DD292156A5 - DEVICE AND METHOD FOR CONTROLLING A MACHINE FOR ELECTROEROSIVELY CUTTING A PRE-ASSEMBLED PATH OF AN ELECTRODE-FORMING WORKSTICK - Google Patents

Abstract

The invention relates to an apparatus and a method for controlling a machine for electrical discharge cutting of a workpiece (2) with a wire electrode (1) according to a predetermined path, wherein a numerical control for cutting an angle or arc is provided such that the points first the track, where changes must be made, calculated, on reaching these points, the relative movement between workpiece (2) and wire electrode (1) interrupted and at least one parameter of the operating conditions is gradually changed until reaching a predetermined value, the change in each stage a change of a further parameter reflecting the conditions in the working gap is effected and the subsequent change of the first parameter is then made when the further parameter reaches a prescribed threshold. After cutting the angle or arc, corresponding stepwise changes of the two parameters are made to reach the initial working conditions and relative movement between workpiece (2) and wire electrode (1). Wire electrode; numerical control; Cutting angles and curves}

Description

^lo ~ tga / 2

where R is the radius of the wire, g is the gap width and α is the angle to be cut.

A method according to any one of claims 9 to 15, characterized in that the step of calculating comprises an arithmetic operation consisting in maintaining the distance I ₀ passed through by holding each first parameter at its final value V according to equation

I ₀ = τ · r

where τ is the angle in radians corresponding to an arc to be cut and r is the radius of curvature of the arc.

For this 3 pages drawings

Field of application of the invention

The invention relates to an apparatus and a method for controlling a machine for electrical discharge cutting of a workpiece with a wire electrode from a piece of raw material.

Characteristics of the known prior art

It is known to automatically change at least one processing parameter in certain processing torques in a machine for electrical discharge cutting with a wire electrode. The most common way is to change the processing speed as a parameter. This is generally done either gradually without interruption of the

Wire feed or in a single step after the wire feed has been interrupted, assuming that gradual parameter changes during a wire feed interruption are generally too time consuming and increase processing time. Therefore, for example, according to US Pat. No. 4,725,706, an abrupt change in operating conditions is always made.

From the Swiss patent 654233 it is known to modify a parameter influencing the geometry of the workpiece to be electroeroshred by a wire electrode, such as the frequency of pulses of a pulse generator, and to measure the average working voltage, the discharge frequency being varied according to a predetermined time function, by working with steps of predetermined duration. In addition, the parameter is continuously reduced to advance the wire, and lowered the feed rate already at the approach to the top of an angle. The change of the processing speed leads to relatively long processing times.

Object of the invention

The aim of the invention is to provide an apparatus and a method for controlling a machine for electrical discharge cutting with a wire electrode, which allow a faster, more precise and therefore economical work.

Explanation of the essence of the invention

The object of the invention is to provide a device and a method for controlling a machine for electrical discharge cutting with a wire electrode, which make it possible to automatically change at least one processing parameter faster processing with good precision, especially in the angular ranges of the piece produced and ' To avoid wire breakage.

According to the present invention, therefore, there is provided an apparatus for controlling a machine for electroerosion cutting a workpiece constituting an electrode with a wire electrode, comprising a generator connected between the two electrodes and outputting voltage impressions, and numerical control for the relative movement between the workpiece and wire guides; the properties of the pulses emitted by the generator and the injection pressure of a working fluid in the electrical erosion gap generated by the wire electrode, characterized by

a numerical control element for controlling the change of at least one first working parameter,

a computer unit element which is programmed to determine the locations of the path on which a change of the first parameter must be made and which is adapted to output corresponding signals to the numerical control element,

an organ for determining a second parameter providing an image of the states prevailing in the electric discharge gap and outputting a signal to the numerical control when the value of the second parameter reaches a predetermined threshold, this signal being a renewed change of the first Parameter or terminating the execution of the change when the first parameter has reached a desired final value or returned to its initial value or when a predetermined number of changes of the first parameter have been performed, causes, and

means associated with the numerical control for effecting a change of the first parameter by a predetermined amount.

It is useful if the numerical control controls the change in the frequency of the voltage pulses of the generator. At this time, the numerical control may also control the change of the injection pressure of the working fluid into the electric discharge gap, in particular, the means for changing the first parameter by a predetermined amount by a solenoid valve connected to the numerical control element. Advantageously, the numerical control can effect a constant value change of the first parameter (s). The organ for determining a second parameter that provides an image of the ruling in the electric discharge machining gap states can be a circuit that includes a measuring element for the working voltage Urr> -DO de m We rkstück and the wire electrode. In particular, in this case the saddle is a 'Mi can ttelwe RTEs order m and an average Referenzspa include the operating voltage and the difference Usa between said mean value> ung Umr and for comparing the thus obtained value of Usa to a predetermined Bezugsschwellenwet ltkreis means for determining. The circuit may include means for integrating the value of Usa for a predetermined duration, eg, 1i50ms.

Further, according to the present invention, there is provided a method of controlling an electric discharge machine with a wire electrode and along a predetermined path while varying at least a first working parameter V to a certain end point V when the wire electrode reaches the apex of an angle or arc, whereafter the value V is returned to its initial value V at the end of the cutting of the angle or arc, comprising the following steps:

- Calculation of places of the railway where the changes have to take place,

- stopping the Relativbewe _b JNG between the wire electrode and the piece to be machined, when the wire reaches one of these sites,

- Continuously determining a second parameter which provides an image of conditions prevailing in the working gap and, when the value of the second parameter reaches a predetermined threshold. Changing the value V of the at least one first parameter by a predetermined amount Q, which change again changes the second parameter,

- re-changing the one or more parameters when the value of the second parameter again reaches the predetermined threshold, this change being performed in stages until the desired final value reaches V or until a predetermined number of stepwise changes have been made,

- Resumption of the relative movement between the wire electrode and We ^ piece, wherein the relative movement by the of

the value V attained for the first parameter (s) is determined,

Changing the value of the first parameter or parameters in successive stages in the opposite direction when the wire reaches the next predetermined position according to the first step, the duration of these stages being determined as before by continuous comparison with a reference threshold of a second parameter provides an image of the conditions prevailing in the working gap, until each first parameter has reached its initial value V o to ^r

predetermined number of stages of change has been performed.

Here, the increments and decrements applied with respect to each first parameter are suitably the same. In particular, the predetermined threshold value achieved by the second parameter at the end of each stage may be its value for cutting along a straight line. The first parameters that can be varied are the frequency of the

Pulses and the injection pressure of working fluid into the working gap. The working voltage between the too

piece and processed the wire electrode prevails continuously gemes sen w can ground. In this case, the mean value Um of the operating voltage can be determined, the difference Usa to the mean reference voltage Umr calculated, and the value thus obtained for Usa compared to a predetermined threshold value.

According to the invention, it is possible to achieve a satisfactory geometry regardless of the concavity or convexity or the interruption of the straightness of the cutting path by projecting or projecting angles.

The parameter to be modified is progressively varied, this modification being, for example, a reduction.

The relative movement between the wire electrode and the workpiece is stopped but the generator continues to deliver Impulso to the wire electrode;

this pause causes a change in a second parameter that asymptotically approaches a predetermined limit;

if the second parameter has reached a reference value (in the course of its change against an asymptote), the value ν of the parameter to be modified, for example the pulse frequency of the generator, is changed by a predetermined quantity Q;

this change causes a change in the second parameter, which progressively assumes its reference value restoring the gap;

again, the first parameter is decreased by a quantity O 'and this is done successively until the first parameter has been lowered to a value V (X) after performing η decrements;

- The relative movement between wire electrode and workpiece is resumed; the modified parameter is generally a factor of the discharge power, its reduction in this case causes a lowering of the feed speed which is automatically realized by the servo drives which effect the relative movement;

- in this area, which is reduced to a distance I _{0 of} the railway, is being worked;

without stopping the relative movement, the value of the first parameter is increased by a given quantity Q ", which causes a change in the second parameter, which then progressively assumes its reference value, equilibrating in the gap; successive increases are made;

if the first parameter again assumes the value V, the processing becomes under the initial operating conditions

continued.

In this way, a double progressive change is made in steps of the operating conditions of operation, this change being achieved either by setting reference processing conditions or by returning to the processing equilibrium after each elementary change. The duration of the stages is determined by the time needed to return to reference working conditions or balance. The quantities Q, Q ', Q "can be equal or different for each realized decrement or increment (elementary change) .These quantities can be sufficiently small (and the stages sufficiently numerous) for the change to be gradual and practically continuous In particular, the operating values and the corresponding reduced value V (x) of the first parameter are such that the bending of the wire is sufficiently reduced to be acceptable, ie, so that geometrical errors caused in the region of this bend disappear or become acceptable within the desired precision However, this is not a consequence of

Change of operating conditions that has been realized.

It will be understood that the above described procedure can also be used to increase a parameter (for example, the mechanical tension of the wire).

The duration of the steps of changing the discharge frequency (or other parameter affecting the geometry, for example, affecting the power of the pulses) is not predetermined. The average working voltage at a predetermined threshold word can be stabilized after each of these changes before the subsequent change. It is therefore the

Times are needed for sequential stabilization that determines the duration of these stages. The change will

performed at steady state operating conditions. It is therefore not necessary to correct the feed rate as a function of successive values for the average working voltage so as to maintain a constant discharge distance since the feed rate is zero. When approaching the top of an angle, the feed rate is reduced only after the peak.

Surprisingly, this mode of operation, in which the advancement of the wire is interrupted and the steady-state operating conditions are varied stepwise, makes it possible to produce an angle or arc under automatic parameter change as compared to processes performed by a process that undergoes a change of the wire feed makes considerable reductions.

The first parameter is generally one of the factors of the operating conditions, in particular the power of the pulses generated by the generator, such as the frequency of these pulses, the intensity of the working current, the discharge time or the discharge voltage. Its change may preferably take place by changing one or more other parameters,

For example, a change in the injection pressure of the working fluid and / or the mechanical tension of the wire and / or the take-off speed of the latter verbundcii be. This makes it possible in particular to increase the effect of the frequency decrease on the bending of the wire.

The second parameter that provides a picture of the processing conditions is, for example, the average working voltage, the control voltage (difference between the mean reference voltage and the average working voltage) or the average ignition duration of the discharges.

The predetermined threshold reached by the second parameter at the end of each stage may be the initial value at the moment of stopping the wire. In this case, the first stage is suppressed and the first lowering of the parameter V is performed while stopping the relative movement between the wire and the workpiece. The quantities by which the first parameter is varied at each stage, as well as the number η or η + 1 at stages, the

Final value V (X), which is reached by the parameter before cutting an angle, or the predetermined threshold,

which is achieved by the second parameter at the end of each stage, for example, are experimentally determined by selecting the values that will provide a satisfactory geometry for the workpiece when cutting a sheet or angle at the highest possible speed but avoiding short circuits. These values are stored on an element suitable as an information carrier, which is connected to the computer unit of the numerical control of the EDM machine. The values represent a function of the material of the workpiece and the material and geometry of the wire electrode.

In the case of cutting a sheet, the larger the radius of curvature, the less necessary it is to lower the operating conditions and thus the parameter to obtain a good geometry. It is possible to stop at an intermediate stage corresponding to a reduced value V (x) of the first parameter instead of performing the stages until reaching V (X). In other words, V (x) and η are a function of the radius. The larger the latter, the closer V (x) will be to the initial value V of the parameter and the smaller will be η. On the other hand, the smaller the radius, the closer v "(x) to V (X) corresponding to the intersection of an angle and the larger n.

The term "angle" refers to both a sharp and a rounded angle, the radius of curvature of which is of the order of the radius of the wire electrode or less. The locations of the path where the numerical control according to the invention employs are determined by the computer unit which is suitable Is programmed and configured to provide a first signal that initiates the beginning of this automatism, ie stopping the relative movement of wire and workpiece, and then generates a second signal when the wire has a length I _{0 of} the web under machining with the first parameter reduced to the value V (x) The second signal causes the beginning of the successive increase of the operating conditions The calculation of I ₀ is different for an arc and an angle.

embodiment

The invention is explained below with reference to an embodiment shown in the accompanying drawings.

Fig. 1 shows schematically a machine for electrical discharge cutting by means of a wire electrode, which is provided with a device according to the present invention.

2 and 3: show the cutting conditions in the range of an angle or an arc. 4 shows a diagram according to which the pulse frequency is plotted against the time for cutting an angle or a sheet for explaining the method.

According to Fig. 1, a wire electrode 1 is provided, which is continuously drawn by two wire guides 3,4, which are each arranged on one side of a workpiece to be machined 2, by means of means not shown, which hold the wire simultaneously under predetermined mechanical tension , Two sliding contacts 5, 6 form the power supply, the working current being generated by a pulse generator 30, while two injection nozzles 7, 8 direct working fluid jets along the wire electrode 1 and into the gap which is cut in the workpiece 2 by the wire electrode 1

 The wire guide 3, the injection nozzle 7 and the upper sliding contact 5 are arranged in a movable working head 10 along a vertical axis Z and generally simultaneously (in a relatively small lame) in a horizontal plane spanned by the orthogonal axes U and V, whereby the Wire electrode 1 can be tilted while the lower wire guide 4 is fixed. The horizontal movement (U, V) is realized by means of a cross-movable table 11, which is actuated by a servo drive comprising two motors 12 and 13. The lower working head 15, which receives the wire guide 4, the injection nozzle 8 and the lower sliding contact 6, is fixedly arranged with respect to the machine bed.

The workpiece 2 is attached via a device, not shown, on a table, which itself over the bottom of a

Working container (not shown), which is filled with working fluid, is arranged. The work container is along two

horizontal and mutually orthogonal axes X and Y via a cross-movable table 25 movable, which is actuated by a two motors 23,24 comprehensive servo drive.

The cutting path is stored on an information carrier 16, which transfers its data to a computer unit 14 of a numerical controller 9. The latter comprises a suitably programmed unit, at least one memory and a reader as well as a device 18 (processor or interpolator) which controls the relative displacements between the workpiece 2 representing an electrode and the wire electrode 1 by operating the motors 12, 13, 23, 24 as well how other working parameters such as the injection pressure of the working fluid controlled by the action of solenoid valve 31,32 and controls. The computer unit 14 determines the cutting path and transmits control signals to the interpolator 18 intended to effect shifts in the two directions X, Y by means of the two motors 23,24.

The pulse generator 30 is connected to the two electrodes 1,2 such that it emits successive voltage pulses which are intended to trigger discharges in the work area. It receives control signals from the numerical controller 9 (via a connection, not shown) to control, for example, a change in the given value of the pulse frequency or the voltage or the duration of the sparks.

A dielectric fluid reservoir supplies the working container and injector injection circuits 7, 8 to the working heads. The injection circuits are shown schematically in Fig. 1 by a pump 35 and solenoid valves 31 and 32 connected to liquid distributors 33 and 34.

In the illustrated embodiment, it is assumed that two parameters, namely the pulse frequency and the injection pressure, are changed. The parameter which provides a picture of the machining conditions is the average working voltage Um obtained by integration over a certain number of successive measurements by means of a voltage measuring element circuit 21, the output of which is applied to a subtracter 22 which supplies the differential voltage Usa between Um and Um mean reference voltage Umr forms, where Usa is in a comparator member einos computer unit element 20 with a predetermined reference threshold value.

It has been determined experimentally that the geometry is satisfactory and short circuits are avoided at an angle at a reduced frequency F (X) = F / 8, where F is the pulse frequency at normal operating conditions, i. H. the working conditions used for cutting before reaching the angle or arc with a small radius of curvature. It has also been experimentally determined that at least eight consecutive reductions must be made to reduce the nominal frequency F to the reduced frequency F / 8 in order to intersect an angle of satisfactory geometry.

In addition, experimentally, the relation between the radius of curvature r of a sheet and the size of the correspondingly reduced frequency F (x) or the number η of elementary changes has been determined (and stored in an element 17 of the information carrier 16).

The reduction in the injection pressure of the working fluid is preferably in fewer steps than for the frequency

to make the change in injection pressure zero during the last change in frequency, thereby reducing the risk of shorting, since the wire will then have a minimum bend. This also avoids the maximum that the wire is moved on restarting (since it is no longer made of smooth material), this also applies during the production of the angle.

The control of the pressure via a control member 19 which is controlled by the computer unit 14 and itself outputs control signals to the pulse generator 30 and to a battery of four Elcktromagnetventilen 36,37,38,39, which are connected in parallel to the bypass of the high-pressure pump 35, the working fluid from a reservoir to the distribution network for the injectors 7.8 and the working container supplies.

Furthermore, an integrated automatic control circuit is provided, which makes it possible to control the speed of the relative movement between the wire electrode 1 and workpiece 2 with respect to Umr. Therefore, the speed at which the wire electrode 1 passes through the distance I ₀ , automatically set.

4 shows the reduction of the pulse frequency in eight stages in a phase A during which there is no relative movement between wire electrode 1 and workpiece 2 to intersect an angle. The latter takes place during a subsequent phase B with the frequency F (X) under relative movement between wire electrode 1 and workpiece 2. After cutting the angle, a phase C follows, during which the nominal operating frequency F in likewise 8 stages under relative movement between wire electrode 1 and workpiece 2 is finally reached again. Obviously, the steps have different lengths of time.

When cutting a sheet, not all eight stages are required depending on its radius of curvature (shown in dashed lines).

In the case of an angle u, the computer unit is preferably programmed so that I ₀ is calculated as follows:

_ Wire radius R + gap g ° ~ tga / 2

(Vgl.Fig.2).

In the case of an arc with the radius of curvature r, the computer unit is preferably programmed so that I ₀ is calculated as follows:

Io = τ r,

where τ is the angle expressed in radians for this arc (see Fig. 3).

In this way, I ₀ depends only on the geometry and diameter of the wire and not on the electrode material and the thickness of the workpiece.

The determination of I ₀ and the delivery of signals A and B to the means 18 for effecting the progressive changes in the operating conditions which serve to return to work equilibrium are effected by the computing unit 14 without the intervention of a programmer. The computer unit 14 analyzes the web placed in the memory (programmed in particular to follow the evolution of the radius of curvature of the web) and performs simple calculations as exemplified above.

It is particularly surprising that, arguably the stages of change for the operating conditions have no predetermined duration, the making of an angle or arc is faster than when the changes are made according to stages of predetermined duration. It is possible according to the invention to make an angle of 90 ° in a workpiece of SOmm thickness in 47 s, in contrast to 121 s, which are needed if the frequency is lowered every 7.5 s and then raised again (in both cases the same Distance I ₀ required).

The parameter changes can be changed to constant sizes for each level of change, but it is also possible to change the sizes from one level to another.

Another advantage is the simplification in terms of both machine equipment and programming. It is no longer necessary, for example, the discharge distance corresponding to

To adjust pulse frequency changes. The automatic change of the parameters is very easy. A determination of the deflection of the wire and the width of the cut gap (requiring the cutting to break) and the calculation of a reduced speed according to a formula linking the radius of curvature, the gap and the deflection, the latter being of the type of wire, the Thickness of the workpiece, the distance between the wire guides, the injection pressure, the working current power, the mechanical tension of the wire, etc. is dependent, is no longer necessary. An interruption of the processing is just as necessary as a special circuit with

Sensors for measuring the deflection and the gap. According to the invention, work is carried out independently of the largest part of these factors, in particular also independent of the workpiece thickness and the wire sag. These do not affect the change of the feed rate. They also do not require correction of the stored cutting path to compensate for errors in geometry due to deflection.

Another advantage is that a combination of other measures is easily possible, such as protective measures that serve to reduce the operating conditions such as in the presence of a wire break risk, the operating conditions are restored when the wire break risk is eliminated.

If two curves of small radius of curvature follow each other directly or follow a very acute angle curvature, it is possible to cause the two automatisms to overlap, since each automatism has three easily separable phases A, B and C, as stated above. Each of phases A and C may either be interrupted or turned on at any stage.

The invention can be used only to change the operating conditions such that an angle or arc is cut under suitable operating conditions, but also to halt or resume processing, for instance in the case of voluntary stop or drop in operating conditions as a result of the occurrence of breakage or short circuit risk. Above all, it is also possible to prevent short circuits when starting up the machining again.

Claims (15)

An apparatus for controlling a machine for electroerosion cutting along a predetermined path of an electrode forming workpiece (2) with a wire electrode (1), the machine comprising a generator (30) connected between the two electrodes (1,2) is and voltage pulses outputs, and a numerical control (9,14,16,18) for the relative movements between the workpiece (2) and wire guides (3,4), for the properties of the generator (30) output pulses and for the Injection pressure of a working fluid into the electric discharge gap, characterized by a numerical control element (19) for controlling the change of at least one first working parameter, a computer unit element (20) programmed to determine the locations of the path on which a change of the first parameter must be made and adapted to output corresponding signals to the numerical control element (19), an organ (21,22) for determining a second parameter which provides an image of the conditions prevailing in the electric discharge gap and outputs a signal to the numerical control element (19) when the value of the second parameter reaches a predetermined threshold, said signal causing the first parameter to be changed again or to cease execution of the change when the first parameter has reached a desired final value or returned to its initial value or when a predetermined number of changes of the first parameter have been made; means associated with the numerical control (19) for effecting a change of the first parameter by a predetermined amount. 2. Apparatus according to claim 1, characterized in that the numerical control element (19) controls the change in the frequency of the voltage pulses of the generator (30). 3. Apparatus according to claim 2, characterized in that the numerical control element (19) also controls the change of the injection pressure of the working fluid in the electric discharge gap. 4. Apparatus according to claim 3, characterized in that άαΰ, the means for changing the first Parameter is formed by a predetermined size by a battery connected to the numerical control (19) of a plurality of solenoid valves (36,37,38,39). 5. Device according to one of claims 1 to 4, characterized in that the numerical control element (19) causes changes with a constant value of the first or the first parameter. 6. Device according to one of claims 1 to 5, characterized in that the organ (21,22) for determining a second parameter which provides an image of the conditions prevailing in the electric discharge gap, a circuit which is a measuring element (21) for the working voltage Um between the workpiece (2) and the wire electrode (1) contains. 7. Apparatus according to claim 6, dadu rch in that the circuit means (22) for determining a mean value by the Arbeitsspannu ng un d represents the difference Usa between said mean value Um and an average reference voltage Umr and for comparing the thus obtained value of Usa with a predetermined reference threshold. 8. Apparatus according to claim 7, characterized in that the circuit comprises means for integrating the value of Usa for a predetermined duration. A method of controlling an electric discharge machine with a wire electrode and along a predetermined path while varying at least a first working parameter V to a predetermined final value V when the wire electrode reaches the apex of an angle or arc, whereafter the value V is set to its value Initial value V is returned at the end of the cutting of the angle or arc, characterized by the following steps: - Calculation of places of the railway where the changes have to take place, Stopping the relative movement between the wire electrode and the piece to be machined, when the wire reaches one of these positions, continuously determining a second parameter providing an image of conditions existing in the working gap and, when the value of the second parameter reaches a predetermined threshold, changing the value V of the at least one first parameter by a predetermined amount Q, which Change again the second parameter changes - re-changing the one or more parameters when the value of the second parameter again reaches the predetermined threshold, this change being performed in stages until the desired final value reaches V or until a predetermined number of stages Changes have been made, Resuming the relative movement between the wire electrode and the workpiece, the relative movement being determined by the value V achieved by the one or more parameters (n), Changing the value of the first parameter (s) in successive stages in vice versa, when the wire reaches the next predetermined position according to the first step, the duration of these stages being determined as before by continuous comparison with a reference threshold value of a second parameter providing an image of the conditions prevailing in the working gap each first parameter has reached its initial value V or until a predetermined number of stages of change have been performed. 10. The method according to claim 9, characterized in that the increments and decrements, the with respect to each first parameter are the same. A method according to claim 9 or 10, characterized in that the predetermined threshold value achieved by the second parameter at the end of each stage is its value for cutting along a straight line. 12. The method according to any one of claims 9 to 11, characterized in that the first parameters that are varied, the frequency of the pulses and the injection pressure of working fluid in the working gap. 13. The method according to claim 12, characterized in that continuously the working voltage, which is between the piece to be machined and the wire electrode, is measured. 14. The method according to claim 13, characterized in that the average value U m of the working voltage determines the Difforenz Lisa calculated to the mean reference voltage Umr and the value thus obtained for Usa is compared with a predetermined threshold. 15. The method according to any one of claims 9 to 14, characterized in that the step of Calculating comprises an arithmetic operation consisting in maintaining the distance I ₀ passed through by holding each first parameter at its final value V according to the equation