Method, apparatus and system for flexible electrochemical processing

Conventional electrochemical machining process requires fixed shaped tool cathodes, which makes retooling time consuming and expensive. Flexible tool cathodes include elastically deformable cathodes that can deform in two or three dimensions and can adapt to the contour of the workpiece while the workpiece is moving relative to the flexible tool cathode. That is, the flexible tool cathode can perform tracing. Certain flexible tool cathodes can be also used for special configurations such corners and edges. The flexible tool cathodes can be used to polish, finish, or shape the workpiece through electrochemical processes.

RELATED APPLICATION

The subject matter of the present disclosure may be related to U.S. applications Ser. Nos. 12/567,829 and 12/567,835 both entitled “SYSTEMS AND APPARATUS RELATING TO ELECTROCHEMICAL MACHINING” and filed on Sep. 28, 2009, which are hereby incorporated in their entirety by reference.

One or more aspects of the present invention relate to method, apparatus and system for flexible electrochemical processing.

BACKGROUND OF THE INVENTION

Traditionally, machining methods such as turning, grinding, drilling, and milling involve application of mechanical forces. In these methods, a hard tool is used to machine the workpiece, and thus, the tool needs to be harder than the workpiece. However, in some applications, it is desirable that the workpiece itself be made of hard materials. For example, blades of turbine engines have stringent requirements including hardness since they are subjected to harsh operating environments. When the workpiece itself is hard, conventional mechanical machining is typically not feasible.

Electrochemical machining (ECM) is commonly used as an alternative method of machining hard workpieces. In ECM, an electrically conductive hard workpiece is machined with a tool, which is also electrically conductive. During ECM, the tool acting as a cathode is located relative to the workpiece acting as the anode, such that a gap is defined therebetween, and the gap is filled with flowing electrolyte such as sodium nitrate aqueous solution. A high density direct current with low voltage is applied between the cathodic tool and the anodic workpiece to cause electrolytic dissolution of the workpiece. The dissolution action takes place in an electrolytic cell formed by the cathodic tool and the anodic workpiece separated by the flowing electrolyte. The eroded material or sludge, a form of metal hydroxide, is removed from the gap with the flowing electrolyte. The anodic workpiece generally assumes a contour that matches the contour of the cathodic tool. The sludge can be filtered from the electrolyte and the clean electrolyte can be reused.

In ECM, the tool does not wear. Also, the rate of machining is independent of the hardness of the workpiece. Thus, soft metals such as copper and brass may be used as the tool to shape workpieces of hard or tough metals such as carbon steel, inconel, titanium, hastelloy, and kovar or alloys thereof, and the tool cathode may be used repeatedly. This is advantageous since shapes, even complex ones, can be formed on soft metals with relative ease and used to shape workpieces of hard metals and alloys.

ECM does have its drawbacks. Specialized tool must be constructed for each desired shape in the conventional ECM. In an industry like power generation, even a small gain in efficiency such as one percent represents significant operation cost savings. Thus, turbine manufacturers are constantly redesigning turbine blades and other turbine parts to achieve incremental efficiency gains. Using the conventional ECM in such a circumstance requires regularly producing new tools, which can be very expensive. Thus, it would be desirable to provide electrochemical processing methods, apparatuses, and systems that can flexibly adapt to workpieces of different shapes to reduce costs and time associated with the conventional ECM.

BRIEF SUMMARY OF THE INVENTION

A non-limiting aspect of the present invention relates to a flexible electrochemical tool to perform a flexible electrochemical process on a workpiece. The flexible electrochemical tool may comprise a strip sheet metal elastically deformable in two dimensions (2D), a machine ram, and a plurality of support connectors connected with the machine ram at upper ends thereof and connected with the strip cathode at lower ends thereof along a length of the strip cathode. The plurality of support connectors may include at least one fixed support connector whose lateral position is fixed relative to the machine ram. Each support connector may be arranged to vary in stroke as the strip cathode elastically deforms. Also, each support connector may include a rotating coupler arranged to couple the lower end of the support connector and arranged to turn as the strip cathode elastically deforms.

Another non-limiting aspect of the present invention relates to a method to perform a flexible electrochemical process on a workpiece. In the method, a flexible electrochemical tool may be positioned such that a flexible cathode of the flexible electrochemical tool cathode engages a surface part of the workpiece submerged in a work tank filled with electrolyte. After the workpiece has been engaged, power and electrolyte flow may be initiated to start the electrochemical machining process. Then the flexible cathode may be traced toward a first or second end of the workpiece. Tracing may involve maintaining the power and the electrolyte flow while the flexible cathode is being moved relative to the workpiece.

Another non-limiting aspect of the present invention relates to a system to perform a flexible electrochemical process on a workpiece. The system may comprise a work tank, a flexible electrochemical tool with a flexible cathode, a machine ram, a plurality of clampings, and a controller. The work tank may be filled with electrolyte. The flexible cathode may be capable of continually adapting to a surface contour of a workpiece. The machine ram may be arranged to move the tool cathode. The plurality of clampings may be arranged to secure the workpiece within the work tank. The controller may be arranged to position the tool cathode such that the tool cathode engages a surface part of the workpiece submerged in work tank filled with electrolyte. Or the tool cathode contains electrolyte channels to supply the electrolyte to the electrolytic cell without submerging the workpiece. The controller may also be arranged to initiate power and electrolyte flow after the workpiece has been engaged to start the flexible electrochemical process. The controller may further be arranged to trace the tool cathode toward a first or a second end of the workpiece. Tracing may involve maintaining the power and the electrolyte flow while the tool cathode is being moved relative to the workpiece.

Another non-limiting aspect of the present invention relates to a cornering flexible electrochemical tool to perform a flexible electrochemical process on a workpiece. The cornering flexible electrochemical tool may comprise a cathode, a machine ram, and an elastomer in between the cathode and the machine ram to provide an adaptive backing so that the cathode adapts to a shape of a corner of the workpiece. The corner of the workpiece may be formed by two side surfaces that extend substantially in straight directions from a corner point forming a concave surface with a corner angle θ. The cathode may be pre-bent at an angle α for corner angles that is greater than θ. The difference provides the necessary compression of the cornering tool cathode to workpiece corner.

Another non-limiting aspect of the present invention relates to an edging flexible electrochemical tool to perform a flexible electrochemical process on a workpiece. The edging flexible electrochemical tool cathode may comprise a strip cathode, a machine ram, and an elastomer in between the cathode and the machine ram to provide an adaptive backing so that the cathode adapts to a shape of an edge of the workpiece. An edge of the workpiece may be formed by two side surfaces that extend substantially in straight directions from an edge point1060forming a convex surface with an edge angle φ. The strip cathode may be pre-bent at an angle β for edge angles that is less than φ. The difference provides necessary compression of the edge cathode to the workpiece edge.

Another non-limiting aspect of the present invention relates to a flexible electrochemical tool to perform a flexible electrochemical process on a workpiece. The flexible electrochemical tool may comprise a sheet cathode, a machine ram, and a plurality of support connectors. The sheet cathode may be arranged to elastically deform in three dimensions (3D). The plurality of support connectors may be connected with the machine ram at upper ends thereof and connected with the sheet cathode at lower ends thereof along an upper surface of the sheet cathode. Each support connector may be arranged to vary in stroke as the sheet cathode elastically deforms. Each support connector may also include a lower coupler arranged to couple the lower end of the support connector with the flexible sheet cathode as the sheet cathode elastically deforms.

Another non-limiting aspect of the present invention relates to a flexible electrochemical tool to perform a flexible electrochemical process on a workpiece. The flexible electrochemical tool may comprise a sheet cathode, a machine ram, an elastomeric backing and at least one sensor. The sheet cathode may elastically deform in 3D. The machine ram may be arranged to move the flexible electrochemical tool and to apply a compression force. The elastomeric backing may be arranged to provide an elastic backing to the sheet cathode. The sensor may be arranged to measure a surface height of a workpiece when the flexible electrochemical tool is engaged with a workpiece. The sheet cathode may continually adapt to a contour of the workpiece when the tool cathode is moved while being engaged with the workpiece.

Another non-limiting aspect of the present invention relates to a method to perform a flexible electrochemical finishing process. In the method, a flexible electrochemical tool may be positioned such that a flexible cathode of the flexible electrochemical tool engages a surface part of a workpiece submerged in a work tank filled with electrolyte or sprayed with electrolyte from the tool cathode. After the workpiece has been engaged, power and electrolyte flow may be initiated to start the flexible electrochemical finishing process. The workpiece may be flexible electrochemical finished as the flexible electrochemical tool is moved toward a first or a second end of the workpiece. The flexible electrochemical finishing process may comprise finishing the workpiece so as to correct surface errors of the workpiece, surface errors being defined as deviations in a surface height of a surface part that it outside a predetermined tolerance limit for the surface part. The flexible electrochemical finishing process may be performed while the tool cathode is engaged with the workpiece and moving relative to the workpiece.

Another non-limiting aspect of the present invention relates to a system to perform a flexible electrochemical finishing process on a workpiece. The system may comprise a work tank, a flexible electrochemical tool including a flexible cathode, a machine ram, a plurality of clampings, and a controller. The work tank may be filled with electrolyte or the workpiece is sprayed with electrolyte from the tool cathode. The flexible electrochemical tool may include a strip cathode, and the flexible electrochemical tool may be capable of continually adapting to a surface contour of a workpiece. The machine ram may be arranged to move the flexible electrochemical tool. The plurality of clampings may be arranged to secure the workpiece within the work tank. The controller may be arranged to position the flexible electrochemical tool such that the flexible cathode engages a surface part of the workpiece submerged in work tank filled with electrolyte or sprayed with electrolyte from the flexible electrochemical tool. The controller may also be arranged to initiate power and electrolyte flow after the workpiece has been engaged to start the a flexible electrochemical finishing process. The controller may further be arranged to finish a first or a second end of the workpiece. The a flexible electrochemical finishing process may comprise finishing the workpiece so as to correct surface errors of the workpiece, surface errors being defined as deviations in a surface height of a surface part that it outside a predetermined tolerance limit for the surface part. The workpiece may be flexible electrochemical finished while the flexible electrochemical tool is engaged with the workpiece and moving relative to the workpiece.

The invention will now be described in greater detail in connection with the drawings identified below.

DETAILED DESCRIPTION OF THE INVENTION

As described herein, flexible electrochemical processes in accordance with embodiments of the present application may provide a relatively high speed way to finish, polish, and/or shape workpieces. In one or more non-limiting aspects, flexible electrochemical tools are provided which can adapt to the general contours of the different workpieces.

InFIG. 1, a conventional ECM system is illustrated. The ECM system100includes a power supply102, a tool104and a workpiece106respectively acting as a cathode and an anode of an electrolytic cell, an electrolyte pump108, and an electrolyte tank110. The shape of the tool104is fixed. In operation, the tool104and the workpiece106are positioned such that a relatively narrow inter-electrode gap112is defined by the space therebetween. The power supply102is used to apply a voltage across the workpiece106and the tool104.

The system100includes an electrolyte system to pump a continuous stream of pressurized electrolyte into the gap112in which the electrolyte is pumped from an electrolyte tank110by the pump108and delivered to hollow electrolyte channels114formed within the tool104. The channels114direct the electrolyte toward the workpiece106. From the channel114, the electrolyte exits the tool cathode104and flows through the gap112at a relatively high rate and pressure.

The workpiece106is shaped by metal removal of the workpiece metal by electrochemical dissolution of the anodically polarized workpiece106. During the ECM operation, the electrolyte moving through the gap112removes the electrochemical dissolution material from the workpiece106diminishing the shape error of workpiece106. The metal removal rate is generally inversely proportional to the separation between the cathode and the anode. As the tool104is moved closer to the workpiece106, the separation, i.e., the gap112, between the cathodic tool104and the anodic workpiece106along the lengths of the tool104and the workpiece106, the gap tends to a steady-state value, and the workpiece106generally takes on the contour of the tool cathode104.

As noted, the tool104is uniquely shaped to fabricate the corresponding workpiece106in the conventional ECM. Fabricating many tool104, each with its unique shape and with necessary electrolyte channels114configured therein, can be costly. When a tool104is used to fabricate relatively small number of corresponding workpieces106, the cost issue can become exaggerated.

In the above-mentioned related application, a cathode tool is described which provides a significant level of interchangeability.FIG. 2illustrates an example of a related tool300that can be used on variety of workpiece shapes due to its flexibility, which is unlike the conventional tool. The tool300illustrated inFIG. 2includes a cathode302, spacer pads304, an elastomeric backing306, conducting strips308, and an electrolyte channel310. A machine ram312can move or position the tool cathode300so as to engage a workpiece in a desired manner.

The cathode302is a relatively thin and flexible, electrically conducting material. As illustrated inFIG. 3, the elastomeric backing306provides shape compliance to allow the cathode302to deform, and thus conform to the surface contour of the workpiece106. The elastic deformation is such that the outer surface of the cathode302is at a desired distance from the surface of the workpiece106allowing efficient ECM processes to be carried out. In this way, the related tool300allows a general three-dimensional surface to be polished without requiring numerous cathode dies.

When the workpiece106has a larger surface area than the tool300, the tool300is applied to different parts of the workpiece106one part at a time. For example, the tool300, and more specifically the cathode302compresses down to embrace a surface part of the workpiece106. After the surface part is embraced, the electrolyte and power are turned on to polish the engaged surface part underneath the cathode302. After the surface part is polished, the tool300is lifted and shifted to embrace a new surface part, and the new surface part is polished. This cycle of intermittent polishing continues until the entire surface of the workpiece is polished.

FIGS. 4aand4billustrate a two-dimension (2D) flexible electrochemical tool (FEC tool)400which can be used to perform a flexible electrochemical process (FEC process) according to an embodiment of the present invention. For ease of reference, directions X, Y and Z are indicated in these figures. “X” refers to side-to-side or lateral direction; “Y” refers to up-down or vertical direction; and “Z” refers to in-out direction. It should be appreciated that when components or embodiments are described as being in a particular position or moving in a particular direction, they are for descriptive purposes and are not meant to be limiting. For example, when a component such as a FEC tool is described as moving vertically, it does not necessarily mean that in the actual implementation, the component must move in the direction of gravity.

One of several motivations behind the present invention is to enable the workpiece to be traced. As used in this document, tracing refers to a capability to perform a flexible electrochemical process while the workpiece and the FEC tool are in motion relative to each other. As an example, a workpiece can be polished by tracing. It should be appreciated that compared to the intermittent polishing process described above with the related application, the continuous polishing process should be faster. Tracing also has other desirable qualities as will be shown throughout this document.

The phrase “flexible electrochemical process” (also FEC process) is introduced in the above paragraph. The FEC process broadly encompasses processes such as polishing, finishing, and shaping among others. The FEC processes should be distinguished from the conventional ECM process in which a fixed cathode die is used to remove chunks of metal from the workpieces. The FEC processes described in this document, unless otherwise specifically stated, generally refer to the removal of layer or layers of metal from the workpiece metal surface using the flexible electrochemical tools.

It should also be appreciated that the workpiece and the FEC tool can be moved relative to each other by moving one, the other, or both. Thus, unless explicitly indicated otherwise, statements such as “A moved relative to B” should be taken to be equivalent to statements such as “B moved relative to A”, and “A and B moved relative to each other”, and thus should be taken to encompass all possibilities of relative movement. Also statements such as “A moved towards/away from B” and “A and B moved towards/away from each other” should also be taken to indicate relative movements.

To facilitate smooth tracing motion, some amount of lateral rigidity is desirable. Lateral rigidity provides a more stable lateral motion on curved part surfaces. The FEC tool embodiment illustrated inFIGS. 4aand4bprovides such lateral rigidity and vertical flexibility.FIG. 4aillustrates an uncompressed state of the FEC tool400andFIG. 4billustrates a compressed state. The FEC tool400can include a strip cathode402, a machine ram412, and a plurality of support connectors420. As will be explained further below, the plurality of support connectors420can comprise at least one fixed support connector and at least one non-fixed support connector. The machine ram412can be connected with the support connectors420at the upper ends of the support connectors420. The lower ends of the support connectors420can connect with the strip cathode402along a length of the strip cathode402.

The strip cathode402is preferably elastically deformable, i.e., flexible, in 2D. The explanation is as follows. InFIG. 4a, the strip cathode402in the uncompressed state is positioned lengthwise in the lateral direction. That is, the strip cathode402is linear in the X or lateral direction in the uncompressed state. In the compressed stated as seen inFIG. 4b, the strip cathode402elastically deforms or flexes in the Y or vertical direction at different points along its length in the lateral direction, and the amount of deformation in the Y direction can be different along different X positions as the strip cathode402conforms to the surface contour of the workpiece106.

An analogy is that of a windshield wiper conforming to a curvature of a car windshield as the wiper and the windshield are moved relative to each other. One of several advantages of the FEC tool400is that the strip cathode402can bend more easily for a 2D curved line. Also, the strip cathode402can spring back easily when the workpiece106surface changes shape. Note that the support connectors420can be oriented in the Y direction, normal to the X direction, when the sheet cathode402is in the uncompressed state.

When the FEC tool400is under compression as shown inFIG. 4b, it is seen that vertical lengths of the different support connectors420are different, i.e., their strokes are different to adjust to the curvature of the workpiece106. Thus, the support connectors420can vary their strokes as the strip cathode402elastically deforms in 2D. But regardless of the strokes, it is preferred that the support connectors420apply a uniform pressure. This provides vertical flexibility so that the strip cathode402can adapt to the curved surface of the workpiece106. As seen inFIG. 4b, note that as the strip cathode402elastically deforms, the stroke of the support connectors420vary so as to change the length of the support connectors420.

In one embodiment, the support connectors420are air cylinders, and each cylinder420may include a vertical sliding bearing427and a spring428. The bearing427and spring428enable the cylinder420to provide elastic support so that the flexible strip cathode402can conform to the surface contour of the workpiece106.

The air cylinder420can further include a piston429which can be moved in the vertical direction by any well known drive mechanism, and the air pressure within the cylinder420can be accurately controlled through, for example, controllers (not shown inFIGS. 4aand4b). Using the constant air pressure and drive mechanisms, the stroke of each cylinder420can be set according to the contour of the workpiece106, which in turn allows the deformation of the strip cathode402to be controlled. As will be demonstrated further below, the ability to shape the strip cathode402provides an advantageous capability to finish the workpieces106. The drive mechanism can be incorporated in the machine ram412. The cylinder420, through the drive mechanism, may vary its stroke based on the workpiece contour so as to elastically deform the strip cathode402and to embrace the workpiece surface.

As mentioned, it is preferred that the pressure exerted by the plurality of cylinders420, i.e., the plurality of support connectors420, be substantially equal. That is, the support connectors420can have variable stroke and substantially constant pressure, which the constant air pressure in the cylinder ensures. Alternatively, hydraulic or electromagnetic mechanisms among others may be used as support connectors420.

Each support connector420can include a rotating coupler450that couples the lower end of the corresponding support connector420with the strip cathode402. An example of a rotating coupler450is a journal bearing. As seen inFIG. 4b, the journal bearings450turn to couple the vertically extending support connectors420with the curved strip cathode402with its elastomeric backing strip445.

As the strip cathode402is elastically deformed in 2D, it is shortened in the lateral direction. Sliding couplers430above outer support connectors420allow the cylinder translation460to link the vertically extending support connectors420with the curved and thus, laterally shortened strip cathode402. The support connectors420connected to the sliding couplers430are referred to as non-fixed support connectors since they are allowed to slide in the lateral direction, preferably within some limits, relative to the machine ram412.

On the other hand, it may be preferred that there be at least one fixed support connector420whose lateral position is fixed relative to the machine ram412. InFIGS. 4aand4b, the center support connector420is shown to have its lateral position fixed through a fixed connection440to the machine412. This is merely an example and not a limitation. Any of the support connectors420including the non-center support connectors may be fixed.

The rotating couplers450and sliding couplers430allow the non-fixed support connectors to extend or contract substantially vertically straight from the machine ram412and be linked with the curved and laterally shortened strip cathode402. The fixed support connector extends or contracts as the strip cathode402deforms substantially vertically straight from the mounting machine ram412due to its fixed connection. The rotating couplers450allow the both fixed and non-fixed support connectors420to adapt substantially normally to the workpiece surface contour as seenFIG. 4b.

It is assumed that during the flexible EC process operation, the FEC tool400is moved in the Z direction—in and out of the paper inFIGS. 4aand4b—relative to the workpiece106. The FEC tool400can trace the surface of the workpiece106along the Z direction. As the FEC tool400is driven in the Z direction, all support connectors420, both fixed and non-fixed, can take varying strokes to provide lateral rigidity for the driving force as well as vertical freedom for cathode flexibility. For clarity, electrolyte and power connections are not shown.

FIG. 5illustrates an example flexible cathode structure that includes the strip cathode402as viewed from the bottom. Note that inFIG.5, since the view is from the bottom, the Y direction is now in and out of the paper as indicated. Preferably, the strip cathode402is flat, relatively long and thin. The relative dimensions are not necessarily to scale. The structure500is preferably much longer than it is wide so that in practice, the cathode402can considered to be a strip, i.e., a line cathode. The cathode402can be formed from a flexible sheet metal with rubber strip backing.

The structure500can also include a plurality of inlets510to supply the electrolyte and a plurality of outlets520to allow the electrolyte to flow out. Spaces between a plurality of insulation spacers530define the outlets520in this embodiment. The structure500also includes one or more insulating standoffs535. The standoffs535and spacers530guide the electrolyte from the inlets510to the outlets520and prevent electrolyte leaks and escapes to the sides and the back. The insulating standoffs535and the insulation spacers530are all preferably at a predetermined thickness (into and out of page) so as to provide a well-defined inter-electrode gap between the cathode402and the workpiece106(not shown inFIG. 5).

Preferably, insulating coating540is applied to areas where the electrolyte flow is not stable. Unstable electrolyte flow can cause undesirable surface roughening. These areas usually include electrolyte inlets510and outlets520. Thus, as shown inFIG. 5, insulating coatings540are applied in the area corresponding to the inlets510, and520, and the center of the structure500is exposed where the electrolyte is relatively stable.

FIG. 6illustrates a flexible 2D FEC tool600according to another embodiment of the present invention. The FEC tool600can include many similar components as the FEC tool400such as the strip cathode structure500and the machine ram612. The FEC tool600can also include the plurality of support connectors620which comprises at least one fixed support connector (connected to fixed connector640) and at least one non-fixed connector (connected to rotating couplers630). The support connectors620can be coupled to the cathode structure500at the lower ends thereof through rotating couplers650, for example, journal bearings. Further, the support connectors620can vary their strokes and also apply a uniform pressure while varying the strokes.

However, instead of the sliding couplers, rotating couplers630can couple the non-fixed support connectors620at the upper ends thereof. For ease of reference, rotating couplers650and630are respectively referred to as the lower and upper rotating couplers. The upper rotating couplers630allow the outer non-fixed support connectors to rotate to accommodate the laterally shortened strip cathode402as it elastically deforms. The non-fixed support connectors may not necessarily extend or contract vertically straight from the machine ram612. The lower rotating couplers650allow the both fixed and non-fixed support connectors to adapt substantially normally to the workpiece surface contour.

Operation wise, both FEC tools400and600are excellent to perform a continuous EC process such as the continuous EC polishing process. While the FEC tool400,600is moved to trace the surface of the workpiece106, the FEC tool400,600continually adapts to the surface contour of the workpiece106to carry out the EC polishing process.

FIG. 7illustrates system to perform a flexible EC process such as polishing, finishing and/or shaping according to an embodiment of the present invention. The system700may be described as an example of an electrolyte system in which the electrolyte may fill a work tank720such that the workpiece106such as a steam turbine bucket and a FEC tool704are submerged during the EC process. Alternatively without submerging the FEC tool704and workpiece106, the electrolyte can be supplied through fluid conduits to the cathode inlets and out from cathode outlets of the FEC tool704. The FEC tool704can be connected to a machine ram712. The workpiece106may be supported in the work tank720by left and right clampings732,734. A controller760may control the system's operation automatically or under manual instructions from an operator. The controller760may be implemented through various combinations of hardware, software, and firmware components such as a computer, storage devices, communication units, and numerical control programs.

For sake of clarity, components such as the electrolyte reservoir, electrolyte pump, electrolyte filter, power supply, controllers, drive mechanism, pipes, hoses, and fittings are not shown. Also connections between the controller760and other components are not shown so as to reduce obscuring the information.

FIG. 8illustrates a flow chart of an example method to perform a flexible EC polishing process according to an aspect of the present invention. In the method800, the controller760may position the FEC tool704to engage a surface part of the workpiece106in step810. For example, the FEC tool704may be positioned at a known starting location such as right or left end of the workpiece106near the right or the left clamping734,732. In step820, the controller760may initiate power and electrolyte flow to start the EC polishing process.

In step830, the FEC tool704traces the workpiece106under the control of the controller760until an end side of the workpiece106is reached. For example, if the FEC tool704initially engages the workpiece106near the right clamping734, the first tracing motion would be to move the FEC tool704towards the left clamping732until the FEC tool704reaches the left clamping732. While the FEC tool704is moving, the controller760causes the power and electrolyte to be maintained, i.e., the EC polishing process is carried continuously out. Also as the FEC tool704is moved, it continually adapts to the surface contour of the workpiece106.

When the end side is reached, the controller760can determine whether or not the EC polishing process is complete in step840. The criteria for determining whether the ECM polishing is finished may depend on the particular circumstances. As an example, tracing the entire surface of the workpiece106once may be deemed complete. In another circumstance, tracing only a portion of the entire surface may suffice. If a very smooth surface is desired, then the workpiece106may be traced more than once.

If in step840it is determined that the EC polishing process is not complete, then in step850, the controller760may cause the FEC tool704to be repositioned. For example, the controller760may instruct the right and left clampings732,734to rotate the workpiece106in the “w” direction. After the FEC tool704is repositioned, step830can be repeated to trace the workpiece106until the other end is reached. During the tracing step, the EC polishing process can be continuously carried out. The controller760may continue the loop of steps830,840, and850until when in step840, it is determined the EC polishing process is complete. Then in step860, the controller760may cause the FEC tool704to be disengaged from the workpiece106.

While the FEC tools400and600are extremely useful, there are circumstances where workpiece contour is very aggressive such as corners and edges. In these circumstances, alternative designs of the flexible FEC tool may be desirable.FIGS. 9aand9billustrate a flexible FEC tool900according to another embodiment of the present invention. The FEC tool900may be referred to as a cornering FEC tool900.FIG. 9aillustrates a side view of the FEC tool900, andFIG. 9billustrates the FEC tool900as viewed from the position A inFIG. 9a.

The structure of the FEC tool900can be similar to the structure500illustrated inFIG. 5. The FEC tool900may includes the plurality of inlets910, a plurality of outlets920, a plurality of insulation spacers930and standoffs935, insulating coatings940, and a cathode902, which can be a strip or a sheet cathode, as seen inFIG. 9b. These components may serve similar functions of the components of the structure500illustrated inFIG. 5and thus will not be described further.

But as seen inFIG. 9a, the FEC tool900can include an elastomer950, which provides an adaptive backing so that the FEC tool900can adapt to a corner of the workpiece106. Note that inFIG. 9b, the elastomer950is such that it is more backfilled near the edge of the cathode902then at the center. This helps to provide a tight sealing for the electrolyte and workpiece pressure for corner adaptation.

Generally, the corner of the workpiece106can be viewed as two side surfaces962and964that extend substantially in straight directions from a corner point960forming a concave surface, and the corner angle θ can be viewed as the angle made by the side surfaces962and964. The corner angle θ can be a right angle as illustrated inFIG. 9bor can be other angles. The angle θ is not specifically limited. In many instances, corner angles that range between 80° and 100° are typical.

The bottom layer of the FEC tool900can be pre-bent to a blunt angle α that is not necessarily the same as the corner angle θ. Preferably, the pre-bent angle α should be at least substantially equal to the corner angle θ of the workpiece, α−θ≈0. However, it is even more preferred that the relationship α−θ>0 holds, i.e., the blunt angle α of the FEC tool900is preferred to be greater than the corner angle θ of the workpiece106.

In operation, this cornering FEC tool900may be first squeezed into the workpiece corner part to have a tight fit under the force of the machine ram912. When the FEC tool900is squeezed into the corner, compression along the edge of the cathode902is likely to be lower than the compression in a center thereof when α−θ≈0, i.e., when they are substantially at the same angle. When α−θ>0, the compression along the edge will increase which thereby minimizes the compression difference between the edge and the center of the cathode902. That is, the compression difference tends to zero as the difference α−θ increases. Eventually, the compression on the edges may become greater than in the center as α−θ continues to increase.

Thus, in one aspect, the cornering tool900is such that the difference α−θ is greater than or substantially equal to a minimum angular difference and less than or substantially equal to a maximum angular difference. The minimum and maximum angular differences may be determined being the range of angular differences in which the pressure differences between the edge and center of the FEC tool remains within a predetermined tolerable range. Minimum and maximum angular differences respectively being 10 and 45 degrees are satisfactory in some EC processes.

FIGS. 10aand10billustrate a FEC tool1000according to another embodiment of the present invention. The FEC tool1000may be referred to as an edging FEC tool1000.FIG. 10aillustrates a side view of the FEC tool1000.FIG. 10billustrates a cross-section view of the structure of the FEC tool1000as viewed from the position A along the line A-A. Generally, an edge of the workpiece106can be viewed as two side surfaces1062and1064that extend substantially in straight directions from an edge point1060forming convex surface, and the edge angle can be viewed as the angle φ made by the side surfaces1062and1064.

The edging FEC tool1000may includes the plurality of inlets1010, a plurality of outlets1020, a plurality of insulation spacers1030and standoffs1035, insulating coatings1040, and a cathode1002, which can be a strip or a sheet cathode, as seen inFIG. 10b. These components may serve similar functions as described with respect to similar components ofFIGS. 5 and 9b, and thus will not be described further.

Like the cornering FEC tool900, the edging FEC tool1000can include an elastomer1050, which provides an adaptive backing so as to adapt to the edge of the workpiece106. The edges may be more backfilled than the center where the bent occurs. The cathode1002is pre-bent. However, the pre-bent angle β is preferably more severe than an edge angle φ of the workpiece106such that the relationship β−φ≈0 holds. It is even more preferred that the relationship β−φ<0 holds.

In operation, this edging FEC tool1000may be first squeezed into the workpiece edge part to have a tight fit between the flexible cathode1002and the part edge. As the FEC tool1000is swept through the edge, FEC tool opening and side curvature vary to keep a tight contact with the part surface near the edge area.

For a pre-bent angle β of the cathode1002, the FEC tool1000can be used on edges with angles φ that range from φ0to φ1, i.e., φ0≦φ≦φ1, such that within the edge angle range φ1-φ0, the compression difference between any parts of the cathode1002is within the predetermined tolerance limits. For example, the range φ1-φ0is preferably 10 degrees or less. That is, an edging tool1000that is nominally designed for a particular angle φ may be used for edging surfaces whose angles are within few degrees of the nominal angle. Even more preferable is that the range φ1-φ0be 2 degrees or less

Note that a zero tool angle can be provided for very sharp angles. In the zero angle edging tool1000, the pre-bent angle β of the tool1000from the edge point1060made by side surfaces1062and1064may be zero for at least a part when the tool1000is not engaged with the workpiece106. The backfill provided by the elastomer1050deforms accordingly to provide the necessary pre-bent angle and fitting. The edging FEC tool1000can be used for continuous or intermittent polishing as the workpiece106is driven relative to the FEC tool1000.

Up to this point, examples of 2D FEC tool have been illustrated and described.FIGS. 11aand11billustrates a flexible three dimensions (3D) FEC tool according to an embodiment of the present invention. The FEC tool1100can include a sheet metal1102, a machine ram1112, and a plurality of support connectors1120. The machine ram1112can be connected with the support connectors1120at the upper ends thereof. The lower ends of the support connectors1120can connect with the sheet cathode1102at various parts along an upper surface of the sheet cathode1102.

The sheet cathode1102is preferably elastically deformable in 3D. InFIGS. 11aand11b, the sheet cathode1102can be part of a layered structure that includes an elastic backing1145in the XZ plane. As seen, when the sheet cathode1102is in the uncompressed state, the support connectors1120can be oriented in the Y direction which is normal to the XZ plane of one sheet cathode1102. When a surface part of a workpiece106(not shown) compresses the layered cathode1102, along different XZ points, the sheet cathode1102can deform elastically in the Y direction to conform to the surface contour of the workpiece106, either concave or convex. The sheet cathode1102can deform to continually conform to a 3D surface contour of the workpiece106such that the surface of the workpiece106is traced as it is moved in relation to the sheet cathode1102. In this manner, the workpiece106can be continually polished.

The support connectors1120can vary their strokes as the sheet cathode1102elastically deforms in 3D, which can also change the lengths of the support connectors1120. But regardless of the strokes, it is preferred that the support connectors1120apply a uniform pressure, thus providing vertical flexibility to enable the sheet cathode1102to adapt to the curved surface of the workpiece. In one embodiment, the support connectors1120comprise air cylinders similar to the cylinders420illustrated inFIG. 4a. That is, while not shown inFIGS. 11aand11b, each cylinder1120may include a vertical sliding bearing1127and a spring1128providing elastic support, and may include a piston1129which can be moved to change the stroke of the cylinder1120. The amount of piston movement, and therefore the amount of stroke depends on the surface height of workpiece contour when compressed. The drive mechanism can be incorporated in the machine ram1112. Further, it is preferred that the pressure exerted by the plurality of support connectors1120be substantially equal regardless of the length of the stroke.

At the upper end thereof, at least one support connector1120may be coupled to the machine ram1112by an upper coupler1130which is rotatable in two orthogonal directions such as a ball joint. At the lower end thereof, each support connector1120may have a lower coupler1150connecting the sheet cathode1102with the support connector1120.

In one embodiment, the lower coupler1150can include a mechanism that is rotatable in two orthogonal directions (e.g., a ball joint) connected to a pad, and the pad may be attached to the sheet cathode1102.

Such rotatable lower couplers1150allow the support connectors1120to adapt substantially normally to the workpiece surface contour.

In another embodiment, the lower coupler1150may include the pad, but not include the rotatable mechanism. It is described above that the sheet cathode1102can be a layered structure. The structure preferably includes an elastic backing1145(e.g., elastomeric backfill or rubber backing) on the sheet metal1102, and the pad is attached to the elastic backing1145. Thus, even when the lower coupler1150does not include the rotatable mechanism, the connection of the support connector1120with the sheet cathode1102is not necessarily rigid, and some amount of adaptation normal to the workpiece contour can still occurs.

It should be noted that even in the embodiment that does include the rotatable mechanism, the sheet cathode1102still preferably includes the elastic backing1145.

In the flexible EC operation, the FEC tool1100can be compressed between the workpiece106below and the machine ram1112above. The support connectors1120at the same pressure can take different strokes according to the surface part of the workpiece106. The lower couplers1150can connect the sheet cathode1102with the support connectors1120, and the upper couplers1130can allow an appropriate contact angle to be maintained between the support connectors1120and the machine ram1112. As the FEC tool1100traces the surface part, the support connectors1120can take different strokes and provide lateral rigidity for the driving force. Electrolyte and power connections are left out for clarity. Note the center support connector with air cylinder need not have the ball joint on the top. This center support connector provides the main lateral rigidity. However, this is not a strict requirement as will be shown below.

It is preferred that the upper couplers1130have a predetermined limited angular range of rotation in the two orthogonal directions. In this instance, each support connector1120may have the corresponding upper coupler1130such that they are all non-fixed support connectors1124. However, it is also possible that at least one support connector1120includes a fixed connection1140at its upper end such that the support connector1120is fixed in the XZ position relative to the machine ram1112. InFIG. 11a, the fixed support connector1120can be the center support connector. But this is merely an example and not a limitation. Any of the support connectors1120may be fixed.

The embodiments of the flexible FEC tools illustrated inFIGS. 4a-11bare excellent choices to trace the workpiece106for the flexible EC polishing process. In other words, the FEC tool704used in the flexible EC polishing method illustrated inFIG. 8maybe any of the FEC tools400,600, and1100. When aggressive surfaces such as corners and edges require EC polishing, the workpieces900and1000may be used.

However, most if not all of the FEC tool embodiments may also be used for flexible EC finishing. For the purposes of this document, finishing refers to a process in which surface errors of the workpiece are corrected. In this context, a surface error is defined as a deviation in the contour of a surface part of the workpiece, e.g., deviation in the surface height, which is beyond a predetermined tolerance range allowed for that surface part.

Finishing provides a way to perform a precise near net shape forming. Using forming processes such as casting or forging provides the final workpiece shape without surface finishing. Achieving near net shape by precise forming has been desired for many decades to reduce or even eliminate the need for final machining since this would simplify the process and reduce the cost.

Processes such as investment casting and net forging can provide precise forming to meet tight tolerances as small as 0.001″ or one mil. However, this is accomplished at great costs and can be done only for limited part sizes. It is fundamentally difficult for a forming process to handle bulk material while holding the surface precision. Generally, displacing the bulk material makes it difficult to keep surface accuracy. Also, forming costs increase exponentially as the part tolerances get tighter and the part size gets larger. If final machining is still needed for tighter tolerances, the original purpose of net shaping is defeated or diminished. Despite the reduced stock removal, a precise and expensive 5 axis computed numerically controlled (CNC) machining may still be required to contour the 3D surfaces.

In addition, the surface quality from the forming processes usually cannot meet most part specifications. The surface roughness is usually high after the surface part is directly solidified or forged. In most instances, there is a layer of rough surface grains on any casting or forging due to the slow surface cooling rate. Surface oxidation is often a problem without finishing.

To relieve the cost pressure of stringent forming precision and allow bulk material and surface to be handled economically at the same time, it becomes necessary to implement efficient surface finishing after near-net shaping. In this way, conventional machining for large stock removal can be simplified or even revolutionized to handle only the surface materials since near net forming eliminates the need of rough machining. Rough and fast handling can usually be better done by forming to save materials and energy.

In an aspect of the present invention, the flexible FEC tools are provided which can be used for the flexible EC finishing a workpiece after near-net forming. For example, the workpiece106such as nozzles or buckets of a turbine may be initially near-net formed through investment casting. The flexible FEC tools, much like the FEC tools400,600,900,1000, and1100described above, can adapt to the near-net formed airfoil surface without the CNC motion. As the flexible cathode traces through the workpiece surface, the FEC process can polishes the surface and also correct residual errors, i.e., the surface can be finished.

To enable the flexible EC finishing, the FEC tool preferably includes a sensor that can detect a surface height of the workpiece. A controller can compare the detected height with stored part geometry. Depending on the correction that is necessary, the controller can control any combination of the factors that affect the material removal such as the electrical voltage, the tracing speed, the electrolyte flow and pulse parameters if pulse power is applied.

There are numerous advantages to the flexible EC finishing. For example, the flexible EC finishing can remove the need to provide a powerful machine spindle. Also, there is little to no tool wear even for hard and resistant inconel. The mechanical load can be low while providing high structural accuracy. Further, fast polishing and finishing can be accomplished with relatively few flexible FEC tools.

FIGS. 12aand12billustrate a flexible 3D FEC tool1200according to another embodiment of the present invention. The FEC tool1200can be used for flexible EC polishing like the FEC tool1100. But in addition, the FEC tool1200can also be used for flexible EC finishing.FIG. 12ais a side view andFIG. 12bis a bottom view of the FEC tool1200.

The FEC tool1200can include a sheet cathode1202, spacers1230, an elastomeric backing1250, and an electrolyte inlet channel1210. A machine ram1212can position the FEC tool1200so as to engage the workpiece in a desired manner. Preferably, the cathode1202is a relatively thin and flexible, electrically conducting material, such as copper and stainless steel. The elastomeric backing1250can provide an elastic backing to allow the cathode1202to deform, and thus conform to the surface contour of the workpiece. The elastic deformation can be such that the outer surface of the cathode1202is at a desired distance from the surface of the workpiece106allowing efficient ECM processes to be carried out. In this way, the FEC tool1200can continuously polish a general 3D surface without requiring numerous cathode dies.

FIG. 12billustrates a cathode structure of the FEC tool1200. Spaces between a plurality of insulation spacers1230define a plurality of outlets1220to allow the electrolyte flowing in from the inlet channel1210to flow out. The spacers1230guide the electrolyte from the inlet1210to the outlets1220. The spacers1230form the inter-electrode gap between the cathode1202and the workpiece. In this particular embodiment, the spacers1230closer to the center may be formed more thickly than the spacers1230closer to the edge or periphery to allow a better electrolyte flow from the center. Insulating coatings1240can be formed in areas corresponding to the spacers1230where the electrolyte flow is not likely to be stable.

The FEC tool1200may be used in the ECM polishing operation illustrated inFIGS. 7 and 8. That is, the FEC tool1200can be applied to different parts of the workpiece106and the tooling structure can be compressed between the workpiece surface part below and the machine ram above. The elastomeric backing1250can provide the necessary elastic compression to seal the electrolytic cell that conforms to the surface of the workpiece. As the workpiece106and the tooling cathode1200are moved relative to each other, the sheet cathode1202can conform to the contour of the surface part engaged with the sheet cathode1202, and the ECM polishing can be carried out by turning on the current and flowing the electrolyte between the gap of the workpiece surface and the sheet cathode1202.

But in addition, the FEC tool1200can also include a sensor that senses the surface height of the workpiece106during tracing, i.e., during the continuous movement. InFIGS. 12aand12b, the sensor1260is implemented as a combination of a stylus1262, a linear scale1264, and a spring1266. This combination is but merely one of many ways to implement the sensor1260. Other implementations of the sensor include LVDT linear variable differential transformers and capacity sensors.

The stylus1262can be disposed substantially at a center of the FEC tool1200as shown, but this is not a limitation. The stylus1262may be placed elsewhere. For example, the stylus1262may be placed in “front” of the cathode1202in the tracing direction. Further, the number of sensors is not limited to one, i.e., multiple sensors may be provided. For example, two styluses1262, one on each side of the cathode1202in the tracing direction.

It should be noted that sensors may be added to the FEC tools400,600, and/or1100for use in the flexible EC finishing as illustrated inFIGS. 13aand13b. InFIG. 13a, the cathode structure500is reproduced and inFIG. 13b, the bottom view of the FEC tool1100is reproduced. In these figures, sensors1360drawn as doubled squares are distributed around each cathode structure. Other components such as inlets, spacers, couplers and outlets are not numbered for clarity. While more sensors1360are desirable, there may be some practical considerations such as cost that may factor into how many sensors may actually be deployed. It suffices to say that with the type of modifications illustrated inFIGS. 13aand13b, the FEC tools400,600and1100can also be used for flexible EC finishing workpieces.

FIG. 14illustrates a flow chart of a method to perform flexible EC finishing according to an aspect of the present invention. Note that the method1400of performing the flexible EC finishing share many of the steps in common with the flexible EC polishing method800illustrated inFIG. 8. Like the flexible EC polishing process, the flexible EC finishing process can adapt to the contour of the workpiece surface. But instead of simply conforming to the existing surface contour, the cathode tool's elastic deformation can be actively controlled so that the workpiece is contoured to a desired final shape. InFIG. 14, it is assumed that the workpiece106, such as a bucket of a steam turbine, has been initially near-net shaped through another process such as investment casting, forging, or even through conventional ECM using a fixed shaped FEC tool. The system700illustrated inFIG. 7will be used in conjunction to describe the example flexible EC finishing method1400in which the FEC tool704may be any of the FEC tools400,600,1100, and1200with sensors.

In the method1400, the controller760may position the FEC tool704to engage a surface part of the workpiece106in step1410. Preferably, the FEC tool704is positioned at a known starting location of the workpiece106. In step1420, the controller760may initiate power and electrolyte flow to start the ECM process.

In step1430, the controller760can cause the FEC tool704to trace the workpiece106until an end side of the workpiece106is reached. During the trace, the workpiece106is subjected to the flexible EC finishing process.FIG. 15illustrates a flow chart of an example process to carry out the step1430to perform the flexible EC finishing process during a trace of the workpiece according to an aspect of the present invention. The process illustrated inFIG. 15can be assumed to be continually performed as the FEC tool704is moved from one end to the other end of the workpiece106.

In step1510, the height of the surface part may be detected through a sensor such as the sensor1260,1360. The sensor1260,1360may provide the detected result to the controller760. In step1520, the controller760may compare the detected surface height against a stored model for that particular surface part of the workpiece106. In step1530, the controller760may determine whether or not the difference between the detected height and the stored model height is within tolerance limits.

It should be noted that tolerances for workpieces can range from very general to very specific. In one instance, the same tolerance limits may apply for the entire surface of the workpiece or even across several workpieces. In another instance, different surface parts within one workpiece may have tolerance limits that only apply to the particular surface parts. Indeed, different tolerance limits may apply to the same workpiece under different circumstances. For example, a workpiece manufacturer may offer different levels of guarantees for the same workpiece. For the highest guarantee level commanding the highest price, very tight tolerances may be applied in finishing the workpiece. For other guarantee levels, correspondingly greater deviations from the stored model may be tolerated.

If the difference is within the tolerance limits, then the process can proceed to step1550to determine whether this trace is completed, e.g., the controller760may determine whether the end side of the workpiece106has been reached. If the trace is not complete, then the process may repeat from step1510.

If however it is determined in step1530that the detected height is outside the tolerance limits, the controller760may take corrective action or a combination of actions in step1540. A non-exhaustive list of corrective actions includes the following. First, the shape of the cathode1202may be altered. As described above, the FEC tools400,600and1100can include drive mechanisms which can be controlled to vary the strokes of the support connectors420,620, and1120. Depending on the corrections that may be necessary, the controller760may alter the support connector strokes so as to vary the amount of the EC processing taking place under different portions of the cathode. For example, for a surface part that require relatively more or less removal of material, the stroke of the support connector420,620,1120may be controlled to decrease or increase the inter-electrode gap. Since the workpieces are already near-net shaped, drastic active shaping of the flexible cathode is unlikely to be necessary.

Second, the tracing movement can be altered. For example, the controller760may cause the FEC tool704to move slower or even faster as needed. The speed of movement may correspond to the amount material removal required to bring the surface part height to within tolerance limits. Generally, slower speed will enable more EC processing to be performed to the surface part. Indeed, it may even be that the direction of the trace may be reversed for a short distance before proceeding back in the original tracing direction.

Third, current may be increased or decreased as necessary. If a surface part requires relatively more or less EC processing, the controller760may cause the power supply to increase or decrease the current flow as the FEC tool704passes over the surface part. The controller760may also control the electrolyte pump to increase or decrease the electrolyte flow as needed.

Of course, the controller760may combine any of the described corrective actions to effectuate the ECM finishing. After the corrective actions are taken in step1540, the controller760may proceed to step1550.

If at step1550it is determined that the particular trace run is complete, then the process1430is exited and the method resumes in step1440inFIG. 14. In this step, the controller760may determine whether the entire flexible EC finishing process is completed. As noted with respect to the flexible EC polishing method ofFIG. 8, the criteria for determining whether the flexible EC finishing process is completed may depend on the particular circumstances.

If in step1440the controller760determines that the flexible EC finishing is not complete, then in step1450, the controller760may reposition the FEC tool704, for example, by causing the clampings732,734to rotate the workpiece106. After the FEC tool704is repositioned, the controller760may proceed to step1430to repeat the flexible EC finishing as the FEC tool704moves in the other direction. The controller760may repeat the loop of steps1430,1440, and1450until in step1440, it is determined the flexible EC finishing is complete. Then in step1460, the FEC tool704can be disengaged from the workpiece106.

In addition to flexible EC polishing and finishing processes, the flexible tracing may be used to finally shape the workpieces as well. It has been described that in industries such as power generation, even an incremental gain in efficiency can represent significant cost savings. It has also been described that with the conventional ECM, retooling can be an expensive process. Retooling, that is manufacturing a new FEC tool, may also take significant amount of time.

The flexible EC processes described above can be used to mitigate such costs and time. For example, there may be circumstances in which a relatively small change in the design of an existing workpiece is shown to provide beneficial efficiency improvements. In these circumstances, the workpiece may be initially fashioned using existing molds, dies, or FEC tools. The initially fashioned workpiece may be subjected to the flexible EC shaping process with the new design. In effect, the changes between the old and the new design can be treated as errors to be corrected through the flexible EC finishing process. Such process can be used to start manufacturing the newly designed workpieces while molds, dies, and FEC tools specific to the new design can come on line. This can enhance the chances of a manufacturer to be the “first-to-market”.

Such process can also be used to relatively quickly test a new design. For example, computer modeling may indicate that a tweaked design is promising, but a real-world test is required for confirmation. Instead of incurring the time and expense of retooling for testing purposes, the flexible EC finishing process can be used to fabricate the testing workpiece.