TITLE:

AN ELECTROCHEMICAL DISCHARGE MACHINE AND METHOD OF OPERATIONS THEREOF

TECHNICAL FIELD

The current invention generally relates to electrochemical discharge machines, and in particular to electrochemical discharge machines having an active feedback system and method of operation of the electrochemical discharge machines for precise machining in microscale.

BACKGROUND

Electrochemical discharge machining is a non-traditional machining process that can be used for fabricating or machining micro scale features such as micro-holes, micro-channels and 3- dimensional (3D) patterns on various materials. As the name implies, electrochemical discharge machining involves electrolysis in combination with electric discharge for the machining process. Since the material removal process in electrochemical discharge machining does not involve any sophisticated technologies such as laser beam or electron beam, it is more economical. From the materials perspective, it is versatile as both electrically conducting as well as non-conducting materials can be machined.

A typical electrochemical discharge machine (ECDM) includes an electrolytic cell having a small electrode (tool electrode) and a much larger counter-electrode (anode) in an electrolyte. On applying a pulsating voltage across the electrodes, a gas film forms at the smaller electrode upon electrolysis. When the pulsating voltage applied is above a critical value an electrochemical discharge occurs through the gas film formed at the smaller electrode through the electrolyte. The electrochemical discharge releases high thermal energy which can be utilized for material removal from a workpiece. However, the electrochemical discharge machining process is quite complex involving electrochemistry, heat transfer, melting and evacuation of material from the workpiece. For example, the gas film can be quite unstable as a result the electric discharge becomes unpredictable resulting in inconsistent machining.

Various feed and/or feedback mechanisms have been incorporated in ECDM to maintain a gap between the tool electrode and the workpiece for stable electrochemical discharge for machining. The known feed and/or feedback mechanisms, such as gravity feed, force feed and constant velocity feedback mechanisms, however have their own limitations. For example, in gravity feed mechanism, the gravitational force/pull of the tool electrode is considered to maintain the gap. As it is not an active feedback mechanism physical contact between the tool electrode and the workpiece cannot be avoided, which may cause wear and tear of the tool electrode. In force feed mechanism, a high- resolution force sensor detects the force of contact between the workpiece and the tool electrode. However, the force feed mechanism is a passive mechanism as the feedback provided is only upon contact. In constant velocity feedback mechanism, a constant velocity of the tool electrode is maintained with respect to the workpiece. As the constant velocity feed mechanism does not account for surface indentations it may at best work well for a completely flat surfaced workpiece. In pursuit of continuous improvement in ECDM system, a feedback mechanism that addresses some of the shortcomings in the known art is desirable.

Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.

SUMMARY

In one embodiment of the invention, an electrochemical discharge machining system is provided. The electrochemical discharge machining system comprises an electrolytic cell, wherein the electrolytic cell comprises an electrolytic bath mounted on a base comprising an electrolyte. The electrolytic cell further comprises an anode immersed in the electrolyte and a tool electrode attached to a movable arm and operable to move in and out of the electrolytic bath. A workpiece to be machined is in contact with the electrolytic bath and positioned between the anode and the tool electrode and is maintained at a pre-determined gap with the tool electrode. The electrochemical discharge machining system further comprises an active feedback system, wherein the active feedback system comprises a capacitance sensor, wherein the capacitance sensor measures a change in capacitance between the tool electrode and the anode, or between the tool electrode and the electrolyte, or both to provide a capacitance feedback signal. The active feedback system further comprises an analogue digital convertor, wherein the analogue digital convertor processes the capacitance feedback signal from the capacitance sensor and provides an active feedback signal. The change in capacitance is correlated to the pre-determined gap. The electrochemical discharge machining system further comprises a driver system, wherein the driver system comprises a microprocessor. The microprocessor processes an input file and provides an input signal, wherein the input file is based on a pattern to be machined on the workpiece. The electrochemical discharge machining system further comprises a digital controller, wherein the digital controller processes the input signal and the active feedback signal from the active feedback system and provides a driver signal to drive the movable arm, or the base or both so as to maintain the pre-determined gap between the tool electrode and the workpiece.

In yet another embodiment, a method for operating an electrochemical discharge machining system is provided. The method comprises providing an electrolytic cell, wherein the electrolytic cell comprises an electrolytic bath mounted on a base comprising an electrolyte, an anode immersed in the electrolyte, a tool electrode attached to a movable arm and operable to move in and out of the electrolytic bath and a workpiece to be machined in contact with the electrolytic bath, wherein a distance between the workpiece and the tool electrode is maintained at a pre-determined gap. The method further comprises measuring a change in capacitance between the tool electrode and the anode, or between the tool electrode and the electrolyte, or both to generate an active feedback signal, wherein the change in capacitance is correlated to the pre-determined gap. The method further comprises generating a driver signal from the active feedback signal and an input signal, wherein the driver signal drives the movable arm, or the base or both so as to maintain the pre -determined gap, and wherein the input signal is based on an input file comprising a pattern to be machined on the workpiece.

These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF DRAWINGS

The advantages of embodiments of the disclosure may be more readily ascertained from the description of certain examples of the embodiments of the disclosure when read in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an exemplary electrochemical discharge machining system in accordance with embodiments of the present invention;

FIG. 2 is a flowchart of a method of operating the electrochemical discharge machining system of FIG.l;

FIG. 3 is a schematic diagram depicting cross-sectional side views of machining zones of electrolytic cells having differing distance between a tool electrode and a workpiece; and FIG. 4 is a plot of capacitance against depth or recess in a workpiece machined by a tool electrode.

Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.

DETAILED DESCRIPTION

The following description and example illustrate some exemplary embodiments of the disclosed invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present invention.

The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.

As used herein, the term “feedback mechanism” refers to a mechanism in-built in the ECDM system whereby feedback from steps performed as part of an operation is considered while conducting later steps in the operation. The feedback mechanism is also otherwise termed as “feedback system”.

As used herein, the term “active feedback mechanism” refers to a mechanism in-built in the ECDM system whereby real-time feedback from a prior step performed as part of an operation is considered while conducting the next succeeding step in the operation. The active feedback mechanism, is also otherwise termed as “active feedback system”.

FIG. 1 is an electrochemical discharge machining system 100 in accordance with an exemplary embodiment of the invention. The electrochemical discharge machining system (ECDMS) 100 comprises an electrolytic cell 102, an active feedback system 104 and a driver system 106.

Referring to FIG. 1, the electrolytic cell 102 comprises an electrolytic bath 108 in which an electrolyte 110 is provided. In one embodiment, walls 108a of the electrolytic bath 108 is made of acrylic. Without any limitation, any suitable non-conducting, thermally stable material may be utilized to form the walls of the electrolytic bath 108. The electrolytic bath 108 is mounted on a base 112. The base 112 can be a fixed base or a movable base. The base 112, if movable, is capable of translational motion in X-Y axes plane.

The electrolyte 110 comprises an aqueous alkaline solution. In one embodiment, the aqueous alkaline solution is selected from the group consisting of potassium hydroxide, magnesium hydroxide, sodium hydroxide, calcium hydroxide and any combinations thereof. In preferred embodiments, the electrolyte 110 is sodium hydroxide, potassium hydroxide or a mixture of both. The concentration or molarity of the electrolyte 110 may be suitably adjusted for optimizing machining condition and/or machining parameters.

The electrolytic cell 102 includes an anode 116. The anode 116 may be completely immersed or partially immersed (as shown in FIG.l) in the electrolyte 110, of the electrolytic bath 108. In one embodiment, the anode 116 is a rectangular strip immersed in the electrolyte 110. The anode 116 may be of any shape such as rod-shaped, or rectangular strip. In one embodiment, the anode 116 comprises graphite, stainless steel, gold, platinum, silver or any combinations thereof. In one preferred embodiment, the anode 116 is composed of stainless steel.

The electrolytic cell 102 comprises a tool electrode 118 attached to a movable arm 120. The tool electrode 118 typically have lower surface area than the anode 116. A smaller surface area correspondingly provides for discharge at lower voltage. In one embodiment, a wire-shape is preferred, although other shapes having smaller surface area may be utilized. The tool electrode 118 comprises copper, tungsten, tungsten carbide, brass, bronze, stainless steel, titanium, aluminum, graphite or any combinations thereof. In a preferred embodiment, the tool electrode 118 comprises tungsten-carbide.

The tool electrode 118 is operable to move in and out of the electrolytic bath 108 through controlled motion of the movable arm 120 to which it is attached to thus making a translational motion in Z-axis plane. The controlled motion of the movable arm 120 can be controlled by automated control system, as in the present disclosure. In one embodiment, the movable arm 120 forms part of a computer numerical controller (CNC) machine capable of translational motion in 3-dimensional X- Y-Z axes plane. In preferred embodiments, the movable arm 120 is a rotating spindle to which the tool electrode 118 is attached to. The rotating spindle translates its rotating motion to the tool electrode 118 by which any irregularities on the electrode surface is nullified during electrochemical discharge. In a preferred embodiment, the movable arm 120 is a rotating spindle and operable to move in Z-axis plane. A workpiece 122 to be machined is placed in the electrolytic bath 108, and positioned between the anode 116 and the tool electrode 118. The workpiece 122 may be placed in the electrolytic bath, immersed or partially immersed in the electrolyte 110 depending on a part of the workpiece 122 to be machined. As will be appreciated only the part of the workpiece 122 in contact or touch with the electrolyte 110 will be machined. Optionally, the workpiece 122 may be placed over a magnetic strip 124 to minimize any inductance effect and influence the electric discharge to be aligned with the magnetic field of the magnetic strip 124.

In one embodiment, the electrolyte 110 is in constant circulation, circulation of the electrolyte for instance helps in removing debris from the workpiece 122 during machining. The electrolyte 110 may be flowed into the electrolytic bath 108 through an inlet (not shown) placed close to the tool electrode 118. As will be appreciated, circulation of the electrolyte 110 may require additional components such as circulation pump, storage means and the like and forms part of the ECDMS 100.

The workpiece 122 can be an electrically conducting material such as a metal. In certain other embodiments, the workpiece 122 is semi-conducting or non-conducting. Non-limiting examples of the workpiece 122 comprises a metal, silicon, silicon carbide, glass, ceramic, aluminum oxide, silicon nitride, quartz, polymer, polymethyl methacrylate (PMMA), polycarbonate (PC) or any combinations thereof.

A gap 126 between the workpiece 122 and the tool electrode 118 is to be maintained at a predetermined gap. The gap 126 is critical in obtaining a stable electrochemical discharge whereby quality of machining of the workpiece 122 using ECDMS 100 is achieved. As used herein, the term “gap” refers to the closest distance between the tool electrode 118 and the workpiece 122 at a location of machining within the electrolytic bath 108. The pre-determined gap, as used herein, refers to the closest machining distance between the tool electrode 118 and the workpiece 122 which is determined prior to each step of operation of the ECDMS 100, and will be discussed in detail with reference to the method of operation as shown in FIG.2. In an embodiment, where the electrolytic bath 108 is mounted on the base 112 which is movable, the translational motion of the base 112 in X-Y axes plane is controlled along with the movable arm 120 to adjust the pre-determined gap between the tool electrode 118 and the workpiece 122.

The electrolytic cell 102 comprises a power source 128. The power source 128 could be an alternating current (AC) or a direct current (DC) capable of providing a pulsating voltage across the tool electrode 118 and the anode 116 to initiate the electrochemical discharge process. The electrochemical discharge machining system 100 comprises the active feedback system 104. The active feedback system 104 comprises a capacitance sensor 130 and an analogue digital convertor 132.

The capacitance sensor 130 is an electronic device that utilizes the property of capacitance to detect the presence or absence of an object, or a position of the object without being directly in contact. The capacitance sensor 130 works on the principle that the capacitance between two parallel capacitor plates is inversely proportional to the distance between them. During machining, a distance between the tool electrode 118 and the workpiece 122 varies and the gap between them can be correlated to the change in capacitance.

A typical capacitance sensor comprises an excitation port that supplies a voltage and a sensing port which senses a change in capacitance with respect to the voltage applied. By placing the ports suitably, a change in capacitance can be measured which corresponds to the distance between the two ports. The capacitance sensor 130 measures the change in capacitance between the tool electrode 118 and the anode 116, or between the tool electrode 118 and the electrolyte 110, or both. As it is anticipated, that the change in capacitance measured at the anode 116, or the electrolyte 110 to be similar due to them being in electrical contact, one of the ports is placed in proximity to the anode 116, or the electrolyte 110 and the other port at the tool electrode 118. The change in capacitance measured by the capacitance sensor 130 is correlated to the gap 126 between the tool electrode 118 and the workpiece 122. In one embodiment, the excitation port is placed in proximity to the tool electrode 118 and the sensing port is placed in proximity to the electrolytic bath 108 comprising the electrolyte 110.

In one embodiment, a constant capacitance is applied in parallel between the tool electrode 118 and the anode 116, or between the tool electrode 118 and the electrolyte 110, or both and a change in capacitance is measured with reference to the constant capacitance. The capacitance sensor 130 provides a capacitance feedback signal based on the measured change in capacitance. The capacitance feedback signal from the capacitance sensor 130 is in analogue format.

The analogue digital convertor 132 processes the capacitance feedback signal from the capacitance sensor 130 and provides an active feedback signal. As will be appreciated, the capacitance feedback signal from the capacitance sensor 130 comprises raw data or is in analogue format which is converted to a digital signal format by the analogue digital convertor 132. The digital signal format can advantageously be read by other components of the ECDMS 100. The digital signal may be subjected to further signal processing such as noise reduction, amplification of the signal and the like, as known in the art. In one embodiment, the analogue digital convertor 132 is a capacitance to digital converter (CDC). Suitable CDCs include AD714x, AD715x, and AD774x families from Analog Devices; FDC 2xlx from Texas Instruments; or PcapOx from ScioSense.

The ECDMS 100 comprises the driver system 106. The driver system 106 comprises a microprocessor 134 and a digital controller 136.

The microprocessor 134 processes an input file to provide an input signal. As will be appreciated, the microprocessor 134 may include various other components, for example, power supply, required software and other associated components required to process the input file.

The input file, in one embodiment, is based on a pattern to be machined on the workpiece 122. The input file may additionally include data on the workpiece 122 such as its material properties; machining parameters such as machining speed; closest distance to be maintained between the workpiece 122 and the tool electrode 118 given the material properties and the pattern to be machined; for machining the pattern on the workpiece 122. With reference to machining speed, for instance, a material having high shear modulus or “hardness” such as quartz may require a slower machining speed than a material that has low shear modulus or less hardness, such as PMMA. Further, the machining speed may also depend on the pattern to be machined. Non-limiting examples of machining parameters or machining conditions include terminal voltage, frequency of pulse, duty cycle of the pulse, concentration of the electrolyte 110 and the rotational speed of the spindle to which the tool electrode 118 is attached to.

The digital controller 136 processes the input signal and the active feedback signal from the active feedback system and provides a driver signal to drive the movable arm 120 on which the tool electrode 118 is attached to, and/or the base 112 such that the pre-determined gap between the tool electrode 118 and the workpiece 122 is maintained. In another embodiment, the digital controller 136 sends the driver signal to the movable arm 120 for Z-axis plane motion and to the base 112 on which the electrolytic bath 108 is placed for the X-Y axes plane motion such that the pre-determined gap between the tool electrode 118 and the workpiece 122 is maintained. By maintaining the predetermined gap during the entire machining process, the electrochemical discharge released is utilized for material removal from the workpiece 122 to form the pattern. Additionally, the digital controller 136 may send the driver signal to other components of ECDMS 100 such as electrolyte circulation pump, rotating spindle and the like to maintain the pre-determined gap.

Capacitance measurement between two objects is technically possible only if a dielectric medium separates the two objects. In ECDMS 100, even though the tool electrode 118 and the workpiece 122 are in contact with the electrically conducting electrolyte 110, a dielectric medium, (i.e., the dielectric film of hydrogen gas at the tool electrode 118), is formed due to electrolysis as a result of which the tool electrode 118 and the workpiece 122 acts as capacitors. As compared to prior art systems, the ECDMS 100 of the present invention is advantageous as it provides real-time active feedback based on which the motion of tool electrode 118, or the electrolytic bath 110, or both may be controlled whereby the pre-determined gap is maintained.

The active feedback system 104 of the present disclosure avoids any physical contact between the workpiece 122 and the tool electrode 118 thus minimizing damage to the tool electrode 118 and resulting in precise machining of the workpiece 122. As the feedback system 104 relies on change in capacitance, any wear and tear of the tool electrode 118 is accounted for while determining the predetermined gap. Thus, the active feedback system 104 helps the digital controller 136 to compensate for the tool loss occurring due to long-periods of machining. The active feedback system 104 of the present invention is particularly advantageous as it provides precise positioning of the tool electrode 118 with respect to the workpiece 122 at the machining location due to real-time determination of the pre-determined gap. The quality of machining by ECDMS 100 of present disclosure is better in terms of surface roughness or “over cut” The surface roughness of the workpiece thus machined, in one embodiment, is less than 500 nm, more preferably in the range of 500 nm to 200 nm, and most preferably less than 200 nm. The improved machining achievable by ECDMS 100 minimizes “over cut” and is particularly helpful for high aspect ratio features whereby uniform dimension is maintained along the machined feature. The active feedback system 104 avoids contact between the workpiece 122 and the tool electrode 118 by adjusting the terminal voltage, in one instance, to prevent over cut. The electrochemical discharge machining system 100 of the present disclosure is capable of patterning features having an aspect ratio in the range of 1 :0.5 to 1:10. The aspect ratio can be defined as the ratio of width to height of a specific feature in the pattern. The real-time information of the pre-determined gap is helpful while machining through-hole patterns and to precisely know when it is complete.

FIG. 2 is a flowchart 200 of a method for operating the electrochemical discharge machining system 100 of the present invention. Referring to FIG.2, the method comprises providing the electrolytic cell 102 (as shown in FIG.l), at step 210. The electrolytic cell 102 comprises the electrolytic bath 108 mounted on the base 112 comprising the electrolyte 110, the anode 116, the tool electrode 118 attached to the movable arm 120, and the workpiece 122 in the electrolytic bath 108, wherein a distance between the workpiece 122 and the tool electrode 118 is maintained at a predetermined gap. On applying current through power supply 128, electrolysis occurs with the release of hydrogen gas bubbles at the tool electrode 118 and oxygen gas at the anode 116. Upon increasing current density, more and more hydrogen gas bubbles are formed which coalesce to form a gas film (dielectric film) over the tool electrode 118 which restricts the contact of the tool electrode 118 and the electrolyte 110 and eventually resulting in electrochemical discharge. To utilize the electrochemical discharge for machining, the workpiece 122, and the tool electrode 118 is to be maintained at the pre-determined gap.

At step 220, a change in capacitance between the tool electrode 118 and the anode 116, or between the tool electrode 118 and the electrolyte 110, or both is measured to generate an active feedback signal. The active feedback signal is generated by the the active feedback system 104, wherein the active feedback system 104 comprises the capacitance sensor 130 and the analogue digital convertor 132. As discussed previously, the capacitance sensor 130 measures a change in capacitance that can be correlated to the changing distance (gap) between the tool electrode 118 and the workpiece 122 during machining.

At step 230, a driver signal is generated by processing the active feedback signal and an input signal to drive the movable arm 120, or the base 112 or both. The driver signal is generated by the driver system 106 comprising the microprocessor 134 and the digital controller 136. The microprocessor 134 generates the input signal, which is based on the input file comprising the pattern to be machined on the workpiece 122. The input signal factors in material properties, and the pattern to be machined on the workpiece 122 amongst other parameters, as discussed earlier. The microprocessor 134 sends the input signal to the digital controller 136. The digital controller 136 processes the input signal and the active feedback signal from the active feedback system 104 and translates it to the driver signal. The driver signal is read by the movable arm 120, or the base 112 or both to maintain the pre-determined gap. Additionally, the driver signal may be read by other components of ECDM 100 such as a rotating spindle, electrolyte circulation pumps to perform machining while maintaining the pre-determined gap.

FIG.3 is a schematic diagram 300 depicting cross-sectional side views of machining zones of electrolytic cells 302, 304, 306, 308, 310 and 312 having differing distance or gap between a tool electrode 320 and a workpiece 322. The differing distance or the gap can be correlated to the change in capacitance and the working of the ECDMS 100 of FIG.l can be described with reference to FIG. 3.

The electrolytic cells 302 to 312 includes an electrolytic bath 324 comprising an electrolyte

326, and an anode 328. At the onset of machining, a gap 330a exists between the tool electrode 320 and the workpiece 322 in the electrolytic cell 302, and the tool electrode 320 is not in contact with the electrolyte 326 and this position can be considered as home position. The next step in the operation is depicted by electrolytic cell 304.

Referring to FIG. 3, at the home position as shown by electrolytic cell 302, the digital controller 136 (of FIG.l) receives the input signal from the microprocessor 134 (of FIG.l) and the active feedback signal from the analogue digital converter 132 (of FIG.1 ) to generate the driver signal. The driver signal drives the movable arm 120 (of FIG.l), or the base 112 (of FIG.l) or both to maintain a pre-determined gap to be maintained at the next step of operation as shown by electrolytic cell 304, of FIG.3, and the process is continued to form the pattern on the workpiece 322.

The electrolytic cell 304 includes a gap 330b between the tool electrode 320 and the workpiece 322. In electrolytic cell 306, a gap 330c exists which is ideal for electrochemical discharge essential for machining. The electrolytic cells 308, 310 and 312 have gaps 330d, 330e and 330f, respectively. The gap 330d of the electrolytic cell 308 corresponds to a pattern having low aspect ratio compared to the gap 330e of the electrolytic cell 310. The gap at any time is maintained between 20 microns (pm) and 30 pm.

FIG. 4 is a plot 400 of capacitance with respect to depth. FIG. 4 in conjunction with FIGG further illustrates how capacitance measurement can be correlated to the gap between a tool electrode and workpiece of ECDMS. The depth, as used herein, refers to a recess formed on machining of the workpiece 322 by the tool electrode 320. In other words, it is the distance or length traversed by the tool electrode 320 within the workpiece 322. The depth can be measured by measuring a distance from a top surface of the workpiece 322 to the machined part or the position of the tool electrode 320 within the workpiece 322. The capacitance versus depth corresponding to gaps 330a-f (of FIGG) is plotted as shown in FIGG. A point 402 on the plot 400 corresponds to when there is no change in capacitance i.e., while machining a pattern having constant depth.

As shown in FIG. 4, the capacitance at the home position is the lowest. As the gap decreases the capacitance increases drastically such that machining occurs as shown by electrolytic cells 302 to 306. By considering, the capacitance at each “gap” position, the pre-determined gap can be arrived upon to form the pattern on the workpiece.

It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms "including" is used as the plain-English equivalent of the respective term "comprising" respectively.