GB2163277A

GB2163277A - Controlling the tool electrode in an electrical discharge machine tool

Info

Publication number: GB2163277A
Application number: GB08519826A
Authority: GB
Inventors: Masanori Konno; Kiyoshi Imai; Toshihiro Dobashi
Original assignee: Amada Co Ltd
Current assignee: Amada Co Ltd
Priority date: 1984-07-20
Filing date: 1985-08-07
Publication date: 1986-02-19
Also published as: GB8519826D0; GB2163277B

Abstract

A tool electrode control apparatus for an electrical discharge machine tool comprises an electrical discharge condition judgment circuit (39) which judges the condition of the electrical discharge between a tool electrode (27) and a workpiece (W) from the waveform of the electrical discharge and outputs a pulse signal to indicate that the electrical discharge is abnormal, an electrode up-down motion command circuit (43) which outputs a pulse signal instructing the tool electrode to move up or down when the number of abnormal electrical discharge waveforms reaches a pre-set value, a working gap detection circuit (59) which detects when the tool electrode is at its lowest machining point so that the gap between the tool electrode and the workpiece is at a minimum, an electrode control circuit (45) which causes the tool electrode to move up and down in accordance with the output from the working gap detection circuit (59) and the electrode up-down motion command circuit (43), and a peak hold circuit (73) which detects the lowest tool electrode position when machining starts and stores this position in memory. The speed at which the electrode is moved may be increased if the advance or retreat of the electrode has continued for a predetermined time. A "runaway point" to which the electrode is retracted may be preset. <IMAGE>

Description

SPECIFICATION A method of and apparatus for controlling the tool electrode in an electrical discharge machine tool This invention relates to a method of and apparatus for controlling the tool electrode in, for example, an electrical discharge machine tool used for machining workpieces such as steel workpieces and, in particular, to a control device which, when the waveform of the electrical discharge between a tool electrode and workpiece becomes abnormal, is capable of rapidly eliminating the abnormality.

In a typical electrical discharge machine tool which machines workpieces such as steel workpieces, the tool electrode and the workpiece are placed opposite each other, separated by a very small gap, in an electrically insulating dielectric fluid such as kerosene.

Voltage pulses are repeatedly applied between the tool electrode and the workpiece to machine the workpiece by melting and removing metal from its surface. In this type of electrical discharge machine tool, electrical discharges are repeated at the pulse frequency and the tool electrode gradually sinks into the workpiece. At this time, if the gap between the tool electrode and the workpiece is too large, an electrical discharge will not take place.

Conversely, if the gap is too small, a short circuit will occur. Therefore, the electrical discharge machine tool contains a servomechanism which accurately controls the tool electrode position to ensure that the correct gap is maintained between the tool electrode and the workpiece.

In an electrical discharge machine tool of the type described above, when fine metal waste products removed from the surface of the workpiece by the machining and products of decomposition of the dielectric fluid are present between the tool electrode and the workpiece, the electrical insulation between the tool electrode and the workpiece is reduced and normal discharges do not take place, causing loss of machining accuracy.

Consequently, various methods have been employed to remove metal waste products from between the tool electrode and the workpiece. For example, if it is possible to open a hole for the dielectric fluid in the tool electrode or the workpiece, dielectric fluid can be ejected or sucked out through the hole. However, if it is not possible to open a hole for the dielectric fluid in the tool electrode or the workpiece, it is necessary to use another method such as injecting the dielectric fluid under pressure into the gap between the tool electrode and the workpiece or periodically moving the tool electrode up and down to create a pumping action which removes waste products of the machining from between the tool electrode and the workpiece.

When machining the workpiece to form a blind or dead-end hole such as a hole with a bottom, use of the injection and pumping actions together is effective in removing waste products of machining from the hole. However, in the method which involves periodically moving the tool electrode up and down, even if the waveform of the electrical discharge between the tool electrode and the workpiece is normal, the need to move the tool electrode up and down unavoidably causes a drop in machining efficiency.

In addition, in an electrical discharge machine tool of the type described above, the unit distance through which the tool electrode is moved in a given time is kept to a relatively small value to keep the machining process stable, and the motion is slow. Consequently, if, for example, waste products of machining cause a short circuit between the tool electrode and the inside of a deep hole, the tool electrode cannot be pulled away at high speed, so that considerable time is required before the short circuit is eliminated. Furthermore, when, as described above, the tool electrode must be pulled back through a relatively large distance, considerable additional time is required before the tool electrode can be returned to its original position.

In addition, in an electrical discharge machine tool of the type described above, threedimensional machining is sometimes performed by moving the tool electrode in the X, Y and Z directions with respect to the workpiece. In this type of machining, the tool electrode is controlled in accordance with a main programme which has been previously stored in a computer control unit which controls the electrical discharge machine tool.

Because it is not easy to guide the tool electrode along a path that differs from the path preset by the programme while machining is in progress, if it becomes necessary to remove waste products of machining from between the tool electrode and the workpiece or to eliminate a short circuit, all that is done conventionally is to move the tool electrode up and down, so that effective pumping action cannot be expected when, for example, machining a big hole.

Accordingly, it is an object of the present invention to enable the provision of a method of and an apparatus for controlling the tool electrode in an electrical discharge machine tool, whereby the above disadvantages may be overcome or at least mitigated.

A first aspect of the invention provides a method of controlling a tool electrode of an electrical discharge machine tool, which method comprises raising and/or lowering the tool electrode if an abnormality is detected in a discharge between the tool electrode and a workpiece.

A second aspect of the invention provides apparatus for controlling a tool electrode of an electrical discharge machine tool, which apparatus comprises means for detecting an abnormality in a discharge between the tool electrode and a workpiece, and means responsive to the detecting means for raising and/or lowering the tool electrode, as well as an electrical discharge machine tool whenever provided with such apparatus.

The invention thus enables the provision of a method of and apparatus for detecting the waveform of the electrical discharge between a tool electrode and a workpiece, judging whether it is normal or abnormal, and moving the tool electrode up and down.

The invention further enables the provision of a method of and apparatus for starting the up-down motion of the tool electrode when the tool electrode is at its lowest point during the machining process, and ending the updown motion when the tool electrode has returned to the said lowest point.

In a preferred aspect, the apparatus of the invention comprises an electrical discharge condition judgment circuit, which judges the condition of the electrical discharge between a tool electrode and workpiece in an electrical discharge machine tool, an electrode up-down action command circuit which commands the up-down motion of the tool electrode in accordance with the output from the electrical discharge condition judgment circuit and an electrode control circuit which controls the updown motion of the tool electrode in accordance with the commands from the electrode up-down action command circuit.In addition, a working gap detection circuit which detects when, during the machining process, the tool electrode has reached its lowest point so that the gap between the tool electrode and the workpiece is at a minimum, and a peak hold circuit, which detects the position of the tool electrode at the start of the up-down motion and stores this information, are preferably provided, in which case the electrode control circuit is also responsive to the output from the working gap detection circuit.

The invention still further enables the provision of a method of detecting the machining condition between the tool electrode and the workpiece which makes it possible to advance or withdraw the machining electrode rapidly and continuously.

In order to achieve this object, in a preferred aspect of the method of the invention, when the advance or retreat of the tool electrode has continued for a certain length of time or a certain number of times, the unit distance of tool electrode motion can be increased up to an upper preset limit.

The invention also enables the provision of a method of removing waste products during three-dimensional preprogrammed machining.

To this end, in a preferred aspect of the invention the tool electrode is temporarily moved to a preset point positioned far from the workpiece along the Z-axis, and the tool electrode is moved up and down.

For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a side elevational view of an electrical discharge machine tool in accordance with the invention, Figure 2 is a block diagram of a first apparatus in accordance with the invention, Figure 3 is a waveform diagram illustrating the invention, Figure 4 is a block diagram of a second apparatus in accordance with the invention, Figure 5 is a flow chart illustrating the use of a computer when carrying out the invention, and Figure 6 is a perspective view of a tool of the machine tool of Fig. 1 being controlled in accordance with the invention.

Referring to Fig. 1, the electrical discharge machine tool 1 comprises a box-shaped frame 3, a XV cross-table device 5 which is mounted on the base 3, an upright hollow column 7 mounted on the rear of the base 3, and a machining head 9 which is vertically movably mounted on the hollow column 7. The XV cross-table device 5 comprises a Y-axis motion table 13, which is guided by a guide table 11 on the upper surface of the base 3 and is free to move in the Y direction, and an X-axis motion table 15, which is mounted on top of the Y-axis motion table 1 3 and is free to move in the X direction. The details are not shown in this figure, but the Y-axis motion table 1 3 is moved in the X direction by a Y-axis servomotor 1 7 which is mounted on the guide table 11.The X-axis motion table 1 5 is also moved in the X direction by a X-axis servomotor 1 9 which is mounted on the Y-axis motion table 1 3. Mounted on the X-axis motion table 1 5 is a machining tank 21, inside which is a work table 23 which holds a workpiece W.

The machining head 9 is moved up and down by a servomotor 25 which is mounted on the upper part of the column 7. It has a tool electrode 27, for electrical discharge machining of the workpiece W, which can be freely removed and replaced.

In the above configuration, the tool electrode 27 and the workpiece W are held close to each other, and electrical discharge machining of the workpiece W is performed by electrical discharges which take place between them. At this time, the workpiece is moved under control in the X and Y directions as necessary. In addition, the tool electrode 27 is moved in the Z direction. Consequently, it will be understood that even with an electrode of simple shape, it is possible to perform electrical discharge machining of complicated threedimensional shapes.

Referring to Fig. 2, a very small working gap 29 is maintained between the tool electrode 27 and the workpiece W. At both ends of the tool electrode 27 and the workpiece W, a machining power supply 31, a current limiting resistor 33 and a transistor 35 are connected in series. In addition, at both ends of the said tool electrode 27 and the workpiece W, a detection resistor 37 is connected for the purpose of measuring the voltage between the tool electrode 27 and the workpiece W.

An appropriate voltge is obtained from this detection resistor 37 by voltage division and fed to the input for an electrical discharge condition judgment circuit 39, which judges the condition of the electrical discharges.

Also, connected to the transistor 35 is a pulse generator 41, which controls the ON-OFF action of the transistor 35.

In the configuration described above, when a pulse current is sent from a pulse generator 41 to the transistor 35, ON-OFF action of the transistor 35 corresponding to this pulse current takes place, and an electrical discharge occurs in the discharge gap 29 between the tool electrode 27 and the workpiece W. The discharge voltage waveform in the working gap 29 is detected by the electrical discharge condition judgement circuit 39, and the condition of the electrical discharge is judged. As will be explained in more detail below, the electrical discharge condition judgment circuit permits discrimination among four conditions: no electrical discharge, abnormal electrical discharge, normal electrical discharge and polluted condition.In this embodiment, the output of the electrical discharge judgment circuit 39 is used when an abnormal discharge or polluted condition occurs in the working gap 29.

As is shown in Fig. 2, the output from the discharge condition judgment circuit 39 and the output from the pulse generator41 are connected to an electrode up-down motion command circuit 43, which outputs a signal that commands the tool electrode 27 to move up and down. The output of this electrode updown motion command circuit 43 is connected to an electrode control circuit 45, which controls the said servomotor 25 to move the tool electrode 27 up or down.

The electrode up-down motion command circuit 43 is comprised mainly of an OR gate 47, counter 49, comparator 51, AND gate 53 and preset counter 55.

In more detail, when an abnormal discharge condition occurs in the working gap 29, that is to say when the insulation condition between the electrodes is not normal, a pulse 39a which indicates that an abnormal discharge condition exists is output.When a polluted condition exists in the working gap 29, a pulse 39b which indicates that a polluted condition exists is produced. These pulses are connected from the discharge condition judgment circuit 39 to the two inputs of the OR gate 47. The output of this OR gate 47 is connected to the counter 49, and the output of the counter 49 is connected to one of the inputs of the comparator 51. The counter 49 counts the number of inputs from the OR gate 47 and outputs the cumulative total A to the comparator 51. The comparator 51 compares the count value A to a set value B which was preset from another input terminal.When A > B a pulse signal is output. This pulse signal is connected to one input terminal of the AND gate 53; the other input terminal is connected to the output of the preset counter 55. The input of the preset counter 55 is connected to the output of the pulse generator 41. When the cumulative number of pulses input from the pulse generator 41 reaches the set count C, a pulse signal is output to the AND gate 53.

The output of the pulse generator 41 is also connected to the base of the transistor 35.

The output of the AND gate 53 is connected to the electrode control circuit 45. This circuit drives the servomotor 25.

In addition, the output of the preset counter 55 is connected to a delay circuit 57. The output of the delay circuit 57 is connected to the counter 49 and the reset input of the preset counter 55.

In the configuration described above, when a current pulse travels from the pulse generator 41 to the transistor 35, the transistor 35, as shown in Fig. 3(a), goes ON or OFF. When transistor 35 goes On or OFF, the working gap 29 between the tool electrode 27 and the workpiece W sycnhronizes with the ON-OFF action of the transistor 35 and an electrode discharge takes place, with one of several possible voltage waveforms, depending on the condition between the electrodes, as shown in Fig. 3(b).

The discharge condition judgment circuit 39 compares the voltage level E, at the leading edge of the pulse applied between the electrodes and the voltage level E2 upon oscillation of a detection pulse, time t later than the leading edge, with the two reference voltage levels V, and V2, and thus, as shown in Figs. 3(d) through (g), discriminates among the four conditions of no pulse, abnormal pulse, normal pulse and polluted condition.

Specifically, when the voltage level E, at the leading edge of the pulse applied between the electrodes is larger than the reference voltage level V1, and in addition the voltage level E2 is smaller than the reference voltage level V2, the discharge is judged to be normal.

When the voltage level E, at the leading edge of the said applied voltage pulse and the said voltage level E2 are both smaller than the reference voltage level V2, it means that the insulation in the gap between the electrodes has dropped, and the condition of the discharge is judged to be abnormal. When the voltage level E, at the leading edge of the said applied pulse and the said voltage level E2 are both larger than the reference voltage V1, the gap between the electrodes is too large and it is judged that no discharge is taking place.

When the voltage level E, at the leading edge of the applied pulse is of magnitude between V, and V2, there is not sufficient insulation between the electrodes and a polluted condition is judged to exist due to waste products of machining in the gap between the electrodes.

When an abnormal pulse condition or polluted condition exists, there are waste products of macining in the gap between the electrodes, causing the insulation to be inadequate, so it is necessary to remove the waste products.

Consequently, when the discharge condition judgment circuit detects a waveform which indicates an abnormal discharge or a polluted condition, and output pulses 39a and 39b are output to the OR gate 47, ali pulses of both types are input from the OR gate 47 to the counter 49, and the number of the pulses is counted by the counter 49. The cumulative count value A counted by the counter 49 is output to the comparator 51. In the counter 51 the cumulative count value A is compared to the set value of B. When A > B a pulse signal is output from the comparator 51 to the AND gate 53.

Meanwhile, when the number of pulses from the pulse generator reaches the count value C set in the preset counter 55, a pulse signal is input from the preset counter 55 to the AND gate 53. At the same time, a pulse signal which resets the counter 49 and the preset counter 55 is input through the delay circuit 57. Consequently, if the total number of abnormal discharge condition waveform pulses and polluted condition waveform pulses reaches a fixed value within a certain time, pulse inputs enter both of the input terminals of the AND gate 53, and a pulse signal is output from the AND gate 53 to the electrode control circuit 45. The electrode control circuit 45 in turn controls the servomotor 25, causing the tool electrode 27 to move up or down by an appropriate amount.

This creates a pumping action between the tool electrode 27 and the workpiece W, removing waste products from the working gap 29 and making it possible for normal discharges to occur.

Fig. 4 shows a second embodiment of this invention. it is the embodiment shown in Fig.

2 with the addition of an working gap detection circuit 59 and a tool electrode setting circuit 61 which sets the position of the end the up-down movement of the tool electrode 27 to the lowest point reached in the electrical discharge machining process.

The working gap detection circuit 59 has a resistor 63, a capacitor 65 and a comparator 67 which compares the voltage V3 produced by averaging the voltage obtained by voltage division with the detection resistor 37, the averaging being done by the averaging resistor 63 and the capacitor 65, with the set voltage V0 which corresponds to the time when the working gap 29 is a minimum.

When the two are equal, the comparator 67 outputs a pulse signal. The output of the comparator 67 is connected to one of the inputs of the AND gate 69. The output of the AND gate 53 is connected to the other input of this AND gate 69. The output of this AND gate 69 is connected to the electrode control circuit 45.

In the configuration described above, the up-down motion of the tool electrode 27 starts when the tool electrode 27 is at its lowest point during the machining process.

This means that the pumping effect created by the up-down motion of the tool electrode 27 will be large. Accordingly, the working gap 29 is cleaned rapiddly.

The tool electrode setting circuit 61 has a linear scale 71 which detects the present position of the tool electrode 27, a peak hold circuit 73 which stores the lowest position of the tool electrode 27 in memory among the position detection signals from the linear scale 71 during the electrical discharge machining process, and a comparator 75 which compares the input value from the peak hold circuit 73 with the input value from the linear scale 71 which indicates the present position of the tool electrode 27 When the two agree, the comparator 75 outputs a stop signal to the electrode control circuit 45. That is to say, in this configuration the tool electrode 27, which is moved up and down to create pumping action, is stopped at the lowest point reached during the machining process.This insures that the pumping effect created by the up-down motion of the tool electrode 27 will be large, so that the working gap 29 is cleaned out rapidly, and at the same time it is easy to start the subsequent electrical discharge machining.

As explained above, when the working gap 29 between the tool electrode 27 and the workpiece W is polluted by waste products from machining, the working gap 29 can be cleaned by moving the tool electrode up and down with respect to the workpiece W. After the working gap 29 has been cleaned in this way, electrical discharge machining is started again. By gradually advancing the tool electrode 27 in the direction of the workpiece W, electrical discharge machining is performed.

However, in order for electrical discharge machining to be performed efficiently, it is necessary for the tool electrode 27 to be controlled as it goes through various actions such as advancing, stopping and retreating corresponding to the electric discharge condition of the working gap 29. In this case, if the advance or retreat of the tool electrode 27 continues over a certain time or over a certain number, if the unit distance which the tool electrode 27 is moved by each command from the computer control device remains fixed, it becomes difficult to move it quickly.

Therefore, in this embodiment, there is a means such that when advance or retreat commands for the tool electrode 27 continue, the unit distance which the tool electrode 27 moves is increased within a preset limit to move the tool electrode 27 quickly.

In more detail, when the electrical discharge machine tool is controlled by a computer, and interrupts to the main program occur and a motion command calculation program is started on a regular cycle, processing proceeds according to the flow chart shown in Fig. 5.

In step S1, based on the result of the immediately preceding machining condition detection, it is judged whether the next command should be an advance, a stop or a retreat.

When it is judged that the next step should be an advance, in step S2 it is investigated whether the previous motion command was for an advance or a retreat. If it was other than an advance (either a stop or a retreat), then the current advance is not a continuation, so the number of continuous advances counter CF is cleared and the unit distance AX is set to the initial value AXO If the preceding motion command was for an advance, then in step S3 it is investigated whether the number of continuous advances counter CF has reached the set value CFma,. If the counter CF has not reached the set value Coma, the counter CF is incremented by 1 and the unit distance AX is not changed.If the counter CF has reached the set value CFn,aX, it is judged that the advance commands have continued for the set time.

In this case, in step S4 it is investigated whether or not the unit distance AX has reached its maximum value AXmaX. If the maximum value AXmax has been reached, then the unit distance is not increased. If the maximum value AXmax has not been reached, then in step S5 the unit distance AX is increased by the set value AX.

In step S6 a command for an advance by the unit distance AX is output.

In step S7 a flag which indicates the direction of the preceding motion command is set to advance.

If, in the step S1, it is judged that the next command should be for a stop, then in step S8 the flag which indicates the direction of the preceding movement is set to stop.

If, in the step S1, it is judged that the next command should be for a retreat, then processing proceeds in a manner analogous to the case of an advance. That is to say, it is investigated whether the preceding motion command was for an advance or a retreat; if it was not a retreat then the number of continuous retreats counter CB is cleared, and the unit distance AX is set to the initial value AXO.

If the preceding motion command was for a retreat, it is investigated whether or not the number of continuous retreats counter CB has reached the set value CBrnax. If the counter CB has reached the set value CBrnax, the unit distance AX is increased by the set value Ax.

On the other hand, if the unit distance AX has reached its maximum value AXm2x then it is not increased. If the counter CB has not reached the set value CBmax, the counter CB is incremented by 1 and the unit distance AX is not changed. Next, a command for retreat by the unit distance AX is output, and the flag indicating the direction of motion is set to retreat.

Thus, when the machining condition in the working gap is normal the unit distance is small while if it is abnormal it is possible, for example by continuation of retreat commands, to control the motion of the tool electrode so that it is moved a large distance. Consequently, when an abnormal condition such as a short circuit occurs during electrical discharge machining, the abnormal condition can be removed rapidly by increasing the speed of motion of the tool electrode.

Referring to Fig. 6, when electrical discharge machining of a complicated shape is performed by moving a tool electrode 27 of simple shape which is installed in an electrical discharge machine tool in the X, Y and Z directions with respect to the workpiece W, the position of the tool electrode 27 is controlled by a computer. The path of the tool electrode 27 and other conditions are programmed in advance, and in addition a target position called "runaway point" is preset; no matter where the tool electrode 27 is located it can be moved quickly to and from the runway point at a preset speed either along a straight line in space or by independent movements in the three coordinate directions. Commands for round-trip movement of the tool electrode 27 to and from the runaway point can for example be given at fixed intervals.

Also, the back-and-forth motion of the tool electrode can be triggered automatically based on the result of detection of the condition of the electrical discharge in the working gap between tool electrode 27 and the workpiece W, thereby improving machining efficiency.

In this embodiment, the program coordinates are designated X, Y and Z and the runaway point is set at point P (O, 0, Z,) on the Z-axis. As the electrical discharge machining progresses, if the direction of back-andforth motion of the tool electrode 27 changes, the position of the runaway point is changed in the program.

Also, depending on the shape to which the workpiece W is being machined, it sometimes happens that there will be an obstacle be tween the tool electrode 27 and the runaway point. In this case, when the back-and-forth motion of the tool electrode 27 is started by a command, it retreats along its path by a distance which is present in the program and then it travels to and from the runaway point.

In this way a collision between the tool electrode 27 and the obstacle is avoided.

That is to say, when electrical discharge machining of a complicated shape is done by moving the tool electrode 27 in the X, Y and Z directions with respect to the workpiece W, when a command is given for the tool electrode 27 to move back and forth, the tool electrode 27 moves to and from a runaway point which is spaced from the workpiece.

Consequently, the tool electrode 27 moves a considerable distance and stirs up considerabie movement of the dielectric fluid, removing the waste products of machining from the working gap. Accordingly, there will be few occurrences of abnormal electrical discharges between the tool electrode and the workpiece, improving the machining efficiency.

Claims

1. A method of controlling a tool electrode of an electrical discharge machine tool, which method comprises raising and/or lowering the tool electrode if an abnormality is detected in a discharge between the tool electrode and a workpiece.

2. A method according to Claim 1, which comprises starting to raise and lower the tool electrode when the tool electrode is at the lowest point thereof during electrical discharge machining and finishing raising and lowering the tool electrode when the tool electrode returns to the said lowest point thereof.

3. A method according to Claim 1 or 2, which comprises detecting whether there is an abnormality in a waveform of the discharge between the tool electrode and the workpiece.

4. A method of controlling a tool electrode of an electrical discharge machine tool, which method comprises advancing or retreating the tool electrode in accordance with a character istic of an electrical discharge between the tool electrode and a workpiece, and increasing the speed at which the tool electrode is advanced or retreated if the tool electrode has been advanced or retreated continuously for a predetermined length of time and/or for a predetermined number of steps.

5. A method according to Claim 4, wherein the characteristic of the electrical dis charge between the tool electrode and the workpiece is detected at fixed time intervals, after each of which the tool electrode is ad vanced or retreated through a unit distance or kept still, and, when advancing or retreating of the tool electrode has continued for a predetermined length of time or a predeter mined number of the said fixed time intervals, the unit distance through which the tool electrode is moved each time is increased.

6. A method according to Claim 5, wherein the unit distance is not increased beyond a preset maximum value.

7. A method of controlling a tool electrode of an electrical discharge machine tool, which method comprises moving the tool electrode, either in accordance with a characteristic of an electrical discharge between the tool electrode and a workpiece or after a predetermined time interval, to a position outside the workpiece, and reciprocating the tool electrode.

8. A method according to Claim 7, wherein the tool electrode is moved in three dimensions with respect to the workpiece during electrical discharge machining.

9. A method according to Claim 7 or 8, further comprising varying the said position outside the workpiece.

10. Apparatus for controlling a tool electrode of an electrical discharge machine tool, which apparatus comprises means for detecting an abnormality in a discharge between the tool electrode and a workpiece, and means responsiveto the detecting means for raising and/or lowering the tool electrode.

11. Apparatus according to Claim 10, wherein the detecting means is adapted to produce a signal if a said abnormality is detected and the means for raising and/or lowering the tool electrode is adapted to raise and/or lower the tool electrode when a predetermined number of the said signals have been produced.

1 2. An electrical discharge machine tool whenever provided with apparatus in accor dance with Claim 10 or 11.

1 3. A method of controlling the tool elec trode of an electrical discharge machine tool, substantially as hereinbefore described with reference to any one of Figs. 2, 4, 5 and 6 of the accompanying drawings.

14. Apparatus for controlling the tool elec trode of an electrical discharge machine tool, substantially as hereinbefore described with reference to, and as shown in, Fig. 2 or 4 of the accompanying drawings.

1 5. An electrical discharge machine tool, substantially as hereinbefore described with reference to, and as shown in, Figs. 1 and 2 or 4 of the accompanying drawings.

16. Any novel feature or combination of features described herein.