Abstract:
An electrode used as a tool in an electrochemical machining process to generate raised areas or ridges in the walls of a predrilled hole in an electrically conductive workpiece has an electrically conductive cylinder partially coated with an electrically insulating material in a pattern defining the raised areas to be formed. The pattern may comprise a plurality of spaced apart rings. An electrochemical machining method of drilling bulbs in the walls of a predrilled hole uses the electrode of the invention to greatly increase process efficiency.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a tool and a method used for electrochemical machining. More particularly, this invention relates to a tool and method for forming features in predrilled holes using electrochemical machining. 
     A specialized adaptation of electrochemical machining, known as shaped-tube electrochemical machining (STEM), is used for drilling small, deep holes in electrically conductive materials. STEM is a noncontact electrochemical drilling process that can produce holes with aspect ratios as high as 300:1. It is the only known method which is capable of manufacturing the small, deep holes used for cooling blades of efficient gas turbines. 
     The efficiency of a gas turbine engine is directly proportional to the temperature of turbine gases channeled from the combustor of the engine and flowing over the turbine blades. For example, for gas turbine engines having relatively large blades, turbine gas temperatures approaching 2,700° F. are typical. To withstand such high temperatures, these large blades are manufactured from advanced materials and typically include state-of-the-art type cooling features. 
     A turbine blade is typically cooled using a coolant such as compressor discharge air. The blade typically includes a cooling hole through which the air passes. A further design advancement has been the addition of internal ridges in the cooling hole to effect turbulent flow through the hole and increase cooling efficiency. Cooling features within the hole such as turbulence promoting ribs, or turbulators, thus increase the efficiency of the turbine. 
     The cooling holes commonly have an aspect ratio, or depth to diameter ratio, as large as 300:1, with a diameter as small as a few millimeters. The turbulators extend from sidewalls of the hole into the air passage about 0.2 mm., for example. 
     The method currently used for drilling the cooling holes in turbine blades is a shaped-tube electrochemical machining (STEM) process. In this process, an electrically conductive workpiece is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes function as cathodes in the electrochemical machining process, while the workpiece acts as the anode. As the workpiece is flooded with an electrolyte solution from the drilling tubes, material is deplated from the workpiece in the vicinity of the leading edge of the drilling tubes to form holes. 
     Turbulated ridges are formed in the cooling holes by a modification of the standard shaped-tube electrochemical machining (STEM) process for drilling straight-walled holes. One common method is termed cyclic dwelling. With this technique, the drilling tube is first fed forward, and then the advance is slowed or stopped in a cyclic manner. The dwelling of the tool that occurs when the feed rate is decreased or stopped creates a local enlargement of the hole diameter, or a bulb. The cyclic dwelling, for which cyclical voltage changes may be required, causes ridges to be formed between axially spaced bulbs. These ridges are the turbulators. 
     The cyclic dwelling method is very low in process efficiency compared to shaped-tube electrochemical machining (STEM) drilling of straight-walled holes because of the lengthy required time for drilling each bulb individually by cyclic tool dwelling. The dwell time required to form a single bulb can be greater than the time for drilling an entire straight-walled hole. 
     U.S. Pat. No. 5,306,401 describes a method for drilling cooling holes in turbine blades that uses a complex tool resetting cycle for each turbulator in the hole. This method also has low process efficiency, having even longer operating times for drilling the turbulator ridges than the cyclic dwelling method because of the time required to reset the electrode tool. 
     In addition, both the cyclic dwelling method and the method disclosed in U.S. Pat. No. 5,306,401 require that additional equipment be used with a standard STEM machine for control of machine ram accuracy, electrolyte flow and power supply consistency, since these are crucial to hole quality. Failure to control the dimensions of the turbulated holes often leads to part rejection, adding significant manufacturing costs for the machining process. 
     Accordingly, there is a need in the art for a new and improved method for manufacturing turbulators that has a relatively short machining cycle time. There is an additional need for an improved method of manufacturing more complex features such as spiral or helical ridges and the like. There is an additional need for a method utilizing relatively simple and easily implemented manufacturing techniques. In particular, there is a need for a method that does not require complex lateral or vertical displacement of the electrode. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrode for use in an electrochemical machining process, particularly shaped-tube electrochemical machining (STEM), and a method of machining bulbs and ridges in a predrilled hole using the electrode. The electrode and methods of the invention provide for convenient, cost effective machining of features in holes with large aspect ratios. This is accomplished by simultaneous machining of the bulbs using a shaped-tube electrochemical machining (STEM) process with a modified electrode. 
     The electrode of the present invention has an electrically conductive body with an external surface partially coated with an insulating material in a pattern defining raised areas to be formed on the internal surface of a predrilled hole in a workpiece. The electrode may be solid or hollow. 
     The electrochemical machining process of the present invention forms a raised area in a surface of a predrilled hole in a workpiece. The process includes the steps of positioning, in the holes, an electrode coated with an insulating material in a pattern defining the raised area to be formed in the hole, and machining at least one bulb in the interior surface of the hole by passing an electric current between the electrode situated in the hole and the workpiece while circulating an electrolyte solution through the hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a conventional shaped-tube electrochemical machining (STEM) electrode; 
     FIG. 2 is a schematic representation of an electrode coated with an insulating dielectric material in a pattern defining raised areas or ridges to be machined in a predrilled straight-walled hole, in accordance with the principles of one embodiment of the present invention; 
     FIG. 3 is a schematic representation of the hole shown in FIG. 2 after the raised areas have been formed with an electrode of the present invention; 
     FIG. 4 is a schematic representation of an electrode of the present invention that is solid, situated in a hole and includes a locator to position the electrode in the hole; 
     FIG. 5 is a cross-sectional view through a locator; 
     FIG. 6 is a schematic representation of an electrode of the present invention that is hollow, and situated in a blocked hole; and 
     FIG. 7 is a schematic representation of another embodiment of an electode in accordance with the instant invention. 
    
    
     DETAILED DESCRIPTION 
     A better understanding of the invention may be gained by reference to the drawings. FIG. 1 is a schematic view illustrating a conventional shaped-tube electrochemical machining (STEM) electrode  10  and the operation of electrode  10  in electrochemically machining a hole  8  having interior walls  9  in an electrically conductive workpiece  20 . Electrode  10  of the prior art is a hollow metal tube  11  coated on an exterior surface with an insulating dielectric material  12  except at the end proximate to electrically conductive workpiece  20 , where a band  14  of exposed metal is disposed. During the drilling operation, an electrolyte solution is continuously circulated through the body of electrode  10  and hole  8  while an electrical potential is applied between electrode  10  and workpiece  20 . The solution is pumped to an inlet  16  at the end of electrode  10  opposite the end composed of band  14  of exposed metal, through the body of electrode  10 , and through an end hole  18 , which end hole  18  is enclosed by band  14  of exposed metal, through hole  8  and out of the upper end of hole  8 , to be collected in a sump (not shown). The direction of electrolyte circulation is shown generally by arrows  13  and  15 . 
     Electric current passes between band  14  of exposed metal electrode  10  and a portion of wall  9  of hole  8  directly adjacent to band  14  of exposed metal, resulting in removal of metal from that area of wall  9 . Electrical insulation by dielectric material  12  blocks the current flow in coated areas  17  on the exterior surface of electrode  10 , so that no deplating occurs in the areas of wall  9  opposite coated areas  17 . The electrolyte solution dissolves the deplated metal and carries the dissolved metal out of hole  8 . Because of the geometry of the exposed conductive surface of electrode  10 , a current is established primarily in a lateral direction toward wall  9 . Current density decreases as the distance between wall  9  and band  14  of exposed metal of electrode  10  increases due to material dissolution, limiting the depth drilled. In addition, operating conditions such as total machining time, pulse amplitude, pulse on-time, and pulse off-time determine the total electrical charges passing through the machined areas, which operating conditions in turn determine the amount of metal removal. As is known, these parameters, along with the nature and concentration of the electrolyte and the operating voltage determine the diameter of hole  8 . 
     The conventional method of forming raised areas such as ribs or ridges in hole  8  is to remove metal from areas of hole  8  adjacent to the desired location of the raised area to form bulbs  32  by a modified shaped-tube electrochemical machining (STEM) process. The cyclic dwelling method of the prior art uses a cyclically varying feed rate to form bulbs  32  of diameter greater than that of the straight portion  30  of the hole. FIG. 1 shows the cyclic dwelling method schematically. The feed rate is relatively fast when drilling straight portion  30  of the hole, and relatively slow when drilling bulbs  32 . Similarly, cyclic variation of voltage can cause formation of bulbs, or enhance the bulbing process. However, cyclic variation of voltage requires a sophisticated power output. 
     The electrode and methods of the present invention provide for convenient, cost effective machining of features in holes with large aspect ratios. Examples of the features that may be produced are turbulators in cooling holes in turbine airfoils, rifling in gun barrels, and grooves in air bearing shafts. 
     With the improved electrode and machining process of the invention, it is possible to machine as many bulbs as desired, in whatever configuration desired, while achieving a significant reduction in process time. Furthermore, no variation of process parameters such as feed rate or voltage are needed; therefore, costly sophisticated controls for the instrument are not required. 
     FIG. 2 depicts an electrode  100  in accordance with one embodiment of the invention in a predrilled hole  101  having a straight wall  102 , of an electrically conductive workpiece  110 . FIG. 3 shows electrode  100  in the same hole  101  after bulbs  120  and intervening raised areas, or ridges  122 , have been created. In the embodiment shown in FIGS. 2 and 3, electrode  100  comprises a hollow cylindrical electrically conductive cylinder  105  coated with an electrically insulating coating  103  in a pattern having intervening areas  104  of exposed metal or conductive material on the exterior surface. The pattern of insulating coating  103  defines raised areas or ridges to be machined in predrilled hole  101 . In this embodiment, the pattern is a series of rings  106 . The (+) and (−) designations indicate pulsed voltage through the body of electrode  100  and workpiece  110 . 
     As shown in FIG. 3, areas of exposed conductive material  104  on the surface of electrode  100  define areas where bulbs  120  are formed by removal of metal from wall  102  of hole  101 . Raised areas or ridges  122  are created in wall  102  of hole  101  where no deplating occurs in the vicinity of insulated portions  106  of the surface of electrode  100 . 
     FIGS. 2 and 3 depict an embodiment of the invention where electrode  100  consists of cylinder  105 , having a body composed of an electrically conductive material. The diameter of cylinder  105  may be as small or as large as necessary to fit the predrilled hole. However, the outside diameter of cylinder  105 , measured over the coated surface, typically ranges between about 1 mm to about 8 mm. The thickness of coating  103  is typically in the range between about 0.15 to about 0.2 mm thick. 
     Cylinder  105  allows for pumping of an electrolyte solution into hole  101  through an inlet  112  at the end of electrode  100  extending outside hole  101  and out of end hole  114  at the other end of electrode  100 . Inlet  112  and end hole  114  facilitate uniform electrolyte flow through the areas being machined. Electrode  100  may also have electrolyte outlets  116  along the exposed surface of electrode  100 . Outlets  116  in addition to end hole  114  may be desirable where relatively large areas are being machined. The size of outlets  116  determines the added amount of electrolyte supplied to machining areas, which in turn determines surface quality of the bulbs  120  as well as metal removal uniformity. 
     The operation of a shaped-tube electrochemical machining (STEM) instrument with an electrode of the present invention is similar to that with a conventional electrode. Current is provided by coupling electrode  100  to a negative terminal of a STEM power supply (not shown) and workpiece  110  to a positive terminal. Electrode  100  is positioned inside smooth-walled hole  101  obtained from a previous drilling step. An electrolyte solution, which solution may be the same electrolyte as used in the first drilling step, is pumped into an end of hole  101  under pressure. Where electrode  100  is hollow and may contain outlets  116  for the electrolyte, the solution is pumped into inlet  112  of electrode  100 . In this embodiment, the electrolyte flows into inlet  112  and out through outlets  116  along the side surface of electrode  100  and end hole  114 . All raised areas or ridges as defined by the pattern of the coating of electrode  100  may be formed in hole  101  simultaneously. 
     The body of electrode  100  of the invention is composed of a conductive material, preferably titanium because of titanium&#39;s resistance to electrolytic action. The outer surface of the electrode body is covered with an electrically insulating coating  103  in a pattern that leaves some areas of the surface exposing the conductive material of the body. Coating  103  is made of a dielectric material, which dielectric material should preferably be smooth, of even thickness, tightly adhered to the surface of the body and free of pinholes or foreign material. Exemplary dielectric materials suitable for electrode  100  of the present invention include polyethylene, polytetrafluoro-ethylene, ceramics, and rubbers. A preferred method for fabricating electrode  100  of the present invention is disclosed in a commonly assigned U.S. patent application entitled A PROCESS FOR FABRICATING A TOOL USED IN ELECTROCHEMICAL MACHINING filed concurrently herewith under Ser. No. 09/187,664, the entire contents of which is incorporated by reference herein. 
     The pattern in coating  103  on the electrode body of the present invention defines raised areas or ridges  122  to be formed in predrilled hole  101 . A preferred pattern is at least one ring  106  or band circumferentially disposed on the external surface of electrode  100 . A more preferred pattern is a series of rings or bands  106  circumferentially disposed on the external surface of electrode  100 . The present invention, however, contemplates employing any pattern configuration desired. Examples of other configurations that may be employed are lines, rings or bands longitudinally disposed along the external surface of electrode  100 . Additional configurations that may be employed are steps or staircases, and one or more spirals or helices, as shown in FIG.  7 . The geometric components of the pattern may also be disposed orthogonally or obliquely, relative to a longitudinal axis  107  of electrode  100 . 
     FIG. 4 illustrates another embodiment of the invention where an electrode  140  is solid and may include a locator  144  at one end. The function of locator  144  is to position electrode  140  in hole  101  properly, such that electrode  140  is coaxial with the walls of hole  101 . Locator  144  is preferably composed of the same material(s) as an insulating coating  141  in other areas on the exterior surface of electrode  140 , differing only in the thickness of coating  141 . The outside diameter of electrode  140  measured at locator  144  is less than the inside diameter of hole  101 . This outside diameter should be sufficiently small that electrode  140  may be easily inserted in hole  101 , but sufficiently large so that electrode  140  fits snugly within hole  101 . Locator  144  preferably comprises a coating of greater thickness compared to coating  141  on other parts of electrode  140 . For example, the thickness of the coating  141  is typically in the range between about 50 to about 75 microns, while locator  144  typically comprises a thickness in the range between about 100 to about 150 microns. 
     FIG. 5 depicts a cross-section of a locator  150  in a non-circular hole  151 . Locator  150  should have at least three points on a surface in contact with wall  154  of hole  151 , and should allow for free flow of electrolyte through hole  151 . Exemplary locator  150  has four arms  152  in contact with wall  154  of hole  151 . Electrolyte flows through spaces  156  between arms  152 . No metal is exposed between arms  152 . 
     A locator is preferably disposed near the end of electrode  100  inserted in hole  101 . Where the cross section of hole  101  is not circular, it may be desirable to provide additional locator(s)  145 , to aid in centering electrode  100  in hole  101 . A preferred position for such an additional locator  145  is at a midsection of electrode  100  as shown in FIG.  6 . 
     The electrode and method of the invention may be used with a workpiece having blind (i.e. non-through) holes or through holes. As described above, uniform electrolyte flow is important for ensuring surface as well as metal removal uniformity. In one embodiment of the invention, uniform electrolyte flow through a blind hole is provided for. This is illustrated in FIG.  3 . The electrolyte solution is preferably passed through the interior of a hollow electrode  100 , into hole  101  and out of the opening at the upper end of hole  101  and is collected in a suitable sump (not shown). 
     For through holes, or holes with more than one opening, some measure is preferably taken to ensure uniform electrolyte flow inside hole  101 . Through holes are commonly used in gas turbine blades. For example, the cooling holes that are frequently manufactured in such blades using shaped-tube electrochemical machining (STEM) have an inlet and an outlet for the flow of coolant. 
     One method to ensure uniform electrolyte flow in a through hole is to block one end of the hole. FIG. 6 illustrates this method, with a through hole blocked with a plug  162  of suitable material, for example, rubber. Using this method, the electrolyte solution may be passed through a hollow electrode  100  such as that depicted in FIGS. 2 and 3. The outlet(s) for the solution may be located either along the side or at the lower end of electrode  100 . Where the electrode is solid and the predrilled hole is a through hole, electrolyte solution may be pumped in one end of the hole and out the other end. 
     FIG. 4 shows the second method to ensure uniform electrolyte flow in a through hole where the electrode is solid. Electrode  140  consists of a solid body  145  coated with a suitable dielectric material  141  in a pattern, leaving areas where electrically conductive material of the body is exposed, and a locator  144 . Using this method, electrolyte is pumped, for example, from the lower end of hole  101 , around electrode  140 , and out of the upper end of the hole  101 . 
     EXAMPLE 
     A straight-walled hole was drilled in a workpiece made up of two pieces of stainless steel clamped together. The hole was drilled at the interface where the two pieces were joined using a standard STEM apparatus and a conventional electrode similar to that shown in FIG.  1 . After the straight drilling was completed, an electrode according to the present invention, such as that illustrated in FIGS. 2 and 3, was connected to the STEM apparatus, and placed within the predrilled hole. A set of bulbs was simultaneously electrochemically machined in the hole, leaving raised areas, or ridges, between the bulbs. The spacing of the rings of insulating material in the pattern on the electrode correlated with the spacing of the ridges in the hole, and the width of the rings correlated with the width of the ridges. 
     While only certain features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.