US3084245A - Electro-discharge machining of metals and the like - Google Patents

Description

tates This invention relates to the electro-discharge machining of metals for reproducing from or on a block of metal the form or surface contour of a model by electrically cutting or eroding the block of metal with an electric spark or are from the model and, more particularly, to liquid hydrocarbon compositions in which such electric machining is carried out with the model and the block of metal immersed in the hydrocarbon composition for enhancing the speed and efficiency of the operation. This application is a continuation-in-part of copeuding application Serial -No. 13,411, filed October 22, 1959.

Techniques or processes of the character to which this invention relates have to do with the well known procedure of electro-discharge machining or (as hereinafter sometimes referred to) electro-erosion, whereby there is obtained in a block of metal a reproduction of the desired surface contour or formation of a model of the desired finished article by electrically eroding or cutting away the metal of the block of raw material with a spark or are engendered by the model as an electrode. Thus, generally, such techniques involve the producing of an electrical discharge between a model and the block or unfinished piece of metal onto which it is desired to impress or erode a reproduction (including a negative reproduction) of the configuration of the model. Such erosion or cutting is provided by having the model as, for example, the cathode of an electrical circuit and the block of metal to be formed as the anode of the circuit.

With such an arrangement, as is well understood in this art, electric discharge between the model-cathode and the rough block-anode erodes or cuts away those portions of the block-anode to correspond with or reproduce the surface of the model, and, particularly, as the model is advanced toward the block of metal being worked during the operation of the process. That is, the impressed electric current provides that the block of metal or other material being worked is constantly eroded, as the anode, while the model or cathode is moved toward the metal being worked, and, consequently, progressively is introduced toward or into the metal being worked to tiorm a reproduction of the model in the surface of the rough block.

Such metal working techniques are well known and, as will be understood, provide a method for rapidly and readily forming in a block of metal various shapes and configurations which would be quite difiicult to obtain by other more classic means of metal working such as turning, grinding, cutting, etc.

Such an electro-erosion forming operation may be carried out, as is well understood, with the anode and cathode surrounded by air or various other media. Certain enhanced results are achieved if the medium surrounding the anode and cathode at least in the portions thereof where the eroding or forming action occurs, is a liquid medium or a medium other than air, advantageously circulated around the electrodes. Such a liquid medium has been found to produce a number of advantages in operation. For example, by ionization when electrons initially are emitted by the cathode, it liberates suppleice mentary electrons which give up kinetic energy at the point of impact on the anode or block being formed; similarly, a surrounding liquid medium absorbs heat energy liberated during the operation and also may aid materially in the physical elimination of particles of metal eroded by the electrical operation from the anode.

As noted above and as is well understood in this art, it is particularly desirable, from the standpoint of enhancing the economies of a commercial operation involving electro-erosion, to increase the speed with which the eleotr-o-eroded reproduction can be achieved (i.e., the speed with which satisfactory results are obtained in advancing the cathodic model toward the anodic metal to be formed), as well as by reducing as much as practicable the erosion or consumption of the model electrode in use, and, just as some enhancing of these desirable qualities may be achieved by utilizing a liquid medium in which the electrodes are immersed during the electroerosion operation, so also may such advantages of a liquid bath be further enhanced by the particular choice of composition of the liquid bath.

According to this invention, then, a hydrocarbon liquid medium is provided, particularly adapted for use in an electro-erosion operation of the character described, and utilizing a dielectric hydrocarbon liquid which, particularly, permits augmenting the speed of erosion and permits, for the same consumption of energy, an increase in the yield of the process as well as substantially diminishing the consumption or erosion or necessary wastage of the cathodic driving model or electrode. Such liquid medium comprises primarily hydrocarbons having a carbonto-hydrogen weight ratio of equal to or greater than 7, which hydrocarbons are readily obtained from various treatments of certain petroleum fractions and otherwise in accordance with this invention as noted in more detail below; and, preferably, such liquid hydrocarbon medium includes aromatic and solvent-extracted cuts or fractions of petroleum distillates and/or monocyclic monoalkyland polyalkyl-benzenes of the general formula C H and having the above noted carbonhydrogen weight ratio, all treated in accordance herewith to produce a liquid dielectric medium adapted for achieving the enhanced results disclosed in an electroerosion operation of the character to which this invention relates.

With the foregoing and other objects in view, as will be apparent from the disclosure herein and from the following description and the appended claims, several illustrative embodiments of materials and steps embodying and for practicing this invention may now be described in somewhat greater detail.

As has been noted, there are certain advantages to be obtained, with an electro-erosio-n operation of the character to which this invention relates, if the electrodes are surrounded by a liquid dielectric medium, instead of air or other gaseous media. Among these, as noted, may be counted the enhancement of an ionizable liquid media, the aid in controlling and dissipating heat engendered at the eroding surface by the electric discharge, and the fluid dissipation of metallic particles removed from the surface of the rough block or workpiece being eroded or formed into the desired final configuration. It has been found, however, and particularly in accordance with this invention, that certain important enhanced advantages are obtained in connection with the utilization of liquid compositions embodying this invention in such electro-erosion operations, and that such advantages are substantially unexpectedly enhanced overrthe results achieved with other liquid preparations previously utilized in such operations, and particularly by comparison with similar hydrocarbon substances having a carbon-to-hydrogen weight ratio of less than 7, at which a point a discernibly sharp diiference in desirable characteristics occurs (although, perhaps, for reasons which may not now be completely understood) by comparison even with hydrocarbons having less than such ratio but incidentally other characteristics (such as viscosity, boiling point, flash point, etc.) which might suggest operative or desired applicability thereof for the purposes in accordance herewith.

Thus, particularly satisfactory results have been obtained in accordance herewith by using, as the liquid immersion medium for electro-crosion process of the character described, aromatic hydrocarbons having a carbon-hydrogen ratio in excess of 7, which hydrocarbons are extracted, in known manner, from various petroleum distillates and treated, in accordance herewith, to produce the particularly adapted liquid medium for achieving the enhanced results hereof. Similarly, satisfactory or preferred results have been achieved in accordance herewith by using, particularly, aromatic hydrocarbons belonging to the family of monocyclic monoalkyl and/ or polyalkyl benzenes, as noted above, with the desired carbon-hydrogen weight ratio. Other desirable characteristics of hydrocarbons for use in connection herewith will be understood and are well known in the art of electrodischarge machining (and, particularly, for so-called inside machining where the entire workpiece is immersed in the dielectric bath, as compared with outside machining where merely a film of dielectric liquid is maintained between the electrode and a workpiece forming a spark gap therewith).

Also, as will be understood, some of the characteristics of the hydrocarbon liquids in accordance herewith and/or the dielectric baths including such hydrocarbons may vary somewhat depending upon or in accordance with the particular machining operation being used and/ or such characteristics thereof as the particular type of apparatus being used, the desired speed of cutting, the configuration being cut, etc. For example, in most instances one would not desire to utilize a hydrocarbon having an excessively high vapor tension such as associated with hydrocarbons boiling above about 260 C. at atmospheric pressure. Similarly, as will be understood, hydrocarbon baths in accordance herewith may comprise one or another of individual hydrocarbons disclosed, or mixtures thereof, or mixtures of various hydrocarbons which, individually and alone, may not come within the definitions hereof provided that such mixture thereof in the bath as used is in accordance with the ranges of weight ratios of carbon to hydrogen, etc., in accordance herewith.

Also, as will be understood, such hydrocarbon materials may be obtained from a variety of sources and by a variety of diiferent means or processes. For example, they may be produced by thermal or catalytic decomposition or conversion of hydrocarbons, by chemical synthesis techniques, by physical separation or distilla tion, or by combinations thereof. Satisfactory results in accordance herewith have been achieved with hydrocarbon materials prepared, in known manner, by such well known cracking processes as catalytic platforming, or reforming or hydrofining, by steam cracking, etc., whereby cuts or fractions are produced rich in alkylsubstituted aromatic hydrocarbons. Similarly, various distillation separation or fractionating techniques have been used, in known manner, as well as solvent extractions such as with fur fural, phenol, sulfurous anhydrides, nitrobenzene, dimethyl-formamide and its analogues, etc. It may be particularly noted that, in modern commercial or industrial practice, various of the alkylbenzenes are also being produced more and more by the direct alkylation of various simpler aromatic hydrocarbons, such, for example, as dodecy-lbenzene by alkylation of benzene with :tetramers of propylene, with the production of, in addition to the desired product, a non-aromatic fraction and fractions alkylated to various degrees such as etc.-, benzenes, which materials, suitably selected in ac cordance herewith satisfactorily provide hydrocarbon liquids for electro-discharge machining, and have been utilized by themselves or admixed with other materials or concentrated by solvent extraction and/or distillation, as noted above.

As purely illustrative of materials and treatments in accordance herewith and for achieving the enhanced results hereof in electro-erosion processes regarding the above noted increase in speed of the cutting or eroding or forming cycle, the reduced consumption of the model electrode, etc., may be noted several particularly satisfactory examples, although these following examples are set forth, as will be understood, merely as further illustrative of the invention and not by :way of limitation thereof.

For example, satisfactory results according to this invention were achieved by taking a fraction of Middle East crude petroleum having been distilled and separated to provide a cut of "a viscosity (at 210 F.) of 3.9 centistokes, a density (at 115 C.) of 0.894, and a refractive index (11 of 1.4842. Such distillate was initially dewaxed to a pour point of -12 C., and then was submitted to a fur-fural extraction using 180 volumes of furfural for each volumes of the petroleum cut.

According to such treatment, there was obtained, after a separation of the furfunal phase and after a stripping or distillation of the solvent, a residual material for use in accordance with this invention having the characteristics, among others, as follows:

PRODUCT A Density at 15 C 1.031 Refractive index 11 1.5751 Viscosity (centistokes):

at 100 F 9 2.19

at 50 C 44.0

at 210 F 6.77 Flash point (L-uchaire), C Four point, C -12 Weight ratio of C/H 9.2

As noted, satisfactory results were obtained with the foregoing preparation in an electro-erosion machine of known construction, with the foregoing preparation as an immersion fluid or dielectric medium, instead of other petroleum or lubricating oils heretofore utilized, to produce the enhanced results noted in the electro-erosion or forming of a metal article from hard steel.

As illustrated of the enhanced advantages achieved by utilizing compositions embodying and for practicing this invention, as compared with conventional lubricating oil media, for an electro-erosion operation of the character described, one may note centain comparisons re.- garding such considerations as the impressed potential, the speed of forming the desired article in the electroerosion operation, and the amount or rate of consumption of the model electrode or forming anode with different liquid baths. For example, comparisons have been made as between the Product A noted above and either or both of a more conventional liquid bath for an electroerosion operation composed of, for example, materials resenting the following characteristics:

Product Product B 0 Density at 15 C 0.ss7 0.874 Refractive Index 1. 463 1. 465 Viscosity (Gentistokes):

At 100 F 20.2 40, 6 At 50 0.-.. 14

3. 9 6.1 l2 l2 6. 4 G. 3

Merely for comparing and emphasizing the enhanced advantages of a composition such as Product A in accordance with this invention with compositions such as Product B or Product C as previously used as liquid immersion media for an electro-erosion process of the character to which this invention relates, one may note that, in electro-erosion apparatus as above noted, comparative data were obtained in connection with forming a block of extra-hard steel (with a carbon content of about 1%) by measuring, among other things, the volumes of metals eroded or removed from the steel block (as the anode) per unit time for comparable or equivalent operations involving separately the several liquid media mentioned, such measurements forming, as will be understood, an expression of the speed of cutting or erosion in the forming operation, and such data were obtained as a function of the electric potential impressed upon the system. Thees data are tabulated in the following Table I:

Table 1 Product A Product B Product As will be understood from the foregoing, the speed of cutting or erosion is, quite desirably, approximating 40% to 50% higher with the same impressed electrical potential when the surrounding liquid medium is one in accordance with the provisions of this invention, as compared with the previously known Products B and C. As will also be understood, however, the speed of cutting or erosion is not, in a commercial or mass-producing operation, necessarily the only element of important consideration.

Thus, for optimum efficiency in an electro-erosion operation of the character to which this invention relates, one should consider the consumption or rate of consumption of the cathodic electrode model, because, indeed, the overall efficiency may not, necessarily, be enhanced if the speed of cutting is raised or increased only with a sacrifice in the original model, as by a concomitantly increased rate of consumption thereof requiring, of course, eventual or frequent replacement of the model.

Utilizing compositions embodying and for practicing this invention, however, it is noted that, for any given electrical potential, the products embodying this invention, while producing a faster forming or cutting or eroding rate, also achieve a status of operation where the anodic model is less consumed.

Comparative data illustrating this enhanced advantage according to this invention may be noted as follows:

Table 11 Electric Potential (kw.)

Consumption of Cathode: 100x mm loss at cathode mm eliminated at anode Product A- Product B Product 0.

mam

6 erosion process according to this invention along both the lines of increased rate of cutting and decreased rate of cathode consumption.

As further illustrative of the enhanced advantages achievable with compositions embodying and for practicing this invention as oil immersents for the cutting electrodes of an electro-erosion device of the character described and as mentioned above, one may note a number of comparative data collected with regard to an other composition in accordance with this invention, which will be here designated as Product D.

Thus a distillation cut of crude petroleum having a viscosity of approximately 7.2 centistokes at 200 F. was de-waxed'and treated with an extracting treatment of furfural. The portion of this cut soluble in furfural Was separated by solvent extract and by distillation. To volumes of this separated product was added some 20 volumes of kerosene to adjust the viscosity thereof to a desirable range, and the resulting admixture (as Product D) exhibited the following characteristics:

PRODUCT D Density at 15 C. 0.965 Viscosity (centistokes):

at F. 34.9

at 200 F. 4.6 Flash point (Luchaire), C. 55 Pour point, C. 9 Weight ratio of C/H 8.10

As noted, with other compositions embodying and for practicing this invention, an enhanced comparative result is shown over other lubricating oil immersion media for electro-erosion systems. For example, comparative data are noted below in Table III comparing the above noted Product D with the previously noted Product B as follows (the units of notation being the same as above):

As a further comparison of the enhanced results attributable to compositions embodying and for practicing this invention, one may note the comparative results achieved with, for example, a lubricating oil composition such as that described above as Product C, as compared with another product according to this invention. For example, satisfactory results are obtained with a composition (which may be herein designated as Prodnot B) being produced from a dc-waxed petroleum crude submitted to an extraction treatment by a substance such as phenol, with the extracted products in the phenol phase being mixed with a 10% hydrocarbon extract from a cut of kerosene washed with liquid sulfuric anhydride. The designated mixture (Product E) has the characteristics noted below:

. PRODUCT E Density at 15 C. 0.988 Viscosity (centistokes):

at 100 F. 73.4 at 210 F. 6.4 Pour point, "C 20 Weight ratio of C/H 8.48

When comparative runs were made using and comparing the results of the above mentioned Product C, for example, with the above recited Product B, it was found that the Product E in accordance with this invention produces (in accordance with the units of notation previously mentioned) a speed of cutting or formation (at a 6 kw. potential of approximately 685 as com- 6 pared with the aforementioned speed of cutting or Prodnot C of 515 (both calculated in terms of mm. per min. as above noted). Furthermore, the consumptions of the model or cathodic electrode, expressed as shown in Table II, are 15 with the Product C, andonly 11 with the Product E of the present invention.

One interesting property of the hydrocarbon compositions embodying and for practicing this invention should be emphasized, as will be noted from the foregoing, in that the utilization of the various compositions herein in an electro-erosion operation or technique of the character described as dielectric media for immersion of the various electrodes produces a certain superficial hardening, perhaps by cementation on the particular part or by tempering the particular metal to the electric heating thereof and the bathing thereof in the liquid medium disclosed, so that a definite metallurgical advantage is obtained. Actually, such enhanced advantages may include the aspect of the superficial hardening to a certain extent through the surface of, for example, steel treated in accordance with the foregoing, and the tempering thereof, as one would expect from the intense heating by the electrode discharge of rough steel in a bath of oil, as one might temper a steel member by heating it and then quenching it in a bath of oil. It is interesting to note the comparative data which, according to the foregoing, has been developed from various satisfactory uses of, for example, Product A as compared with Product B in an operation embodying and in accord ance with this invention. For example, comparative data in Table IV below, illustrative of the invention, is set forth as between a Product A in accordance with the invention and the previous Product B:

Naturally, the metallurgical qualities of any material treated in accordance with this invention may be expected to be altered as the material is heated, in connection with the electro-erosion thereof, in the presence of a circulating liquid in accordance with this invention, and, as will be well understood, the superior treatment of any material in accordance with this invention will, as is well understood, be calculated so as to accommodate whatever treating or other metallurgical or crystalographic changes may be undergone in accordance with the electro-erosion heating as immersed in one or another of the materials or compositions set forth herein.

As will be understood, of course, the electro-erosion treatment according to this invention, dependingupon, to some extent, the particular composition of the metal being treated, may, indeed, have no eflect whatsoever, metallurgically, on the metal being treated. Actually, as will be noted from the foregoing and as well understood in this art, the surface hardening mentioned above depends, on a large measure, on the viscosity, among other factors, of the composition in which the metal being treated is immersed during the electro-erosion treatment. For example, it is quite possible, by the appropriate admixture of various substances in the immersing liquid, to regulate to a substantial extent the amount of temper or superficial hardness of the final workpiece. As illustrative of the foregoing, one may note the following example, which is to be comprehended within this invention, but is merely set forth herein as illustrating the tempering or surface hardening effect (or the avoidance thereof) achievable as a concomitant or auxiliary efiec-t of the electro-erosion process in accordance with the instant in- ,vention.

For example, taking Product A as previously described and fractionating this particular hydrocarbon substance by distillation in a vacuum, it is possible to recover a head fraction containing approximately 25% of the total product previously described-which head fraction is to be designated here as Product F.

As illustrative of such an operation, a number of comparative data were obtained with regard to these various Products A and F, and mixtures thereof. As further illustrative, one might note that an equal mixture of such Product A and Product P (which mixture should be considered here as Product G) was utilized in applying this invention to a block of extra-hard steel, with satisfactory results, and with the comparative results, all indicating the enhanced advantages attributable to liquid media embodying and for practicing this invention as, for example, are set forth in Table V:

As additionally illustrative of other materials with which satisfactory results have been achieved in accord ance herewith may also be noted the following:

PRODUCT H An alkyl-substituted mono-aromatic hydrocarbon corresponding approximately to the formula 0 1-1 boiling between about 205 225 C. at atmospheric pressure, and having a carbon-hydrogen weight ratio of about 8.05 was used as the dielectric liquid bath for an electro ero'sion machining operation on a block of extra hard steel (at about 1% carbon) with a copper electrode under power input of the order of 1 ,kw. at a frequency of about 3600 hertz, to achieve satisfactory cutting conditions and a desirably fine surface finish. By comparison with comparable results obtained in working under the same conditions but using as the dielectric bath an ordinary light petroleum or kerosene fraction usually employed in such an operation, the cutting speed here was increased by a factor of about 1.65 that of the conventional technique, while the wastage or consumption of the copper electrode was only about half that experienced with the conventional hydrocarbon outside the ranges in accordance herewith.

PRODUCT I accordance herewith using hydrocarbons near the minimum recited ratios, comparisons of the results achieved are noted below in Table VI, in which the cutting speed is noted in mmfi/ minute and the electrode consumption in terms of the percentage ratio of volume of material lost at the cathode to volume eliminated at the anode:

Table VI Speed Consump tion Dodecylbenzene 288 a 16 Deodorlzed Kerosene 31 '9 Considering the results obtained with deodorized kerosene as a base index of 100, the foregoing indicates that the utilization of the dodecylbenzene (with a carbon-hydrogen weight ratio only just above the minimum of 7 disclosed herein) gives a cutting speed index of 150, but an electrode-consumption index of only 52.

PRODUCT J A dielectric bath in accordance herewith was formed from a residue fraction of the distillate from a catalytic reforming process having the following characteristics:

Such hydrocarbon product was utilized in an electr c-discharge machining operation as noted withthe two previous examples, and satisfactory results were achieved with a cutting speed of 265 mm. /minute, and With an electrode consumption of only 19%. By comparison with the above noted deodorized kerosene and results as a base index of 100, Product I produced the results of a speed index of 140 and an electrode-consumption index of only 61.

PRODUCT K A light fraction of alkylated products provided from the manufacture of dodecylbenzene by alkylation of henzene with tetramers of propylene was utilized and evi denced the following characteristics:

Density at 15 C 0.862 A.S.T.M. distillation, C.:

Initial 94 166 20% 187.5 50% 205 80% 276 90% 35 6 Final 370 Aromatic content (wt. percent) 100 Weight ratio of C/H 7.9

In the utilization of this product K as above, a cutting speed of 260 mm. minute was achieved with an electrode consumption of only 19%, which, by comparison with the above noted index of deodorized kerosene, results in a speed index of 135 but a consumption index of only 61. By contrast, and as further exemplifying the sharp or pronounced change in desirable characteristics of hydrocarbons at the minimum weight ratio value of 7 in accordance herewith, a heavier alkylated fraction provided by the same manufacture of dodecylbenzene, but having a density of 0.876 and distilling between 325 C. and 380 C., and a carbon-hydrogen weight ratio of only 6.95 produced results appreciably and significantly inferior in the same electro-discharge machining operation, and not greatly or significantly better than a kerosene bath.

Whereas, as will be understood, this invention is not limited to the utilization of the particular materials set forth in the various illustrative examples above, it may be noted that the alkyl benzene materials within the weight ratio disclosed have produced particularly favorable results especially when it is desired to obtain a fine finish on the surfaces being machined, as well as when it is particularly desired to utilize relatively low power input (of the order of 1 kw.) for the machining operation. If, on the other hand, it is desired to utilize a higher power input for machining, other aromatic extracts may be preferred, such as those obtained in the course of solvent refining of petroleum distillates during lubricating oil manufacture, as exemplified by such materials as noted above as Product A or Product B, etc. In such cases, also, it may be desired to adjust the viscosity of such heavier materials to a more convenient range by an admixture of diluent for the dielectric bath since, as will be understood, such extracts from processes of manufacturing lubricating oils normally produce viscosities in a more or less high range.

According to the foregoing, as will be noted, a definite increase in the efiiciency and/or other optimum utilization of electro-erosion processes of the character to which this invention relates results from utilization, as an immersion medium for the opposed electrodes of such a process, the compositions embodying and for practicing this invention, and, also, as noted in the foregoing, such liquid compositions are, indeed, available from the various hydrocarbon and/or petroleum fractions disclosed. Accordingly, in such eleotro-erosion operations, it now becomes possible, in connection with the utilization of compositions embodying and for practicing this invention, to achieve a degree of speed and/ or eficiency substantially beyond that which may have been obtainable by the previously utilized liquid media noted.

While this invention has been described and exemplified in terms of its preferred embodiments, those skilled in the art will appreciate that modifications can be made without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. In a method of the character described for the electric discharge machining of a block of metal by cutting the metal with an electrical discharge from an electrode, the steps which comprise immersing said block of metal and said electrode in a dielectric bath comprising aromatic petroleum hydrocarbon, said hydrocarbon bath having a carbon-to-hydrogen weight ratio in excess of 7, and maintaining said metal in said bath during said electric dis charge machining thereof.

2. In a method of the character described for the electric discharge machining of a block of metal by cutting the metal with an electrical discharge from an electrode, the steps which comprise immersing said block of metal and said electrode at least in the area of electrical discharge therebetween in a dielectric bath comprising solvent extracted, aromatic petroleum hydrocarbon, said hydrocarbon bath having a carbon-tohydrogen Weight ratio in excess of 7, and maintaining said metal in said bath during said electric discharge machining thereof.

3. In a method of the character described for the electric discharge machining of a block of metal and the like by cutting the metal with an electrical discharge from an electrode model of the final configuration to which the block of metal is to be machined, the steps which comprise immersing said block of metal and said electrode model in a dielectric bath consisting essentially of aromatic petroleum hydrocarbon, said hydrocarbon bath having a carbon-to-hydrogen weight ratio in excess of 7, and maintaining said metal in said bath during said electric discharge machining thereof for tempering said metal in said bath and under the influence of heat from said electric discharge.

4. In a method of the character described for the electric discharge machining of a block of metal and the like by cutting the metal with an electrical discharge from a cathode model of the final configuration to which the block of metal is to be machined, the steps which comprise immersing said block of metal and said cathode model at least in the area of electrical discharge therebetween in a dielectric bath consisting essentially of solvent extracted aromatic petroleum hydrocarbon, said hydrocarbon bath having a carbon-to-hydrogen weight ratio in excess of 7, maintaining said metal in said bath during said electric discharge machining thereof for tempering said metal in said bath and under the influence of heat from said electric discharge, and adjusting the viscosity of said hydrocarbon bath by admixture therewith of a difierent liquid hydrocarbon dielectric for controlling the extent of said tempering action, said carbon-to-hydrogen weight ratio of said bath still being in excess of 7 after said admixture of said different hydrocarbon.

"5. In a method of the character described for the electric discharge machining of a block of metal and the like by cutting the metal with an electrical discharge from an electrode, the steps which comprise immersing said block of metal and said electrode in a dielectric bath comprising a furfural-extracted aromatic petroleum hydrocarbon fraction, said hydrocarbon bath having a carbon-to-hydrogen weight ratio in excess of 7, maintaining said metal in said bath during said electric discharge machining thereof, and adjusting the viscosity of said hydrocarbon bath by admixture therewith of a difiFerent liquid hydrocarbon dielectric for controlling the effect thereof on said metal during said machining, said carbon-tohydrogen weight ratio of said bath still being in excess of 7 after said admixture of said different hydrocarbon.

6. In a method of the character described for the electric discharge machining of a block of metal and the like by cutting the metal with an electrical discharge from an electrode, the steps which comprise immersing said block of metal and said electrode in a dielectric bath comprising a phenol-extracted aromatic petroleum hydrocarbon fraction, said hydrocarbon bath having a carbonto-hydrogen weight ratio in excess of 7, maintaining said metal in said bath during said electric discharge machining thereof, and adjusting the viscosity of said hydrocarbon bath by admixture therewith of a different liquid hydrocarbon dielectric for controlling the eflect thereof on said metal during said machining, said carbon-tohydrogen weight ratio of said bath still being in excess of 7 after said admixture of said different hydrocarbon.

7. The method of electric discharge machining as recited in claim 1 in which said dielectric bath comprises a monocyclic alkylated benzene hydrocarbon having the general formula C H 8. The method of electric discharge machining as re cited in claim 1 in which said dielectric bath comprises dodecylbenzene.

9. The method of electric discharge machining as re- References Cited in the file of this patent UNITED STATES PATENTS 2,628,330 Williams Feb. 10, 1953 2,818,490 Dixon et al Dec. 31, 1957 2,884,313 Browne Apr. 28, 1959 2,962,577 Webb Nov. 29, 1960

Claims (1)

1. IN A METHOD OF THE CHARACTER DESCRIBED FOR THE ELECTRIC DISCHARGE MACHINING OF A BLAOCK OF METAL BY CUTTING THE METAL WITH AN ELECTRICAL DISCHARGE FROM AN ELECTRODE, THE STEPS WHICH COMPRISE IMMERSING SAID BLOCK OF METAL AND SAID ELECTRODE IN A DIELECTRIC BATH COMPRISING AROMATIC PETROLEUM HYDROCARBON, SAID HYDROCARBON BATH HAVING A CARBON-TO-HYDROGEN WEIGHT RATIO IN EXCESS OF 7, AND MAINTAINING SAID METAL IN SAID BATH DURING SAID ELECTRIC DISCHARGE MACHINING THEREOF.