FR2531359A1 - Method and device for electrical-discharge machining. - Google Patents

Abstract

In a machine for electrical-discharge machining, and especially electrical-discharge machining using a moving wire electrode 1 which defines, together with the workpiece 2 to be machined, a machining gap G, electrical pulses are passed into a fluid filling the machining gap. This fluid is constituted by water vapour or a mixture of water vapour and a water-based liquid, or alternatively a mixture of water vapour, liquid water and a gas in order to improve the efficiency and the stability of machining.

Description

Process and device for EDM machining
The present invention relates to machining by electroerosion, and more particularly to a new and improved method and apparatus for machining by electroerosion a conductive part with an electrode-tool using a fluid medium based on water. .

In EDM machining processes, it is well known that the machining agent plays several important roles. These roles include the function of electric discharge agent, removal of work products and cooling action. It is commonly used, as an electroerosive machining agent, a hydrocarbon, such as kerosene, in a conventional manner, to satisfy these conditions. However, such a liquid has the disadvantage of being flammable and therefore involves a fire hazard. At the present time, as a variant, water is commonly used, in particular in circulating wire EDM machines. If a liquid based on distilled water satisfactorily fulfills the functions required from an EDM machining agent, it leaves much to be desired with regard to its ability to remove accumulations of material, the stability and the machining precision when compared to a conventional flammable hydrocarbon liquid.

The present invention is directed to a new and improved EDM machining process which increases the efficiency of machining and the performance of a process using a water-based liquid over conventional processes.

The present invention also relates to a new and improved EDM apparatus for implementing the method described.

According to the invention3 there is provided, according to a first aspect, a method of machining by electro-erosion of a part, this method comprising the following steps a) there is a tool electrode remote from the part vis-à-vis screws to define a machining interval between them; b) is delivered in this machining interval a machining fluid constituted at least in part by water vapor; c) passing a succession of electrical pulses in the interval between the tool electrode and the workpiece to remove material from the workpiece by electro-erosion; and d) a relative movement of the tool electrode and of the workpiece is carried out to advance the removal of material by EDM from the workpiece.

The machining fluid can contain, in addition to 11 water in its vapor phase, water in the liquid phase and must have a specific resistance in a range between 10 and 105 ohms-cm. and preferably between 10 and 104 ohms-cm.

The specific resistance of the working fluid formed by the steam-water-liquid mixture is controlled by the respective control of the proportions of water vapor and liquid water. The water-liquid can be an aqueous solution of an electrolyte.

The machining fluid can consist of a mixture of water vapor, a gas such as air and liquid water and must, once again, have a specific resistance in a range between 10 and 10 ohms -cm, and preferably between 102 and 10 ohms-cm. Here again, the liquid water can be an aqueous electrolyte solution. The specific resistance of the mixed machining fluid is again controlled to the desired extent, by controlling the proportion of steam, gas and liquid water.

This special machining fluid can, according to a specific characteristic of the invention, be delivered in the machining interval using the following measures, connecting a nozzle, by a fluid line, to a sealed container partially filled of a predetermined quantity of liquid water, the nozzle is oriented towards the machining interval, the liquid water is heated in the container to produce a pressurized vapor which constitutes said vapor of water above the surface of the liquid water in the container and the pressurized steam is allowed to flow in the form of a jet at high speed in the pipe, to be injected from the nozzle in the machining interval.

The present invention also provides, according to a second aspect, an apparatus for the machining by electro-erosion of a part with a tool electrode disposed facing this part and at a distance therefrom, beyond an interval d machining, this apparatus comprising a) means for supplying, in the machining interval, a machining fluid constituted at least in part by water vapor; b) energy supply means for passing a succession of electrical pulses through said interval between the electrode-tool and the workpiece for removing material by electro-erosion of the workpiece; and c) means for moving relatively relative to each other the tool electrode and the workpiece to advance said removal of material from the workpiece by EDM.

Specifically, the means a) comprise at least one nozzle device adapted to be oriented towards the machining interval and communicating, by means of a conduit with a fluid-tight container and partially filled with liquid water, and means for heating the liquid water in said container, so that a pressurized vapor is formed which constitutes at least in part the machining fluid which flows in said pipe means and, from there, is injected by said nozzle device in the machining interval.

The conduit means may comprise mixing means comprising a first inlet communicating with the container for admitting into the mixing means water vapor coming from the container, such that the water vapor and the liquid water mix in these mixing means and the mixture forming at least part of the machining liquid is forcefully transmitted by the nozzle device to be injected into the machining interval.

The ducting means can also comprise another mixing means having a first inlet communicating with the container for supplying steam from the container to this second mixing means and, in addition, a second inlet for admitting a gas into said second mixing means, so that the water vapor and the gas are mixed in this second mixing means and the mixture forming at least part of the machining liqu-ide is force-fed through the nozzle to be injected therein in the machining interval.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description, made with reference to the appended drawing in which - Figure 1 is a sectional view schematically illustrating its embodiment of the invention. , in application to a circulating wire EDM device; and - Figure 2 is a schematic sectional view illustrating a variant of the embodiment of Figure 1
Referring to the figures, an embodiment of the invention will be described with reference to the machining process by electrical discharge with a circulating wire in which the invention can advantageously be exploited.

However, it should be clear that the invention is applicable to other machining processes by electrical discharge, without circulating wire or by forming cavities for drilling, stamping or other similar operations.

In FIG. 1, an electrode wire 1 is shown along a workpiece 2 in relation to EDM machining. The electrode wire 1 is unwound from a store roll (not shown), transported axially between a pair of wire guide members 3 and 4 and delivered to a receiving means (not shown). The guide members 3 and 4 are positioned so as to define a linear rectilinear linear path between them, along which the electrode wire 1 moves axially as shown in the direction from bottom to top, as indicated by the arrow. electrode 1 is axially driven by a traction drive unit (not shown) provided between the upper guide member) l and the receiving device 'and the desired tension on the circulating wire 1 and its speed of movement are determined by this drive unit, in conjunction with a braking unit (not shown) provided between the magazine roller and the lower guide member 3. A gap G is formed between the circulating wire electrode 1 and the piece 2 and supplied with a special machining fluid, described below.

A power supply for machining by electric discharges5 is connected to the wire electrode 1 and to the part 2 to apply a succession of electric pulses between them, in order to produce electric discharges of EDM in the interval G to remove material from part 2.

The part 2 is securely mounted on a work table 6 which is driven horizontally, according to a cross slide device by a pair of motors 7 and 8 in response to control signals delivered by a machine advance control unit 9, for example a digital control unit (NC). The motor 7 is designed to move the work table 6 in the direction of the X axis and the motor 8 to move this work table ó along a Y axis perpendicular to the X axis. The data corresponding to a cutting pattern prescribed are stored in the control unit 9 and converted into signals to move the part 2. transversely and relatively to the electrode wire 1 flowing along the rectilinear linear path mentioned previously, between 1-the guide organs 3 and 4, according uz travel path in the XY plane corresponding to the prescribed cutting pattern. During this cutting operation, a groove 10 is formed behind the electrode wire, gradually, as the part 2 is moved.

As shown below and next to the part 2, there is a nozzle 11 oriented towards the machining opening G to deliver a machining fluid F. Preferably, this nozzle is also arranged above and next to the upper surface of the part 2, so as to be oriented towards the machining interval G.

According to the present invention, the machining fluid F consists, at least in part, of a water vapor produced in a water vaporization unit 13 comprising a sealed container 14 and containing water -liquid 15 which is introduced therein from a water reserve 16 by an intake pipe 17. The liquid water 15 can be distilled water having a specific resistance between 10 and 105 ohms-cm .

 as obtained from tap water or city water, in an ion exchanger cartridge. The container 14 is equipped, in its floor part, with the aid of a heating device 18 supplied with energy by a supply 19 for vaporizing the liquid water 15 which is contained therein. The water vapor is collected above the surface of the liquid water 15 in the container 14 and transmitted to the nozzle 11 via the outlet pipe 20. The water vapor is delivered by the nozzle 11 at machining interval G.

The water intake pipe 17 is equipped with a valve 21 connected to a float 22 in the container 14, so that the valve 21 is closed to stop the supply of water from the source 7 when the float 22 reaches a predetermined level in the liquid water 15. In this way, the quantity of non-vaporized liquid water 15 in the container 14 is kept constant. The container 14 is also equipped with a decompression outlet 23 which can open to the atmosphere and the opening of which is controlled by an electromagnetic valve 24 to control the vapor pressure in the container 14 and therefore the flow steam in the nozzle 11. The valve 24 is designed to be controlled, electromagnetically, by a control circuit 25 whose input is electrically connected to the machining interval G in order to detect an interval variable such as a voltage, a current, a resistance and / or an interval impedence. The valve 24 is thus automatically controlled by the control circuit 25 according to an interval signal, so that the volume flow rate of the water vapor in the nozzle 11 can vary optimally depending on the state or condition of the interval. For example, a decrease in the interval impedence causes an increase in the volume flow rate of water vapor in the machining interval G and vice versa.

In accordance with an additional characteristic of the invention, the water vapor is supplied in the form of a mixture with an aqueous liquid in the working interval G. For this purpose, the device shown comprises a chamber 26 formed in the nozzle unit 11. The mixing chamber 26 has a first inlet 27 constituted by a converging end portion of the pipe 20 received in the nozzle 11 for injecting the jet of water vapor into this nozzle 1 -1. The mixing chamber 26 has a second inlet 28 which communicates via a line 29 with an aqueous liquid 30 contained in a sealed container 31. The aqueous liquid 30 is sent into the container 31 from a supply 32 by l 'via an intake duct 33 and can here also be constituted by distilled water obtained by passing city or tap water through an ion exchange resin cartridge.

The aqueous liquid 30 can have a specific resistance in a range between 103 to 105 ohms-cm.

The water supply 31 can be the same as the water supply 16. The intake duct 33 is equipped with a valve 34- mechanically connected to a float 35, so that the valve 34 is closed to interrupt the water supply from the source 32 when the float 35 reaches a determined level. In this way, the amount of liquid water in the container 31 is kept constant.

A compressor 36 is also connected to the container 31 to apply pressure to the liquid water 30 which is contained therein. The compressed air in the space above the surface of the water in the container 31 causes the liquid water to rise in the pipe 29 and its admission into the mixing chamber 26 in the nozzle 1 1
The inlet 27 into the mixing chamber 26 as mentioned, communicates with the source of water vapor via the conduit 20 and constitutes an injection nozzle for injecting the water vapor into the nozzle 15.

A negative pressure zone 37 communicating with the second inlet 28 and the pipe 29 is created around the nozzle 27. As a result, the liquid water which rises in the pipe 29 and through the inlet 28 , is sucked into the zone 37 and effectively mixed with and sucked into the water vapor flowing from the pipe 20 into the nozzle 11. TJn homogeneously mixed flow of water vapor and liquid water, which constitutes the machining fluid F, is thus produced in the nozzle li and delivered in the region of the machining interval G. The machining fluid F must have a specific resistance of between 10 and 105 ohms- cm. and preferably between 10 and 104 ohms-cm.

Line 29 is provided with a valve 38 which is designed to be controlled electromagnetically by the previously mentioned control circuit 25. The valve 38 is thus automatically controlled by the control circuit 25, as a function of the interval signal, so that the proportion of liquid water mixed with the steam varies as desired, depending on the condition It should be noted that the valve 38 and the valve 24 must also be manually adjustable in order to establish such an optimal proportion, which can be fixed, in the event that it is not intended to be sensitive. to the control circuit, in correspondence with an optimal interval condition determined by a predetermined machining condition.

The valve 33 can be completely closed to allow only water vapor, coming from the vaporization unit 13, to be delivered in the form of a jet in the machining interval G.

The valve 24 must be adjusted to establish an optimal volume flow of water vapor to be delivered, with or without mixing with the liquid water, in the machining interval G, according to a condition of pre-established machining and can be controlled by circuit 25 to vary the proportion in the mixed machining fluid F, or the quantity, of water vapor in the machining interval optimally, depending on the condition variable interval.

The valve 38 must be adjustable to establish an optimal quantity and proportion of the liquid water in the mixed machining fluid F, as a function of a pre-established machining condition and can be controlled by the circuit 25 to vary the proportion and quantity, optimally, depending on the variable interval condition.

The proportion of water vapor and liquid water in the mixture determines a specific resistance of the machining fluid F delivered to the machining interval.

In a typical operating mode of the apparatus, the valve 38 can normally be closed. The valve 24 must then be adjusted to allow the water vapor alone to be transmitted at a given volume flow rate in the machining interval G. The control circuit 25 controls the state or the condition of the interval and, based on an interval signal indicating a characteristic interval state or condition such as the development of an interval short circuit or arc which requires removal. a higher proportion of the water content, acts on the valve 36 to allow a mixture of liquid water with the water vapor, which mixture is delivered at the interval G until the suppression is obtained, or during a given period of time in which the deletion should be obtained.

When the water-based fluid which has hitherto been used exclusively in its liquid phase in conventional methods of machining by electrical discharges, is used in its vapor phase, it has been observed that it effectively retains the properties required as an electric shock and cleaning agent, and its additional function as a cooling agent is greatly improved. This results in a faster cooling action of each individual discharge location and, consequently, greater constriction of each individual discharge column, both of which contribute to an improvement in material removal by machining by electric discharges. In addition, the improved cooling effect allows to overcome the problem of accidental wire breakage, and therefore to use a thinner electrode wire which provides increased cutting precision and allows the wire to travel under higher tension. , which ensures greater cutting stability. For these reasons, the efficiency of the cut, its precision and its stability are improved.

Example
A part made of a SKDîî material (Japanese industrial standard) and 20mm thick was machined by electroerosion with an electrode wire made of copper and 0.2mm in diameter. The electrode wire was continuously driven to circulate axially at a speed of 2m per minute in the workpiece, while a succession of EDM machining pulses of pulse duration T os of 4 micro-seconds , interval between impulse
on T off of 10 micro-seconds and peak current I p of 20 amperes was transmitted between the electrode wire and the workpiece in the machining fluid. When the machining fluid consisted of water vapor produced in a spray unit and sent in the machining interval at a volume flow rate of 1.5 liters per minute in a nozzle assembly as described and shown generally above, the machining of the part has been obtained with a removal rate as low as 1.6mm per minute. When the working fluid was constituted by a mixture containing by volume three parts of water vapor and one part of liquid of specific resistance. 2x103 ohmscm and transmitted at a volume flow of 1 liter per minute, the machining of the part was obtained with a removal rate of 1.9mu per minute. No breakage of the electrode wire occurred in the above two cases. When the machining fluid consisted of liquid water alone, in accordance with the traditional practice in machining by electric discharges, the part has was machined with a removal rate of 1mm per minute.

In FIG. 1, the pipe is also provided with another mixing chamber 40 in which the water vapor for the vaporization unit 13 is mixed with a gas such as air. The gas from a source 41 is forced by a pump 42 into the line 20 of the mixing chamber 40 vla a valve 43 and a nozzle 44. The valve 43 is controlled by the control circuit 25,. according to the interval signal to control the proportion of gas in the mixture according to the interval condition.

In FIG. 2 a nozzle 111 has been shown, modified and arranged coaxially with an electrode wire 1 circulating vertically upwards, from the lower guide member 3 towards the upper guide member (not shown) through the workpiece 2. As in the embodiment of FIG. 1, the nozzle 111 comprises a mixing chamber 126 having a first fluid inlet 127 and a second fluid inlet 128 arranged coaxially with the first fluid inlet 127 and at electrode wire 1 and communicating with a volume of liquid water 30 contained in the container 31. The liquid water 30 can here again be distilled water or an aqueous solution of an electrolyte such as sodium chloride or nitride of potassium. The first inlet of fluid can communicate with the vaporization container 14 shown in FIG. 1, but can also be any other traditional construction device for manufacturing water vapor. In the mixing chamber 126, a jet of water vapor sent by the first inlet 127 is mixed with the water-liquid drawn from the second inlet 128, the mixed machining fluid F being injected into the interval It should be noted that instead of water vapor alone, a mixture of water vapor and air or any other gas such as oxygen, carbon dioxide, nitrogen or argon, can be injected through the first fluid inlet 120 into the mixing chamber, so that the machining fluid F can be composed of a combination of water vapor, gas and liquid water. In this case, a steam / gas mixing chamber of traditional construction is provided in the line 120, coming from the water vaporization unit 13 in FIG. 1.

The machining fluid F must have a specific resistance between 10 and 105 ohms-emo and preferably between 102 and 10 ohms-cm. The proportion of water vapor, gas and aqueous solution, or liquid water, in the working fluid F must be controlled to ensure that the specific resistance of the mixed fluid F is within a such a range and must preferably be also controlled by the control circuit 25 as described above.

Claims (18)

Claims 1. Method for machining a part by EDM, characterized by the following steps a) there is an electrode-tool (1) with respect to the part (2) and at a distance therefrom to define them a machining interval G; b) this machining interval G is supplied with a machining fluid constituted at least in part by steam; c) passing a succession of electrical pulses in said interval between the tool electrode (1) and the workpiece (2) to remove material from the workpiece by electro-erosion; and d) said tool electrode and said workpiece are moved relatively to each other to advance the removal of material by EDM from the workpiece. 2. Method according to claim 1, characterized in that said working fluid contains a part of liquid water and has a specific resistance between 10 and 105 ohms-cm. 3. Method according to claim 2, characterized in that said specific resistance is between 102 and 104 ohms-cm. 4. Method according to claim 2, characterized in that the specific resistance of said machining fluid is controlled by controlling the respective proportions of water vapor and liquid water in this fluid. 5. Method according to one of claims 2, 3 or 4, characterized in that said liquid water is an aqueous electrolyte solution. 6. Method according to claim 1, characterized in that said machining fluid is a mixture of water vapor, a gas and liquid water and has a specific resistance between 10 and 105 ohms-cm. 7. Method according to claim 6, characterized in that said liquid water is an 'aqueous --- electrolyte solution. 8. Method according to claim 7, characterized in that the specific resistance of said machining fluid is controlled by controlling the proportion of said water vapor, said gas and said aqueous solution. 9. Method according to claim 1, characterized in that step b comprises the following measures: - a nozzle (11) is connected via a fluid line (20) to a hermetically closed and partially filled container with a predetermined amount of liquid water (15); this nozzle (11) is oriented towards the machining interval - (G.); the liquid water is heated in this container (14) to produce a vapor pressure constituting said water vapor above the surface of said liquid in said container (14) and allowing said vapor to flow in the form of a jet in said pipe (20) and through said nozzle (11) in said machining interval. 10. The method of claim 9, characterized in that further mixing said water vapor with a water-based liquid to produce said machining fluid before its admission into said machining interval (G). 11. Method according to claim 10, characterized in that said water vapor is mixed with said water-based liquid in a mixing chamber (26) formed in said nozzle. 12. The method of claim 1, 2, 6 or lot characterized in that said electrode (1) is an electrode wire suitable for traveling axially along a straight linear path through said piece (2) with which it defines said interval of machining. 13. The method of claim 12, characterized in that said nozzle (111) is arranged substantially coaxially with said straight linear path; 14. The method of claim 12, characterized in that said machining fluid is transmitted to the machining interval in the same direction as the direction of movement of said electrode wire along said path. 15. Device for machining a workpiece by EDM with a tool electrode (1) arranged facing the workpiece at a distance therefrom on the other side of a machining interval (G), this apparatus being characterized in that it comprises a) means for supplying said machining interval with the aid of a machining fluid constituted at least in part by steam; b) energy supply means (5) for passing a succession of electrical pulses in said interval between the tool electrode (1) and the part (2) to remove material from said electro-erosion piece (2); and c) means for moving relatively relative to each other said tool electrode and said workpiece to advance said work of removing material from the workpiece by EDM. 16. Device according to Claim 15, characterized in that the said means (a) comprise at least one nozzle device (11) capable of being oriented towards the said machining interval (G) and of communicating by driving means (20 ) with a sealed container (14) partially filled with liquid water (15) and means (18) for heating the liquid water in this container to produce a vapor pressure above the surface of the liquid water in this container, such that the pressurized steam forming at least a portion of said machining fluid is forced to flow into said pipe means and from there is injected from the nozzle device (11) in said machining interval (G). 17. Device according to claim 16, characterized in that said conducting means comprise a mixing means (26) having a first inlet (27) communicating with said container for supplying steam from this container to the mixing means and a second inlet (28) for sending a water-based liquid to said mixing means (26) such that said water vapor and said water-based liquid are mixed in said mixing means (26) and the mixture forming at least part of said working fluid is forced through the nozzle device (11) to be injected into said working interval (G). 18. Device according to claim 17, characterized in that said conducting means comprise another mixing means (40) having another first inlet communicating with said container for supplying said water vapor from said container to said second mixing means and a another second inlet (44) for sending a gas into said second mixing means, so that the water vapor and said gas are mixed in this second mixing means and the mixture forming at least a portion of said machining fluid is forced through the nozzle device (11) to be injected into said machining interval (G).