GB2206829A

GB2206829A - Travelling-wire electrical discharge machining apparatus

Info

Publication number: GB2206829A
Application number: GB08715266A
Authority: GB
Inventors: David Murray Harley
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1987-06-30
Filing date: 1987-06-30
Publication date: 1989-01-18
Also published as: GB8715266D0

Abstract

Apparatus is provided which will form an internal recess having a part-circular cross section in a metallic workpiece. The workpiece 11 is supported and rotated about an axis 13 by a support member 12. A circular guide member in the form of a grooved wheel 14 is rotatable about an axis 16 perpendicular to the axis of rotation 13 of the workpiece. Wire 17 is supplied in a continuous length and passes around the guide member 14 whilst a power source produces an electrical discharge between the wire 17 and the workpiece 11. The guide member 14 is fed towards the workpiece 11 whilst the workpiece is rotated to form a recess 10 having the required part-circular section and dimensions. With the axis 16 positioned not to intersect the axis 13, an annular recess with part-circular cross section can be formed (Fig. 3). Alternatively, using additional wire guides (30, Fig 4) an undercut groove may be formed. <IMAGE>

Description

TRAVELLING-WIRE ELECTRIC DISCHARGE MACHINING APPARATUS Travelling wire electric discharge machining is one form of what is commonly known as "spark-erosion" machining, which is commonly used for producing a high quality and accurately-dimensioned finish on a workpiece. Spark erosion machining using solid electrodes was originally used for forming or finishing a three-dimensional contour on a workpiece, and involved making a machining electrode having a similar contour to that required on the workpiece and advancing the electrode towards the workpiece in a dielectric medium as an electric discharge is produced between the workpiece and the electrode.

The effect of the discharge is to erode away material from the workpiece. The process is slow, and therefore is used to produce a suitable final finish and shape on a workpiece which has been previously machined by other means. A problem with this technique has been that the electrode is also eroded, though suitable choice of materials makes this erosion far slower than that of the workpiece.

One solution to the problem of electrode erosion has been to use a wire as the electrode and to feed wire continuously past the discharge position so that eroded wire is continuously being replaced by new wire. This technique is commonly used for forming or finishing straight cuts in a workpiece. However the technique has not been suitable for forming surfaces curved in more than one direction, particularly if these are internal surfaces.

It is an object of the invention to provide apparatus for machining internal recesses having part-spherical regions.

According to the present invention there is provided travelling-wire electrical discharge machining apparatus for forming internal recesses having part-spherical regions in a metallic workpiece, which apparatus includes support means for supporting and rotating a workpiece about an axis a circular guide member supported for rotation about an axis perpendicular to the axis of rotation of the workpiece, wire supply arranged to pass a continuous length of wire around part of the guide mernbcr, energising means for producing an electric discharge between the wire passing around the guide member and adjacent surfaces of the workpiece, and feed means for feeding the guide member into the recess until a recess of the required shape and dimensions is formed.

The expression "part-spherical" is used to denote that the surface may comprise or include part of a spherical surface or section up to and including a hemisphere.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of part of machining apparatus according to a first embodiment of the invention; Figure 2 is a different view of part of the apparatus of Figure 1; Figure 3 shows another shape of recess which may be formed by the apparatus of Figures 1 and 2; and Figure 4 is a sectional yiew of part of the apparatus according to a second embodiment of the invention.

The first embodiment to be described is concerned with the formation of a simple hemispherical recess 10 in the surface of a workpiece 11. The workpiece is mounted in a holder 12 which is rotatable about an axis 13 passing through the centre of the recess 10. A guide member 14 in the form of a grooved wheel of radius slightly smaller than the finished radius of the aperture, is supported on a shaft 15 for rotation about an axis 16 perpendicular to and passing through the axis of rotation 13 of the workpiece 11. A continuous length of wire 17 is fed around the groove in the wheel 14 from a feed spool 18 to a take-up spool 19.

Feed means, not shown, are provided to feed the guide wheel 14 into the workpiece in the direction of the arrow 20.

Figure 2 shows another view of part of the apparatus of Figure 1. The shaft 15 is cut away to allow the guide wheel 14 to move into the recess up to its centre, and the shaft itself does not rotate. A source of direct current 22 is connected between the guide wheel 14 and the workpiece 11, the connections being through some form of brush and slip-ring contact, and the workpiece and at least the lower half of the guide wheel are immersed in a dielectric liquid such a paraffin up to a level indicated by the line 21.

An electric discharge is struck between the wire 17 passing around the lower half of the guide wheel 14 and the workpiece 11 and this erodes away small quantities of material from the workpiece 11. The workpiece itself is rotated about the axis 13, at, say, 100rpm, so that the relative movement between the guide wheel and the workpiece describes a part-spherical surface. The radius of the surface is determined by the radius of the guide wheel to the bottom of the groove carrying the wire 17, plus the diameter of the wire 17 plus the width of the discharge gap between the wire and the workpiece.

The guide wheel is fed into the workpiece to a depth necessary to provide a surface of the required shape and dimensions. The recess may take the form of any portion of a spherical surface up to a hemisphere. Further feed of the guide wheel 14 into the workpiece will result in the formation of a circular bore with a hemispherical bottom.

Other shapes of recess may be formed using the same apparatus. For example if, as shown in schematic form in Figure 3, the wheel 14 is positioned away from the intersection of axes 13 and 16 then an annular groove with a part-spherical bottom may be formed. Other variations will be apparent.

More complex recesses may be formed by modification of the support for wheel 14 and the wire feed arrangements.

Figure 4, for example, illustrates schematically the formation of an internal undercut groove. In this case additional guides 30 are necessary to keep the wire 17 away from the workpiece, and the guide wheel is fed into the workpiece in the direction of the arrow 31. In this case the workpiece 11 is rotated about a horizontal axis 13 so that the guide wheel 14 may be fed vertically into the groove in the workpiece. Clearly other shapes may be machined using the apparatus with suitable modification of the wire path and the feed direction.

Claims

Claims:

1. Travelling-wire electric discharge machining apparatus for forming internal recesses having part-spherical regions in a metallic workpiece, which apparatus includes support means for supporting and rotating a workpiece about an axis, a circular guide member supported for rotation about an axis perpendicular to the axis of rotation of the workpiece, wire supply means arranged to pass a continuous length of wire around part of the guide member, energising means for producing an electric discharge between the wire passing around the guide member and adjacent surfaces of the workpiece, and feed means for feeding the guide member into the recess until a recess of the required shape and dimensions is formed.

2. Apparatus as claimed in Claim 1 in which the guide member is rotatable about an axis passing through the axis of rotation of the workpiece.

3. Apparatus as claimed in Claim 2 in which the guide member is located at the intersection of the said two axes.

4. Apparatus as claimed in any one of Claims 1 to 3 in which the guide means comprise a wheel having a groove in its periphery which engages the said continuous length of wire.

5. Apparatus as claimed in any one of the preceding claims in which the feed means is operable to feed the guide member in a direction parallel to the axis of rotation of the workpiece.

6. Apparatus as claimed in any one of Claims 1 to 5 in which the feed means is operable to feed the guide member in a direction parallel to the axis of rotation of the guide member.

7. Travelling-wire electrical discharge machining apparatus substantially as herein described with reference to the accompanying drawings.