Surface preparation apparatus and method

Abstract: The present invention is an apparatus and method which utilise electrical discharge machining equipment and techniques to prepare an electrically conductive surface for thermal spray coating or for other purposes. The apparatus includes one or more electrodes (16), with each electrode having a working face (36), defining a plane. The one or more electrodes are mounted on the leading end of a shaft (14) and can form part of an electrode assembly (12). The shaft (14) is at least moveable axially by a shaft actuating mechanism (52) and, in some applications, may also rotate the shaft. Each electrode (16) is moveable by a gap setting mechanism (18) in response to and in a direction different than the axial movement of the shaft (14) to form a voltage gap between each working face and the surface being prepared. A supply of dielectric fluid is disposable in the voltage gap. An electrical discharge circuit (120) conducts an electric current through the dielectric fluid in the gap to produce a gap voltage. The shaft actuating mechanism (52) is controlled by a gap voltage control system.

Description: The present invention is related to surface preparation techniques, more particularly, to thermal spray coating a surface and, even more particularly, to an apparatus and method for preparing an electrically conductive surface for receiving a thermal spray coating using electrical discharge machining technology. Thermal spray coating techniques are used in a number of applications and are particularly attractive in the automobile manufacturing industry where weight reductions are desired to obtain increased fuel efficiency. For example, it has been proposed to use a thermal spray coating of an iron based material to replace the cast iron sleeve currently used in the cylinder bores of various types of aluminium engine blocks. Such a thermal spray coating can provide the wear characteristics required for the surface of the aluminium cylinder bores and eliminate the weight associated with the cast iron sleeve. In order to obtain optimum adherence of the thermal spray coating, the aluminium bore surface must be properly prepared or roughened before being coated. Grit blasting is one known technique which can be used to prepare the aluminium bore surface for thermal spray coating. However, grit blasting typically requires a thorough cleaning of the blasted aluminium surface followed by the application of an intermediate bonding coat before the iron based thermal spray coating is applied. In addition to the added expense of having to perform these intermediate steps, it is very difficult, if not impossible, to remove all of the grit from the blasted surface and such bonding coats typically contain environmentally unfriendly materials. The problems associated with grit blasting can be avoided by using an ultra high pressure water jet technique to prepare a surface for thermal spray coating. However, such water jet processes are relatively expensive and difficult to run and maintain. The water jet process is typically so noisy that it must be performed in an enclosed and acoustically shielded area. In addition, the process consumes a great deal of energy to compress large quantities of high purity water. Furthermore, with water jet spraying, the degree to which the surface is prepared or roughened is relatively difficult to control and is more dependant on the condition of the material forming the surface being sprayed. It has been proposed in DE-B 1 293 515 to improve the adhesion of sprayed metal coatings on metal substrates by roughening the substrate surface by electric spark erosion using a small-diameter electrode. To avoid the formation of deposits on the roughened surface which then need to be removed, e.g. by grit blasting, it is proposed to perform the spark erosion under an inert gas shield. There is still a need for a relatively inexpensive apparatus and method capable of accurately controlling the preparation of an electrically conductive surface to optimise the adhesion of a thermal spray coating thereto, particularly when the surface being prepared is a curved interior or exterior surface. There is a further need for such an apparatus and method which is capable of preparing an electrically conductive surface that does not require a separate cleaning operation to be relatively free of contaminants, such as grit, and which provides for optimum adherence of a thermal spray coating without the need of an intermediate bond coating. The present invention provides an apparatus and method as set forth in claims 1 and 7 respectively which utilise electrical discharge machining (EDM) equipment and techniques to prepare an electrically conductive surface for thermal spray coating. It may also be advantageous to utilise the principles of the present invention to prepare a surface for a purpose other than thermal spray coating. An apparatus for the fine machining of curved surfaces, e.g. of bores and shafts, having the features set forth in the preamble of claim 1 is disclosed in DE-B 1 045 574. Another apparatus, particularly for the electroerosion of the interiors of bores, which is similar in some respects is disclosed in DE-C 44 21 246. Both forms of apparatus include an axially moveable shaft having a conical leading end which engages directly or indirectly with the electrodes so that axial movement of the shaft causes radial displacement of the electrodes to adjust the gap between the working face of the electrodes and the surface of the workpiece, but in either case is any provision made for ensuring that the working face of the electrodes remains aligned with the surface being worked. An embodiment of the apparatus of the present invention includes a plurality of electrodes each having a working face which defines a plane. The electrodes are mounted on the leading end of a shaft or bar and form part of an electrode assembly or tool. The shaft is movable axially by a shaft actuating mechanism, and it desirable for the shaft actuating mechanism to also rotate the shaft about its central longitudinal axis. Each electrode is moveable by a gap setting mechanism in response to the axial movement of the shaft, caused by the shaft actuating mechanism. The movement of each electrode, caused by the axial movement of the shaft, is in a direction different than the axial movement of the shaft. Each electrode is moved by the gap setting mechanism in a direction substantially perpendicular to the plane of its corresponding working face to form a voltage gap between its working face and the electrically conductive surface being prepared. The gap setting mechanism moves each of the electrodes simultaneously to form a uniform voltage gap, between each electrode working face and its corresponding electrically conductive surface, at substantially the same time. It is also desirable for the gap setting mechanism to move each electrode (i.e., each working face) a fraction of the distance the shaft is moved axially by the shaft actuating mechanism. The apparatus embodying the present invention also includes a supply of dielectric fluid which is disposable in the voltage gap formed between the working face of each electrode and its corresponding electrically conductive surface. The dielectric fluid being supplied can include any suitable type of dielectric fluid conventionally used in electrical discharge machining. Although, it is desirable for a water based dielectric fluid to be used in order to reduce, if not eliminate, the degree of cleaning required before the thermal spray coating is applied. An electrical discharge circuit is used to conduct an electric current through the dielectric fluid in the gap to produce a gap voltage between each electrode working face and its corresponding electrically conductive surface. The shaft actuating mechanism is controlled by a feed-back or closed-loop gap voltage control system, in response to changes in the gap voltage. The control system includes a sensor for sensing the gap voltage. In the prior proposal to use EDM equipment and techniques to prepare an electrically conductive surface for thermal spray coating, the surface preparation process has not been controlled using a gap voltage control system, such as that employed in conventional electrical discharge machining operations. That is, until now, the system normally used to control a machining gap, for a conventional electrical discharge machining operation, has not been used to control the movement of an electrode during a surface preparation process, especially when the electrode is being moved in a direction different than that of the shaft mounting the electrode. Because each electrode is moveable in response to movement by the shaft, the control system enables the optimum voltage gap, between each electrode working face and the surface being prepared, to be maintained, even as the electrode wears down with use. During the surface preparation process minor fluctuations in the actual gap voltage, from a preset gap voltage, are likely to occur. Typical EDM gap voltage control systems may overreact in causing the gap setting mechanism to move each electrode back to the desired voltage gap. By adapting the gap setting mechanism to move each electrode only a fraction of the distance moved by the shaft, any major change in the actual gap voltage, from the preset gap voltage, caused by the gap voltage control system overreacting will be minimised and significant interruptions in the overall surface preparation process reduced. Therefore, with such a gap setting mechanism, an electrically conductive surface can be prepared efficiently and the preparation process controlled with a high degree of accuracy and precision using simpler and less expensive shaft actuating mechanisms and voltage gap control systems. In addition, by using such a gap setting mechanism, each electrode working face can be moved toward and away from the corresponding surface being prepared with a relatively smooth action. It is desirable to maintain the same voltage gap between the surface being prepared and the entire working face of each electrode so that the electrically conductive surface is more uniformly prepared. To help insure such uniform spacing between each electrode working face and the corresponding surface being prepared, the present apparatus includes an aligning mechanism which maintains each working face in a substantially parallel relation with the electrically conductive surface. Some workpieces have multiple surfaces which need to be prepared. For example, a single engine block can have 4, 6, 8 or more cylinder bores. For such a workpiece, one apparatus can be used for each surface. With engine blocks, the relative location of the cylinder bores on the same block can vary, within acceptable tolerances, from one block to another. While this variance does not affect the performance of the engine block, it can make it difficult to prepare two or more of the cylinder bores at one time, without having to move and set-up each apparatus anew for each block. To make it easier to prepare the variable multiple surfaces on such workpieces at one time, it is desirable to mount the one or more electrodes on each apparatus using a flexible coupling mechanism which allows movement in a plane perpendicular to, but not along, the longitudinal axis of the shaft. In this way, all the electrodes on each apparatus can be shifted together to compensate for the misalignment, rather than having to move and set-up each apparatus for every workpiece. It is desirable for the apparatus to include at least one dielectric supply line which directs dielectric fluid from the supply of dielectric fluid into the voltage gap. One example of such a dielectric supply line is a bore formed through at least one electrode, with a dielectric exit of one or more openings in communication with the voltage gap. However, the present apparatus can be used to advantage regardless of whether the workpiece, containing the surface to be prepared, is submerged in a bath of dielectric fluid using conventional sinker techniques or, as disclosed in detail herein, the dielectric fluid is directed to fill and flush through the voltage gap. For preparing circular surfaces, the present apparatus includes a shaft actuating mechanism which is capable of rotating the shaft about an axis of rotation. Such a shaft actuating mechanism can be used to prepare an inner curved surface, such as the inside diameter of a circular bore, or an outer curved surface, such as the outside diameter of a shaft or tube having a circular cross section. When an inner curved surface is being prepared, the working face of each electrode faces outward from the axis of rotation of the shaft. When an outer curved surface is being prepared, the working face of each electrode faces inward toward the axis of rotation of the shaft. In each case the plurality of electrodes is disposed in a circle in the electrode assembly. The apparatus may include pairs of opposing electrically conductive surfaces which are in close proximity and which move relative to one another during the preparation of the electrically conductive surface. An example of such a pair of opposing electrically conductive surfaces may be the ball and race of a metallic ball bearing assembly. To prevent, or at least significantly inhibit, the electrical erosion of one or both surfaces in each pair of such opposing electrically conductive surfaces, it is desirable for the apparatus to be adapted so as to electrically isolate as many pairs of such surfaces as practicable from the electric current supplied by the electrical discharge circuit. One embodiment of an electrode assembly or tool which may be used with the above described apparatus, or any other suitable electrical discharge machine having an axially moveable shaft, includes the above described one or more electrodes and the gap setting mechanism. The gap setting mechanism is mountable on the axially moveable shaft. One exemplary gap setting mechanism includes at least one first wedging surface disposed at an angle from the shaft when the electrode is mounted thereon. Each first wedging surface is in slidable engagement with a corresponding second wedging surface on each of the electrodes such that axial movement of the shaft causes one wedging surface to slide by the other. This relative sliding movement between the first and second wedging surfaces causes each electrode to move in a direction different than the axial movement of the shaft. For ease of maintenance, it is desirable for each electrode to include an electrode adapter which mounts a replaceable electrode insert, where the working face of the electrode is formed on the electrode insert. In this way, a worn out working face can be replaced, without having to take the time or incur the cost to replace the entire electrode, by only replacing the electrode insert. With such an electrode assembly, it may desirable for at least one dielectric supply line to be formed through each electrode, with a dielectric exit of one or more exit openings being formed in each electrode working face. In this way, dielectric fluid can be supplied directly to the working face of each electrode through the dielectric supply line. When the apparatus of the invention is used in a method for preparing for metal spraying a pre-existing electrically conductive internal or external surface of circular cross-section, the method includes the step of axially moving the shaft and moving each electrode, in response to the axial movement of the shaft, in a direction different from the axial movement of the shaft to form a voltage gap between each working face and its corresponding electrically conductive surface. The present method further includes disposing a dielectric fluid in the voltage gap and conducting an electric current through the dielectric fluid to thereby produce a gap voltage across the gap. Movement of each electrode, and thereby its corresponding voltage gap, is controlled by controlling the axial movement of the shaft in response to changes in the gap voltage. It is desirable for the step of disposing the dielectric fluid in the voltage gap to include flushing the dielectric fluid through and out of the voltage gap. While it may include other subsequent operations, the present method can include the step of thermal spray coating the electrically conductive surface. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1A is a top view of one embodiment of an apparatus for preparing an electrically conductive surface according to the present invention; Fig. 1B is a top view of one embodiment of an electrode assembly mounted on the shaft of the apparatus of Fig. 1A at break line 1; Fig. 2A is a partially sectioned side view of the apparatus of Fig. 1A taken along lines 2A-2A; Fig. 2B is a partially sectioned side view of the electrode assembly of Fig. 1B taken along lines 2B-2B; Fig. 3 is a bottom view of the electrode assembly of Fig. 1B; Fig. 4 is an end view of the apparatus of Fig. 1A with the electrode assembly of Fig. 1B mounted on the shaft thereof; Fig. 5 is a sectional view of the electrode assembly of Fig. 1B taken along lines 5-5; Fig. 6 is a sectional view of the apparatus of Fig. 1A taken along lines 6-6; Fig. 7 is a sectional view of the flexible coupling mechanism on the apparatus of Fig. 1A taken along lines 7-7; Fig. 8 is a sectional view of the flexible coupling mechanism of Fig. 7 taken along lines 8-8; and Fig. 9 is a sectional view of the flexible coupling mechanism of Fig. 7 taken along lines 9-9. Referring to Figs. 1-4, one embodiment of an apparatus 10 is shown for preparing an electrically conductive surface, such as the inner wall of a cylinder bore for an engine block (not shown), for receiving a thermal spray coating. The apparatus 10 includes an electrode assembly or tool 12 mounted on the leading end of a drawbar or shaft 14 made, for example, from 360 brass round stock. The electrode assembly 12 includes three two-piece electrodes 16 and a gap setting mechanism 18. Each electrode 16 includes a replaceable electrode insert 20, made of brass, bolted or otherwise removably mounted to an electrode adapter 22. The gap setting mechanism 18 includes a wedge-shaped electrode housing or wedge 24, made of AMPCO-18 and having a central longitudinal opening 26 for receiving a reduced diameter section 28 of the shaft 14 therethrough. The wedge 24 has three dove-tail slots 30 formed along its length and inclined radially outward from its trailing end towards its leading end. References herein to a trailing, rear or back direction refer to the left side of Figs. 1A and 2A, and references to a leading, forward or front direction refer to the right side of Figs. 1A and 2A. Each dove-tail slot 30 is operatively adapted to slidably receive a key-shaped cross section of one of the electrode adapters 22 lengthwise therein. Each dove-tail slot 30 has a first wedging surface 32 in slidable engagement with a second wedging surface 34 located on the underside of its corresponding electrode adapter 22. Thus, as described in greater detail below, when the wedge 24 is moved axially the first wedging surfaces 32 slide by the second wedging surfaces 34 and the electrodes 16 move radially outward. Each electrode insert 20 has a working face 36 which defines one side of a voltage gap between the corresponding electrode 16 and the surface being prepared. To help maintain the same voltage gap (i.e., a uniform spacing) between the surface being prepared and all the surface area of each working face 36, the electrode assembly 12 includes an aligning mechanism 38 which maintains each working face 36 in a substantially parallel relationship with, for example, the inside wall of a cylinder bore. The aligning mechanism 38 includes three nylon centring bars 40 interspersed one between every two adjacent electrodes 16. Each centring bar 40 is spring-loaded into a corresponding slot 42 formed in the electrode housing 24 and includes two nylon rollers 44. The rollers 44 are forced against the inside wall of a cylinder bore by two sets of springs 46, such as crest-to-crest or belleville springs. Each set of springs 46 is disposed in a recess formed on the underside of its corresponding bar 40. Each centring bar 40 is mounted with two bolts 48, so as to only be moveable radially outward a preset distance in its slot 42. The centring bars 40 are shown in the figures in their fully compressed state. The amount of radial extension possible for each centring bar 40 is dependent on the space 50 between the head of each bolt 48 and the bottom of the counter bore in which the bolt 48 is disposed (see Fig. 2B). The apparatus 10 includes a shaft actuating mechanism 52 which can alternately and simultaneously rotate and axially move the shaft 14 along and about its central longitudinal axis 54, respectively. The actuating mechanism 52 includes a motor 56 for rotating the shaft 14 and a ball screw assembly 58 (shown diagrammatically) for effecting the axial movement of the shaft 14. The ball screw assembly 58 is driven by a servomotor and controlled using a rotary encoder according to conventional practice, neither of which form a basis for the present invention and are therefore not disclosed herein in detail. The shaft rotating motor 56 drives a pulley 60 through a gear belt 62. The pulley 60 is keyed to rotate the trailing end of a tubular drive shaft 64, made from 1018 steel tubing. Rotation of the drive shaft 64 is transmitted to the drawbar 14 through three radially oriented lock screws 66. Each lock screw 66 is made of an electrically insulative material, such as a suitable plastic, and disposed through the wall of the drive shaft 64 and into a longitudinal slot 68 formed in the drawbar 14. The actuating mechanism 52 is mounted on a bracket 72 which can be translated to move the electrode assembly 12 in position in a cylinder bore and then fixed in place. To position the electrode assembly 12 in a cylinder bore, the bracket 72 mounting the shaft actuating mechanism 52 is moved. Once the electrode assembly 12 is so positioned, the shaft actuating mechanism 52 is activated to actuate the ball screw assembly 58 and the motor 56 to prepare the cylinder bore surface for thermal spray coating. The drive shaft 64 is mounted to bracket 72 through an electrically isolated bearing assembly 74. The bearing assembly 74 includes two longitudinally spaced ball bearings 76 mounted between the outer surface of the drive shaft 64 and an electrically insulative housing 78 made, for example, of a type C canvas phenolic. The motor 56 is mounted directly to the housing 78. A tubular spacer 80, made of 1018 steel tubing, is disposed around the outside of the drive shaft 64 and between the two ball bearings 76. A ring shaped spacer 82 made, for example, of a C.R. 1018 steel is disposed around the outside of the drive shaft 64 and between the pulley 60 and the trailing bearings 76. The pulley 60 and the bearing assembly 74 are locked in place on the outside of the drive shaft 64 against an annular flange 84 extending out from the drive shaft 64 by a retaining nut 86 threaded on the trailing end of the drive shaft 64. Two bushings 69 and 70 made of a suitable bearing metal, such as brass, are mounted one at either end on the inside diameter of the drive shaft 64 to help maintain a clearance between the drawbar 14 and the drive shaft 64, while allowing the drawbar 14 to freely move axially relative to the drive shaft 64. The drawbar 14 is mounted at its trailing end to the ball screw assembly 58 by a rotary coupling assembly 88. The rotary coupling assembly 88 enables the drawbar 14 to be moved axially while being rotated. The rotary coupler 88 includes two half-couples 90 and 92 made of 8620 steel and threaded one into the other. The one half-couple 90 is bolted to a bracket 94 which is mounted so as to be moved by the ball screw assembly 58. The other half-couple 92 includes a self-aligning ball bearing 96 to which the trailing end of the drawbar 14 is secured by a retaining nut 98 threaded thereon. With the electrodes 16 prevented from moving axially, as described in detail later on, axial movement of the drawbar 14 caused by the actuation of the ball screw assembly 58 causes each first wedging surface 32 of the electrode housing 24 to slide by its corresponding second wedging surface 34. When the drawbar 14 is pulled back or pushed forward, so too are the first wedging surfaces 32. The wedging surfaces 32 and 34 are disposed at an angle from the axis 54 of the drawbar 14 so that this axial movement of the electrode housing 24 causes the electrode 16 to move radially out from and in toward the axis 54, respectfully. In this way, the voltage gap formed between each electrode working face 36 and the inside surface of, for example, a cylinder bore being prepared can be controlled by actuating the ball screw assembly 58. In addition, depending on the angle used, the radial distance travelled by the electrodes 16 can be a fraction of the axial distance travelled by the drawbar 14. Satisfactory results have been obtained when the angle chosen, about 7° from the axis 54, results in the electrodes 16 moving radially about an eighth of the distance moved axially by the drawbar 14. The electrode assembly 12 is mounted to rotate with the drive shaft 64, as described in detail below, when the motor 56 is activated. By rotating the electrode assembly 12, the working face 36 of each electrode 16 can effectively form a voltage gap with all of the inside surface of a cylinder bore being prepared. It is desirable for each electrode 16 to be longer than the cylinder bore is deep to insure preparation of its entire inner surface in one operation. A supply of dielectric fluid is flushed through three dielectric supply lines 100, into the voltage gap and out the cylinder bore, taking eroded particles of the workpiece with it. Each dielectric supply line 100 includes a teflon hose 102 sheathed with a braid of bronze wire and having an electrically conductive connector at each end. As well be discussed later on, the sheath of bronze wire braid around the hose 102 is used as a path for conducting electricity to the electrodes 16. The leading connector is coupled to the trailing end of a supply bore 104 formed longitudinally in the corresponding electrode adapter 22. A plurality of exit openings 106 are formed radially through each electrode adapter 22 and the corresponding electrode insert 20 to connect the supply bore 104 to its working face 36. The trailing connector is connected to a tubular slip shaft 107, made of copper, which is interlocked for rotation with the drive shaft 64 and the drawbar 14 by the lock screws 66. A nylon isolator sleeve 105 is disposed between the drive shaft 64 and the slip shaft 107. The electrically insulative screws 66 and sleeve 105 electrically isolate opposing electrically conductive surfaces of the drawbar 14 and the drive shaft 64. Each supply line 100 is in communication with an annular supply groove 108, formed on an inside diameter of a housing 110, made of 6061-T651 aluminium, through an elbow bore 112 formed through the slip shaft 107. A pair of O-rings 114, disposed on either side of the supply groove 108, seal the space between the slip shaft 107 and the housing 110. The annular supply groove 108 is in communication with an external supply of dielectric fluid (not shown) through a connecting bore 116 formed through the aluminium housing 110. The aluminium housing 110 is bolted to the leading end of the electrically nonconductive bearing housing 78 through a washer shaped isolator plate 118, made of nylon. Because it is mounted to the isolator plate 118, electrical erosion between the aluminium housing 110 and the copper slip shaft 107 can be prevented. An electrical discharge circuit 120 is used to conduct an electric current through the dielectric fluid in the gap between each working face 36 and the inside surface of, for example, a cylinder bore, to produce a gap voltage therebetween. This circuit 120 includes the slip shaft 107, the connectors and sheath of braided brass wire on each hose 102, and each electrode 16. An external electric current is supplied to the circuit 120 through a pair of brush assemblies 122 mounted in the aluminium housing 110 on either side of the copper slip shaft 107. Each brush assembly 122 includes a spring loaded graphite brush 124 which maintains electrical contact with the side of the slip shaft 107 as it rotates. A drain bore 126 (shown in phantom in Fig. 4) is formed through the side of the housing 110 between the corresponding brush assembly 122 and the back O-ring 114. Drain bore 126 prevents dielectric fluid, that may leak from the supply groove 108 through the back O-ring 114, from building up and interfering with the operation of the brush assemblies 122. Referring to Figures 7-9, to make it easier to prepare the inside wall of two or more cylinder bores using two or more of the apparatus 10, it is desirable for each apparatus 10 to include a flexible coupling mechanism or coupler 128 which allows movement of the electrode assembly 12 radially from the axis 54 in an X-Y plane, but not along the axis 54. The flexible coupler 128 includes a central steel plate 130 sandwiched between a first and second steel block 131 and 132. The plate 130 and each block 131 and 132 define a central bore through which the drawbar 14 is disposed. The central plate 130 includes three first pins 134 and three second pins 135 press-fit or otherwise mounted to extend out beyond opposite sides of the plate 130. The pins 134 and 135 are interspersed, one next to the other, so as to be equally spaced circumferentially around the drawbar 14. Each first pin 134 extends into a slot shaped cavity 136 formed in the first block 131, and each of the second pins 135 extends into a slot-shaped cavity 137 formed in the second block 132. Plate 130 and blocks 131 and 132 are spring loaded together with three coil springs 138, equally spaced circumferentially around the outside of elements 130-132. The ends of each spring 138 are bolted one to each block 131 and 132. The coil springs 138 are of sufficient strength to prevent the axial separation of the elements 130-132. The bolts anchoring either end of each spring 138 also secure the blocks 131 and 132 to the leading end of the slip shaft 107 and a brass flange 140, respectfully. To electrically isolate and thereby prevent electrical erosion within the flexible coupler 128, it may be desirable for a layer of insulative material to be disposed between the first block 131 and the central plate 130. The same electrical isolation may be obtained by using a two piece first block 131, with the two pieces separated by a layer of insulative material (indicated by phantom line 139). It is also desirable, for electrical isolation, to position a space 141 between the leading end of the first block 131 and the insulator sleeve 105, the drive shaft 64 and the bushing 70. The trailing end of electrode adapter 22 is mounted to the flange 140 so as to prevent axial movement of the electrodes 16 but to allow the electrode adapters 22 to move radially outward from and inward to the axis 54 in response to rear and forward movement of the wedge 24, respectively. Referring to Figure 5, such freedom of radial movement can be obtained by mounting each electrode adapter 22 between two slide pins or dowels 142, with each dowel 142 being disposed in one of two corresponding grooves formed on either side of the trailing end of the electrode adapter 22. Each dowel 142 is mounted to the flange 140 by a bolt secured washer 144 disposed in a transverse slot formed in the corresponding dowel 142. To allow the entire electrode assembly 12 to shift radially, as permitted by the flexible coupler 128, the electrode housing 24 is mounted on the leading end of the drawbar 14 so as to radially float thereon. For example, the electrode housing 24 can be mounted using a pair of spring biased flanges 146 and 147 which are disposed at either end in the central opening 26 of the housing 24 and loosely fit around the reduced diameter section of the drawbar 14. The leading flange 147 is bolted to the electrode housing, while the trailing flange 146 is not. A coil spring 148 is disposed between the flanges 146 and 147 and is preloaded by tightening a pair of jam nuts 150, threaded on the leading end of drawbar 14 with a nylon washer. The amount of force required to radially shift the electrode assembly 12, with regard to the drawbar axis 54, can be controlled by the torque applied to the jam nuts 150 and the strength of the coil spring 148. The shaft actuating mechanism 52 is controlled by a conventional feedback or closed-loop gap voltage control system, in response to changes in the gap voltage during the surface preparation process, compared to a preset gap voltage desired for the gap between each electrode working face 36 and the inside surface of, for example, a cylinder bore. The control system includes a conventional gap voltage sensor. Readings from this sensor are used by a computer controller to actuate the ball screw assembly 58 to axially move the drawbar 14, and thereby the electrode working faces 36 (as described above) to maintain the preset gap voltage. Rotation of the drive shaft 64, and thereby the electrode assembly 12, by activating motor 56, can be controlled manually or by computer control using conventional equipment, as desired.

Claims: An apparatus for metal spray coating a pre-existing electrically conductive internal or external cylindrical surface of circular cross-section, said apparatus comprising: an electrode assembly (12) comprising a plurality of electrodes (16) each having a working face (36) defining a plane facing said surface and extending parallel to the axis thereof a shaft (14) having a leading end mounting said electrode assembly; a shaft actuating mechanism (52) operatively adapted for axially moving and rotating moving said shaft; a gap setting mechanism (18) operatively adapted for simultaneously moving said electrodes in response to the axial movement of said shaft (14) by said shaft actuating mechanism (52), in a direction perpendicular to said shaft and to said surface to form a voltage gap between said working face and said surface; a supply of dielectric fluid disposable in said voltage gap; an electrical discharge circuit (120) operatively adapted for conducting an electric current through the dielectric fluid disposed in the gap, thereby producing a gap voltage between said electrodes and said surface; and a gap voltage control system for controlling said shaft actuating mechanism (52) in response to changes in the gap voltage between said electrodes (16) and said surface, characterised in that: said electrode assembly (12) further includes centring bars (40) interspersed between every two adjacent electrodes (16), said centring bars extending in the axial direction of said surface and being spring-loaded to contact said surface to maintain each working face (36) of said electrodes substantially parallel with said surface. An apparatus according to claim 1, wherein said centring bars (40) are each provided in each end region thereof with a roller (44) arranged to run circumferentially on said surface and forced into contact therewith by said spring-loading. An apparatus according to claim 1 or 2, wherein said electrode assembly includes three electrodes (16) and three centring bars (40) interspersed between them. An apparatus according to any preceding claim, further comprising a flexible coupling mechanism (128) adapted to allow movement of the electrode assembly (12) in a plane perpendicular to, but not along, the longitudinal axis of said shaft (14). An electrode assembly for use with an electrical discharge machine, having an axially movable shaft (14), to prepare for metal spray coating a pre-existing electrically conductive internal or external cylindrical surface of circular cross-section, said electrode assembly (12) comprising; a plurality of electrodes (16) each having a working face (36) defining a plane facing said surface and extending parallel to the axis thereof; a gap setting mechanism (18) operatively adapted for simultaneously moving said electrodes in response to the axial movement of said shaft (14) by said shaft actuating mechanism (52) in a direction perpendicular to said shaft and to said surface to form a voltage gap between said working face (36) and said surface, characterised in that said electrode assembly (12) further includes centring bars (40) interspersed between every two adjacent electrodes (16), said centring bars extending in the axial direction of said surface and being spring-loaded to contact said surface to maintain each working face (36) of said electrodes substantially parallel with said surface. An electrode assembly as claimed in claim 5, wherein said centring bars (40) are each provided in each end region thereof with a roller (44) arranged to run circumferentially on said surface and forced into contact therewith by said spring-loading. A method in which the electrically conductive internal surface of a cylinder of an internal combustion engine is prepared for metal spray coating by treating the surface by electrical discharge machining using an apparatus as claimed in any of claims 1 to 4, in which the electrodes (16) are longer than the cylinder bore is deep so as to prepare the entire internal surface in one operation, and rotating the electrode assembly during the treatment. A method according to claim 7, which further comprises the step of thermal spray coating the prepared surface.

Inventors: HABEL MICHAEL JOSEPH, PETERSON LARRY ALLEN, HABEL, MICHAEL JOSEPH, PETERSON, LARRY ALLEN
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1996
Main classification: B23H7
IPC codes: B23H9, B23H7