PARTIALLY CONDUCTIVE CATHODE FOR ELECTROCHEMICAL MACHINING

Abstract: The cathode includes a machining surface, at least a portion of which comprises alternate layers or lamina of conductive and non-conductive material whose spacing and thickness are selected to reduce over-cutting on the workpart surface.

Description: The present invention relates to electrochemical machining and, in particular to a cathode construction for use in such machining to reduce overcutting of close tolerance workparts. Electrochemical machining (ECM) has been used in the past to machine close tolerance surfaces on gas turbine engine parts, such as the airfoils and roots of blades and vanes. A problem experienced in ECM'ing such parts has been over-cutting or excessive removal of material from certain areas on the work-parts. In particular, it has been difficult to maintain tolerances for the 45° pressure angle and adjacent radii on the root form of certain compressor blades as a result of over-cutting during the ECM process. There is a need to reduce over-cutting in this application so that required tolerances can be maintained on a consistent basis. Cathode structures for ECM'ing the leading and trailing edge radii of an airfoil are disclosed in U.S. Patents 3,849,273 and 3,970,538. Cathodes and ECM processes for forming airfoil members are also disclosed in U.S. Patents 3,714,017; 3,803,009; 4,052,284; 4,057,475; and 4,256,555. Composite cathode structures for ECM'ing gears, spiral lands and other workparts are illustrated in U.S. Patents 3,268,434; 3,429,798; and 3,499,830. Other patents involving ECM processes and cathodes include U.S. Patents 2,650,979; 3,288,699; 3,326,785; 3,410,781; 3,566,068 and 4,100,388. DE-A-1,515,195 discloses a cathode for electrochemically machining a workpiece, said workpiece having a close tolerance surface intermediate interconnecting surfaces, said cathode having a working surface comprising a plurality of distinct conductive regions. According to the present invention there is provided a cathode as disclosed in DE-A-1,515,195, characterised in that said working surface comprises two outer conductive surface portions for positioning in spaced opposing machining relation to said intermediate surfaces and an inner partially conductive surface portion which is located between said outer conductive surface portions for positioning in spaced opposing machining relation to said close tolerance surface and is made up of a plurality of conductive layers and non-conductive layers which are so located and dimensioned that in use, said outer conductive surface portions have a greater surface conductivity than said inner partially conductive surface portion, the conductivity of which is determined by the dimensioning of the conductive and non-conductive layers. It should be noted that the conductive regions of the cathode disclosed in the above-mentioned DE-A-1,515,195 are insulated from each other by thin insulating layers so that the cathode surface has a uniform, high surface conductivity. Conversely, the partially conductive surface portion of the cathode of the present invention is made up of layers or lamina of conductive and non-conductive material whose dimensions determine the surface conductivity of the cathode. By appropriately selecting the thickness and spacing of the layers the desired non-uniform surface conductivity is achieved. In particularly the thickness and spacing of the layers can be selected to reduce over-cutting of the workpart surface. Also contemplated in accordance with a preferred embodiment is such a cathode machining surface which has an inclination or angular orientation different from that of the workpart surface to be machined to reduce over-cut. DE-A-1,515,195 also discloses a method of electrochemically machining a workpiece having a close tolerance surface intermediate interconnecting surfaces, comprising advancing a cathode towards the workpiece, current being carried between the workpiece and the cathode by an electrolyte in a gap therebetween. In another aspect, the present invention provides a method of electrochemically machining a workpiece having a close tolerance surface intermediate interconnecting surfaces, in which a cathode according to the invention is advanced over the close tolerance surface of the workpiece whilst passing current between the workpiece and the cathode via an electrolyte in a gap therebetween, said cathode being disposed with one of said outer conductive surface portions (54/56) at a leading portion thereof, in spaced opposing machining relation to one of said intermediate surfaces, the other of said outer conductive surface portions being disposed at a trailing portion thereof, in spaced opposing machining relation to the other of said intermediate surfaces, and said inner partially conductive surface portion (70f, 72f) being disposed in spaced opposed machining relation to said close tolerance surface. Preferred embodiments of the cathode and method of electrochemical machining of the present invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an end elevation of an ECM apparatus for machining the root form of a compressor blade of a gas turbine engine; Figure 2 is an end elevation of one of the cathodes shown in Figure 1 with the insert detached; Figure 3 is a plan view of the cathode of Figure 2; Figure 4 is a side elevation of the detached insert of Figure 3; Figure 5 is an enlarged partial end elevation of the cathode adjacent a root form machined; Figure 6 is a perspective view of the composite laminated cathode insert used to reduce over-cutting; Figure 1 illustrates an electrochemical machining apparatus for machining the root form 10 of a gas turbine engine compressor blade 12 having an airfoil 14 attached to the root form. An electrolyte chamber 16 is formed between the cathodes 20 and the root form. In particular, each cathode includes a working cathode member 22 on opposite sides of the root form as shown and spaced therefrom. The cathodes are shown in Figure 1 in the fully advanced position at the end of machining the root form. Below the root form is a seal member 24 having surfaces 26 across which the cathode members slide and seal as the cathodes are advanced towards the root form during machining. Above the root form is a seal plate or cover 30 through which the root form extends as shown for machining. Each cathode member 22 is affixed to a cathode holder 24 which, itself, is affixed to a ram member 28 driven by a conventional ball screw assembly. At the top, the cathode holders 24 and ram members 28 are guided in movement by inclined surfaces 31 on plate members 32 of the machine. The compressor blade is fixtured with the root form between the cathodes as shown by a fixture plate 34 disposed in the seal plate 30. The airfoil 14 is located in chamber 36 formed by chamber-forming members 38, 40 held together by multiple machine screws 42. Members 38, 40 in turn, are located in an opening 43 in a machine top plate 44 with seals 45, 46 providing electrolyte seals to prevent leakage. As is known, electrolyte, such as an aqueous salt solution, is caused to flow through the electrolyte chamber between the root form surfaces to be machined and the respective opposed facing cathode members 22. A source of electrolyte and pump (not shown) provide such electrolyte and form no part of the invention. Likewise, means known to the art are provided to make the root form anodic relative to the cathodes so that metal is removed electrolytically therefrom when current is flowed through the electrolytic cell formed by the root form, cathodes and electrolyte. Figures 2 - 5 illustrate the cathode structure of the invention in detail. In particular, each cathods 20 includes the holder 24 having an elongate shank 40 with keyway 42. The shank 40, as mentioned, is affixed to the ram member 28 by machine screws (not shown) with the keyway 42 receiving a key 46 (Fig. 1) to insure proper alignment. The working cathode member 22 may be a separate component affixed to the holder shank 40 by suitable fastening means such as machine screws or it may be integral with the holder shank. Each cathode member 22 includes a generally central slot 50 adapted to receive a primary cathode insert 52. As is apparent, e.g. see Figs. 1, 4 and 5, the profile of the working surfaces 54,56 of each cathode working member 22 and insert 52 is configured generally similar to the profile of the root form although not identical as will become apparent. The primary cathode insert 52 includes a recessed pocket 57 adapted to receive a secondary composite laminated cathode insert 60 as shown best in Fig. 5. The primary insert 52 includes a pair of spaced counterbores 62 which align coaxially with cylindrical holes 64 in the secondary insert 60 for receiving pins 66 for fixedly and accurately holding the secondary insert 60 in the pocket 57, Fig. 5. The secondary cathode insert 60 is located relative to the root form so as to machine the 45° pressure surface 68 thereof which is a high tolerance surface required for fixturing in a support disc or hub as is well known. The pressure surface 68 may have a straight, conical or other surface profile. As shown, the secondary insert 60 is of composite construction comprising alternate conductive layers or laminations 70 and non-conductive layers or laminations 72. For illustrative purposes only, the conductive layers 70 are brass whereas the non-conductive layers 72 are plastic, such as nylon 101 available from E. I. DuPont de Nemours & Co., Wilmington, Delaware. Importantly, the thickness and/or spacing between the conductive and non-conductive layers 70,72 is selected to reduce observed excessive over-cutting on the convex radius 74 on the root form and the 45° pressure surface 68. For the root form shown in Fig,. 5 and insert 60 shown in Fig. 6, the four intermediate, alternating brass and plastic layers 70a,72a have a thickness which is equal and one-third the thickness of the top brass layer 70b and bottom plastic layer 72b, which are of equal thickness. The bottom brass layer 70c is equal in thickness to the intermediate, alternating brass and plastic layers. The brass layers 70 carry electrical current by virtue of physical contact with the primary insert 52, which is made of copper, at their inner edges 70d and end edges 70e and also at the respective top and bottom surfaces of the top brass layer 70b and bottom brass layer 70c. Another important feature of the secondary insert 60 is the angle of inclination of working surface 80 thereon. In particular, the angle of inclination is selected so as to be less than the 45° inclination of pressure surface 68. The spacing between surfaces 68 and 80 increases as one proceeds from right to left (relative to Fig. 5) along the pressure surface 68 or along the center line of the root form. For illustration only, an angle of 38° 15' has been found suitable for the working surface 80. The laminations of the secondary insert 60 are typically initially held together in stacked relation by an epoxy cement therebetween. The working surfaces 54,56 of the primary inserts 52 and the outer edges 70f,72f adjacent the root form and constituting the working surfaces 80 of the secondary inserts 60 are machined to desired profile when assembled on the working cathode members 22. The semi or partially conductive nature of the secondary insert 60 provided by the alternating conductive and non-conductive layers 70,72 with the reduced angle of inclination of the working surface 80 has enabled electrochemical machining of the root form while maintaining high tolerances required for the 45° pressure surface 68 and radii adjacent the pressure surface. Although certain preferred features and embodiments of the invention have been described hereinabove and in the drawings, it is to be understood that modifications and changes may be made therein without departing from the scope of the invention as defined in the appended claims.

Claims: A cathode (22) for electrochemically machining a workpiece (14), said workpiece having a close tolerance surface (68) intermediate interconnecting surfaces, said cathode having a working surface (80, 54/56) comprising a plurality of distinct conductive regions (70f, 54, 56) characterised in that said working surface (80, 54/56) comprises two outer conductive surface portions (54/56) for positioning in spaced opposing machining relation to said intermediate surfaces and an inner partially conductive surface portion (70f, 72f) which is located between said outer conductive surface portions (54/56) for positioning in spaced opposing machining relation to said close tolerance surface (68) and is made up of a plurality of conductive layers (70a, 70b, 70c) and non-conductive layers (72a, 72b) which are so located and dimensioned that in use, said outer conductive surface portions (54/56) have a greater surface conductivity than said inner partially conductive surface portion (70f, 72f), the conductivity of which is determined by the dimensioning of the conductive (70) and non-conductive (72) layers. A cathode (22) as claimed in claim 1, wherein one of said outer conductive surface portions (54/56) is convex in profile. A cathode (22) as claimed in claim 1 or claim 2, wherein an outer non-conductive layer (72b) of said inner partially conductive surface portion (70f, 72f) has a greater thickness than the other non-conductive layers (72a) of said inner partially conductive surface portion. A cathode (22) as claimed in claim 3, wherein said outer non-conductive layer (72b) of said inner partially conductive surface portion (70f, 72f) has a working surface portion (72f) which is inclined with respect to the remainder (70f) of said partially conductive surface portion such that in profile, said working surface subtends on obtuse angle. A cathode (22) as claimed in any preceding claim, wherein said outer conductive surface portions (54/56) are directly electrically connected so as to ensure that in use, they are at the same potential. A cathode (22) as claimed in claim 5, wherein said conductive layers (70a, 70b, 70c) of said inner partially conductive surface portion (70f, 72f) are directly electrically connected to said outer conductive surface portions (54/56) so as to ensure that in use, said conductive layers and outer conductive surface portions are all at the same potential. A cathode (22) as claimed in any preceding claim comprising an insert member (60) which defines said inner partially conductive surface portion (70f, 72f) and is inserted into a recess in a metal body (52) which defines said outer conductive surface portions (54/56). A method of electrochemically machining a workpiece (14) having a close tolerance surface (68) intermediate interconnecting surfaces comprising the steps of providing a cathode (22) as claimed in claim 1 and advancing the cathode over the close tolerance surface of the workpiece whilst passing current between the workpiece and the cathode via an electrolyte in a gap therebetween, said cathode being disposed with one of said outer conductive surface portions (54/56) at a leading portion thereof, in spaced opposing machining relation to one of said intermediate surfaces, the other of said outer conductive surface portions being disposed at a trailing portion thereof, in spaced opposing machining relation to the other of said intermediate surfaces, and said inner partially conductive surface portion (70f, 72f) being disposed in spaced opposed machining relation to said close tolerance surface. A method as claimed in claim 8, wherein a cathode (22) in accordance with claim 3 is provided and is advanced with said outer non-conductive layer (72b) of greater thickness trailing said other non-conductive layers (72a) of said inner partially conductive surface portion (70f, 72f). A method as claimed in claim 8 or claim 9, wherein the width of said gap is less at said leading conductive surface portion (54/56) than at said partially conductive surface portion (70f, 72f). A method as claimed in any of claims 8 to 10, wherein said workpiece is a turbine blade (14) and said close tolerance surface (68) is a surface of the root of the turbine blade. A method as claimed in claim 10, wherein a cathode as claimed in claim 4 is used to machine the pressure surface (68) of the root of a turbine blade (14), said inner partially conductive surface portion (70f, 72f) of said cathode (22) being disposed opposite the surface portion of the turbine root from which the pressure surface is being formed, the leading edge of the cathode (22) being located opposite a concave portion of the turbine blade root and the trailing edge of the cathode being located opposite a convex portion of the turbine blade root.

Inventors: HINMAN, EDMUND R.
Applicants: EX-CELL-O CORPORATION
Application year: 1986
Main classification: B23H3
IPC codes: B23H3, B23H9