GB2034752A

GB2034752A - Electrochemical Machine for Removing Metal

Info

Publication number: GB2034752A
Application number: GB7928419A
Authority: GB
Original assignee: BURVIC TOOL CO Ltd
Current assignee: BURVIC TOOL CO Ltd
Priority date: 1978-11-01
Filing date: 1979-08-15
Publication date: 1980-06-11

Abstract

The apparatus comprises an electrode (23) held in a nozzle (34) of an electrode assembly (24). The electrode (23) has a plate-like end portion (47) into the surface of which are embedded diamond particles. The electrode (23) is connected to a voltage source delivering a square waveform by way of a cable (36), and a tube (27) supplies a flow of electrolyte to the electrode (23) through the nozzle (34). <IMAGE>

Description

SPECIFICATION Electrochemical Machine The invention relates to the removal of surface metal from a workpiece by an electrolytic method and one aspect of the present invention provides a machine for effecting such removal.

One method of producing a mould such as a mould for plastics articles is to use the technique of spark erosion in which an electrode having an outer form of the same shape as that required of the mould is disposed over a metal blank. The electrode and blank (or workpiece) are immersed in a bath of dielectric fluid and a voltage applied between the electrode and workpiece to produce sparking therebetween. The sparking action removes metal from the blank and the voltage between electrode and blank is automatically maintained constant by varying the separation between electrode and blank so that at the end of the process a depression is produced in the blank which corresponds exactly (though in reverse) to the shape of the electrode.

The interior surface of the mould produced by spark erosion is not smooth but comprises thousands of minute indentations produced by the sparks. Furthermore the action of heating the surface (by each spark) followed by quenching or cooling (by the dielectric fluid) produces a very hard surface. Before the eroded workpiece may be used as a mould the surface must be made smooth not only because the surface of the moulded article would be unsatisfactory but also because release of the article from the mould would otherwise be difficult. This smoothing of the eroded mould has hitherto usually been performed manually with scraping and polishing tools, an operation which may take up a very significant portion of the total time involved in producing a mould.

It is an aim of the present invention to provide a method and apparatus for easily and relatively rapidly removing the hard, uneven surface of a metal workpiece which has been spark eroded.

According to a first aspect of the present invention there is provided a method of removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, in which an electrode is moved over said surface at a predetermined separation from the surface whilst an electrolyte is caused to flow over said electrode and an alternating or pulsating voltage is applied between the electrode and the workpiece.

Preferably the voltage applied has a substantially square waveform. The period in which the voltage falls to a low value or to zero may advantageously be substantially 10% of the period of the voltage waveform.

According to a second aspect of the present invention there is provided apparatus for removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, which apparatus comprises an electrode, means for maintaining a predetermined separation between an outer surface of the electrode and the workpiece surface, means for providing a flow of electrolyte over or around said electrode, and means for providing an alternating or pulsating voltage between the electrode and the workpiece.

The means for maintaining said predetermined separation may comprise an insulating material partically covering said electrode outer surface.

The insulating material may comprise a plurality of diamond particles of uniform dimensions attached to the electrode surface so as to leave portions of the metallic surface of the electrode exposed to the electrolyte. The diamond particles may be attached to the electrode surface by an electrolytic plating process in which the surface of the electrode is chromium plated.

The insulating material may however comprise a coating of a plastics material such as PTFE (poly tetra fluor ethylene) with portions of the underlying metallic electrode exposed to liquid electrolyte for example by holes, grooves or gaps provided in the coating.

Preferably means are provided for automatically reducing and/or maintaining the voltage between electrode and workpiece at a zero level in the event of a short circuit occurring between electrode and workpiece. Means may also be provided for varying the flow of electrolyte and a drain may also be provided for removing used electrolyte.

The present invention will hereinafter be further described by way of example with reference to the accompanying drawings in which: Fig. 1 is a perspective view of apparatus constructed in accordance with the present invention; Fig. 2 is a perspective view of an electrode assembly of the apparatus of Fig. 1; Fig. 3 is a perspective "exploded" view of the assembly of Fig. 2; Fig. 4 is a section through a nozzle of the assembly of Fig. 2 and Fig. 3; Fig. 5 is a plan view of the apparatus of Fig. 1; and Fig. 6 is a circuit diagram for an electronic supply and control device of the apparatus.

The apparatus comprises a stand 10 manufactured from sheet steel and having an integral projecting table portion 1 The stand 10 is mounted on two horizontally disposed steel bars 12 each of which has a respective short leg 1 3 at a front end and a respective castor 14 at a rear end as that the stand is normally fixed in position, but may be tilted onto the castors alone; for movement if desired.The table portion 11 has a tray 1 5 of plastics material with a flat periphery 1 6 supported by and attached to a corresponding upper surface of the table 11, and a flat central working area 1 7. Between regions 16 and 1 7 of tray 1 5 is a depressed portion 18 (Fig. 5) serving as a sink for expended electrolyte and having a drain hole 1 9 which leads by way of a tube to a waste container housed within the stand 10 and accessible by way of a door (not shown) in the rear of the stand 10. On an upper surface 20 of stand 10 there is disposed a control unit 21.A terminal 22 is electrically connected to an electrode 23 of an electrode assembly 24 whilst a further output terminal 25 is electrically connected to a stainless steel base plate 26 resting on the working area 1 7 of plastics tray 1 5.

The electrode assembly 24 is supplied with liquid electrolyte through a tube 27 connected, by way of an orifice 28 in the stand 10, with a pump (not shown) disposed within stand 10 and operative to pump electrolyte through tube 27 at a rate controlled by unit 21 and variable by means of a knob 29. The current passing between the electrode 23 and a work piece is also controlled by the unit 21 and determined by the position of a switch 30. A warning lamp 31 lights when a short circuit occurs between electrode 23 and workpiece or plate 26 and simultaneously the output is reduced to zero (voltage and current) and maintained in that state until the electrode is removed from the workpiece in which event the circuit is automatically re-set. An "on" button 32 and an "off" button 33 are provided on the front of the control unit 21.

The electrode assembly 24 is shown in Figs. 2 and 3 and comprises the electrode 23 fitted into a nozzle 34 held in a tubular plastics holder 35. The electrolyte supply tube 27 passes, together with a plastics covered wire cable 36, through the tube 35. One end 37 of cable 36 is soldered into a hole bored into the hexagonal head 38 of a screw 39 whilst the other end of the cable is connected to the output 22 of the control unit 21. The tube 27 extends through a central bore in screw 39 and through a cylindrical member 40 and a screw threaded portion 41 of that member, and into an internally threaded bore 42 (Fig. 4) of nozzle 34.

An annular brass ferrule 43 is disposed on tube 27 and is disposed within cylindrical member 40 so that when screw 39 is screwed into member 40 (which is internally threaded) the ferrule is compressed axially and consequently distorted to grip tube 27. A rubber or neoprene sealing washer 44 is disposed between nozzle 34 and member 40, and fits over screw threaded portion 41 , to prevent leakage of electrolyte fluid.

As may be seen in Fig. 4, the nozzle 34 has a central bore 43 to receive the shaft of electrode 23. A steel ball 44 is located in a bore extending radially of bore 43 and is supported by a small rubber or neoprene block 45 which serves as a resilient mounting. When the shaft of electrode 23 is inverted into bore 43 the ball 44 is urged against the shaft thereby providing a frictional grip which holds the electrode sufficiently firmly while also allowing angular displacement of the electrode about its longitudinal axis. Although not illustrated, a depression, preferably an annular groove could be provided in the shaft of electrode 23, for engagement with ball 44.A groove 46 of V-shaped cross-section is provided in the surface of bore 43 and extends axially along said surface from bore 42 to the front of nozzle 34, and, with the shaft of electrode 23 in place, serves as a channel for the passage of electrolyte.

The end portion 47 of electrode 23 has a flat shape with a pointed end and all sides are coated with a layer of minute diamond particles, of uniform diameter, of the order of several microns.

The diamond particles are attached by and simultaneously with the chromium plating of the end portion 47. The diamond particles in effect provide a layer of insulating material which prevents a short circuit between the portion 47 and a workpiece under treatment. At the same time, however, metal (chromium) surface of the end portion is still exposed to the electrolyte fluid by virtue of the gaps between the individual diamond particles.

In Fig. 6 is shown an electrical circuit forming part of the control unit 21. An essential part of the circuit as a whole is an integrated circuit 48 which is a standard 555 timer operated in a stable mode to produce a voltage at an output point 49 which is of square waveform in which the voltage alternates between a high level of about 1 0V, and a zero level, the length of time for which the voltage is zero being approximately 10% of the period of the waveform. The proportion of time for which the voltage is zero is determined by the arrangement of resistors R and R, npn transistor T, and diode D1 connected between a high (10V) supply voltage line 50 and "discharge" terminal 51 and "trigger" terminal 52 of the 555 timer 48.The output from terminal 49 is taken to the base of npn transistor T2 connected in emitter follower mode between supply line 50 and a return line 53. Transistor T2 has a resistor R2 connected in its emitter line and the emitter of transistor T2 is electrically connected to the respective base of each of transistors T3,T4,T5, T6, T7,T8,T9, T10 each of which is connected in emitter follower mode with a respective one of resistors R3, R4, R5, R6, R7, R8, R9, R10 connected in the respective emitter line.Each of transistors T3 toT10 is connected between voltage supply 50 and a line 54 which provides the output 25 of the control unit which is connected to the plate 26.

The line 53 provides the output 26 which is connected to electrode 23. Switch 30 of the control unit is situated along rail 54 so as to divide transistors T3 to T10 into two banks of four parallel transistors so that a switching element 55 is moved from position 56 to 57 the current delivered to output 25 is increased by the sum of the current outputs of transistors T3 to T6 which in this case means that the current output is approximately double from 1 OA to 20A.

Line 54 is connected to supply line 50 by way of a high value (100us) resistor R" and also between diodes D, and D2 of a diode pump circuit having a smoothing capacitor C, and variable resistor R,2. The voltage tapping from variable resistor R,2 is connected by way of a protection resistor R,3 to the base of a control transistorT provided with a resistor R,4 in its collector line.

The collector of transistor T" is connected by way of a protection resistor R1s to the base of a switching transistor T,2 which has lamp 31 connected in its collector circuit. The base of transistor Tor 1 is also connected by way of a protection resistor R16 to the base of a transistor T,3 with a resistor R,7 in the collector circuit. The collector of transistor T,3 is connected by way of a base protection resistor R,8 to the base of a transistor T14 having a resistor R,g in the collector line. The collector of transistor T14 also connected to a "reset" terminal 58 of integrated circuit 48. A supply terminal 59 of timer 48 is connected to supply line 50 whilst a "ground" terminal of timer 48 is connected to line 53.Threshold and "control" terminals 61 and 62 of timer 48 are connected to line 53 by way of capacitors C2 and C3, and it will finally be noted that terminals 52 and 61 are electrically connected together.

The operation of the apparatus as a whole will now be described. A spark eroded workpiece to be cleaned (i.e. have the hard eroded surface removed) is placed on plate 26, the apparatus is switched on by pressing button 32, and the flow of electrolyte to the electrode assembly is adjusted by turning knob 29. The electrolyte is a dilute solution of potassium nitrate which is pumped through tube 27 to nozzle 34 where it passes along channel 46 to the shaft of electrode 23 to which it adheres by a natural surface tension effect and thus reaches and flows over and around the flattened end portion 47 of the electrode. The end portion 47 of the electrode is then placed in contact with and moved over the surface of the workpiece under treatment, with the application of only a very gentle pressure.

Electrolysis takes place between workpiece and electrode at a rate determined by the magnitude of the electric current which in turn is determined by the position of switch 30 of control unit 21.

During the electrolysis metal is removed from the workpiece and but for the flow of electrolyte would be deposited on the electrode portion 41.

Deposition on the electrode is also prevented by the nature of the voltage (and therefore also current) waveform between electrode 23 and workpiece in that deposit and sludge removal from the electrode by the electrolyte flow is facilitated during the periods of zero current. It has been found advantageous in optimising metal removal whilst preventing deposition on the electrode of the period for which the current is zero is approximately 150,us whilst the period for which the current (and thus voltage) is a maximum is 1500 500/15. The operation is continued until the surface of the workpiece is smooth and clean. Very considerable savings in time are achieved over conventional (manual) methods of smoothing metal surfaces and furthermore the final finish is often superior.Should the shaft of electrode 23 touch the workpiece being treated to produce a short circuit the voltage and current supply to the plate 26 is almost instantaneously reduced to zero and maintained in that state until the electrode is removed from the workpiece. At the same time warning lamp 31 lights up.

Referring to Fig. 6, it will be understood that transistor T" is normally in a conducting state by virtue of the positive voltage tapped from resistor R12 and supplied to the base of T,1. Since T11 is conducting the voltage at the base of transistor T.2 is low and thus transistor T12 does not conduct and lamp 31 is "off". Should a short circuit occur the voltage at line 54 falls to zero, and this voltage drop is communicated to the base of transistor T,1 which is consequently switched off causing transistor T,2 to conduct thereby lighting warning lamp 31.Simultaneously transistor T,3 is switched from its normal conducting state to a nonconducting state so that transistor T14 is switched on, the voltage at terminal 58 is reduced from a high value to a low value thereby reducing the output at terminal 49 to zero. When the electrode is removed from the workpiece the voltage at the junction between D, and D2 is raised to a level sufficient to switch transistors T" and T13 into conducting states. This occurs by virtue of the provision of resistor R" which conducts current from the line 50 through diode D2 and resistor R,2 to line 53. Thus, resistor R" in effect provides the automatic re-setting when the electrode is removed from the workpiece.

Other embodiments of the invention apart from that referred to above are, of course, possible. In a further embodiment (not illustrated) it has been found advantageous to provide a six position switch for varying the current supply between 1 and 20 amperes. This is particularly useful where spark erosion has produced a relatively smooth, fine workpiece surface and enables a low value of current to be selected, this being all that is required to effect the necessary polishing. In this case the output transistors T3 to To are connected generally as illustrated in Fig. 6 but with the emitters of the transistors connected together in pairs T3 to T4,T5to T6,T7to T8,T9to T10 and each pair of emitters connected through a respective one of equal value resistors to one end of a line of five current limiting resistors connected in series. The six position switch has a movable contact which can be brought into electrical connection with the beginning or end of the resistor line or any of the junctions between the resistors of the line to draw off a variable current which is supplied to lead 37 of the electrode assembly.

Claims

1. A method of removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, in which an electrode is moved over said surface at a predetermined separation from the surface whilst an electrolyte is caused to flow over said electrode and an alternating or pulsating voltage is applied between the electrode and the workpiece.

2. A method as claimed in claim 1 in which the applied voltage has a substantially square waveform.

3. A method as claimed in claim 2 in which the period during which the voltage falls to a low value or zero is substantially 10% of the period of the voltage waveform.

4. Apparatus for removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, which apparatus comprises an electrode, means for maintaining a predetermined separation between an outer surface of the electrode and the workpiece surface, means for supplying a flow of electrolyte over or around said electrode, and means for providing an alternating or pulsating voltage between the electrode and the workpiece.

5. Apparatus as claimed in claim 4 in which the means for maintaining the predetermined separation between the surfaces of electrode and workpiece comprises electrically insulating material partially covering said electrode outer surface.

6. Apparatus as claimed in claim 5 in which the insulating material comprises diamond particles of substantially uniform dimension attached to the electrode surface so as to leave portions of the outer surface of the electrode exposed to the electrolyte.

7. Apparatus as claimed in claim 6 in which the diamond particles are attached to the electrode surface by an electrolyte plating process in which the surface of the electrode is chromium plated.

8. Apparatus as claimed in claim 5 in which the insulating material comprises a plastic coating on the electrode, the coating having apertures therein to expose the underlying outer electrode surface to the liquid electrolyte.

9. Apparatus as claimed in any of claims 4 to 8 further comprising means for automatically reducing the voltage between electrode and workpiece to substantially zero level in the event of a short circuit occurring between electrode and workpiece.

10. Apparatus as claimed in any of claims 4 to 9 further comprising means for varying the rate of flow of electrolyte.

11. Apparatus as claimed in any of claims 4 to 10 in which the means for providing a voltage between the electrode and workpiece is operative to deliver a square voltage waveform in which the voltage periodically alternates between a high value and zero.

12. Apparatus as claimed in claim 11, in which the ratio of the period for which the voltage is at the high level to the period for which the voltage is at the zero level is approximately 10:1.

13. Apparatus as claimed in any of claims 4 to 12 in which the electrode comprises a stem portion, a flat working portion at one end of the stem, and a nozzle into which the other end of the stem is fitted, the flow means being operative to deliver liquid electrolyte to said other end of the stem such that the electrolyte flows along the outside of the stem to the working portion of the electrode.

14. Apparatus as claimed in any of claims 4 to 13 further comprising means for varying the current flow between electrode and workpiece.

1 5. Apparatus for removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 6. A method of removing metal from the surface of a workpiece, thereby cleaning and smoothing said surface, substantially as hereinbefore described with reference to the accompanying drawings.