GB2247204A

GB2247204A - Flaw of dielectric fluid in electrical discharge machining

Info

Publication number: GB2247204A
Application number: GB9018520A
Authority: GB
Inventors: Stuart Apsley Bridges
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1990-08-23
Filing date: 1990-08-23
Publication date: 1992-02-26
Also published as: GB9018520D0

Abstract

An apparatus for electrical discharge machining has a tubular electrode 12 within an enclosing sleeve 15, 16 and spaced from the sleeve by an annular gap. Pressurised di-electric fluid is passed through the gap and is discharged through the tubular bore in the electrode so as to ensure that clean di-electric fluid encloses the electrode and reduces the tendency to arc and the tendency for a central pillar to develop. <IMAGE>

Description

FLOW OF DIELECTRIC FLUID IN ELECTRICAL DISCHARGE MACHINING This invention relates to the flow of dielectric fluid in apparatus for electrical discharge machining (EDM).

In one form of conventional EDM a tubular electrode is used to drill a hole in a work piece and pressurised dielectric fluid is fed through the bore of the tubular electrode so as to discharge into the machining gap.

Machining debris is picked up by the dielectric fluid and is flushed away by way of the annular space between the electrode and the wall of the hole being drilled.

The presence of this debris in the annular space can result in arcing between the electrode and the wall of the hole. This in turn leads to a degradation of the surface quality of the hole wall.

Also, in certain machining conditions, a thin pillar of unmachined material remains in the bore of the electrode. This pillar can, as drilling continues, become unstable and short out the electrode. This obviously upsets machining conditions and is particularly severe at the point of breakthrough on the underside of the work piece. At breakthrough the flow of dielectric fluid from the bore of the electrode is substantially lost thereby causing arcing between the electrode and the remainder of the work piece. This affects machining stability severely and increases the time taken to complete breakthrough.

According to a first aspect of the present invention there is provided apparatus for use in electrical discharge machining with a tubular electrode, which apparatus is adapted to generate a pressure differential to effect the flow of dielectric fluid from the machining gap along the bore of the tubular electrode.

Preferably enclosure means is disposed about the electrode, which enclosure means is adapted to receive pressurised dielectric fluid which can then flow into the machining gap thereafter to be discharged along the bore of the electrode.

A preferred feature is that said enclosure means comprises annular sleeve means adapted to form a seal with the work piece around the electrode, which sleeve means has an inlet for pressurised dielectric fluid, a chamber for said dielectric fluid being formed between the electrode and the sleeve means.

According to a second aspect of the present invention there is provided a method of electrical discharge machining using a tubular electrode comprising the step of generating a pressure differential to cause the flow of the dielectric fluid from the machining gap along the bore of the tubular electrode.

According to a third aspect of the present invention there is provided apparatus for electrical discharge machining comprising a tubular electrode, the position of which relative to the work piece is adjustable to form a machining gap, dielectric supply means for supplying dielectric fluid into the region of the machining gap, power supply means connected in use between the electrode and the work piece and arranged when operative to produce machining pulses for electrical discharge machining through the work piece, and means for generating a pressure differential to effect the flow of dielectric fluid from the machining gap along the bore of the electrode.

Embodiments of the invention will now be described in more detail by way of example. The description makes reference to the following diagrammatic drawings in which: Figure 1 is a cross-sectional view through a first embodiment of a device according to the present invention, and Figure 2 is a cross-sectional view through a second embodiment of a service according to the present invention.

In the embodiment of figure 1 there is shown a machining head 10 of an EDM machine tool and a work piece 11. The head 10 comprises a tubular electrode 12 slidably mounted in an annular guide bush 13 which has an inner insulation layer. Surrounding the guide bush 13 is a flow adaptor 14 comprising an annular upper sleeve 15 within which is slidably disposed an annular lower sleeve 16.

The lower sleeve 16 surrounds and is spaced from the electrode 12 and has on its lower axial face an annular sealing element 17. The end 18 of the upper sleeve 15 remote from the sealing element 17 is secured to a clamp arm 19 of the machine tool and an internal annular projection 20 of the upper sleeve receives the guide bush 13. A coil spring 24 is also disposed within the upper sleeve and acts between the projection 20 and the lower sleeve 16 so as to urge the lower sleeve axially away from the upper sleeve and thereby extend the telescopic arrangement of the lower sleeve and upper sleeves 15, 16. A port 21 is provided in the upper sleeve 15 to which port 21 is connected a supply line 22 for dielectric fluid.

The general features of a conventional EDM machine tool have not been shown or described, such as means for adjusting the position of the electrode with respect to the work piece to form the machining gap, power supply means for connection between the electrode and the work piece and control means.

In operation, dielectric fluid is introduced under pressure into the chamber 23 formed by the sleeves 15, 16. The fluid pressure adds to the force of the spring 24 to urge the lower sleeve into sealing engagement with the work piece 11. During machining the dielectric fluid flows into the annular space between the electrode and the wall of the hole being drilled. Debris in the machining gap is picked up by the dielectric fluid and is discharged with the fluid along the bore of the electrode 12 by virtue of the pressure differential generated by the pressure of the dielectric fluid.

The figure 2 embodiment has many similar features to the figure 1 embodiment and so like reference numerals have been given like parts. A single annular sleeve 25 surrounds the guide bush 13 and electrode 12 to form a chamber 23. The sleeve is attached to an external clamp 26 which is operable so as to move the sleeve 25 axially towards or away from the work piece 11. The sleeve 25 is moved towards the work piece 11 to form an annular seal between the sleeve and the work piece by virtue of a sealing element 27.

Once the face seal is formed with the work piece, dielectric fluid can be introduced under pressure through the port 21. When machining, the dielectric fluid flows into the annular space between the electrode and the wall of the hole being drilled. Debris in the machining gap is picked up by the dielectric fluid and is discharged with the fluid along the bore of the electrode 12 under the pressure differential generated by the pressure of the dielectric fluid.

With the above methods of operation, machining debris is not discharged between the electrode and the surface of the hole being drilled. Arcing between the electrode and the surface of the hole is therefore eliminated. A better quality for the surface of the hole is thus ensured.

It has also been found that this effective reversal of flow of dielectric fluid also reduces the tendency for a pillar of unmachined material to remain within the bore of the electrode. This therefore reduces the likelihood of the pillar shorting out the electrode when it becomes unstable and also improves machining stability at breakthrough, which stability is generally impaired by the presence of a pillar.

It will of course be clear to the skilled reader that the necessary seals apart from those generated by the sealing elements 17 and 27 are formed by the arrangements in order to ensure the pressure of the dielectric fluid in the chamber is effective to discharge the fluid along the bore of the electrode.

It will be appreciated that there are many alternative ways of forming a pressure differential so as to establish such a reverse flow for the dielectric fluid.

One simple way would be to use a tubular electrode, flood the work piece with dielectric fluid by, say, immersing the work piece in a dielectric bath and connecting the bore of the electrode to suction means.

Dielectric fluid and any machining debris would therefore be drawn away from the machining gap and up the electrode.

Claims

1 Apparatus for use in electrical discharge machining with a tubular electrode, which apparatus is adapted to generate a pressure differential to effect the flow of dielectric fluid from the machining gap along the bore of the tubular electrode.

2 Apparatus as claimed in claim 1 wherein enclosure means is disposed about the electrode, which enclosure means is adapted to receive pressurised dielectric fluid which can then flow into the machining gap thereafter to be discharged along the bore of the electrode.

3 Apparatus as claimed in claim 2 wherein said enclosure means comprises annular sleeve means adapted to form a seal with the work piece around the electrode, which sleeve means has an inlet for pressurised dielectric fluid, a chamber for said dielectric fluid being formed between the electrode and the sleeve means.

4 Apparatus as claimed in claim 3 wherein the sleeve means comprises an annular upper sleeve and an annular lower sleeve slidably disposed within the upper sleeve, and spring means to urge the lower sleeve axially away from the upper sleeve.

5 A method of electrical discharge machining using a tubular electrode comprising the step of generating a pressure differential to cause the flow of the dielectric fluid from the machining gap along the bore of the tubular electrode.

6 Apparatus for electrical discharge machining comprising a tubular electrode, the position of which relative to the work piece is adjustable to form a machining gap, dielectric supply means for supplying dielectric fluid into the region of the machining gap, power supply means connected in use between the electrode and the work piece and arranged when operative to produce machining through the work piece, and means for generating pressure differential to affect the flow of dielectric fluid from the machining gap along the bore of the electrode.

7 Apparatus for electrical discharge machining substantially as hereinbefore described with reference to figure 1 or figure 2 of the accompanying drawings.

8 A method of electrical discharge machining substantially as hereinbefore described with reference to figure 1 or figure 2 of the accompanying drawings.