and for a fixed (but adjustable) on duration and also to provide high voltage discharge initiation pulses of relatively short fixed duration coinciding with the commencement of the low voltage pulses, in that there is provided a current controller (51, 53) for controlling the machining pulse current to a fixed (but adjustable) level and in that said servo-control means (68, 70, 22) is arranged to operate on a continuous basis to maintain a desired distance between the electrode (12) and the workpiece (30). Apparatus as claimed in Claim 2 in which there is provided a high voltage switch circuit (55) and at least one low voltage switch circuit (54), said pulse generator circuit (37) providing said low voltage machining pulses to said at least one low voltage switch circuit, and including an edge triggered circuit (48) for controlling said high voltage switch circuit. Apparatus as claimed in Claim 3 in which said pulse generator circuit includes a digital counter (44), circuit means (40 to 43) for periodically presetting said counter to a count state representing a required on duration or a required off duration, and pulse source means (45) connected to clock the counter to time out said on and off durations. Apparatus as claimed in Claim 3 in which there are a plurality of low voltage switch circuits (54) in parallel with one another and controlled by a current selection circuit (51) such that selected combinations of switch circuits (54) can be turned on to determine the magnitude of the machining current. Apparatus as claimed in any preceding claim in which gap-voltage detection means (67) is provided to detect a short-circuit condition in which the electrode is touching the workpiece, and inhibiting means for inhibiting the pulses of machining current under the control of said gap voltage detector.

Inventors: EL-MENSHAWY, MOHAMMED FAWZY
Applicants: SPARK TEC (EUROPE) LIMITED
Application year: 1988
Main classification: B23H1
IPC codes: B23H9, B23H7, B23H1