CA2502593A1

CA2502593A1 - Method and apparatus for producing a turbine root by edm and electropolishing

Info

Publication number: CA2502593A1
Application number: CA002502593A
Authority: CA
Inventors: Shadi Mohaisen; Alan Davis; David Stewart Conochie
Original assignee: HPG Ltd
Current assignee: Individual
Priority date: 2005-03-29
Filing date: 2005-03-29
Publication date: 2006-09-29

Abstract

A method for producing a turbine root from a piece of parent material in which the turbine root has an engagement face with retaining elements for retaining the blade to a rotating member includes the steps of rough machining the root from the parent material by Electrical Discharge Machining (EDM) to issue a stock layer along the engagement face, electropolishing the root to remove the stack layer, and stress-treating the root to induce residual compressive stresses along the engagement face.

Description

TITLE: METHOD AND APPARATUS FOR PRODUCING A TURBINE ROOT
BY M AND ELECTROPOLISHING
FIELD OF THE INVENTION
[0001] The present invention relates to producing an article such as a turbine root using Electrical Discharge Machining (EDM) and eleetropolishing.
BACKGROUND OF THE INVENTION
[OD02] Conventional methods for machining the root partton of a turbine blade include grinding, broaching, and milling. These processes can have disadvantages in that customized tooling is generally required to match the particular geometry of a root being processed_ Custom tooling is generally associated with high tooling costs, long lead times, and little or no flexibility with respect to accommodating changes in desired root dimensions.
j0003] U.S. Patent No. A.,888,883 (Cox et al.) teaches a method for producing a turbine blade root in which the root is machined by Electrical Discharge Machining (EDM) rather than by conventional tooling. The EDM
machined portions are then peeved to reduce the effects of surface recast layers and residual tensile stresses.
t0004~ U.S. Published Pat. Appn. No. 20(?4100641345 (Howley) teaches a method for producing a turbine root in which the root is EDM machined into a desired shape, then subjected to glass beading to remove recast material, and then subjected to shot peeving to reduce residual tensile stresses and produce a compressed surtace.
~p005] U.S. Patent No. 4,184,832 (Ahlgrim et a1.) discloses an electropolishing process that provides zonewise electropolishing by dividing a cathode into segments that are electrically insulated from each other and through which current is alternately passed and discontinued to produce a desired electropolishing effect in corresponding portions of a surface to be eleckropocished.
SUMMARY OF THE INVENTION

[ODOBj The present invention provides a process and related apparatus for producing a turbine root by EDM machining the root and then eiectropoiishing the EDM machined surfaces of the root. The process according to the present invention can, with respect to known methods, reduce the amount of EDM machining time required per root, provide more consistent and accurate removal of EDM recast layers, and can improve quality and performance 4f the root. The present invention also provides an apparatus and method far electropolishing a target surface that is non-planar, the target surface having structural projections and recesses extending therefrom. The eleetropofishing method and apparatus can be adapted to consistently and accurately remove a stock layer from a non-planar target surface.
[0007] According to a first aspect of the present invention, a method for producing a turbine root from a place of parent material is provided, in which the turbine root has an engagement face including retaining elements for retaining the blade to a rotating member, and in which the method includes the steps of rough machining the root from parent material by Electrical Discharge Machining (EDM), the EDM step leaving a stock layer along the engagement face. The root is then subjected to electropoltshing to remove 2f? the stock layer. The root is then subjected to stress-treating to induce residual compressive stresses along the engagement face.
[000$j According to a second aspect of the present invention, an apparatus for electropoltshing a target surface is provided, the target surface having a non-planar target; a complementary cathode having a complementary surtace shaped to cooperate with the target surface profile so that when positioned in opposed relation, the target surface and the complementary surface are spaced apart by a uniform gap across the respective areas of the opposed target surface and complementary surface;
an electropolishing solution provided between the target surface and the complementary surface; and, a power supply with a positive terminal connected to the target surface and a negative terminal connected to the complementary cathode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Figure 1 is a perspective view of an embodiment of a turbine blade in accordance with the present invention;
(0010] Figure 2 is a front view of a rotating ring to which the turbine blade of Figure 1 can be assembled;
(0011] Figure 3 is an enlarged front view showing a root portion of the turbine blade of Figuro 1 (n greater detail;
10 (0012 Figure 4 Is a schematic view of an EDM apparatus for machining the turbine blade of Figure 1;
[0013] Figure a is an end view of a portion of the turbine blade of Figur~ 1 after being processed by the apparatus of Figure 4;
(0014] Figure 6 is an enlarged view of a portion of Figure 5 showing an 16 EDM surface in greater detail;
[0016] Figure 7 is an elevation view in partial schematic of an electropolishing apparatus for producing the turbin~ root in accordance with the present invention;
[0015] Figure $ is an enlarged view showing a portion of the apparatus 20 of Figure 7 in greater detail;
[0017] Figure 9 is an elevation view in partial schematic of an alternate embodiment of an electropoiishing apparatus in accordance with the present invention;
[0018] Figure 9 is a top view of a modified embodiment of the 26 apparatus of Figure 9 showing a Complementary cathode in accordance with the present invention; snd (0018] Figure 10 shows the apparatus of Figure 9 after a certain amount of electropoliahing has taken place.

DETAILED DESCRIPTION OF THE INVENTION
[0020] A turbine blade 110 in accordance with the present invention can be seen in Figure 1. The turbine blade 110 has a foil 112 and a root 114 from which the foil 112 extends. The root 114 facilitates assembly of the blade 110 to a disc or ring 115 (Figure 2) that is adapted to rotate at very high speeds during operation.
[0021] The root 114 is generally prismatic, having an upper face 116 from which the toil 112 extends, a lower fiace i 18 opposite the inner face 116, opposed side faces 120 and opposed end faces 122, each of which extend belwe~n the upper and lower faces 118 and 118. The end faces 122 and the upper and lower faces 118, 118 have, in the embodiment illustrated, generally smooth, planar exposed surfaces. The upper and lower surtaces 116 and 118 are generally parallel and may be slightly curved to match the radius of the ring 115 to which a plurality of blades 110 can be assembled.
[0022] The opposed side faces 120 are provided with retaining elements 125 or formations for securing the blade 110 to the ring 115. In the embodiment illustrated, the retaining elements 125 of each side Face 120 include an upper groove 126 and 8 lower groove 128, each of which extends the length of each side face 120. The root 114 has a transverse axis 129 that extends between the opposed end faces 122, In generally parallel relation to the faces 116, 118, and 120. The root 114 has a generally constant cross section in a plane perpendicular to the transverse axis 129. In other words, the root 114 can be generally defined as a solid of translation of a planar shape defined by the opposed end faces 122, translated along the transverse axis 129.
[0023] Each of the upper and lower grooves 128, 128 are, in the embodiment illustrated, defined by respective channels of generally constant cross-section extending between the opposed end faces 122 of the root 114.
The upper and lower grooves 128 and 128 are, in the embodiment illustrated, generally parallel to the upper and lower surfaces 116, 118, and positioned proximate to the upper and lower surfaces 118 and 118 respectively. T'he upper and lower groovas 126, 128 can be separated by an intermediate tongue element 130 protruding from the respective side face 120 of the root 114. Upper and lower tongue elements 132 and 134, respectively, can be provided adjacent the upper and lower grooves 126, 128, opposite the intermediate tongue element 130.
[0024] The retaining elements 125 in the illustrated embodiment provide side faces 120 that are generally non-planar. The tongues 130, 132, 134 and grooves 126, 128 provide the side faces 120 with an exposed surface that is stepped along its height, between the upper and lower faces 116 and 118. The retaining elements 125 are adapted to be engaged by root engaging elements 135 provided in the ring 115 for securely retaining the blade 110 vn the ring 115 (Figure 2). The portion of the side face 120 of the root 114 that is adapted to be engaged by the root engaging elements 135 of the ring 195 generally defines an engagement surface 137 of the root 114..
[0025] To facilitate satisfactory pertormance of the blade 110, the dimensions of the root 114 (particularly along the engagement surface 137) can be precisely matched in relation to the ring 115. The root 114 can also be provided with a crack resistant outer layer 140 extending along and beneath at least the engagement surface 137. Further details of the outer layer 140 are described subsequently herein.
[0426] In accordance with the present invention, a method for producing the root 114 generally includes rough machining the root 114 from parent material by Electrical Discharge Machining (EDM). The EDM process can leave a stock layer 142 along the engagement surface 137, the stock layer generally defined by a layer of material remaining on the root 114 after one manufacturing operation, to be removed in one or more subsequent manufacturing operations. The unfinished root 114 (i.e. a rough root 114a having a stock layer 142 on the engagement surtace 137) can then be subjected to electrvpvlishing to remove the stock layer 942. After electropolishing, the root 114 can be stress-treated to induce residual compressive stresses along the engagement face and provide the crack resistant outer layer 140. Further details of these steps are provided below.
j002rJ A schematic illustration of an EDM apparatus 144 that can be used in accordance with the present invention is shown in Figure 4. A wire 146 extends between a supply spoof 148 and a take-up spool 150, and is positioned to cut through a work piece 152. In the embodiment illustrated, the work piece 152 has a thickness extending between upper and lower surfaces 154 and 156 that generally correspond to the opposed end faces 122 of the root 114 of the blade 110. The wire 146 is arranged to cross the thickness of the work piece 152, and can be generally parallel to the transverse axis 129.
The wire 146 and work piece 152 are oppositely Charged, and as the wire 146 is advanced towards (and through} the work piece 152, a rapid series of sparks between the work piece 152 and the wire 146 is generated due to electrical discharge. The heat from the sparks melts or vaporizes the adjacent material of the work piece 152, providing a thermal erosion process.
An insulating fluid 158 can be supplied In a column around the wire 146 passing through the work piece 152, which can help to control the temperature in the wire 146 and work piece 152 and can help to flush away particulate and debris.
[0028] The EDM process can be directed along a "cutting" path 158 through the work piece 152 to cut the work piece 152 into two pieces 152a and 152b. In the embodiment illustrated, the second piece 152b does not form part of a finished root and can be scrap. The first piece 152a provides a root blank yr rough root 114a having a rough root profile ar perimeter that includes a rough side face 120a. The rough side face 120a of the rough root 114a generally corresponds to the side face 120 of the (finished) root 114 of the blade 110 of the present invention, but is oversized, providing the layer of stock 142 for removal in subsequent processing of the rough root 114a, to prvdu~ the engagement surface 137 and layer 140 of the finished root 114.
X0029] As best seen in Figures 4 and 5, the rough side face 120a has an exposed surface generated by the EDM process and generally defined as an EDM surface 160. The rough side face 120a of the rough root 114a further has an EDM layer 1fi2 that extends beneath the EDM surface 160. The EDM
layer 962 is produced by the EDM process, and at least a portion of the EDM
layer 162 includes what is generally referred to as a "recast' Payer 164. In the embodiment illustrated, and referring nvw also to Figure 6, the recast layer 164 extends from the EDM surface 160 to a depth defined by a recast depth boundary 186 located between the EDM surtace 160 and the finished engagement surface 137. The material located in the EDM layer 162 between the recast boundary 1B6 and the finished engagement surface 137 can include mete) that has been affected by the EDM process but is not part of the recast layer, and can include, for example, what is generally known as a heat affected zone.
[0030] The recast layer 184 generally has undesirable properties that can adversely affect the performance of a product such as the turbine blade 110, manufactured by an EDM process. Such undesirable properties can include, for example, but not limited to, rough surface ftnish, high brittleness, residual tensile stresses, and a tendency to propagate cracks such as stress cracks.
[0034] The stock Payer 142 is generally defined as the layer of material that extends in thickness from the EDM surtace 160 of the rough root 114a to the engagement surface 137 of the finished root 114 of the turbine bl8de 110.
The stock Payer 142 can lee, but need not be, equivalent to the EDM layer 162.
For example, in some processes the stock layer can be thinner than the EDM
layer 162, in which case some of the EDM layer 162 will remain in the finished root 114. The remaining portion of the EDM layer 162 can be treated to alter its metallurgical andlor mechanical properties in, for example, a stress treatment step following removal of the stock layer, examples of which are described subsequently herein.
[0032] in other processes, the stock layer 142 may be thicker than the EDM Payer 162, In which case the entirefy of the EDM layer 162 would be removed from the rough root 114a during removal of the stock layer 142. In _$_ other embodiments of the invention, the stock layer 142 may extend beneath a target surface that has been manufactured by a process other than EDM, so that the stock layer 142 contains no EDM layer 162 at all.
[0033] In the illustrated embodiment, the movement of the wire 146 along the cutting path 159 during the EDM process can be precisely controlled, for example, by a CNC controller, and can accurately trace out the profile of the rough root 114a to provide the stock layer 142 such that the stock 142 layer is of generally constant thickness along the engagement surface 137.
[0034] The thickness of the recast layer 164 can be a function of a variety of factors including the magnitude of the electrical potential across the wire 146 and the work piece 152, the current passed through the wire 148, and the speed at which the wire 146 is moved along its cutting path 159 through the work piece 152. In a typical EDM process, a recast layer 164 having a thickness of about 20 microns is produced. Thinner recast layers 164 can be generated, but to do so can require a longer cycle time for carnpleting the EDM process, or can involve multiple passes of the EDM wire 146 along the cutting path, such as, for example, a second "trim" pass that follows gn initial "rough" EDM pass. The trim pass can be a second EDM
pass in which a relatively thin layer of material is removed from the rough root 114a, and in which the EDM process parameters are adjusted to minimize the thickness of the resultant recast layer 164. In this way, a recast layer 164 having a thickness of about 4-10 microns can ba produced.
X0035] The method of producing the root 114 in accordance with the present invention can advantageously accommodate a relatively thick recast layer 164, as explained in greater detail subsequently herein. For example, the recast layer 164 can have a thickness of about 14-20 microns or about 20-microns or more. This can facilitate manufacture of the root 114 with an EDM process that comprises a single pass, i.e. an EDM step in which the wire 30 146 is directed along the cutting path 159 only once. Alternatively or additionally, accommodating a thicker recast layer 164 can facilitate operating _g.
the EDM process at a faster production rate, whether in a single or multiple pass process.
[0036] In accordance with one embodiment of the present invention, after the EDM step, the rough root 114a can be elechnpolished to remove the 5 stock layer 142_ Removing the stock layer 142 can advantageously remove some or all of the recast layer 184 of the rough root 114a. One embodiment of an eiectropolishing apparatus 180 that can be used in accordance with the present invention is Shawn in Figure 7. The electropolishlng apparatus 180 includes a tank 182 containing en electropalishing solution 184 to provide a 10 bath in which a target surface 185 to be electrapolished can be immersed.
The solution 184 is generally highly ionic, and can contain, for example, a mixture of phosphoric and sulfuric acids.
[0037] In the embodiment illustrated, the rough root 114a can be suspended in the solution 184 so that the EDM surtace 160 of the rough side 15 face 120a is generally immersed In the solution 184. The EDM surface '160 provides a target surface 185 to be treated by the electropolishing process.
The apparatus 180 further includes a cathode 186 positioned adjacent the target surface 185, and a power supply 188 in electrical connection with the cathode 18B and the rough root 114a. More particularly, the c~thade '186 is 20 connected to a negative terminal of the power supply, and the rough root 114a is connected (as anode) to a positive terminal of the power supply 188 so that an electrical potential is gener6~ted across the cathode 786 and the target surface 185.
[0038] The cathode 186 has an opposing portion 187 defined by an 25 area of the cathode 186 that has generally the same vertical and horizontal extent as, and is positioned in opposing relation ta, the target surface 185.
In the embodiment illustrated, the opposing portion 187 of the cathode 186 is of a generally flat, planar configuration. In cases where the target surtace 985 is non-planar, the flat opposing portions 187 provide an electropolish spacing 30 183 between the cathode 18B and the target surface 185 that varies at different points along the target surface 185. More particularly, the distance to the cathode 186 will be greater at the recesses and less at the projections of the target surface 185.
[0039] The cathode 186 can be generally basket-shaped or U-shaped in cross-section, providing two opposing portions 187 on opposite sidewall portions of the cathode 186. The apparatus 180 can also be provided with agitation means 190, such as, for example, an impeller, for stirring the solution 184. The cathode 186 can be of a wire mesh material, having apertures or openings through which fluid flow paths 192 generated by the agitation means 190 can pass.
[0040] In operation, metal (l.e. metal comprising the stock layer 14~) at the target surface 185 is dissolved by the electropolishing process into the solution 184 as metal ions. The process can generally be described as fihe approximate reversal of an elecGro-plating process. The amount of metal removed from the target surface 185 is a function of a variety of factors, Including the current level, voltage I~vel, and the amount of time that the target surface 185 is exposed to the electropla~ng pmCess.
LOt141] The removal of material from the target surface 185 by the electropolishing apparatus 180 will generally not be uniform at all points on the target surtace 185. Rather, the electropolishing of the apparatus 180 will generally remove material preferentially from peaks 195 or high points of the target surface 185, resulting in a leveling or smoothing effect of the target surface 185. The inventors have determined that this smoothing effect of the electropolishing process of the apparatus 1$U can have beneficial effects in terms of localized surface treatment results, but can have detrimental effects an the dimensions of the target surface 1$5 when considering the target surface 185 in its entirety.
[0042] The peaks subjected to smoothing by the electropolishlng process can be generally categorized Into two groups, namely, micro-peaks 19$ and macro-peaks 198. The micro-peaks 196 include surtace asperities 197 located at edges of the grain boundaries of the rnetallurgieal structure of the target surface 185. Smoothing these edges or asperities can _ 11 _ advantageously inhibit stress crack initiation at the grain boundaries, and thereby prolong the service life of the turbine blade 110. This smoothing of the grain boundary edges can occur after relatively little electropalishing treatment of the target surface 785. For example, electropolishing the target surface 185 to remove 4 microns or less in some cases has been found to provide signiftcant beneficial smoothing of the edges of the grain boundaries of the target surtace 185.
[0043] The macro-peaks 9 98 subjected to smoothing by electropolishing include structural projections 200 extending from the rough 10 side faces 120a aFthe rough root 114x, for farming the retaining elements of the blade 110. In the embodiment illustrated, the structural projections include the intermediate, upper and lower tongue elements 130, 132 and 134.
During electropolishing, metal at the target surface 185 along the structural projeckions 200 is generally removed at a faster rate than metal along 15 recesses 202, such as along the base of the grQOVes 126 and 128. Some areas of the target surface 185, such as the corners 204 at the base of the grooves 126, 128, can experience little or no metal removal at all.
[4044] The difference in rates of metal removal by the electropolishing between the structural projections 200 and the recesses 202 of the target ZO surface 185 can compromise the dimensional integrity of the finished root 114. tn other words, the uneven removal of the stock layer from the rough root 114a can provide a finished root 114 that does not have an engagement surface 137 with the precisely desired dimensions, and therefore the root 114 may not fit properly in the ring 115. This poor fit can result in poor 25 performance or premature failure of the blade 110.
[0045] The uneven removal of the metal from the projections and recessed areas of the target surtaae 185 can become more pronounced as the 'treatment by eiectropolishing is increased, or in other words, as the average amount of metal to be removed is increased. Thus at lower removal 30 amounts, electropolishing using the cathode 187 with the flat opposing portions 187 can provide satisfactory results in which the benefits of micro-peak smoothing can be obtained, while the potential problems of macro-peak smoothing are avoided or remain at an acceptably insignificant level. A
suitable process, for example, for producing the root ~! 14 in accordance with the present invention can include producing the rough root 114a by an f=DM
5 process that provides the rough root 114a with a stock layer 158 of about 4-microns thick. The rough root 114a can then be electropolished to remove the 4-6 micron stock layer from the target surface 185. Removal of the stock layer 142 produces a semi-finished root 114b that is generally of the size and shape of the finished root 114. The semi-finished root 114b has semi-finished 10 side faces 120b seml-finished engagement surfaces 137b that correspond generally in size and shape to the respective finished side faces 120 and engagement surfaces 137.
X0046] After the stock layer 142 has been removed by electropolishlng, the semi ~rnlshad root 114b can be subjected to a stress treatment step to 15 provide the generally crack resistant outer layer 140 (Fisures 3 and 5).
The outer layer 140 can be generally free of any internal tensile stresses, and preferably has residual compressive stresses that can resist propagation of any cracks that may, for example, originate at grain boundaries at the surtace 137 of the root 114. The outer layer 140 can extend from the engagement 20 surface 137 to a depth below the surface 137 and defined by boundary 199_ [004Tj According to one embodiment, the stress treatment step includes shot peeving the surface 137b with steel shot. The steel shot can range from S110 to S270, for example, and can be applied manually or with an automated application process. Other shat materials can also be used, for 25 example, wire cut shot or ceramic shot. In some cases, a lower intensity peeving of the electropolished surface 137b may be desirable, and use of glass beads as the peeving media may be optimal.
[0048] In another embodiment, the stress treatment step can use laser penning technology tv treat the surface 137b of the turbine root 114. In laser 30 peeving, a laser beam sends pressure waves beneath a target surface which can provide residual compression stresses relatively deep below the target surface, resulting in an outer layer that can be 1 mm or more in thickness.
[0049] In accordance with the present invention, a further surface finishing step can be provided after the stress treatment step to further enhance the stress fatigue life of the turbine root 114_ The surface finishing step can include vibratory tumble burnishing the root by immersing the root 114 in a container of particulate burnishing media and then agitating the container andlor burnishing media. Alternatively, or in combination, the burnishing media can include a chemical slurry containing a mild acid or the like to provide the desired surface finishing effect to the rook 114.
(005D] An alternative embodiment of an electropolishing apparatus 2B0 in accordance with the present invention can be seen in Figure 9. The apparatus 280 is similar to the apparatus 180, and like features are idenffied with like reference characters, incremented by 100.
[0031] The apparatus 280 includes a tank 282 containing electropolishing s41ut1on 284, a cathode 286 with opposing portions 2$7> and a power supply 288. A pair of target surfaces 985 to be electropolished are pravlded in facing relation towards each opposing portion 287 of the cathode 286, The spacing between the target surface 185 and the opposing portion 287 of the cathode 286 defines an electropolishing gap 289. In the embodiment illustrated, the gap 289 has a non-uniform spacing 283.
[0052] The apparatus 280 is further provided with agitation means 290 in the form of a flow inducing system 310. The flow inducing system 310 is adapted to generate a stream of fluid flow in the solution 284 slang a directed flow path 292 that extends through the electropolishing gap 289.
[0453] In the embodiment illustrated, the flow inducing system 310 includes a pump 312 having an inlet 314 and an outlet 316. A discharge manifold 318 is provided adjacent one end 289a of the electropolishing gap 288, and is connected in fluid communication with the outlet 316 of the pump 312 by means of discharge piping 320. The discharge manifold 318 can have I

nozzles for directing flow along the directed flow paths 292. The cross-sectional fluid conducting area of the manifold can increase relatively gradually in the direction of fluid flow to facilitate generating uniform fluid flow along the directed flaw path 292.
5 [0054 The apparatus 280 is further provided with a flow collector 322 adjacent a second end 289b of the electropolishing gap 289, opposite the first end 288a. The flow collector 322 is adapted to receive solution 284 flowing along the directed flow paths 282, and convey fluid to the inlet 314 of the pump 3'12 via a ooltecting line 324, buffering reservoir 326, and an Intake line 10 328.
X0066] In operation, the flow inducing system 310 generates solution flow in the electropolishing gap 289 along the directed flow path 292. The directed flaw path 292 is advantageously aligned generally parallel to the tines defining the edges of the projections and recesses of the side faces 126 of the 15 root 114. In other words, the directed flow path 292 is generally parallel to the transverse axis 129 of the root 114, and generally parallel to the axes of the elongate projections 200 and recesses 202 (i.e. tongues and grooves) of the target surface 185.
[0056] Provkling a directed flow path 292 parallel to the axis 129 of the 20 elongate, constant cross-section tongues and grooves can promote more uniform removal of the stock layer 142 from the target surface 185.
Furthermore, in the embodiment illustrated, the transverse axis 128 is oriented vertically, and the directed flow path 292 is arranged to convey fluid vertically upwards through the eleCtropolishing gap 289. This vertically upward 25 flow can facif~tate removal and evacuation of gas bubbles that are generated by the eiectropolishing process and which may otherwise adhere to the target surface 285.
[00$Tj Referring now to Figures 10 and 11, the electropolishing apparatus 280 can be modified to provide a further embodiment of an 30 electropolishing apparatus 280'. The etectropolishing apparatus 280' is similar to the apparatus of 280, but includes a complementary conductor or ~romplementary cathode 388 having an opposing portion 387 that cooperates with the target surface 185 to provide an electropolishing gap 389 of generally constant width ar spacing 383.
[0058 More particularly, the complementary cathode 386 has a complementary opposing surface 387 that is shaped to at least partially nest with the non-planar target surface 185. The complementary opposing surface 387 has cathode projections 410 shaped to fit within the target recesses 202 of the target surface 185, and cathode recesses 412 that are shaped to receive the target projections 200 of the target surtace 185_ When in the nested positron, the complementary surtace 387 is adapted to be spaced apart from the target surface by an al~ctrapolishing gap 389 that has a generally uniform width or spacing 3$3. The electropolishing gap 389 is generally defined by the shortest distance measured in a straight line from a selected point on the target surface 185 to the opposing complementary surface 387 of the complementary cathode 386, and is generally uniform, Independent of the location of the selected paint.
[0069a In use, electropoiishing solufian 284 can be provided in the gap 389, and forced to flow along the directed flaw praths 292 extending through the electropofishing gap 389. Metal (i.e. from the stock layer 142) can be dissolved from the target surFace 785 in a reverse electroplating effect.
Because the eiectropvlishing gap 388 has a generally constant width, any preferential metal removal from the structural projections 200 {relative to the structural recesses 202) is minimized or eliminated_ The stock layer 142 is removed generally uniformly from the entirety of the target surface 185, resulting in a corresponding uniform increase in the width of the electropolishlng gap 389. This can faalitate removal of a thicker stock layer 142 from the target surtace 185 without distorting the desired shape of the finished root 114. For example, a stock layer 142 of 10-20 microns or 20-30 microns or more in thickness can advantageously be removed during electropalishlng with the complementary cathode 386. This can reduce the number of passes required when EDM machining the root 114.

[0060] The complementary cathode 386 can be adapted to provide the electrapalishing gap 389 with an initial width that optimizes the removal of the stock layer 142 far the root 914. The initial width 383 of the gap 389 can be, for example, 0.25mm to l0mm or mare. The flow rate of the fluid along the directed flow paths 292 can be optimized to clear away any by-product from the electrapolishing process (including, for example, particulate and gasses) and to facilitate maintaining a predictable and controlled rate of metal removal from the target surface 185.
[0061] Providing a narrawecf gap 38~ (as compared to, for example, the gap 289) can provide additional benefits in that the overall volumetric fluid rate along the directed flow paths 292 is reduced, thus requiring less power andlor a smaller pump for satisfactory operation of the flow inducing means. The narrowed gap 389 can also reduce the power consumed from the power supply 288 for powering the electropolishing process itself, because the resistance across the gap (and hence the 12r power loss) is reduced. I=urther, the narrowed gap 389 can improve overall dimensional control in using the electropolishing process to accurately remove the stock layer 142.
[0062] While preferred embodiments of the invention have been described herein in detail, it is td be understood that this description is by way of example only, and is not intended to be limiting. The full scope of the invention is to be deberrnined from reference to the appended claims.
[0063] For example, the present invention comprehends that the EDM, electropoflshlng, and stress treatment processes used to manufacture the turbine root 114 can be used to manufacture other articles having surtaces with generally parallel elongate projections and recesses, such as, for example, but not limited to, the engagement surface 135 provided in the ring 115 to which the root 114 can be assembled (Figure 2).
~OOB4] The present invention also comprehends that the electropolishing process used to remove a stock layer containing recast produced by an EDM process can also be used to remove stock layers that are free of recast and which are not the product of an EDM process_ Such 7_ stock layers c8~n be left on the finlsfZed part by other initial ar intermediate manufacturing processes such as, for example, but not limited to, milling, broaching, grinding, forging, casting, or water jet machining.

Claims

1. A method for producing a turbine root from a piece of parent material, the turbine root having an engagement face including retaining elements for retaining the blade to a rotating member, the method comprising the steps of:
a) rough machining the root from parent material by Electrical Discharge Machining (EDM) and leaving a stock layer along the engagement face;
b) electropolishing the root to remove the stock layer; and c) stress-treating the root to induce residual compressive stresses along the engagement face.

2. The method of claim 1 wherein the stock layer is about four to eight microns thick.

3. The method of claim 1 wherein step (c) comprises shot peening the engagement face.

4. The method of claim 1 where step (c) comprises laser peening the engagement face.

5. The method of claim 1 further comprising a surface finishing step after step (c) to provide a finished surface along the engagement face.

6. The method of claim 5 wherein the surface finishing step includes vibratory tumble burnishing the root.

7. The method of claim 5 wherein the surface finishing step includes vibratory tumble burnishing the root in a chemical slurry.

8. The method of claim 1 wherein step (b) includes connecting the root to a power supply as an anode, providing a cathode in spaced-apart relation to the root and defining an electropolishing gap therebetween, and providing a stream of electropolishing solution along a directed flow path in the electropolishing gap.

9. The method of claim 8 wherein the engagement face includes elongate projections and recesses extending in parallel along the engagement face, and wherein the directed fluid flow path is generally parallel to the axis of the elongate projections and recesses.

10. The method of claim 9 wherein the cathode is provided with a complementary surface profile adapted to at least partially nest with the projections and recesses of the engagement face, the electropolishing gap having a generally uniform and constant width between the complementary conductor and the engagement face when the complementary conductor and the engagement face are in an at least partially nested configuration.

11. The method of claim 10 wherein the gap is about 0.25mm to 10mm wide.

12. The method of claim 10 wherein the EDM process of step (a) provides a stock layer of about 14-20 microns.

13. The method of claim 10 wherein the EDM process of step (a) is performed in a single pass.

14. The method of claim 9 wherein the electropolishing process of step (b) removes substantially the entire stock layer.