GB2169837A

GB2169837A - Electrochemical machining head

Info

Publication number: GB2169837A
Application number: GB08531639A
Authority: GB
Inventors: Glen Douglas Devey
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-12-27
Filing date: 1985-12-23
Publication date: 1986-07-23
Also published as: GB8531639D0; GB8432667D0; GB2169837B

Abstract

An electrochemical machining or engraving head comprises a moulded block 14 of insulating material recessed at 18 to receive and locate a workpiece and bored to receive and precisely locate an electrode assembly 20 and electrode relative to the workpiece, and is formed with a channel 22 conveying a flow of electrolyte to and through the gap between electrode and workpiece. The channel is formed, as the moulded block is cast, by positioning a flexibly deformable rectangular- cross-section strip of eg polyethylene so that it extends sinuously through the mould, with its ends projecting therefrom and an intermediate point passing between and pinched between a replica workpiece and replica electrode, while the block is cast. After casting, the channel former is removed by pulling on one (and possibly thereafter the other) of its ends. <IMAGE>

Description

SPECIFICATION Electrochemical machining head The present invention relates to electrochemical machining heads and a method of making the same.

In electrochemically machining or engraving a workpiece by means of an electric current made to flow between workpiece and a juxtaposed electrode, through an electrolyte which occupies the space between workpiece and electrode and which is made to flow through the space, it is important both to ensure a correct spacing between the workpiece and the electrode and to provide a suitably designed flow channel configuration for the electrolyte.It has been found, for example, that in the accurate machining of fine grooves the achievement of even groove depth and good definition of the edges of the grooves depends on the provision of a suitably laminar flow of the electrolyte through the space between workpiece and electrode, the flow direction being substantially perpendicular both to the longitudinal direction of the groove and to the direction in which the electrode is spaced from the workpiece.

It is an object of the invention to provide an electrochemical machining or engraving head or jig which will hold a workpiece and an electrode accurately in a desired spaced relationship to one another and which incorporates flow channel means which will promote substantially laminar flow of electrolyte through the space between electrode and workpiece, and to provide a favourable and advantageous method of making such a head or jig.

According to the invention in one of its aspects there is provided an electrochemical machining or engraving head or jig which comprises a cast block of electrically insulating material, the block having a face which is formed with a recess adapted to mate with part of the surface of a workpiece and thereby to locate the workpiece relative to the block, a bore shaped to receive and locate an electrode assembly and electrode with an end of the electrode in a desired spaced relation to a workpiece located in the said recess, and a channel extending, within the block, through between the said recess and the said bore with the recess and the bore opening into the channel on opposite sides thereof, the channel being of substantially elongate cross-section of substantially constant area in the immediate vicinity of the recess and bore and over a distance therefrom, in each direction lengthwise of the channel, which is large compared with the larger transverse dimension of the channel.

According to the invention there is also provided a method of making such a block for a machining or engraving head, the method comprising the steps of: providing a mould body defining a mould space for the block, positioning a replica workpiece in the mould body with part of its surface exposed to the mould space, positioning an elongate and flexibly deformable channel former with its ends clear of the mould space and an intermediate portion in contact with the said part of the replica surface, positioning a replica electrode assembly and electrode to pinch the intermediate portion of the channel former between the replica electrode and the replica workpiece, casting the block in the mould space, removing the cast block from the mould body and the replica workpiece and replica electrode assembly and electrode from the cast block and removing the channel former from the block by applying lengthwise tension to withdraw it lengthwise from the channel it has formed in the block.

The invention will be more fully understood from the following more detailed description of an electrochemical machining or engraving head, a block therefor and a method of making such block, in accordance with the invention, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of part of an electrochemical machining or engraving head including two component machining or engraving head blocks in accordance with the invention; Figure 2 is a perspective view, on a larger scale, of one of the machining or engraving head blocks shown in Fig. 1; and Figures 3, 4 and 5 are respectively a plan, a front elevation and a side elevation (each partly in section) of a moulding jig used, in accordance with the invention, in making engraving head blocks such as are shown in Figs. 1 and 2.

The electrochemical machining or engraving head 11 of which part is shown in Fig. 1 comprises a frame or body portion 12 which supports two head blocks 13 and 14 for sliding linear movement towards and away from one another by means of actuators (not shown) between an open position and a closed position in which their opposed end faces 15 are separated from one another by greater and smaller distances a and b respectively.

The faces 15 of the blocks 13 and 14 are formed with complementary recesses to accommodate between them a workpiece which is to be machined. The shape of the recesses depends, as will be evident, on the shape of the intended workpiece: as shown in Fig. 1, and on a larger scale in Fig. 2, the recess in the face 15 of the block 14 comprises an almost semi-cylindrical section 16 and coaxially therewith an almost semi-cylindrical section 17 and an almost hemi-spherical section 18, and the block 13 has a similar recess of which only an intimation of the section 16 can be seen in Fig. 1. Together, when spaced apart only by the small distance b, the two heads accommodate as a close fit a workpiece which comprises a cylindrical part on which is mounted, by a cylindrical neck of reduced diameter, a spherical end on the surface of which grooves are to be machined.

As shown in Figure 2 (but not in Figure 1), the block 14 is also formed with a stepped bore 19 to receive an electrode assembly 20, which includes an electrode 21, as a close mating fit and thereby to position the electrode accurately in relation to a workpiece clamped between the blocks 13 and 14 in their closed position. The block 14 is further formed with a generally rectangular-section channel 22 which communicates at its ends with electrolyte inlet and outlet connectors 23 and 24 at the upper and lower faces of the block and which, within the block, extends sinuously in smooth curves and passes between the section 18 of the recess and the end of the electrode-accommodating bore 19, which open into the channel from the opposite sides thereof.

In use of the machining head shown in Figs.

1 and 2, in which the block 13, like the block 14, preferably also has associated with it an electrode 21 and is provided with an electrolyte channel 22 and connectors of which one, 23, is therefore shown in Fig. 1, a workpiece to be machined is inserted between the blocks which are then moved into their closed position so that the workpiece is gripped between them and, incidentally, closes off the openings through which the recess sections 18 communicate with the respective channels 22. The connectors 23 and 24 connect the channels 22 to an electrolyte circulating system (not shown) which causes electrolyte to flow through each channel 22 while electric current is made to flow through the electrolyte between the workpiece and each electrode 21 by electrical circuitry (not shown).The blocks 13 and 14 in their closed position accurately locate the electrode ends relative to the workpiece, but it is found that a further requirement for accurate machining of the workpiece is that the flow of electrolyte between the workpiece and the adjacent end of the electrode should be essentially laminar and substantially perpendicular both to the direction in which the electrode is spaced from the workpiece and to the long dimension of the end of the electrode (in the case of an electrode of which the end is of elongate shape so as to effect machining of a long narrow groove on the workpiece surface).If the channel 22 is of elongate substantially rectangular cross section, with its long cross-sectional sides either straight or arcuate, and the electrode is a rectangular-section blade projecting into the channel with its long cross-sectional dimension extending across the channel perpendicular to the lengthwise direction of the channel, as shown in Fig. 2, it is found that suitably laminar flow of the electrolyte along the channel is obtained if the following conditions are provided:: (a) the cross-sectional area of the channel should be substantially constant, and varying in shape only smoothly if at all, for a distance along the channel, in each direction from the electrode, equal to at least three, and preferably four, times the longer cross-sectional dimension of the channel at the location of the electrode; (b) the width of the electrode, i.e. its longer cross-sectional dimension, should be approximately three-quarters of the longer crosssectional dimension of the channel, i.e. of the width of the channel wall through which it protrudes into the channel, and the electrode should be equidistant from the narrow walls of the channel which correspond to its shorter cross-sectional dimension; and (c) the thickness of the channel, i.e. its shorter cross-sectional dimension, should be about ten times the spacing between the end of the electrode and that part of the surface of the workpiece which is exposed at the opposite side of the channel: in other words, the electrode should protrude into the channel by about nine tenths of the shorter dimension of the channel.

It is a further aspect of the invention to provide a method of making a block like the blocks 13 and 14, and in particular to provide a novel and highly satisfactory method of forming the required electrolyte channel in such a block. The method and the means of carrying it out are described with reference to Figs. 3 to 5, which show a moulding jig.

The jig shown in Figs. 3 to 5 comprises a back plate 25 on which are removably secured (by means not shown) a mould base block 26, side walls 27 and 28 and a front wall 29, to form a mould shell. The base block 26 has its upper face recessed to accommodate a dummy workpiece 30 of which the part projecting upwardly above the upper face of the block 26 will form in the block to be moulded a recess like the required recess 16, 17, 18 of the block 14 in Figs. 1 and 2.

A carriage 31, adjustably secured on the back plate 25 by screws 32, carries a dummy electrode assembly 33, and is adjusted so that the end of a dummy electrode 34 comprised by the assembly 33 is in the same relation to the dummy workpiece 30 as is required for the real electrode 21 (in Fig. 1) in relation to a real workpiece accommodated in the recess 16, 17, 18. Before thus positioning the dummy electrode, however, there is interposed between it and the dummy workpiece 30 a strip 35 of polyethylene having the rectangular cross-section and the cross-sectional dimensions which are required for the electrolyte channel. In adjusting the positioning of the dummy electrode 34, the strip 35 is pinched and resiliently deformed locally to a thickness of only about one-tenth of its unstressed thickness, between the dummy electrode and the dummy workpiece 30.The strip extends away from the dummy electrode in both directions and its ends are carried out through apertures 36 and 37 in the side walls 27 and 28 respectively, its path en route being smoothly sinuous in consequence of its resilient deformability. The apertures 36 and 37 are then stopped with any suitable mouldable material, such as that available under the registered trade mark "Plasticene", to form seals 38 and 39 round the strip 35.

The interior of the mould shell, and the dummy electrode assembly 33, 34 and dummy workpiece 30 and channel former strip 35 are then sprayed with a suitable moulding release agent, which may be a PTFE-based release agent, after which the mould shell is filled with a suitable cold casting resin which is then allowed to set as a block 40.

After setting, the mould is stripped and the dummy workpiece and dummy electrode assembly are removed. The channel former strip 35 is then removed by applying a strong steady pull to one of its free ends. Owing to its high Poisson ratio, such endwise tension not only stretches it in the lengthwise direction but also causes its cross-sectional dimensions to decrease, which assists in releasing and removing the strip from the cast block. If the cross-sectional area of the channel produced by the former 35 is reduced locally, due to the pinching effect of the dummy electrode during casting, to such an extent that removal of the strip is difficult and it snaps at the constriction, the part of its length which remains in the block after pulling one of its ends can be removed separately by pulling on the other end.

As described above, one or both blocks 13 and 14 in Fig. 1 may be provided with electrolyte channels and electrode assemblies. If only one is to be so provided, only a dummy workpiece and no dummy electrode assembly or channel former strip will be provided in the mould during casting of the other block, as will be understood. It will also be understood that in using the machining head shown in Fig.

1 either one or two grooves may be machined at a time (one per electrode). After separating the blocks 13 and 14, the workpiece (if it has rotational symmetry) may be rotated in steps to repeat the process and provide a larger number of rotationally spaced grooves on the workpiece.

Claims

1. An electrochemical machining or engraving head or jig which includes a cast block of electrically insulating material having a face which is formed with a recess adapted to mate with part of the surface of a workpiece and thereby to locate the workpiece relative to the block, a bore shaped to receive and locate an electrode assembly and electrode with an end of the electrode in a desired spaced relation to a workpiece located in the said recess, and a channel extending, with the block, through between the said recess and the said bore with the recess and the bore opening into the channel from opposite sides thereof, the channel being of substantially elongate cross-section of substantially constant area in the immediate vicinity of the recess and bore and over a distance therefrom, in each direction lengthwise of the channel, which is large compared with the larger transverse dimension of the channel.

2. A head or jib as claimed in claim 1 and further comprising means for holding a mating workpiece engaged in the said recess, an electrode assembly positioned in the said bore, means connected to the inlet and outlet connectors for providing a flow of electrolyte through the said channel, and means for maintaining an electric current between such workpiece when so engaged and an electrode of the electrode assembly, through such electrolyte.

3. A head or jig as claimed in claim 1 or claim 2, wherein the said channel extends, within the block, sinuously in smooth curves.

4. A head or jig as claimed in any of claims 1 to 3, wherein the channel is of substantially rectangular cross section, with its long cross-sectional sides either straight or arcuate, and the cross-sectional area of the channel is substantially constant for a distance along the channel, in each direction from the located electrode position, equal to at least three times the longer cross-sectional dimension of the channel at that position.

5. A head or jig as claimed in any of claims 2 to 4, wherein the electrode has an end which projects into the channel and is of rectangular cross section with its shorter cross-sectional dimension extending in the lengthwise direction of the channel and its longer cross-sectional dimension extending transversely across the channel.

6. A head or jig as claimed in claim 5, wherein the longer cross-sectional dimensions of the electrode is about three-quarters of the longer cross-sectional dimension of the channel at the location of the electrode, and the electrode is located equidistant from the narrower walls of the channel corresponding to the shorter cross-sectional dimension thereof.

7. A head or jig as claimed in any of claims 2 to 6, wherein the electrode protrudes into the channel by about nine tenths of the shorter cross-sectional dimension of the channel.

8. A method of making a cast block for an electrochemical machining or engraving head as claimed in any of claims 1 to 7, the method comprising the steps of: providing a mould body defining a mould space for the block, positioning a replica workpiece in the mould body with part of its surface exposed to the mould space, positioning an elongate and flexibly deformable channel former with its ends clear of the mould space and an intermediate portion in contact with the said part of the replica surface, positioning a replica electrode assembly and electrode to pinch the intermediate portion of the channel former between the replica electrode and the replica workpiece, casting the block in the mould space, removing the mould body and the replica workpiece and replica electrode assembly and electrode from the cast block and removing the channel former from the block by applying lengthwise tension to withdraw it lengthwise from the channel it has formed in the block.

9. A method as claimed in claim 8, including selecting as the channel former a strip of flexibly deformable material which is of uniform rectangular cross section having longer and shorter cross-sectional dimensions corresponding to the desired longer and shorter dimensions of the channel which it is to form.

10. A method as claimed in claim 8 or claim 9, wherein removal of the channel former from the cast block is effected by applying lengthwise tension at one end of the former so as to stretch the part of the former extending from that end to the point where it was pinched by the replica electrode and the channel is in consequence constricted, increasing the tension until the former snaps at the constriction to allow withdrawal of the said part of the former from the channel, and then applying lengthwise tension at the other end of the former to stretch and withdraw the remainder of it from the channel.