The present invention relates generally to electrochemical machining (sometimes referred to as “ECM” herein) of a work piece, and more particularly relates to a novel method and apparatus for electrochemical machining of a work piece which represents a significant improvement over the present state of the electrochemical machining art.
Electrochemical machining of an electrically conductive work piece is well known and involves a conductive work piece to be machined (anode), a tool (cathode) which is positioned in non-contact, spaced relation to the work piece, and an electrolyte comprising an electrically conductive fluid such as NaCI in H20, for example, flushed between the tool and work piece. The distance or spacing between the anode and cathode is termed the inter-electrode gap or “IEG”. A voltage is applied between the work piece and the cathode whereupon an electric circuit is established between the work piece and the cathode through the electrolyte. With the cathode being continuously advanced toward the work piece and as ions cross the IEG, electrically conductive material (hereinafter “material”) is removed from the work piece as the atoms and molecules leave the work piece and enter into the electrolyte as ions and molecules and are flushed through the IEG. The removal of atoms and molecules from targeted areas of the work piece is what “machines” the work piece into the desired shape. The ECM process may thus be considered the opposite of electrochemical plating where material is added to a work piece through an electrolyte bath.
Besides the atoms and molecules which detach from the work piece material, including their oxides, additional particles formed during the ECM process accumulate in the IEG and are collectively termed “by-products of dissolution”. Although the exact by-products produced during ECM will vary depending on the type of work piece material and electrolyte being used in a particular application, examples of such by-products of dissolution include hydrogen and oxygen gas bubbles formed by hydrolysis of the water in the electrolyte, hydroxyl molecules and various stoichiometric phases of the metal particles with other molecules and atoms. Accumulation and ineffective removal of these by-products of dissolution in the IEG negatively impact many areas of the ECM process including speed, cost, surface finish, and/or dimensional tolerances, for example. Gas generation in the IEG, often misinterpreted as sparking or interference by by-products of dissolution, results in poor surface finish and reduces dimensional precision. Secondarily, the non-gaseous, ionic by-products of dissolution create an electrical insulating layer immediately adjacent to the work piece (referred to herein in its fully characterized form, as identified and defined by the inventor herein, as the Beta Insulating Layer or “BIL”) which inhibits all aspects of the ECM process. While the ECM process has been in practice for many years, the prior art has failed to overcome many of the negative effects caused by gas generation and has failed to recognize the criticality of fully understanding the BIL which is believed to be due, at least in part, by a failure to recognize the exact composition and fluid dynamics of the BIL as well as the interactions between the work piece, IEG, tool and BIL during the ECM process. Although it is impossible to completely eliminate the formation of gases in the IEG and the electrically insulating effect of the BIL, there remains a need for an improved ECM process and apparatus which more effectively controls the formation of gases and the directed removal of the BIL in the IEG during the ECM process.
The present invention controls gas formation in the IEG and fully characterizes and defines the BIL composition and related fluid dynamics during the inventive ECM process and, in so doing, successfully addresses the problems plagued by the prior art ECM processes described above.
In ECM, improvement in the machining rate of the work piece is desirable, however, the prior art has generally associated higher currents with higher machining rates, but have failed to understand or recognize that high currents also generate an undesirable high rate of hydrolysis of the electrolyte. High rates of hydrolysis leads to the formation of an excessive amount of gas bubbles comprising O and O2 molecules emanating from the anodic work piece and H and H2 emanating from the cathodic tool. These gas bubbles interrupt the electrical contact between the anode and the cathode having an electrical insulating effect. Also, if the BIL is not being continuously removed from the work piece during machining, the BIL accumulates in the IEG and gas bubbles ultimately push through the accumulated BIL which creates momentary voids in the BIL which, in turn, allow a momentary and uncontrolled clear current path to the work piece. Since the voids may appear anywhere in the BIL immediately adjacent to the work piece surface, this phenomenon creates pitting in the work piece surface as well as poorly defined edges at the peripheral boundaries of the point of machining. This phenomenon of surface pitting and poor surface edges occurring due to gas bubbles pushing through an accumulated BIL has not been previously recognized by the prior art and hence has not heretofore been adequately addressed.
The present invention addresses the above problem in one manner by minimizing electrolyte hydrolysis and the resultant formation of gas bubbles through proper selection and control of the applied voltage. The present invention addresses the above problem in another manner by constantly and uniformly removing the BIL by direct perturbing of the BIL (also referred to herein as “perturbation” or “mechanical perturbation”) with the tool in an optimally maintained very small IEG which may be in the range of about 0 (infinitely close, but not touching) to about 10 microns. As will be described more fully below, the inventor herein has measured the BIL as forming adjacent to the work piece within this range of about 0 to about 10 microns. The BIL encapsulates the layers, cumulatively measuring 0 to 1 microns, described by the Helmholtz (otherwise referred to as the double layer or electrical double layer) and Gouy-Chapman-Stern (including the screening and adjacent diffuse layers) models, as well as the conventions described as the slipping plane that separates mobile fluid from fluids attached to the surface of the anode and/or cathode. The layer models, inclusively, describe the distribution of ions and charges near the electrode surface whereby a diffuse layer of charge is formed in the electrolyte with the net charge highest near the electrode surface and the concentration of ions diminishing away from the interfacial region immediately adjacent to the surface of the electrode until the distribution of ions becomes homogenous.
More particularly, in one embodiment of the invention, the inventive ECM method includes the step of determining the optimum ECM operating voltage and current at which to machine the work piece. The optimum maximum voltage for a particular type of work piece material may be identified using a potentiostat voltammetry sweep triggered between the work piece material to be machined and a cathode comprising the cathode material used in the inventive ECM system under a set of conditions reflective of those used in the ECM system including the selected electrolyte. The optimum maximum voltage is identified by actively monitoring the current while voltage is gradually increased from zero or near zero volts. As voltage in increased, the current is monitored and the point at which the current begins to increase rapidly defines the Low Machining Potential Voltage (LMPV) at which the actual onset of material dissolution occurs for this particular work piece. The voltage is then continually increased until a decrease in the rate of current increase is detected which defines the High Machining Potential Voltage (HMPV) that should be used during the inventive ECM process on the work piece without initiating excessive hydrolysis of the electrolyte. Thus, both the LMPV (approximate lowest acceptable voltage) and HMPV (approximate highest acceptable voltage) used to machine the work piece is determined. Although it is preferred to use the HMPV to achieve the highest machining rate, any voltage between the LMPV and the HMPV is suitable. Should an even higher voltage be used, the resultant increase in current would be devoted to hydrolysis of the electrolyte, thus further degrading dimensional tolerances.
The prior art, believing the higher the current, the higher the machining rate, have used voltage and currents higher than the optimal voltage and current as identified herein. When the prior art see problems with the resultant machining surface, they attribute it to sparking or interference by the by-products of dissolution. In response to this problem, the prior art, due to a failure to recognize that the higher current is directed at generating more gas rather than machining, reacts by attempting to keep the high voltage and current while putting more effort into removing the by-products of dissolution which they do by a variety of means (e.g., pulsing the electrolyte through the IEG with pressure waves or oscillating the tool relative to the work piece to allow for an oscillating increase in the spacing of the IEG, with or without an in-phase pulsing of the current during maximum IEG spacing). In this instance, the prior art is, in actuality, intermittingly turning off the machining current while simultaneously increasing the IEG spacing in an attempt to forcefully clear away the built up by-products. In the example of prior art U.S. Pat. No. 6,835,299, the oscillating tool has a back and forth movement with respect to the work piece which thus alternately increases and decreases the IEG spacing with a simultaneous pulse off mode of the current during the temporary increase in IEG. According to this patent, this increase in IEG spacing with the machining current off is an attempt to clear away the accumulated by-products which of course is also temporarily turning off the machining process which negatively impacts machining speed and consistency of dimensional accuracy.
In another aspect of the invention, the composition of the insulating layer has been fully characterized and termed the Beta insulating Layer (BIL) herein thereby allowing precise control over the formation and accumulation thereof in the IEG. In an embodiment of the invention, the inventive ECM method constantly and uniformly removes the BIL by constant and direct perturbing of the BIL with the tool in an optimally maintained very small IEG which may be in the range of about 0 to about 10 microns. The result is a fast machining rate with a very smooth machining surface (±0.2 μin) not heretofore attainable using prior art ECM methods. The tool may be configured to constantly and simultaneously pull and push the electrolyte into and out of the IEG which, with constant perturbation of the BIL, results in the constant removal of the BIL in the present invention which prevents the adverse machining effects caused by the uncontrolled build-up of dissolution by-products which the prior art allows to at least temporarily accumulate in between their generally ineffective attempts to sweep away the accumulated by-products.
A more particular description of the invention briefly summarized above is available from the exemplary embodiments illustrated in the drawings and discussed in further detail below. Through this reference, it can be seen how the above cited features, as well as others that will become apparent, are obtained and can be understood in detail. The drawings nevertheless illustrate only typical, preferred embodiments of the invention and are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.
a-2f are simplified side elevational views of an exemplary work piece through stages of preparation for the ECM process;
a is a simplified side elevational view of one embodiment of a tool wheel in proximity to the machining end of the work piece;
b is a cross-section as taken generally along the line 5b-5b in
a is a graph showing Preferred Operating Voltage (POV) varying with surface area;
b is a simplified graphic demonstrating that at a single unadjusted operating voltage there is an overall increased generation of gas in the IEG as the surface area of the work piece expands;
So that the manner in which the above recited features, advantages, and objects of the present invention are attained can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiment thereof that is illustrated in the appended drawings. In all the drawings, identical numbers represent the same elements.
Based on the selected work piece material, an electrolyte 31 is selected for the purposes of conducting current established by a set voltage between the anode and cathode. In the present invention, this electrolyte may take the form of inorganic or organic electrolytes or molten salts, for example. An electrolyte outlet tube 30 is provided for delivering the selected electrolyte 31 to the inter-electrode gap (IEG) 32. Electrolyte 31 is delivered to outlet tube 30 from a reservoir tank 33 which delivers electrolyte via tube 34 to a pump 36 and flow dampener 38 to allow metering of the amount of electrolyte reaching outlet tube 30. Electrolyte is recovered in sump 40 with recaptured electrolyte directed back to reservoir 33 via tube 42.
A power supply 44 is provided which may be in the form of a VAC-VDC converter with a 0-50 VDC variable output receiving power from a 110 VAC plug 46. The DC positive terminal 48 leads via circuit line 50 to holder 20 which delivers a positive electric charge to work piece 16 which thus comprises the positive anode in the ECM system while tool 12 comprises the negative cathode via its connection to ground on motor 14. A digital DC current meter with IR data logger output 52 (available from Agilent Technologies) is connected to circuit line 50 to allow real-time monitoring of current delivered to work piece 16 via output screen 54 displaying a time/current graph of the ECM system in operation capable of displaying current charges in the 0.1 mA level with a maximum of 100 mA and a minimum of zero mA. Voltage output is likewise monitored on a digital volt meter 56 connected to power supply 44.
a-2d illustrate the preparation of an exemplary work piece 16 for machining in an ECM system. In this embodiment, the work piece is made of an electrically conductive material such as the aluminum-doped Silicon Carbide (SiC) which is the subject of commonly owned U.S. Pat. No. 6,616,890, the entire disclosure of which is incorporated herein by reference. It is understood, of course, that the invention may be used to machine any electrically conductive material including but not necessarily limited to all metals, alloys, and composites thereof. In this example, the work piece 16 is first shaped into a blank having a rectangular shape with a high aspect ratio intended to be machined into a surgical blade. SiC is known to be extremely corrosion resistant and durable and is therefore desirable as a material for manufacturing parts that would benefit from these properties such as the aforementioned surgical blades. It is also known, however, to be very difficult to machine with speed and accuracy and is not frequently used as the material of choice for this reason.
The '890 patent describes a method by which the SiC may be doped with aluminum or alumina to make it homogenously electrically conductive allowing it to be machined using the inventive ECM process. While the aluminum-doped SiC is indeed electrically conductive, the inventor herein has found that electrical resistance of a work piece increases with increases in aspect ratio and therefore the electrochemical machining results may be even further optimized by applying what is termed herein a “fugitive electrode” prior to machining. Although the example of work piece 16 herein includes such a fugitive electrode 60 as will be described, it is understood that other chosen work pieces may have sufficient conductivity due to their material composition and/or shape and not require a fugitive electrode.
In
In
As described above, the inventive ECM method includes the step of calculating the optimum inventive ECM operating voltage and current at which to machine the work piece which will vary depending on the material type and characteristics. This may be done by experimentation as illustrated in
The LMPV (Low Machining Potential Voltage) and HMPV (High Machining Potential Voltage) for a particular type of work piece material may be identified using the work piece in the potentiostat system of
As illustrated in
Referring again to
Thus, as voltage is increased, the current is monitored and the point “A” at which the current begins to rapidly increase defines the voltage at which the actual onset of material dissolution occurs (LMPV) for this particular work piece and the corresponding predetermined set of machining conditions. The voltage is then continually increased until a decrease in the rate of current increase is detected at point “C” which defines the maximum voltage (HMPV) that should be used during the inventive ECM process on the work piece without initiating excessive hydrolysis of the electrolyte. Thus, by using this method, both the approximate lowest acceptable voltage (LMPV) and approximate highest acceptable voltage (HMPV) used to machine the work piece is determined. Although it is preferred to use the approximate highest acceptable voltage (HMPV) to achieve the highest machining rate, any voltage between the onset of dissolution voltage and the HMPV is suitable. For higher surface area machining sites, gas generation may be limited by operating the inventive ECM system at a POV near the lower end of the range indicated by line “B” (See also
Once the HMPV has been determined, the inventive ECM process may be initiated using the HMVP as the starting voltage connected to the anode work piece 16 and the negative electrode connected to the cathode tool 12. The work piece 16 is moved into proximity of the planar side surface 12a of tool 12 at a first distance of at least approximately 50 microns. All power is turned on in system 10 and electrolyte is directed out of tube 30. The operator then advances the work piece 16 toward the tool surface 12a into the BIL region of about 0 to about 10 microns from the tool surface 12a defining the IEG 32. As seen in
To begin the machining of the work piece, the work piece 16 is moved to within 10 microns or less of the tool 12 setting the IEG at between about 0 and about 10 microns. An IEG range maintained between about 0 and 10 microns enables the controlled, direct perturbation of an explicitly targeted area of the BIL immediately adjacent to the work piece in a uniform and constant manner (defined by the inventor and referred to herein as “anisotropic machining” of the work piece as will be discussed further below). Machining without the use of a direct perturbing tool and/or an IEG greater than about 10 microns results in an “isotropic machining” of the work piece which is undesirable and the subject of much of the ECM prior art.
As seen in
In this embodiment of the invention, the hierarchy of material removal is as follows:
The above describes the basic chemistry as presently understood, it being noted that there may be other more complex molecular reactions and temporary partial reactions within the chemical dissolution framework taking place as the machining progresses down through the SiC matrix. Without the direct perturbation of the BIL in Step 2, the dissolution reaction quickly proceeds to a Diffusion Limited Reaction (DLR), limited by the electrical resistance encountered due to the unperturbed build up of SiOH in the BIL.
When voltage is applied to the inventive ECM system under a predetermined set of conditions (including selection of the anode, cathode, and electrolyte concentration such as in Example 1 below), the current initially surges beginning with the charging of the Helmholtz layer curve “L 1” which peaks at point “E” (within the first 100 milliseconds of power turn-on) and represents the initial forming of the above-described BIL. Anisotropic machining is best achieved during this initial peak in current, with this current being maintained through the direct perturbation of the BIL. Without direct perturbation of the BIL, the current rapidly settles down to a long term isotropic dissolution (the isotropic machining curve illustrated as section “D” in
When the BIL is directly perturbed as seen in curve section “F” in
A ceramic blade blank approximately (3 mm×7 mm×300 microns) was machined as described below using a work piece made of doped silicon carbide as described in commonly owned U.S. Pat. No. 6,616,890.
A first removable, electrically insulating coating comprising silicone material 58 was applied to the end 16a of the above-described blade blank 16 where machining was intended to occur. The blank 16 was then dipped into a Nickel plating bath to metalize the uncoated areas of the work piece to a plating thickness of 25-50μ. The plating was a continuous, lustrous coating of Ni 60. The plated work piece was then mounted in a brass holder 18 using an electrically conductive epoxy and then coated with a second removable, electrically insulating coating comprising wax 62 which covered the plating 60 as well as the first coating 58.
The brass holder 18 was then mounted into an electrically conductive steel holder of an X-Y mechanical, micrometer position controller 24 which itself was mounted to a tool platform 24 positioned adjacent the tool which comprised a rotatable wheel 12. The end 16a of the work piece to be machined was then moved into spaced relation to the planar side surface of the wheel 12a and an electrolyte feeding tube 30 was then positioned above and slightly behind the work piece with the tube outlet directed at the IEG 32 between the tool and work piece. The electrolyte feeding tube 30 was connected to an electrolyte reservoir 33 having a pump 36 and a dampener 38 operable to deliver the electrolyte through the feeding tube 30 to the IEG 32 space between the tool and work piece. Using the mechanical micrometer controls on the X-Y controller 24, the work piece was moved into a position where the IEG 32 between the work piece 16 and tool 12 was observed to be greater than 10 microns. The work piece was moved toward the spinning tool 12. The first 58 and second 62 coatings were then removed from the end 16a of the work piece where machining was to occur. This was done by passing the work piece end 16a over the spinning wheel 12 (unconnected to the negative terminal of the power source at this point) until the coatings were abrasively removed as depicted in 2f. A digital volt meter 56 and digital ampere (current) meter 52 (with data logging capability) was connected in the manner shown in the electrical schematic labeled
The electrolyte reservoir tank 33 was filled with a two molar concentration of NaCI, for example, in deionized micro filtered water and the electrolyte pump 36 was then turned on. The electrical power to the wheel tool was then turned on via switch 47 which started the wheel tool 12 spinning. A small wave of electrolyte flowing over the end of the work piece 31 was observed which confirmed the work piece 16 was in contact with the electrolyte flowing between the work piece 16 and spinning wheel 12 tool. The work piece end 16a was then moved greater than 10 microns away from the surface of the spinning wheel (cathode) 12. The power source 44 to the anode 16 and cathode 12 was turned on at the HMPV (approximately 6.44V) and the reading on the current meter 52 data logging display was observed at screen 54 and verified as reading in the approximately 0.001 A to 0.002 A range (see
Using the XY controllers (26, 28) on the micrometer 24, the work piece end 16a was slowly moved toward the spinning wheel 12 while assuring that the electrolyte 31 was wetting the wheel tool surface 12a at the location opposite the work piece end 16a. While advancing the work piece end 16a toward the spinning wheel surface 12a, the data logging current monitoring system 52 was monitored. As seen in
Once the blade blank end 16a was machined to the desired depth the controlled shutdown process of the inventive ECM system was initiated. First, the DC voltage was immediately turned off, and the blade blank 16 was quickly moved away from the spinning wheel 12. The spinning wheel 12 was turned off and then the electrolyte flow was turned off.
The blade blank 16 was then removed from the conductive work piece holder 20 and cleaned off to remove the previously applied coating 62 and plating materials 60. Cleaning of the end 16a can consist of cleaning sprays using electrolyte, deionized water, solvents or acid/base compounds of various mixtures and combinations for specific cleaning objectives. The machined blade blank 16 and machined end 16a was then examined under a microscope.
As seen in
The surface is planar as verified by Scanning Electron Microscopy (SEM) and measurement of the surface smoothness using a mechanical profilometer. The machined surface of the blade blank end 16a, a portion of which is depicted in
The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.
Number | Date | Country | |
---|---|---|---|
61353408 | Jun 2010 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 13702926 | Apr 2013 | US |
Child | 14796132 | US |