US2804575A - Arc machining apparatus with periodic power control - Google Patents

Description

Aug. 27, 195? V. E. MATU LAITIS ARC MACHINING APPARATUS WITH PERIODIC POWER CONTROL Filed Oct. 1., 1954 z y i -1 2 F' ff V -AVL/K c I 6' 4 7 QS6 w XX W INVENTOR.

United States Patent 9 ARC MACHINING APPARATUS WITH PERIODIC POWER CONTRGL Victor E. Matulaitis, Franklin, Mich, assignor to Elm; Corporation of Michigan, Clawson, Mich.

Application October 1, 1954, Serial No. 459,703

15 Claims. (Cl. 315-163) This invention relates to improvements in the art of arc-machining sometimes referred to as spark-machining, electrical-discharge-machining, or metal disintegrating.

Briefly, the arc-machining process employs an electrode, which may be cylindrical, annular, in the form of a screw or of other shape, which electrode is fed toward a conductive workpiece while maintaining a gap therebetween across which an electrical discharge is caused to pass, coolant being maintained in the gap at all times. The electrode may be vibrated in certain instances or it may be fed toward the work in such manner that the gap is maintained constant as the machining operation is carried on.

In most instances, the arc-machining process will remove material at a slower rate than conventional machining, and attempts to increase the rate of material removal by increasing the power of each discharge have been only partially successful because increase in cutting speed has been accompanied by a decrease in accuracy, finish and dimensional control.

It is therefore the prime object of my invention to provide an improved arc-machining apparatus and electrical circuit therefor which substantially increases the rate of material removal without sacrifice of accuracy, fineness of finish or dimensional control.

Another object is to improve the stability of operation of arc-machining apparatus.

Another object is to provide an arc-machining circuit wherein a capacitor shunted across the arc gap may be charged through a resistor that, in effect, automatically varies in value from an extremely low value during charging of the capacitor to an extremely high value during discharging thereof.

Still another object is to provide in such a circuit automatically operable time control means operable to delay charging of the capacitor for a period after an electrical discharge has occurred across the gap proportionately to the intensity of the discharge, thus providing time for the gap to deionize after arcing, but avoiding unnecessary delay in charging.

Other objects and advantages of my invention will be apparent from the following description which, taken in conjunction with the accompanying drawings, illustrates a preferred form thereof.

In the drawings in which reference characters have been used to designate like parts referred to herein:

Fig. 1 is a schematic diagram of a basic arc-machining circuit;

Fig. 2. is a graphic representation of the operating characteristics of the Fig. l circuit;

Fig. 3 is a schematic circuit diagram of an improved arc-machining circuit incorporating my invention; and

Fig. 4 is a schematic circuit diagram of a modified simplified arc-machining circuit embodying the invention.

Referring now to Fig. 1, it may be seen that the basic circuit used in a typical arc-machining apparatus comprises a constant potential D. C. source, one side of Patented Aug. 27, 1957 which is connected to a workpiece 10 and the other side to an electrode 12. A resistor 14 is preferably connected in the positive side of the circuit and a condenser 16 is shunted across the gap between the electrode and the work. The electrode 10 may be vibrated axially or it may be fed toward the work without vibration, it being understood that coolant such as water or other solution, preferably a dielectric solution, is on or around the area of the workpiece being eroded. By proper choice of constants for the capacitor 16 and the resistor 14, the flow of current across the gap may be made pulsating in nature.

Operation of the Fig. l circuit may be understood by reference to Fig. 2. Assuming that when the circuit is established, the electrode 10 is spaced a sufficient" distance from the work 12 to prevent a discharge across the gap, the difference of potential between the electrode and workpiece will increase exponetially with respect to time as indicated by the curve O AB. Sufficient lapse of time will permit the potential difference across the gap to reach a value equal to that of the source. However, in the normal operation of arc-cutting apparatus, the spacing of the electrode and work is such that a discharge will occur across the gap before the potential difference thereacross reaches a value approaching that of the source. This point is designated A on the graph.

A discharge across the gap is accompanied by a flow of current derived principally from capacitor 16, which has been previously charged, and as the capacitor discharges, the potential difference across the gap falls rapidly. Usually the capacitor will not completely discharge because at some voltage lower than the discharge voltage, the arc will be interrupted and the capacitor will immediately start to recharge through the resistor 14. Point C on the graph indicates this occurrence. When the capacitor 16 again reaches a voltage high enough to cause a discharge across the gap, the phenomenon is repeated and this repetitive action continues as graphically indicated at A, C, D, E, F, G, etc., as long as the electrode and work are maintained in suitable relation.

The potential difference at which the material-removing discharge occurs, indicated at A, D, F, etc., is a function of and is affected by the material ofthe electrode, the material of the workpiece, the surface roughness of the two, the nature of the coolant used in the gap and,.most important of all, the spacing between the electrode and work. It is generally true that as the gap distance is increased, the potential difference across the gap must also increase to maintain the intermittent arcing or sparking necessary for removal of stock.

It will be noted that the potential difference which exists across the gap at the instant the arc is interrupted, indicated by points C, E, G, etc., is substantially uniform compared to the discharge voltage indicated at A, D, F, etc., and this voltage, besides being affected by the factors noted above, is a function of the value of the capacitor 16.

If the ohmic resistance of resistor 14 is sufficiently high in proportion to the magnitude of the supply Voltage to limit the current to approximately one ampere during a discharge across the gap, the Fig. 1 circuit will function satisfactorily in an arc-machining apparatus. For example, with a D. C. supply voltage of 50 and a resistor of ohms or thereabouts, the circuit will function satisfactorily with almost any value of capacitor between 1 and 30 microfarads, but the rate of stock removalis extremely low. A study of the operating characteristics of the circuit as represented by points A to G in Fig. 2 suggests that disproportionate periods of each individual cycle are spent in charging the condenser; that is to say, the distances CD and EF, as measured on the time scale are very long compared with the distances. AC, DE, and

FG, which represent the time required to discharge the condenser.

It is logical to conclude, therefore, that a decrease in the ohmic value of resistor 14 will permit more rapid charging of condenser 16 and an increase in the rate of stock removal. Such is the ease up to a point. For instance, if in the foregoing example the value of the re sistor 14 is decreased from 100 ohms to, say, 40 ohms, a significant improvement in the rate of stock removal results, such as is indicated by the curve I, K, L, M, N, O, P.

However, progressive decreases in the value of resistor 14 will finally cause an undesirable change in the operating characteristics of the circuit. Again referring to the foregoing example, with a condenser of from 5 to microfarads value, a decrease in the ohmic value of the resistor to about results in a random interruption of the cyclic behavior of the circuit by periods of relatively prolonged arcing such as is shown by the curve WX in Fig. 2. In other words, the circuit becomes unstable with the arcs of short duration, represented by condenser discharges RS, TU, and VW being intermingled with arcs of long duration such as WX.

Generally speaking, in the arc-machining circuit of Fig. 1, there is a minimum critical value of resistance for any chosen value of capacitance at specified applied voltage. If the ohmic value of resistor 14 is less than the critical minimum, the desirable cyclic action of the circuit will be interrupted by relatively long D. C. arcs and operation of the apparatus becomes erratic, speed of stock removal decreases and quality of surface finish diminishes. One reason for this is that the gap is ionized during the discharge and remains in ionized condition for a period of time thereafter, the intensity of ionization and the duration thereof depending upon the intensity of the discharge.

In attaining the objects of my invention, I have replaced the simple resistor 14 of the basic circuit with a variable resistor, the value of which may vary through a wide range-for example, from about two ohms to as much as two or three thousand ohms. In my improved circuit, means is provided for regulating the etfective value of the resistance automatically in such manner that the instantaneous value shall be high during the periods when current is flowing across the gap and low during periods when no discharge is taking place, at which time the capacitor is charging.

In addition, my improved circuit incorporates time regulating means operable automatically to delay the start of charging of the gap shunt capacitance until the gap has deionized, or substantially so.

In the improved circuit, the simple resistor of the Fig. 1 circuit has been replaced by a vacuum tube (or bank of tubes) and the inherent ability of the vacuum tube to become alternately conductive and non-conductive, depending upon the amount of grid bias voltage impressed, has been utilized.

I intend the term vacuum tube as used herein to indicate a tube evacuated to as high a degree as possible, in other words, that type of tube known in the art as a hard tube. Such tubes will not pass high currents (as will the so-called sof or gas filled tubes) but have the advantage that they will handle extremely high frequency currents under excellent grid control.

Referring now to Fig. 3, which is a schematic diagram of my improved circuit, it will be seen that a transformer 18 is provided with a primary 20 and a pair of secondaries 22 and 24. The secondaries are respectively connected across a pair of full-wave rectifiers 26 and 28. Instead of a single transformer with two secondary windings, a pair of similar transformers may be used if desired, it being the object to provide two power sources in series.

Leads 30 and 32 extend from the center taps of the respective secondaries to points 58 and 59 of the circuit, a pair of filter condensers 36 and 38 being connected between the secondary leads and the rectifiers as shown. A

lead 40 connects the anode 42 of a conventional triode vacuum tube 44 with point 46 of the circuit, which point is between the output side of rectifier 28 and condenser 38.

The cathode 48 of tube 44 is connected by leads 50 and 51 with workpiece 10, and by lead 52 with one side of capacitor 16. The electrode 12 is connected by leads 53 and 54 with the other side of the capacitor 16 and by lead 55 with point 58 (the latter being connected to condenser 36 by a lead 56 and to the center tap of secondary 22 by lead 30).

Grid 69 of tube 44 is connected by lead 61 to a grid leak resistor 62, the other side of which is connected to the cathode 48 of tube 44 by leads 5t) and 51. The grid is also connected to a coupling condenser 64, the other side of which is connected to the anode 66 of a vacuum tube 68. Anode 66 of tube 68 is connected with point 46 of the circuit through a resistor 70, and cathode 72 to the workpiece 10 and thus to the cathode 48 of tube 44.

The gap between the electrode 12 and workpiece 10 is shunted by the capacitor 16 and also by a network consisting of a resistor 74 and a condenser 76. The grid 78 of tube 68 is connected to the junction between the resistor 74 and condenser 76 through a source of bias voltage 80.

A rectifier 82 is connected between points 84 and 86 of the circuit, the connection being such that current will pass freely at such times that the voltage across capacitor 16 is greater or tends to become greater than the voltage across capacitor 36, and passage of current will be blocked at such times that the voltage across capacitor 16 is less than that across 36.

Operation of the circuit is as follows:

Let it be assumed that the electrode 12 and workpiece 10 have been brought to a satisfactory space relationship which will be thereafter maintained by manual or automatic servo-means and that coolant, preferably in the nature of a dielectric fluid, is flowing through the gap.

At that instant when a discharge across the gap has just been completed and passage of current through the gap has ceased, the capacitor 16 has been discharged to a minimum voltage condition and tube 44 is biased beyond its cutotf point through the existence of a relatively great negative voltage at grid 60 thereof, as will be subsequently explained. At this same instant, the voltage across condenser 76 is somewhat greater than that across 16, which produces an instantaneous positive signal at the grid 78 of tube 68, caused by the fact that the negative voltage existing between grid 78 and cathode 72 is of minimum value. The tube 68 will, therefore, conduct readily. Inasmuch as any current flowing through tube 68 must flow through resistor 70, anode 66 assumes a relatively negative voltage which becomes immediately effective through capacitor 64 on grid 66 of tube 44 to produce the cutoli bias mentioned above.

Under these conditions, a differential in voltage between condensers 76 and 16 cannot exist, current flowing from condenser 76 into condenser 16 thereby establishing an equilibrium or balanced condition. As this occurs, current flow through resistor 74 gradually diminishes and this reduction in current flow causes grid 78 to become progressively more negative with respect to cathode 72. This negative signal reduces current flow through tube 68 and through resistor with attendant rise in voltage at anode 66. This rise in voltage is transmitted through condenser 64 to grid 60 of tube 44, causing tube 44 to conduct when the rise in voltage is sutficient. It should be noted that the increment of time during which tube 44 is non-conducting is dependent primarily upon the time constant characteristic of the series network which includes resistor 74 and condenser 76.

Immediately that tube 44 begins to conduct, current flow through this tube will be stored in capacitor 16 with attendant rise in voltage at point 84. Because of the presence of resistor 74, the rise in voltage across condenser 76 will lag behind thatacross condenser 16. This results in the creation of a relatively negative signal at grid 78, thereby causing tube 68 to become less conductive. There is then a reduction in current flow through resistor 70 and a rise in voltage at anode 66. This rise in voltage is transmitted through condenser 64 to grid 60 which accelerates the current flow through tube 44 into condenser 16. Thus there is extremely rapid charging of condenser 16 through the rapid triggering action of tube 44.

During the time that capacitor 16 is charging, capacitor 76 is also charging, butthe rise in voltage of 76 will lag that of 16 because of the limiting action of resistor 74.

The voltage across capacitor 16 will, of course, continue to rise until it reaches the critical value required to break down the gap whereupon a discharge will occur between electrode 12 and workpiece 10. At the instant of initiation of the discharge, the voltage across capacitor 16 is still rising, current is starting to flow across the gap and current is flowing into capacitor 76.

Immediately that current starts to flow in the gap, the process is accelerated because of the decreasing resistance characteristic of the gap. In other words, as current flow increases in the gap, the voltage drop across the gap decreases and the tendency for current flow increases. This tendency, together with the low impedance of the short leads which connect the capacitor 16 across the gap, results in rapid discharge of the capacitor 16 causing a rapid drop in voltage at point 84 in the circuit. The capacitor 76 also discharges, but at a slower rate than capacitor 16.

When the condenser 16 has discharged to the point where the gap discharge ceases, the cycle has been completed and is accompanied by the appearance of a relatively positive signal at grid 78. The cycle thusrepeats in the manner described.

It will thus be seen that in the Fig. 3 circuit the tube 44-, in effect, replaces the simple resistor 14 of the Fig. 1 circuit and the shunt capacitance 16 is charged through the'tube. Because the tube will inherently never'become totallynon-conductive, its acts as a variable resistor havingan extremely high value during and just after arc discharge because of the relatively greatnegative bias on grid 60 produced by the action of tube 68 in becoming conductive at the instant of arc discharge.

In similar manner, tube 44 rapidly becomes highly conductive, and thus assumes a very low' value of resistance after the arc discharge has taken place and the arc-gap has had time to deionize. This comes about through the action of the network 74, 76, which, because of the delaying action of the resistor 74, permits condenser 76 to. have momentarily a higher voltage than condenser 16. Current flowing from 76 to 16-then, for a short period after discharge, delays buildup of negativevoltage' on grid 7 8 thereby prolonging conduction through tube 68 and giving the gap time to deionize. Immediately, however, that grid 78 becomes sufficiently negative, tube 68 gradually ceases to conduct and positive voltage rises on anode 66, which voltage is almost instantly effective ongrid 60 through condenser 64; Tube 44 thusis triggered and starts to conduct whereupon capacitor 16 is charged.

It is desired to point out that the time delay feature is of utmost importance to the highly efiicient operation of the circuit because if the tube 44- is rendered instantaneously conductive after completion of a discharge, ionization of the gap permits discharge at relatively low voltage and instability of operation results.

It can be seen that inasmuch as the intensity of the discharge in the gap afiiects both condensers 16 and 76 in proportion, equalization of voltage between them and consequently rendering of tube 68 non-conductive will be delayed or accelerated inversely in proportion to the intensity of discharge. Therefore, careful adjustment of the parameters of the circuit permits the delay period to accommodate itself to the degree of ionization of the gap, but no time is wasted in recharging the condenser 16.

It will, of course, be understood that reference herein to periods of time is in micro-seconds, the duration of the arc and other phenomena being extremely short.

A typical arc-machining apparatus embodying the Fig. 3 circuit has a power supply providing approximately 100 volts between points 46 and 58, and about 40 volts between points 58 and 86. Tube 44 is represented by a bank of fifty-four type 6AS7 or type 6336 triode tubes, and tube 68 may be one type 6AS7 tube. Condenser 16 is usually a variable condenser of five to one hundred microfarads capacity, condenser 76 has about .01 microfarad capacity and resistor 74 a value of one thousand ohms. The other elements have suitable values to suit and are not critical.

The tube bank 44 may be increased to as many as a thousand tubes if such power is required, one tube 68 being required for each group of fifty to sixty power tubes.

It is necessary to use a large number of tubes because of the inability or" presently obtainable tubes to handle relatively large amounts of power. Availability of vacuum tubes of higher power and better operating characteristics would make possible considerable simplification of the circuit. In the Fig. 3 circuit, tube 68 is necessary for the purpose of triggering tube 44, the tube 68 acting primarily as an amplifier to provide sufiicient change in voltage across tube 44 to render the latter conductive or non-conductive, as the case may be. A five volt change in voltage across tube 68 causes approximately one hundred volt change across tube 44, which is about that required to reverse the latters function.

My invention, therefore, contemplates a broader as,- pect than that actually illustrated in Fig. 3. Fig. 4 shows a simplified circuit wherein the tube 68 has been omitted. I11 the diagram, the same reference characters have been used as in Fig. 3 for the same elements. In Fig. 4, the grid 60 of tube 44 is connected to one side of the resistor 74 and one side of the workpiece 10, and the condensers 16 and 76 are connected in parallel through the resistor 74. Assuming that the tube 44 will reverse function in response to the voltage difierential across the resistor 74 (which is effective on the grid 60), tube 44 remains nonconductive until the voltage across condensers 16 and 76 equalizes sulficiently to change the grid voltage positive-then the tube will conduct.

In Fig. 3, the rectifier 82 serves to limit the voltage which can appear across the gap. to substantiallyv that which can appear between points 53 and 86. This rectifier may be omitted without adversely affecting the operating characteristics of the circuit so far as metal removal is concerned, but its presence is desirable for best overall results as more fully set forth in my co-pending application Serial No. 361,730, filed June 15, 1953.

Furthermore, it would be extremely desirable to include in the circuit means'for rendering tube 44 instantaneously non-conductive in the event of a short circuit across the gap, but inasmuch as this feature has been fully. described. and claimed in my co-pending application Serial'No. 338,789, filed February 25, 1953, I have omitted, details. herein for the sake of clarity and. simplification.

While I have shown and described specific embodiments of my invention, it will be understood that this has been done for illustrative purposes and such is not to be construed as an indication that it is the only form the invention can assume. The principles herein set forth are applicable to arc-machining apparatus of varying types and it is intended to limit the scope of the invention only as set forth in the appended claims.

I claim:

1. In an arc-machining apparatus having an electrode, means for causing intermittent electrical discharge across a gap between said electrode and a workpiece for removing material from the workpiece, a capacitor connected across said gap, means for charging said capacitor from a power source including a charging circuit, a variable resistor in said charging circuit, means for automatically varying said resistor such that the resistance value thereof is at maximum value during discharge across said gap, and means for limiting the instantaneous voltage across the gap to a value substantially lower than that of the power source.

2. In an arc-machining apparatus having an electrode adapted to be disposed in spaced proximity to a conductive workpiece, a capacitor connected across the gap between said electrode and the workpiece, a charging circuit for charging said capacitor from a power source such that intermittent electrical discharge will occur across said gap, a variable resistor in said charging circuit, means operable to maintain said resistor at a relatively low value during charging of said capacitor, means operable automatically in response to discharge across said gap for causing said resistor to assume a relatively high value, and means for limiting the instantaneous voltage across the gap to a value substantially lower than that of the power source.

3. In an arc-machining apparatus having an electrode adapted to be disposed in spaced relationship with a conducting workpiece, a source of electrical power, a capacitor, a charging circuit for said capacitor, a discharging circuit for said capacitor including the gap between said electrode and the workpiece, a variable resistor in said charging circuit, means operable automatically in response to discharge across said gap for causing said resistor to assume a relatively high value during said discharge and to assume a relatively low value when said discharge ceases, and means for limiting the instantaneous voltage across the gap to a value substantially lower than that of the power source.

4. The combination of claim 3 wherein said last means includes time-delay means for slowing return of said resistor to low value until said gap has substantially deionized.

5. In an arc machining apparatus having a capacitor connected in shunt across a gap between an electrode and a workpiece, a power source, a charging circuit for said capacitor including a resistor, means for varying the magnitude of resistance value of said resistor, said means being operable automatically in response to current flow in the gap to regulate the value of said resistor in direct proportion thereto, and means for limiting the instantaneous voltage across the gap to a value substantially lower than that of the power source.

6. The combination of claim 5 wherein said automatically operable means includes time-delay means for slowing decrease in resistance value of said resistor after discharge across said gap until said gap has substantially deionized.

7. The combination of claim 5 wherein said resistor comprises a vacuum tube.

8. The combination of claim 5 wherein said resistor comprises a vacuum tube and said automatically operable means comprises a second vacuum tube.

9. In an arc machining apparatus having a capacitor connected in shunt across a gap between an electrode and a workpiece, a power source, a charging circuit for said capacitor including a resistor, means for varying the magnitude of resistance value of said resistor, said means being operable automatically in response to the difference of potential across said capacitor in inverse proportion relatively thereto, and means for limiting the instantaneous voltage across the gap to a value substantially lower than that of the power source.

10. In an arc-machining apparatus having a condenser connected in shunt across a gap between an electrode and a workpiece, a charging circuit for said condenser, 21 triode vacuum tube connected in said charging circuit between said condenser and the power source with the anode of said tube connected to the positive side of said power supply and the cathode and the grid of said tube connected to one side of said condenser, and a network comprising a condenser and resistor in series connected in parallel with said first condenser.

ll. The combination of claim 10 wherein a second triode tube is connected in the circuit with its anode connected to the grid of said first tube, its cathode connected to said network resistor remote from the network condenser and its grid connected to the junction between the network resistor and network condenser.

12. In an arc-machining apparatus, an electrode, means for disposing said electrode in spaced relation to a workpiece such that intermittent electrical discharge across the gap therebetween will erode the workpiece, a first D. C. power source having the positive side thereof connected to the workpiece and the negative side thereof connected to the electrode, a condenser connected in shunt with said gap, a second D. C. power source connected in series with said first source, a vacuum tube connected in the positive lead of said second power source and control means operable automatically in response to discharge across said gap for rendering said tube substantially nonconductive upon discharge across said gap and conductive upon cessation of said discharge.

13. The combination of claim 12 including means connected in the circuit of said first power source for limiting the magnitude of the voltage across the gap in event of short circuit thereof to substantially that of said first power source.

14. The combination of claim 12 wherein said control means comprises a triggering network connected in the grid circuit of said tube.

15. The combination of claim 14 wherein said triggering network comprises a second vacuum tube and control means therefor operable to render said second tube substantially non-conductive during charging of said condenser and conductive in response to discharge of said condenser.

References Cited in the file of this patent UNITED STATES PATENTS 2,310,092 Knowles Feb. 2, 1943 2,483,691 Dawson Oct. 4, 1949 2,495,301 Wengel Jan. 24, 1950 2,515,634 Dawson July 18, 1950 2,628,330 Williams Feb. 10, 1953

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