DE1062362B - Equipment for processing materials by spark erosion - Google Patents

Description

GERMAN

The invention relates to devices for processing electrically conductive materials by means of spark erosion. Such devices, which work with the discharge of a storage for the electrical energy, are often equipped with a charging circuit fed by rectified alternating current. One needs electricity to be more consistent Direction available to the polarity of the working electrode and the workpiece always so to be able to maintain that a minimum of electrode wear is guaranteed.

The electricity used mostly comes from the grid, be it as single-phase or multi-phase electricity, and has a frequency of 50 to 60 Hz. In some devices, electricity is a slightly higher frequency from an alternating current machine is used, and a special dimensioning of the loading circuit is used to avoid the danger contain the formation of arcing.

The working speed of such processes, the so-called removal rate, is usually indicated by the amount of material that is removed from the workpiece in a unit of time. In the best known and most widely used process, it has so far proven impossible ^{to achieve removal rates of more than 100 to 200 mm 3} per minute.

The invention is based on the knowledge that the rate of removal is limited by phenomena is that with the charging current strength during certain periods within the working rhythm of the spark generator are linked and depend to a large extent on its particular design.

First of all, the well-known fact should be noted that the formation of arcing is prevented during spark erosion must be, since an arc would have a destructive effect. There is a tendency to form arcing now at the end of the discharge of the energy storage device, when the transmitted by the spark, from the current supplied at this moment short-circuited source is sufficiently large to cause the spark extend.

Furthermore, an excessively rapid increase in the charging current that follows the extinction of the spark has a voltage at the terminals of the memory result, which brings with it the risk that it will be premature Sparking occurs without any useful effect. This is due to a residual ionization that occurs during the discharge time has not yet completely disappeared and which always promotes arcing.

In most of the common generators for spark erosion attempts have been made to avert these dangers by limiting the charging current to a certain value in order to prevent arcing in the spark gap and the occurrence of a too steep voltage device for processing
of materials by spark erosion

Applicant:

Societe Anonyme dite:

La Soudure Electrique Languepin,

Paris

Representative: Dipl.-Ing. H. Schiffer, patent attorney,
Karlsruhe, Kochstr. 3

Claimed priority:
France 19 July 1957

Alfred Marie Aime Maillet,

Versailles, Seine-et-Oise (France),

has been named as the inventor

to avoid increases at the terminals of the storage unit after the individual discharges.

For this purpose, you have a charging resistor, which was possibly increased by an inductance, in the Charging circuit switched on.

However, this oldest measure, which is used in most cases, does not lead to the abovementioned removal rates of approximately 100 to 200 mm ³ per minute in industrial plants.

The attempt to contain the risk of arcing has also been achieved by connecting one or more capacitors in series with the charging circuit. This solution, which is better than the one previously discussed, because it does not limit the charging current following the discharge of the capacitors, leads to removal rates of the order of 500 mm ³ per minute.

However, facilities based on this principle still require a very precisely functioning control system, that is difficult to realize.

As is well known, every spark releases a number of particles of the material to be processed and forms a small crater in the process. The dimensions of these craters or crater elements form the decisive factor for the surface properties of the machined workpiece and are a function of the energy ¹ J ₂ CE _d ² , which forms the content of a memory with the capacity C at the moment of the sparkover. With a given flashover voltage E _d , ie at a distance between

909 5797351

3 4

see electrode and workpiece, which will not be that large Fig. 1 shows a schematic circuit diagram for a that the accuracy of the erosion is reduced, according to the invention;

and not too small, because there is then no longer possible Fig. 2 shows the variation of the current i and the clamping If the metal generated by the working process voltage e in the charging circuit of Fig. 1 as a function of

To get rid of particles, it is practically the capacity that 5 “. _fr .. _A U ₁₁ -, j ο j · -dj · _r *

j. "· Ι ·. j τ- ■ Ix- j. τ j. 5 -x time £ in the event that the condition Lw =

determines the accuracy of the erosion. The latter is around '^{& 8} ω C

the larger and the surface quality the better, precisely fulfilled;

the smaller the capacitance C is. Fig. 3 can be in the context of a diagram in which the

For a given capacitance C and a given flashover capacitance C is plotted against the frequency f , for which

voltage E _d , the removal rate to io in Fig. 1 shown device a section in

a function of the repetition frequency of the spark to detect the range of capacitance values corresponding to the

and is consequently the greater, the higher the charging conditions for the most favorable spark erosion leads,

speed of such a capacity. In Fig. 1, M denotes a motor which, under intermediate

The invention is based on the object of switching on a speed controller B of any known direction for spark erosion, in which the suitable design drives an alternating current machine 1, the charging speed of the storage device, the frequency of which can thus be varied as far as possible. A complete upgrade is. The working conditions that are created in the pursuit of rectifier network 2 in the bridge circuit consists of this endeavor, lead beyond four mercury vapor rectifier tubes 3, 3 'on the one hand to the additional and surprising advantage, and 4, 4' on the other hand, and directs both of the change that the possibility of regulation the spark work is the same as the phases supplied to the electric machine 1. The network is favorable, further to the fact that the discharge process works via lines in which a self-induction 6 between electrode and workpiece cannot have the character and a resistance 7, with an energy of an arc discharge. memory, namely the one used for spark machining

The invention thus relates to a device capacitor 5, connected. To process electrically conductive materials, the working electrode 8 acts on the workpiece 9 through the spark gap 10.
In the following, the entire input of a rectifier network connected in the circuit, ie that of the alternating current alternating current source, one at the output of the machine 1 and the one in the circuit itself, is denoted by L. Rectifier network lying storage capacitor, R is the total resistance of the circuit, one at a distance that defines the spark gap, 30 Finally, let the total capacitance C of the charging circuit of the workpiece to be processed work - including the capacitor 5 concentrated in this electrode and a discharge circuit, the through the parallel thought.

circuit of the spark gap with the storage capacitors by the action of the rectifier pairs 3-3 'and

is formed. 4-4 'the alternator 1 charges the capacitor 5

The progress sought is according to the invention by 35 in a known manner in always the same direction,

such a dimensioning of the total inductance (L in as indicated by the thin and the thick solid arrows

Henry) of the charging circuit and the angular frequency (co) which is indicated. The capacitor discharges cyclically

AC power source achieves that for a given in a circle made up of himself, the electrodes 8 and 9

Capacitance value (C in Farads) of the storage capacitor and the spark gap 10 consists of those mentioned

,. ^ ₁ . , _r 1. ,, --1.J f-114. 40 electrodes separate from each other.

the equation Lm = at least approximately fulfilled, ", _TTr,, " ..... " , _e . , _

^b ω C If the value of the capacity C of the memory 5 is so

is. This dimensioning results from the knowledge that it is selected that a certain surface quality

for the achievement of the desired effect the circle is achieved, so there is, as already mentioned, one

reactance must be as small as possible and at most some primary prerequisite to achieve a substantial

Ohms. 45 useful energy in the fact that the charging circuit is only a very small one

Furthermore, if the capacitance C is equal to or greater than reactance, which is only a few ohms or

2 microfarads, the total self-inductance is so small still smaller, furthermore in that the equation

be as possible and appropriate between 0.1 and _T 1 ■■ ,. -ujjj t. j-iu. ■ <- aa

η c χτ λ- Lw = —— is at least approximately fulfilled by the fact that

2.5 mH. ω C

The self-induction 50 then advantageously consists of the various variables, in particular the circular

of the circuit only from that of the feeding alternating frequency m, ie the frequency f of the alternating current source

alternator, so that given L and C by varying the speed of the alternator

Equation Lw = —— by adjusting the Um- ° ~. · ■, ο -u u. - ^ j c. ■ j

⁰ ω C oo g _ei such a circuit, the current m is the

speed of the machine to the required value 55 one half of the rectifier bridge 2 for each phase

of w = 2nf is to be fulfilled. interrupted to flow through the other half.

In a further development of the invention, the device can with each change so the current is temporarily

be applied in such a way that when processing going into the corresponding part of the circuit in and

of a workpiece in successive work feeds the capacitor providing the useful energy,

proceeded with increasing processing ⁶ ° Write the voltage of source 1 as & = E cos wt

Accuracy capacitors with from operation to j · j. j η j τλ ·· χ * τ ^. ^r

λ u * ui. j τλ -χ ·· * ■ iij and it is assumed that the damping factor α = - ^^

Operation with decreasing capacity can be used ± - ο ₂ L

and that the frequency of the alternating current source from the charging circuit is very small with respect to w and that the operation to operation is correspondingly increased the initial current and the initial voltage. 65 bar, the charging current results to

For this particular application, input β

directions according to the invention in a special form from - H = - coscoi · (1 - e ~ ^at ).
be educated. The invention is now based on

management examples and on the basis of the drawing. In this expression, Z means the impedance of the

to be purified. 70 circle. The one explained above, found experimentally

5 6

The condition that the circular reactance is very small or zero and III, with the difference that the starting points should be, leads to an impedance Z, which now approaches the value with the end points of the preceding branches to the resistor 22, and thus to a maximum coincide and that their amplitudes are lower. Amperage, which means favorable conditions for useful work A certain time before the end of the second half-period. 5 reaches the terminal voltage of the capacitor 5 on

On the other hand, a test of the above-mentioned point D shows the value of the flashover voltage E _d , so that the relationship that triggers a spark in the discharge circuit for the beginning of the transition state,
(so small t) in the form. During the unloading process, the disappears

β voltage E _d at the terminals of the capacitor 5 according to

H - "ζγ ί cos ω ί ίο the curve piece DD _1. At the same time, the current in the

From the loading circle, as the last part of curve II shows,

it can be written that the factor t in the expression until it reaches a low value. Towards the end of the cos (ω t) , the current intensity flattens out, the discharge slows down and the sinusoidal curve changes, which leads to a rise as considerably as it is from the first part, the closer t is to the origin, ie the time it can be seen from curve II. The next spark has borrowed the starting point. This results in a similar drop in current strength, and so on.
slowing the increase in the charging current, it should be noted that the mutual coordination

which follows a discharge of the storage capacitor (that is, the values L, C and ω in order to achieve a minimum capacitor supplying the spark energy), and the value for the impedance Z is by no means synonymous with a slowed start of the recharge 20 with a tuning of the circuit to electrical this capacitor, which then no longer has the tendency to resonate,
has to discharge prematurely. The circle is incapable of such a phenomenon

Under these conditions, namely to be neglected; because the current only pulsates permissible reactance, the charging current is also practically up, namely after flowing through the two halves of the in phase with the voltage of the source 1. 25 rectifier network in the part of the circuit with the

Should the charging process more than one current change capacitor 5 always in the same direction,
need - here is thought of the charge that the A charging circuit according to the invention with a very

Storage capacitor during the first change to- small value of self-induction eliminates the risk of takes - so the charging current decreases during or arcing from, although the circle is too certain the following changes more and more. The charging 30 points in time of currents of considerable strength through the Capacitor is interrupted as soon as its flow.

Terminal voltage the value of the flashover voltage E _d For example, a charging circuit has been successfully tested

(Fig. 2) at the distance between the workpiece and at which the resistance reaches that of the lower working electrode. At this moment the elements that could be dispensed with were limited: alternating current strikes a spark between the two electrodes 35 source, rectifier, etc .; also the self-induction of the over, and the capacity C discharges within a circle was kept extremely low and beZeit t _d , which is considerably shorter than the charging time t _c . limited to that of the named, indispensable

These processes are illustrated in FIG. The bare elements. The total amount of self-induction Fig. 2 shows in dashed lines (curve I) those through the circle, which are practically on during the experiments the rectifier tubes 3-3 'and 4-4' rectified 4 ° that of the alternator 1 limited, be-voltage the source 1. The solid curve (II) represents 0.3 to 0.8 and 1.2 mH.

represents the charging current, which in the capacitor 5 of FIG. 1 It has been shown experimentally that the most favorable

flows. The dash-dotted curve (III) finally shows working conditions at the lowest values of L then the voltage curve at the terminals of this capacitor when the circle shows the lowest values of the capacitor. These three curves apply to the special 45 ohmic resistance.

■ π- n ■ λ J-T3 - U τ ¹ ii „C - 11.L This surprising result, which the after

Case in which the relationship Lm = —— exactly fulfills _OJ . jj - ^ ■. · Ι -.ü- _t t. -j tj. "O

⁰ ω C state of the art contradicts valid teachings, according to

and consequently the current and voltage are in phase. those to avoid the risk of arcing

The figure clearly shows that curve II, the straight additional self-inductances in the charging circuit

Represents charging current, 5 ° are switched during each change of current, can be explained as follows:

the shaft is unbalanced. It runs so that the circuit according to the invention, which has a low

Abscissa axis is at its origin as a tangent, has self-induction, is the ohmic resistance,

then gradually increases, so that after a certain amount of damage, especially that of the rectifiers, by no means

would prefer wrestled time τ with respect to the maximum voltage "ophlä ^ IPW The DärnnfuiWaktor π - ^R of the reach of source its maximum, but breaks 55 ^nachlassl g ^bar · ^üer damping factor α - - ^ - of

practically related to the source voltage. Your circle is therefore relatively high. Hence the duration of the

The rising edge is flattened and lengthened, while the spark is particularly noticeable when the

the trailing edge is steeper and shortened. capacitor used is large. Now it closes

Also the voltage assumed by the capacitance C spark gap not only the capacitor feeding it

(Curve III) runs tangentially at the origin to the time 60 short, but also the loading circle, so that the in the

axis, increases very slowly at the beginning, then faster, self-induction stored energy also over it

but then slower again, whereby it can also discharge one more, provided that the discharge duration

at least approximately horizontal tangent is reached, is of the order of magnitude of the spark duration, d. i.e. that

as soon as no more charging current flows (charge C ₁ ). the self-induction, as explained, is low.

In the conception of Fig. 2 was assumed (and 65 If the spark is extinguished, the power source

although only for the sake of simplicity) that the clamps begin again, the self-induction in the association

voltage of the capacitor 5 to charge the withstand voltage E _d with the capacitor, which increases

reached at a time that is a little shorter than one of the charging current is delayed and obviously the danger

Period of the source alternating voltage. Decreased during arcing. This applies to

Repeat the second half-period see the branches II 70 special for capacitors for sparking, the

are equal to or greater than 2 microfarads. With the given, applied values of self-induction, the resistance adhering to the circuit, without the need for additional resistors, is sufficient to obtain high values for α. This has the advantage that the losses of the circuit caused by the Joule effect can be kept to a minimum. With regard to the size of the damping coefficient α, the above theoretical explanations cannot be regarded as completely exact. In particular, the rhythm of the charging circuit does not exactly coincide with that of the AC source, and the course of the charging current is not exactly in phase with the rectified voltage of the source. In the meantime, the basic course of the intensity curves and especially their delayed rise from the origin of each current change remains correct. In addition, given values of L and C correspond not only to a single, most favorable value for the frequency of the current source, but rather to a whole range of values on both sides of this most favorable value, that of the minimum reactance

to L

independent of the damping factor α

(Lco =

speaks.

Now the numerical relationships based on experiments can also be given below. According to the invention, they generally relate the capacitance C in the circuit to the self-induction L and the angular frequency ω of the source 1 and contain the conditions for favorable operation when working with the spark gap.

The relationships are as follows:

34.6
-, = - <ω <

34.6
0.5

2fC

C expressed in farads,
and

0.1 < L <2.5 millihenry (2)

or, if the frequency f of the alternating current source is introduced,

2 | / 'C 0.5 | / C

0.1 < L <2.5 millihenry

These conditions are particularly applicable in their entirety to circuits in which the capacity of the Energy storage is greater than 2 microfarads. If the capacity is below this value, a condition can be which approximates the target of minimum impedance are considered sufficient; this is

allow the same efficiency to be ensured, provided that the equivalents listed below are used Conditions are met:

a) approximation of the resistance of the charging circuit in accordance with the limits given above to the angular frequency ω;

b) Limitation of the self-induction L to values which satisfy inequalities (2) and (2 ').

The table below shows the results just explained for capacitance values of the capacitor 5 between 5000 microfarads (for coarse erosion) and 0.01 microfarads (for fine erosion).

2] / L C

0.5 j / Z C

The limit for the value of self-induction is of less practical importance.

The above for the case of a simple circuit with a power supply by rectified alternating current or from an equivalent source and one from the series connection of a self-induction, a resistor and A capacity existing charging circuit conditions set apart are sufficient for that too any more complicated circuits with multiple self-inductances, resistances, or capacitances in Series or parallel connection with spark processing

15th capacity Self induction Permissible frequency f ^ in microfarads in Millihenry the AC power source
in Hz ' 0.5 } [LC 5000 0.1 to 2.5 40 to 160 20th 2000 0.1 to 2.5 40 to 240 1000 0.1 to 2.5 8OMs 320 500 0.1 to 2.5 110 to 440 200 0.1 to 2.5 160 to 640 100 0.1 to 2.5 270 to 1080 25th 50 0.1 to 2.5 390 to 1560 20th 0.1 to 2.5 620 to 2480 10 0.1 to 2.5 870 to 3480 5 0.1 to 2.5 1100 to 4400 2 0.1 to 2.5 1600 to 6400 30th 1 0.5 0.2 1 O 1 U, J.
0.05 35 0.02 0.01 1 ^r 2] [LC

The information in this table is reflected in FIG. 3.

For the values of C, which are plotted in the coordinate system, those frequencies which lead to the best results are in the hatched strip between the two curves α and b. This stripe, which is only a small part of the Cf 'plane, narrowly delimits the range of those frequencies which give a result of interest from all possible frequencies.

It has already been mentioned that the charging of the energy store during the first half-wave of the Charging current, which follows a discharge, can be completed or during one of the following half-waves. With otherwise the same conditions, the charging current increases during the first half-wave the faster, the longer such a half-wave lasts, and it is therefore to the memory quickly To recharge, it is advisable to choose the frequency of the power source as low as the capacitor according to the given formulas only allows. In practice, therefore, memories are sequentially used be used with decreasing capacity, and this method leads according to the invention that Power sources of increasing frequency are used. These changes, on the one hand, those of the spark gap effective capacities, the values of which decrease, on the other hand those of the power source frequencies, which increase, form an essential feature of the invention which provides means for producing these changes. These means consist, for example, in an alternating current machine that can be operated at different speeds, wherein the speed change can be provided continuously or in steps of different heights. Man

can be different with multiple AC machines Speed work or with frequency converters fed directly from the network or other means, which give the same result.

If you cause the electrical discharge machining on a workpiece, for example at a voltage of around 200 volts, successively using capacities of 21.5 and 8 microfarads and working accordingly with the frequencies 957 and 1490 Hz, then the removal rate in the first work phase is 950 mm ³ per minute and in the second 640 mm ³ per minute.

The capacity of the energy store 5, the value of which, as has been explained, by the respective properties the electrical discharge machining to be carried out is determined, must be able to assume different values in succession, which correspond to different frequencies of the power source. This capacity can be achieved in a known manner different individual capacitors are provided, which are connected by known switching means in series, parallel or Parallel series connection can be brought. These switching means can be coupled with those which are used to change the speed of the alternator used as a power source.

Claims (8)

Patent claims: 1. A device for machining electrically conductive materials by spark erosion, which has a charging circuit with an alternating current source connected to the input of a rectifier network, a storage capacitor located at the output of the rectifier network, a working electrode at a distance that defines the spark gap from the workpiece to be machined and a discharge circuit, which is formed by the parallel connection of the spark gap with the storage capacitor, characterized by such a dimensioning of the total inductance (L in Henry) of the charging circuit and the angular frequency (co) of the alternating current source (1) that for a certain capacitance value (C in Farad ) of the storage capacitor the equation Leo = - at least an- is approximately fulfilled. 2. Device according to claim 1, characterized in that the total value of the self-inductance (L) of the charging circuit is between 0.1 and 2.5 mH. 3. Device according to claim 1 and 2, characterized in that the charging circuit is constructed so that its total inductance is practically only due to the internal self-induction of the alternating current source (1) consists. 4. Process for machining electrically conductive materials by spark erosion with one device according to claim 1, characterized in that capacitors are used in successive operations (5) are used with decreasing capacity from operation to operation and that the frequency of the alternating current source is increased in a corresponding manner from operation to operation will. 5. Device according to claim 1 for use according to claim 4, characterized in that for the supply of alternating current is provided by a number of alternating current machines of different speeds that can be optionally switched on. 6. Device according to claim 1 for use according to claim 4, characterized by a device which allows a single existing alternating current machine with different speed to run. 7. Device according to claim 1 for use according to claim 4, characterized in that for The supply of alternating current is supplied by an adjustable frequency converter fed directly from the mains is provided. 8. Device according to claim 1 and one of claims 5 to 7, characterized in that the Switching means for setting the storage capacity and the frequency of the alternating current with one another are coupled. 1 sheet of drawings ® 909 579/351 7.