GB1579679A

GB1579679A - Electro-erosion process using dielectric liquids

Info

Publication number: GB1579679A
Application number: GB27611/77A
Authority: GB
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1976-07-16
Filing date: 1977-07-01
Publication date: 1980-11-19
Also published as: SE7708231L; FR2358237A1; DE2632180A1

Description

(54) ELECTRO-EROSION PROCESS USING DIELECTRIC LIQUIDS (71) We, THE BRITISH PETROLEUM COMPANY LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- The invention relates to electro-erosion processes using hydrocarbon-based dielectric liquids. In these modern versatile processes the machining of electricallyconductive materials is effected by spark discharges between a tool electrode and a workpiece in a working liquid. The particular advantage of these processes is to be found in the great accuracy with which objects of complicated shape can be produced even from very hard materials.

The economy of electro-erosion processes is determined essentially by the wear of the tool electrode and by the volume of material which can be removed in unit time, that is to say the removal performance on the workpiece. These two factors can be influenced by the nature of the working medium. Apart from water and silicone oils, the use of which is only possible or advisable in special cases, hydrocarbon oils especially mineral oil products, are mainly used in practice.

With these hydrocarbon oils, which have a viscosity range from about 1.5 to 30 cSt at 20"C, the efficiency of the initial process, during which the bulk of the removal of the metal from the workpiece takes place, increases as the viscosity increases. On the other hand, however, the finishing operation, which is necessary for smoothing the surface of the metal, can only be carried out with comparatively low-viscosity dielectrics (from 1.5 to about 6cSt at 200C), because in this stage of machining, efficient scavenging must be ensured even with very small working gaps between electrode and workpiece. For this reason, for many fields of application which first of all require as large as possible a removal of metal and then a careful smoothing of the surface, an optimum mode of operation is only possible if the working medium is changed. As a result of this change of dielectric, however, the economy of the process suffers. In practice, therefore, both operations are normally carried out with the same working medium. Because of the finishing operation lower viscosity working media are preferred although this means that operations have, of necessity, to put up with lower removal performances in the main removal stage.

Another disadvantage which is bound up with the use of conventional hydrocarbon oils is the high wear and tear of the tool electrode, which occurs particularly during the main removal stage. This wear and tear increases with the rate of removal and may amount to up to 30 per cent of the amount of metal removed from the workpiece.

Attempts have already been made to improve the removal performance whilst at the same time reducing the wear and tear of the electrode by a careful selection of the hydrocarbons used as the working medium or by the addition of additives.

Thus, for example, from British Patent Specification 1 003 664 polycyclic aromatics with a carbon/hydrogen ratio of 0.8:1 to 1.8:1 are known as working liquids for electro-erosion. The substances described there, however, are compounds in whose molecules homo-aromatic and hetero-aromatics are directly joined to one another. Quite apart from the fact that these compounds in many cases possess an extremely unpleasant odour, their melting points are usually too high in relation to the working temperatures of electro-erosion machines. They are therefore, at most, suitable for use as working liquids for metalworking by electro-erosion after they have been hydrogenated or in the form of eutectic mixtures.

In order to obtain higher rates of removal the use of specially blended spark erosion oils has also already been recommended. Thus, US Patent Specification 3 639 275 describes mineral oii-based dielectric liquids containing certain organic alkaline earth sulphonates and phenols. US Patent Specification 3 648 013 shows the use of conductivity improvers, consisting of chromium salts of special alkylsalicylic acids and alkaline earth salts of selected alkylsulphosuccinic acids.

US Patent Specification 3 708 422 mentions long-chained aliphatic amines as additives.

Apart from the difficulties which are bound up with mixing what are often very small quantities of additives, the blended products described have the disadvantage that, as a result of the losses of oil by evaporation occurring particularly during the main removal stage, the additives rapidly accumulate or are subjected over a period of time to decomposition by the electric discharges, as a result of which negative effects can occur on the erosion process. Also, with the blended products, a sufficient reduction in the wear and tear of the electrode is not obtained.

The problem underlying the present invention therefore consisted in finding hydrocarbon oils which are easy to produce as working fluids for metalworking by electro-erosion, which do not display the disadvantages described above, but instead during the main removal stage permit of increased rates of removal on the workpiece whilst at the same time ensuring a reduced wear and tear of the tool electrode, and yet furthermore are also suitable for use in the finishing operations.

The invention therefore relates to an electro-erosion process using hydrocarbon-based dielectric fluids characterised in that the dielectric fluids contain at least 30 per cent by weight of one or more compounds of the general formula I

in which Rl, R2, R3, R4, Ra, R6, R7 and Ra each signify a saturated straight-chained or branched alkyl radical with 1 to 6 carbon atoms or a hydrogen atom, and n has a value 0 or 1, and when n=0 the sum of the carbon atoms present in the alkyl radicals R, to R6 is at most 6, and when n=l the sum of the carbon atoms present in the alkyl radicals Rl to Ra is at most 10, and to the extent ofO to 70 per cent by weight of known electro-erosion fluids, as hereinafter defined.

When n=O, the compounds of the general formula I have the general formula la

in which R1, R2, R3, R4, R5 and Ra each signify a saturated straight-chained or branched alkyl radical with I to 6 carbon atoms or a hydrogen atom, the sum of the carbon atoms present in the alkyl substituents being at most 6.

If all the substituents R, to R6 are hydrogen atoms, the compound is benzylbenzene (diphenylmethane).

When n=l, the compounds of the general formula I have the general formula Ib

in which R1, R2, R4, Ra, R6, R, and R8 each signify a saturated straight-chained or branched alkyl radical with 1 to 6 carbon atoms or a hydrogen atom, the sum of the carbon atoms present in the alkyl substituents being at most 10.

If all the substituents R, to Ra are hydrogen atoms, the compound is dibenzylbenzene.

Examples of suitable alkyl radicals are methyl and ethyl groups and also straight-chained or branched propyl, butyl, pentyl, and hexyl groups.

Particularly preferred compounds of the general formula I are the monomethyl derivatives of benzylbenzene and dibenzylbenzene. In the benzyl radicals the alkyl radicals can be located in the o, m and/or p position in relation to the methylene group.

In the compounds of the general formula Ib the two benzyl groups on the central benzene ring can be in the o, m or p position in relation to one another.

The electro-erosion fluids used in the invention preferably have a viscosity which does not exceed 40 cSt at 200C. It is true that some compounds of the general formula I possess melting points which are above 20"C. Generally speaking, however, during the production of the possibly substituted benzylbenzenes and dibenzylbenzenes mixtures of isomers which are liquid at ordinary temperature are obtained and can therefore be used without difficulty in electro-erosion machines.

It is also possible by simply mixing compounds of the general formula I of different composition or structure to bring the melting point of the electro-erosion fluid to the value which is required for practical use.

The compounds of the general formula I are technically comparatively easily obtainable by known processes.

By mixing compounds of the general formula I with known electro-erosion fluids it is possible to improve the properties of the latter especially as regards the removal performance and the wear and tear of the electrode. Although, as previously indicated, the proportion of known electro-erosion fluid may be as high as 70 per cent by weight, amounts in the range from 0 to 50 per cent by weight of the total mixture are preferred.

The term "known electro-erosion fluids" means mineral oil fractions or other known hydrocarbon-based fluids.

Suitable mineral oil fractions are, for example, white spirit, kerosene, gasoil and light lubricating oil cuts. These mineral oil fractions, according to their boiling range, have a viscosity in the range from 1 to 40 cSt at 200 C.

It is also possible to mix alkyl-substituted benzenes with the compounds of the general formula I. The preferred alkylbenzenes are those in which the sum of the carbon atoms of the alkyl radicals in the individual molecules is at least 3. The upper limit for the number of carbon atoms is limited by the fact that the viscosity of the mixture obtained should preferably not exceed 40 cSt at 200 C. Examples of suitable alkylbenzenes are described in UK Patent Specification No. 1 531 156.

The electro-erosion fluids used in the invention may contain one or more additives for modifying dertain properties, in quantities of 0.005 to 0.5 per cent by weight of the total mixture, for example oxidation inhibitors, conductivity improvers, dispersant additives and odour improvers.

The electro-erosion fluids used in the invention give increased removal performances accompanied by hitherto unattainable very low rates of wear on the tool electrodes. For this reason products with a viscosity of up to 6 cSt at 200C are not only suitable for the main removal process in which the bulk of the metal removal from the workpiece takes place; they can also be used with advantage in the subsequent finishing operations. The use of a single fluid for both operations is particularly advantageous for the reasons already described.

The electro-erosion fluids of the invention with a higher viscosity are particularly suitable for the main removal process. In this stage of machining, in which high currents are used, the working liquids are exposed to very strong thermal stresses. Therefore, a high flash point, a low tendency to decomposition and low evaporation losses are required. These properties are combined in the higher-viscosity electro-erosion fluids of the invention. Since with respect to high removal performance and low wear and tear of the electrode they do not differ significantly from the less viscous products, it is possible, by mixing suitable components, to adjust the specific properties of the electro-erosion fluid to the optimum desirable for any given application.

In the examples given below, four electro-erosion fluids according to the invention were used (products A, B, AC and BD), two known products (C and D) and also a product described and claimed in UK Patent Specification No. 1 531156 (product E). All the percentages are by weight.

Product A This was a mixture of stereoisomeric monomethylbenzylbenzenes (Structure type la) with the following properties: Viscosity: 3.99 cSt/20"C Density d20: 0.987 g/ml Boiling point according to DIN 51 761: 280 to 2850C Flash point according to DIN 51 758: ' 122"C Product B This was a mixture of stereoisomeric monomethyldibenzylbenzenes (Structure type Ib) with the following properties: Viscosity: 37.2 cSt/20 C Density d20: 1.032 g/ml Boiling point according to DIN 51 761: > 350 C Flash point according to DIN 51 758: 195"C Product C This was a gas oil from a Near East crude refined by furfural extraction and treatment with sulphuric acid: Viscosity: 5.78 cSt/20 C Product D This was a kerosene cut from a Near East crude refined with sulphuric acid.

Viscosity: 2.02 cSt/20 C Product E This was a technical mixture of isomeric di-isopropylbenzenes and (1,3dimcthylbutyl)-benzene.

Viscosity: 1.92 cSt/20 C Product AC This was a mixture of 50 per cent Product A and 50 per cent Product C.

Viscosity: 3.99 cSt/20 C Product BD This was a mixture of 50 per cent Product B and 50 per cent Product D.

Viscosity: 5.22 eSt/20 C The examples show, quantitatively, the removal rate (Vw) and the relative electrode wear obtained with the electro-erosion fluids according to the invention as compared with known mineral oil products and alkylbenzenes. The results were obtained in experiments carried out on an AEG (Registered Trade Mark) electroerosion machine tool Elbomat Type 400 H with impulse generator Elopuls 60 according to VID Richtlinie 3400 using electrolytic copper as the tool electrode and tool steel SEL 2714 as the workpiece. The figures given in the column "Machining setting" are settings for current stage (I), duration of impulse (tl) and duration of pause (two) which are defined for the machine described above.

Example 1 The performance of the Product A according to the invention was compared with that of the known Product C.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in OXo of Vw IA/t0 Product A Product C Product A Product C Intensive metal removal 9/9/3 830 405 7.6 22.8 9/10/2 877 459 5.0 21.7 9/11/1 875 448 1.4 9.5 Main metal removal 6/9/3 504 269 7.6 28.0 6/10/2 501 279 3.3 15.3 6/11/1 494 282 1.1 3.5 Finishing 2/5/1 19.3 6.1 2/6/1 19.0 2.8 2/7/1 18.9 1.4 2/8/1 28.5 1.3 2/9/1 27.6 1.2 2/10/1 20.2 0.8 Example 1 shows that removal rates achieved with the Product A according to the invention were far superior to the values obtained with the known Product C.

Also in the case of Product A the relative electrode wear was considerably less.

In the finishing stage of Example lit was necessary for'the known Product C to use different machine settings than for the Product A according to the invention in order to achieve the best possible results.

Example 2 The performance of the Product A according to the invention was compared with that of Product E.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in % of Vw I/t/to Product A Product E Product A Product E Intensive metal removal 9/9/3 830 511 7.6 13.0 9/10/2 877 557 5.0 10.4 9/11/1 875 532 1.4 4.4 Main metal removal 6/9/3 504 328 7.6 14.3 6/10/2 505i 331 3.3 7.2 6/11/1 494 335 1.1 1.5 Finishing 2/5/1 19.6 4.3 2/6/1 20.6 2.6 2/7/I 19.1 1.8 2/8/1 28.5 1.3 2/9/1 27.6 1.2 2/10/1 20.2 0.8 Example 2 shows that the removal rates obtained with the Product A according to the invention exceeded the figures obtained with the Product E.

Furthermore, with the Product A according to the invention a considerable reduction of the relative electrode wear was achieved. In order, in the finishing stage of Example 2, to achieve optimum values for the Product E it was necessary, as in the case of Example I, to choose somewhat different machine settings than for the Product A according to the invention.

Example 3 The performance of the mixture AC according to the Invention was compared with that of the known Product C.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in /O of Vw l/tito Product AC Product C Product AC Product C Intensive metal removal 9/9/3 590 405 15.0 22.8 9/10/2 620 459 11.8 21.7 9/1 1/1 608 448 5.0 9.5 Main metal removal 6/9/3 370 269 15.4 28.0 6/10/2 386 279 8.1 15.3 6/11/1 381 282 2.6 3.5 Finishing 2/5/1 14.6 19.3 6.1 6.1 2/6/1 19.1 19.0 3.1 2.8 2/7/1 17.4 18.9 1.9 1.4 Example 3 shows that the mixture AC of methylbenzylbenzenes (A) and a mineral oil (C) was superior to the mineral oil (C) in regard to the removal performance and electrode wear under practically all working conditions.

Example 4 The performance of the mixture BD according to the invention was compared with that of the known Product D.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in V0 of Vw l/t/to Product BD Product D Product BD Product D Intensive metal removal 9/9/3 701 372 12.8 15.2 9/10/2 756 432 10.3 18.4 9/11/1 750 414 3.9 75 Main metal removal 6/9/3 418 267 13.0 23.6 6/10/2 427 270 7.2 13.4 6/11/1 435 278 1.9 2.4 Finishing 2/5/1 18.2 18.8 5.9 4.7 2/6/1 22.4 18.8 3.4 2.4 2/7/1 23.5 17.7 1.8 1.2 From Example 4 it can be seen that the mixture BD of methyldibenzylbenzenes (B) and a mineral oil (D) was superior to the pure mineral oil (Product D) under all working conditions, both as regards removal performance and electrode wear.

Example 5 The performance of the mixture BD according to the invention was compared with that of the known Product C.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in V of Vw l/t,/to Product BD Product C Product BD Product C Intensive metal removal 9/9/3 701 405 12.8 22.8 9/10/2 756 459 10.3 21.7 9/11/1 750 448 3.9 9.5 Main metal removal 6/9/3 418 269 13.0 28.0 6/10/2 427 279 7.2 15.3 6/11/1 435 282 1.9 3.5 Finishing 2/5/1 18.2 19.3 5.9 6.1 2/6/1 22.4 19.0 3.4 2.8 2/7/1 23.5 18.9 1.8 1.4 Example 5 shows that the mixture BD according to the invention was far superior under practically all working conditions to a known mineral oil (Product C) of comparable viscosity.

Example 6 The performance of the two products A and B according to the invention was compared for the intensive metal removal stage.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in V of Vw l/t,/to Product A Product B Product A Product B Intensive metal removal 9/9/3 830 829 7.6 8.3 Example 6 shows that the Product B according to the invention during intensive metal removal, for which it is particularly suitable because of its high flash point and its low tendency to evaporation, was comparable in regard to removal rate and electrode wear to the Product A according to the invention.

WHAT WE CLAIM IS: 1. An electro-erosion process using hydrocarbon-based dielectric fluids characterised in that the dielectric fluids contain at least 30 per cent by weight of one or more compounds of the general formula I

in which R R R3, R4, R5, R6, R7 and R8 each signify a saturated straight-chained or branched alkyl radical with 1 to 6 carbon atoms or a hydrogen atom and n has a value 0 or 1, when n=0 the sum of the carbon atoms present in the alkyl radicals R1 to R6 is at most 6, and whe-n n=l, the sum of the carbon atoms present in the alkyl radicals R1 to R8 is at most 10, and from 0 to 70 per cent by weight of known electro-erosion fluids, as hereinbefore defined.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in V of Vw l/t,/to Product BD Product C Product BD Product C Intensive metal removal

9/9/3 701 405 12.8 22.8

9/10/2 756 459 10.3 21.7 9/11/1 750 448 3.9 9.5 Main metal removal

6/9/3 418 269 13.0 28.0

6/10/2 427 279 7.2 15.3 6/11/1 435 282 1.9 3.5 Finishing

2/5/1 18.2 19.3 5.9 6.1

2/6/1 22.4 19.0 3.4 2.8

2/7/1 23.5 18.9 1.8 1.4 Example 5 shows that the mixture BD according to the invention was far superior under practically all working conditions to a known mineral oil (Product C) of comparable viscosity.

Example 6 The performance of the two products A and B according to the invention was compared for the intensive metal removal stage.

Machine Removal rate (Vw) in Relative electrode wear setting mm3/min in V of Vw l/t,/to Product A Product B Product A Product B Intensive metal removal

9/9/3 830 829 7.6 8.3 Example 6 shows that the Product B according to the invention during intensive metal removal, for which it is particularly suitable because of its high flash point and its low tendency to evaporation, was comparable in regard to removal rate and electrode wear to the Product A according to the invention.

WHAT WE CLAIM IS: 1. An electro-erosion process using hydrocarbon-based dielectric fluids characterised in that the dielectric fluids contain at least 30 per cent by weight of one or more compounds of the general formula I

in which R R R3, R4, R5, R6, R7 and R8 each signify a saturated straight-chained or branched alkyl radical with 1 to 6 carbon atoms or a hydrogen atom and n has a value 0 or 1, when n=0 the sum of the carbon atoms present in the alkyl radicals R1 to R6 is at most 6, and whe-n n=l, the sum of the carbon atoms present in the alkyl radicals R1 to R8 is at most 10, and from 0 to 70 per cent by weight of known electro-erosion fluids, as hereinbefore defined.

2. An electro-erosion process as claimed in claim I, characterised in that the

compounds of the general formula I are mainly or exclusively monomethyl derivatives of benzylbenzene or dibenzylbenzene(s).

3. An electro-erosion process as claimed in claim I or 2, characterised in that thc compounds of the general formula I are present in an amount of 50 to 100 per cent by weight, based on the total composition.

4. An electro-erosion process as claimed in any of claims 1 to 3, characterised in that the viscosity of the dielectric fluids does not exceed 40 cSt at 200C.

5. An electro-erosion process as claimed in any of claims 1 to 4, characterised in that the dielectric fluids contain oxidation inhibitors, conductivity improvers, dispersant additives and/or odour improvers in quantities of 0.005 to 0.5 per cent by weight based on the total composition.

6. An electro-erosion process as claimed in claim I substantially as described in the Examples.