Abstract:
A process for producing a pattern in the surface coating of an electrode used in an electrochemical machining process comprises the steps of providing a cylinder having a body composed of an electrically conductive material and a surface coating of an electrically insulating material and exposing the surface coating of the cylinder to a source of light in accordance with the pattern. Locators(s) may optionally be formed on the surface of the cylinder to assist in positioning the electrode in a predrilled hole.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly assigned B. Wei et al., “A Method and Tool for Electrochemical Machining,” U.S. application Ser. No. 09/187,663, U.S. Pat. No. 6,200,439, which is filed concurrently herewith and is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for fabricating a tool used in electrochemical machining. More particularly, the invention relates to a method of producing a desired pattern of insulating material on the surface of the tool. 
     BACKGROUND OF THE INVENTION 
     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 which 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 1500° C. (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 millimeters (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 which occurs when the feed rate is decreased or stopped creates a local enlargement of the hole diameter, or a bulb. The cyclic dwelling causes ridges to be formed between axially spaced bulbs. Cyclical voltage changes may be required. 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 long time required 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 workpieces comprising turbulator blades which uses a complex tool resetting cycle for each turbulator in the hole. It, too, 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, and 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 to the machining process. 
     An improved electrode tool has been developed which provides for convenient, cost effective machining of features in holes with large aspect ratios. Examples of the features which may be produced on workpieces using the tool are turbulators in cooling holes in turbine airfoils, rifling in gun barrels, and grooves in air bearings. The tool is disclosed in aforementioned B. Wei et al., “A Method and Tool for Electrochemical Machining,” U.S. application Ser. No. 09/187,663. 
     With the improved electrode, it is possible to simultaneously 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 is needed, and therefore, costly computer controls for the instrument are not required. 
     The improved electrode of the above referenced patent application is composed of a coated surface in a pattern defining the features to be machined in a predrilled hole in a workpiece. Using a lathe to grind a pattern in the coated surface is difficult due to the small diameter of the improved electrode (as small as 1 mm) and thin walls, where the tube is hollow (as thin as 0.25 mm) In addition, residues of dielectric material left in the ground areas can affect the quality of the features produced using the electrode. 
     Accordingly, there is a need for a new and improved method for fabricating an electrode for use in a shaped-tube electrochemical machining (STEM) process. In particular, there is a need for a method for fabricating an electrode having a surface coated in a complex pattern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for producing a desired pattern on a surface of an electrode for use in an electrochemical machining process. The process includes providing a cylinder having a body composed of an electrically conductive material and a surface coating of an electrically insulating material, and exposing the insulating coating of the cylinder to a source of collimated light in accordance with the pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an improved electrode including an electrically conductive cylinder having an insulating surface coating in a pattern designed to machine raised areas or ridges in a predrilled straight-walled hole. 
     FIG. 2 is a schematic representation of the hole shown in FIG. 1 after the ridges have been formed with the improved electrode. 
     FIG. 3 is a flowchart illustrating a sequence of process steps, in accordance with the present invention. 
     FIG. 4 is a schematic representation of an exemplary apparatus for practicing the process of the present invention. 
     FIG. 5 is a view of an electrically conductive cylinder, an insulating surface coating over the electrically conductive cylinder, and a photomask for protecting the portions of the coating to be left on the cylinder. 
     FIG. 6 is a bottom view of an electrically conductive cylinder, an insulating coating, and a patterned locator. 
     FIG. 7 is a schematic representation of an electrode having an insulating coating patterned in a spiral configuration. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a tool comprising an improved shaped-tube electrochemical machining (STEM) electrode  100 , made by the process of the present invention, positioned in a predrilled hole  101  with a straight wall  102  of an electrically conductive workpiece  110 . FIG. 2 shows the electrode in the same hole after bulbs  120  and intervening raised areas, or ridges  122 , have been created. In FIGS. 1 and 2, electrode  100  comprises a cylinder  105  having a hollow electrically conductive body or tube coated with an electrically insulating coating  103  in a pattern leaving areas of exposed metal or conductive material  104  on the exterior surface of electrode  100 . The pattern of the insulating coating defines the ridges  122  to be machined in predrilled hole  101 . In this embodiment, the pattern of the insulating coating is a series of rings  106  spaced along a longitudinal axis  107 . The (+) and (−) designations indicate pulsed voltage through cylinder  105  of electrode  100  and workpiece  110 . 
     As shown in FIG. 2, 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 . Raised areas or ridges  122  are created in wall  102  where no deplating occurs in the vicinity of the pattern of insulating coating  106  of the surface of electrode  100 . 
     FIG. 2 depicts an embodiment made by the process of the present invention where cylinder  105  includes a hollow tube or body of an electrically conductive material the diameter of which may be as small or as large as necessary to fit the predrilled hole. In one embodiment, for example, the outside diameter of cylinder  105 , measured over the coated surface, ranges from about 0.04-0.3 inches with the thickness of insulating coating  103  being about 0.15-0.2 mm thick. 
     A hollow cylinder  105  allows for pumping of electrolyte solution into predrilled hole  101  through an inlet  112  at an end of electrode  100  extending outside hole  101  and out of an 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 optional electrolyte outlets  116  along the exposed surface of the electrode which may have any desired shape and are shown as rectangles for purposes of example only. Outlets  116  in addition to the end hole  114  may be desirable where relatively large bulbs  120  to be being machined. The size of outlets  116  and/or the portions of cylinder  105  not covered by insulating coating  103  determines the amount of electrolyte supplied to the machining areas, which in turn determines the surface quality of bulbs  120  as well as uniformity of removal of portions of workpiece  110 . 
     FIGS. 1 and 2 illustrate another feature of the improved electrode made by the process of the present invention, locator  118 . The function of locator  118  is to position electrode  100  in hole  101  properly, such that the electrode is coaxial with walls  102  of hole  101 . In one embodiment, locator  118  comprises the same material(s) as insulating coating  103 . The outside diameter of electrode  100  measured at locator  118  is less than the inside diameter of hole  101  and is sufficiently small so that electrode  100  may be easily inserted in hole  101 , but sufficiently large so that locator  118  fits snugly therein. Locator  118  is preferably a coating of greater thickness compared to the coating on other parts of the electrode. For example, the thickness of insulating coating  103  may range from about 100-150 micrometers (μm), while locator  118  may have a thickness ranging from about 200-300 μm. 
     Locator  118  typically permits free flow of electrolyte through hole  101  and, if hole  101  has a bottom surface  115 , locator  118  is preferably positioned during drilling so as not to contact bottom surface  115 . Locator  118  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) at other location(s) along the length of electrode  100  to aid in centering electrode  100  in hole  101 . 
     The operation of a shaped-tube electrochemical machining (STEM) instrument with an improved electrode  100  made by the process of the present invention is similar to that of a conventional STEM electrode. Current is provided by coupling electrode  100  to a negative terminal of the STEM power supply and workpiece  110  to a positive terminal. The improved electrode is placed inside the smooth-walled hole obtained from a previous drilling step. An electrolyte solution which may be the same electrolyte as used in the original drilling step is pumped into an end of hole  101  under pressure. Where the electrode  100  contains outlets  116  for the electrolyte, the solution is pumped into inlet  112  of the electrode. 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 . For further details of the operation of the improved electrode, reference should be had to aforementioned B. Wei et al., “A Method and Tool for Electrochemical Machining,” U.S. application Ser. No. 09/187,663. 
     In accordance with the present invention, electrode  100  is manufactured by selectively removing portions of insulating coating  103  to form the desired pattern by exposing the insulating coating to collimated light (shown as light beam  408  in FIG.  4 ). The process is illustrated in the flow diagram in FIG.  3  and the schematic representations of FIGS. 4 and 5. 
     At step  300  of the inventive process, cylinder  402  is provided with a body comprising an electrically conductive material which may be solid or hollow. Titanium metal is preferred for the body of cylinder  402  because titanium is resistant to electrolytic action. Cylinder  402  may be formed by extrusion or any other known technique. The outer surface of cylinder  402  is then coated with an electrically insulating coating  403  at step  302  which may be applied by spray or dip coating or any other known technique. Insulating coating  403  is preferably smooth, of even thickness, tightly adhered to cylinder  402 , and free of pinholes or foreign material. In addition to possessing these properties, insulating coating  403  should be resistant to attack by the electrolyte solutions used in the STEM process, typically at elevated temperature. For example, the electrolyte may be applied to cylinder  402  at temperatures ranging from about 18 to 32° C. The electrolyte is usually an aqueous acid. Exemplary acids used in an electrolyte solution for a STEM instrument are nitric acid, sulfuric acid, hydrochloric acid, and mixtures thereof, at a concentration of about 16-18% by volume. Exemplary electrically insulating or dielectric materials suitable for insulating coating  403  of the present invention include polyethylene, polytetrafluoroethylene, ceramics, and rubbers. 
     At optional step  304  optional locator  118  (shown in FIGS. 1 and 2) can be formed, for example, by selectively applying a second coating of insulating material to an area by spraying or dip coating or any other of conventional coating method. Locator  118  may have the same composition as insulating coating  403 , or may have a different composition. It is desirable that the locator  118  include a material which is resistant to the electrolyte solution and that adhesion between the coating  403  and locator  118  be sufficient to withstand exposure to the electrolyte solution under operating conditions without delaminating. 
     At step  306 , a desired pattern is then formed in insulating coating  403  by exposing insulating coating  403  to collimated light (shown as light beam  408  in FIG.  4 ). The pattern of insulating coating  403  defines ridges  122  (FIG. 2) to be machined in predrilled hole  101  (FIGS.  1  &amp;  2 ). A preferred pattern is a series of rings or bands  106  (FIGS. 1 and 2) circumferentially disposed on the external surface of electrode  100  (FIGS.  1  and  2 ). However, the present invention contemplates employing any pattern configuration desired. Examples of other configurations which may be employed are steps or staircases, one or more spirals or helices, or rings or bands. The geometric components of the pattern may be longitudinally disposed along the external surface of the electrode, and disposed either orthogonally or obliquely relative to longitudinal axis  406 . 
     Exemplary apparatus  400  shown in FIG. 4 may be used in patterning step  306  of the electrode fabricating process of the invention. Referring to FIG. 4, apparatus  400  includes an electrode  401  including electrically conductive cylinder  402  with electrically insulating coating  403  which may be mounted on a computer numerically controlled (CNC) manipulator  404  for rotation about and/or translation along a longitudinal axis  406  relative to beam  408  of collimated light in order to scan or pass light beam  408  over insulating coating  403 . Manipulator  404  includes a fixture for holding cylinder  402  for translation under light beam  408  parallel to longitudinal axis  406  along scanning direction  410  as illustrated by the double headed arrow in FIG.  4  and for rotation about longitudinal axis  406  as illustrated by arrow  412 . The rotation and translation can be useful for creating complex shapes in insulating coating  403  such as the spiral shape shown in FIG.  7 . 
     A source  414  of collimated light, which may comprise a laser or another collimated light source, is positioned so that light beam  408  is directed to cylinder  402 . A focusing objective  416  can be adjusted to focus light beam  408  on insulating coating  403  of cylinder  402 . A pattern is chosen and a variable aperture  418  is adjusted to narrow light beam  408  to the desired dimensions. The geometric shape and dimensions of the beam are defined by variable aperture  418 . A shutter  420  may be used to interrupt the beam. Apparatus  400  may be operated by a control unit  422 , or may be manually operated. A drive  424  associated with manipulator  404  rotates and translates cylinder  402  in a controlled manner to cause light beam  408  to trace a desired pattern on insulating coating  403 . Light beam  408  may be interrupted by a shutter  420  or by turning off the laser power at those times when it is desired to skip from one point to another without removing insulating coating  403  therebetween. 
     In one embodiment, direct writing can be used for patterning by shaping light beam  408  to expose selected areas of insulating coating  403  to form the desired pattern. Methods of direct writing include using a single beam, using a multiple beam, and using a surrounding beam, for example. Several alternative types of direct writing can be used. For example, cylinder  402  may be mounted in a device, (not shown) similar to one used for laser wire stripping, and having one or more mirrors positioned so as that the reflected beam may impinge on all selected areas of the surface coating simultaneously. Rotation or translation of cylinder  402  may not be required when such a device is employed. As another example, apparatus  400  may include an holographic lens (not shown) which causes the beam of light to be spread in a predetermined pattern along the longitudinal axis  406  of cylinder  402 . The need for translation of the workpiece may be avoided by using such a lens, or one which produces similar spreading of the beam. 
     As another alternative, a photomask may be applied to insulating coating  403  before mounting cylinder  402  in manipulator  404 , and the exposure may be done through the photomask, if desired. FIG. 5 illustrates cylinder  402 , insulating coating  403 , and a photomask  506  which defines an area  508  of insulating coating  403  to be irradiated and protects the remainder of insulating coating  403  which is to be left on cylinder  402 . Photomask  506  should therefore be opaque to be irradiated to the wavelength of light used for the exposure. Photomask  506  may be conformal or nonconformal. An example of a conformal photomask is a concentric cylinder surrounding cylinder  402  and insulating coating  403 . An exemplary nonconformal photomask is a flat sheet of metal or glass. In one embodiment, photomask  506  comprises a commercially available combination of transparent glass with an opaque pattern. In another embodiment, photomask  506  comprises a metal foil stencil. 
     Regardless of which of the above techniques is selected for patterning, light beam  408  is preferably scanned across insulating coating  403  in a scanning direction  410  which is substantially perpendicular to the direction of the light beam  408  to provide substantially complete coverage according to the desired pattern. 
     In one embodiment of the invention, step  306  of exposing insulated coating  403  to light beam  408  comprises scanning a light beam  408  from a light source  414  comprising a laser across insulating coating  403  with the exposure resulting in ablation of the coating in accordance with a desired pattern. Ablation refers to a physical and chemical process of material removal which may include a combination of melting, vaporization, sublimation, or high temperature chemical reactions, among other processes. 
     When light source  414  comprises a laser, light beam  408  may be conditioned to selectively ablate insulating coating  403  and/or the underlying metal of cylinder  402 . Conditioning refers to the process of selecting an appropriate light source  414  having the properties necessary to effect selective exposure of insulating coating  403  and setting the necessary parameters in order that the desired pattern is produced. These properties include wavelength, power, fluence and geometrical shape. The wavelength of the light should match the absorption of the material exposed. For example, most polymers absorb light of 248 nanometers (nm), and a laser having light of wavelength 248 nm is capable of ablating most polymers. The power should be sufficient to form the pattern. Fluence is defined as the intensity of the beam, or energy of the beam per unit area. Geometrical shape refers to the cross-sectional shape of the beam and determines the two-dimensional shape of the area of exposed material. 
     An insulating coating  403  which is to be subjected to ablation may comprise any material which is electrically insulating, resistant to attack by the electrolyte, and which absorbs light of the same wavelength as the light source. When light source  414  comprises a pulsed excimer laser, such lasers typically emit a beam at 193 nm, 248 nm or 308 nm. Other types of lasers, such as CO 2  or YAG may also be used. In this respect, an excimer laser which emits a beam at 248 nm is particularly suitable for use with the method of the invention, as most polymeric materials which are useful as coatings absorb at that wavelength. 
     A fixed laser pulse of sufficient fluence results in the ablation of a fixed quantity of material from insulating coating  403 . Each pulse to which insulating coating  403  is exposed ablates the same quantity of material for a uniform coating. Therefore, the amount of material removed, and ultimately, the thickness of the remaining coating is determined by the number of pulses to which insulating coating  403  is exposed. For an organic polymer coating, 0.3-0.4 μm per pulse are typically removed, although the amount of removal may vary with different coatings, different compositions of the coating, depth of the coating, and type of laser and conditioning of the beam. 
     In addition to being used for patterning insulating coating  403 , light source  414  and the embodiment of FIG. 4 can be used to form and/or pattern locator  118  of FIG.  1 . In an embodiment wherein locator  118  was formed by applying a coating of insulating material over insulating coating  103 , a pattern for the locator may be selected, and a laser beam appropriately conditioned to remove predetermined sections of material of locator  118  to produce a patterned locator. 
     For example, as shown in FIG. 6, a three-armed locator  618  at an end of cylinder  402  may be formed by dipping cylinder  402  in a solution or dispersion of a suitable coating material, before or after a pattern is formed on the insulating coating  403  and drying the coating material. After the coating material is set, the cylinder may be exposed to a conditioned laser such that the coating is selectively ablated in areas  617  in a pattern to form locator with arms  618 . Typically, at least insulating coating  403  will remain on cylinder  402  to insulate the cylinder and prevent metal deplating in that area. In another embodiment, the entire surface of cylinder  402  can be coated with an insulating coating layer sufficiently thick to form locator  118  with the laser additionally being used to reduce the thickness of insulating coating  403  in areas where locator  118  will not be positioned. 
     Likewise, outlets  116  (FIGS. 1 and 2) for the electrolyte solution may be formed in the body of cylinder  402  by exposure to light source  414  such as a laser. As an example, cylinder  402  and insulating coating  403  may be exposed to a laser beam conditioned to remove first insulating coating  403  where it is desired to form an outlet and then perforate the underlying metal of the body. Using this method, an opening is formed in the body of the cylinder which may function as an outlet for the electrolyte solution. An alternative to using light source  414  to form outlets  116  is to use an electrical discharge machine, for example. 
     As an alternative to direct laser ablation, light source  414  may be used to selectively expose an insulating coating  403  comprising a photosensitive material. Several classes of photosensitive, electrically insulating materials which are stable to the acid electrolyte used in the electrochemical machining process are commercially available and may be used as insulating coating  403 . These materials include pre-imidized and precursor polyimide resins, B-staged bisbenzocyclobutene (BCB) and photosensitive epoxy-based resins. The materials may be applied by spray coating or dip coating, followed by an optional low temperature bake to remove solvent. 
     Either direct laser writing or a photomask process may be applied to insulating coating  403 , with light beam  408  then being used to produce a pattern of exposed material in insulating coating  403 . Insulating coating  403  may then be developed in a suitable developer to remove insulating coating  403  in predetermined areas to thereby reveal the underlying electrically conductive cylinder  402 . This process may use either positive acting photosensitive materials, in which material is removed during development in areas exposed to light, or negative acting photosensitive material, in which material is removed during development from areas which were not exposed to light. Developers for both positive and negative processes are well known in the art. 
     As used herein the terms cylinder and cylindrical body include elongated members having cross sections which are not necessarily circular. In addition, the term light includes not only ultraviolet light, but also visible and infrared light. Further, various means of scanning a beam of light across the surface of the cylinder are contemplated, including holding the cylinder stationary while translating the light source. Therefore, these and similar modifications, additions and substitutions are considered to be within the scope of the invention as defined in the following claims.