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Electrochemical machining tool assemblyRelated Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrolytic, Object Protection, Rotary, With Current ControlElectrochemical machining tool assembly description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070175751, Electrochemical machining tool assembly. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] The invention relates generally to electrochemical machining and, more particularly, to monitoring interelectrode gap size and workpiece thickness during electrochemical machining operations. [0002] Electrochemical machining (ECM) is a commonly used method of machining electrically conductive workpieces with one or more electrically conductive tools. During machining, a tool is located relative to the workpiece, such that a gap is defined therebetween. The gap is filled with a pressurized, flowing, aqueous electrolyte, such as a sodium nitrate aqueous solution. A direct current electrical potential is established between the tool and the workpiece to cause controlled deplating of the electrically conductive workpiece. The deplating action takes place in an electrolytic cell formed by the negatively charged electrode (cathode) and the positively charged workpiece (anode) separated by the flowing electrolyte. The deplated material is removed from the gap by the flowing electrolyte, which also removes heat formed by the chemical reaction. The anodic workpiece generally assumes a contour that matches that of the cathodic tool. [0003] For a given tooling geometry, dimensional accuracy of the workpiece is primarily determined by the gap distribution. The gap size should be maintained at a proper range. Too small a gap, such as less than 100 micrometers in a standard ECM operation, could lead to arcing or short-circuiting between the tool and the workpiece. Too large a gap could lead to excessive gap variation, as well as reduction in the machining rate. Monitoring and controlling the gap size between the tool and the workpiece, or directly monitoring the workpiece thickness, is thus important for ECM tolerance control. For example, in machining a turbine compressor blade, the blade thickness should be directly measured during machining, so that a desired thickness can be obtained. [0004] Lack of suitable means for sensing gap size or workpiece thickness may hinder ECM accuracy control. Without such means, many rounds of costly trial-and-error experiments must be run to obtain the gap size changes that occur during the machining process. Gap size can change significantly during the machining process, partly because conductivity of the electrolyte may change in the gap due to heating or gas bubble generation on the tool surface. Variation and inaccuracy in tool feed rate and tool positioning can also contribute to changes in gap size and workpiece thickness. In-process gap detection or workpiece thickness detection is thus important for improving ECM process control. [0005] Recently, an approach for the in-situ measurement of gap size and workpiece thickness has been proposed for ECM process control. In this approach, an ultrasonic sensor is embedded in the ECM tool, and the gap size and workpiece thickness are obtained from ultrasonic time-of-flight measurements. The sensor generates an ultrasonic wave that propagates through the tooling, through the electrolyte in the gap and then through the workpiece. The sensor will receive reflections from the surface of the tool, the front side of the workpiece, and the back side of the workpiece. By comparing the time at which each of these reflected signals is received, the gap size and workpiece thickness can be determined. [0006] However, during conventional ECM operations with a continuous DC voltage, gas bubbles are constantly generated at the cathode, which significantly attenuate the ultrasonic signal propagation through the electrolyte when the ECM voltage exceeds a certain level. Generally speaking, the higher the electrolyte flow rate/inlet pressure, the higher the ECM voltage level may be, while still allowing the ultrasonic measurements to function properly. For example, for an inlet pressure of 150 psi for machining a two square inch sample, the permissible ECM voltage level is about eight volts (8 V). However, ECM voltages are typically in a range of about twelve to about twenty volts (12-20V). In commonly assigned, copending U.S. patent application Ser. No. 09/818,874, entitled "Electrochemical Machining Tool Assembly and Method of Monitoring Electrochemical Machining," it is suggested that the voltage power supply be reduced or regulated to minimize gas bubble generation. Similarly, in commonly assigned, U.S. Pat. No. 6,355,156, Li et al, entitled "Method of Monitoring Electrochemical Machining Process and Tool Assembly Therefor," it is suggested that the DC power supply may be turned off for a brief period of time, such as for the time interval used in pulsed electrochemical machining, so as to minimize the generation of gas bubbles for more accurate measurements. However, adjusting the ECM voltage could potentially compromise ECM machining quality. [0007] Accordingly, it would be desirable to reduce gas bubble generation to improve ultrasonic monitoring of ECM machining operations without compromising ECM machining quality. BRIEF DESCRIPTION [0008] Briefly, in accordance with one embodiment of the present invention, a method of monitoring machining in an electrochemical machining tool assembly is described. The assembly has at least one electrode arranged across a gap from a workpiece. The electrode is energized by application of a potential difference .DELTA.V between the electrode and the workpiece. The method includes exciting at least one ultrasonic sensor to direct an ultrasonic wave toward a surface of the electrode and receiving a reflected ultrasonic wave from the surface of the electrode using the ultrasonic sensor. The reflected ultrasonic wave includes a number of reflected waves from the surface of the electrode and from a surface of the workpiece. The method further includes delaying the excitation of the ultrasonic sensor a dwell time T.sub.d after the occurrence of a reduction of the potential difference .DELTA.V across the electrode and the workpiece. [0009] A method of monitoring machining is also described for a pulsed electrochemical machining tool assembly, where the electrode is periodically energized by application of a number of pulses. For this method, the excitation of the ultrasonic sensor is delayed a dwell time T.sub.d after a transition from a pulse-on state to a pulse-off state. [0010] An electrochemical machining method for machining a workpiece is also described. The electrochemical machining method includes energizing at least one electrode positioned in proximity to the workpiece. The electrode and the workpiece are separated by a gap. The electrochemical machining method further includes flowing an electrolyte through the gap, flushing the electrolyte from the gap, feeding the electrode toward the workpiece, and monitoring at least one of the gap and the workpiece using the ultrasonic sensor. The monitoring includes exciting the ultrasonic sensor to direct an ultrasonic wave toward a surface of the electrode and receiving a reflected ultrasonic wave from the surface of the electrode using the ultrasonic sensor. The reflected ultrasonic wave includes a number of reflected waves from the surface of the electrode and from the surface of the workpiece. The monitoring further includes delaying the excitation of the ultrasonic sensor a dwell time T.sub.d after a reduction of the potential difference .DELTA.V across the electrode and the workpiece occurs. [0011] An electrochemical machining tool assembly is also described. The electrochemical machining tool assembly includes at least one electrode adapted to machine a workpiece across a gap upon application of a potential difference .DELTA.V across the electrode and the workpiece, means for flowing an electrolyte through the gap and for flushing the electrolyte from the gap, means for feeding the electrode toward the workpiece, and at least one ultrasonic sensor adapted to direct an ultrasonic wave toward a surface of the electrode and to receive a reflected ultrasonic wave from the surface of electrode. The reflected ultrasonic wave includes a number of reflected waves from the surface of the electrode and from a surface of the workpiece. The electrochemical machining tool assembly further includes a delay generator adapted to delay the excitation of the ultrasonic sensor a dwell time T.sub.d after a reduction of the potential difference .DELTA.V across the electrode and the workpiece occurs. BRIEF DESCRIPTION OF THE DRAWINGS [0012] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: [0013] FIG. 1 illustrates an electrochemical machining tool assembly embodiment of the invention; [0014] FIG. 2 is a sectional view of the electrochemical machining tool assembly of FIG. 1; [0015] FIG. 3 is an exemplary ultrasonic measurement timing diagram for the electrochemical machining tool assembly of FIGS. 1 and 2; and [0016] FIG. 4 is an exemplary measurement system block diagram of an electrochemical tool assembly embodiment of the invention employing one electrode. DETAILED DESCRIPTION [0017] An electrochemical machining tool assembly 10 embodiment of the invention is described with reference to FIGS. 1-4. As shown in FIGS. 1 and 4, the electrochemical machining (ECM) tool assembly 10 includes at least one electrode 26 adapted to machine a workpiece 12 across a gap 34 upon application of a potential difference .DELTA.V across the electrode 26 and the workpiece. For the example shown in FIG. 1, the workpiece 12 is a rotor blade with a shank portion 14 and an airfoil portion 16. The airfoil 16 has a concave pressure side 18 and a convex suction side 20 joined together at a leading edge 22 and a trailing edge 24. This rotor blade example is purely exemplary, and the ECM tool assembly 10 is equally applicable to other workpieces as well. For the example shown in FIG. 4, the ECM tool assembly 10 has one electrode 26. For the example shown in FIG. 1, the ECM tool assembly 10 includes two electrodes 26, 28 arranged on opposite sides of the workpiece 12. The electrodes 26, 28 are shaped to electrochemically machine the workpiece 12 into the desired shape. Each of the electrodes 26, 28 defines a respective gap 34, 36 with respect to the workpiece 12. For the example shown in FIG. 1, the first electrode 26 has a convex machining surface 30 facing the workpiece 12, and the second electrode 28 has a concave machining surface 32 facing the workpiece 12. Depending on the workpiece 12 being machined, the ECM tool assembly 10 may have more or less electrodes than the example shown in FIG. 2. [0018] The ECM tool assembly 10 also includes means for flowing an electrolyte 38 through the gap 34 and for flushing the electrolyte from the gap 34, for example, as indicated by arrows A in FIG. 1. For the example of FIGS. 1 and 2, the electrolyte flows through and is flushed from gaps 34, 36 in the direction of arrows A. Means for flowing and flushing the electrolyte 38 are known and one example is a pump system 130, which is indicated schematically in FIG. 2. It should be noted that Arrows A indicate only one possible fluid flow direction for the ECM tool assembly 10. Also, to contain the electrolyte 38, the electrode(s) 26 and workpiece 12 may be disposed in a receptacle (not shown) filled with the electrolyte 38. [0019] The ECM tool assembly 10 also includes means for feeding the at least one electrode 26 toward the workpiece 12. For the example shown in FIGS. 1 and 2, the two electrodes 26, 28 are mounted on opposite sides of the workpiece 12, to be movable toward and away from the workpiece 12 along the direction indicated by arrows F. Means for moving the electrode 26 are well known and one example is a typical servodrive system 140 that uses an AC servo motor to drive a ballscrew mechanism to move the electrode, which is schematically indicated in FIG. 2. Movement of the electrode 26 may be controlled by a motion controller in response to feedback data and/or by an operator. [0020] As indicated in FIG. 2, the ECM tool assembly 10 also includes at least one ultrasonic sensor 42, for example an ultrasonic transducer 42, which is adapted to direct an ultrasonic wave toward a surface 102 of the electrode and to receive a reflected ultrasonic wave from the surface of electrode. The reflected ultrasonic wave comprises a number of reflected waves from the surface 102 of the electrode 26 and from a surface 104 of the workpiece 12. For the example of FIG. 2, the sensor 42 is embedded in the electrode 26. Alternatively, the sensor 42 may be positioned on or near an exterior surface of the electrode 26, for example on or near exterior surface 108 of the electrode 26. As indicated in FIG. 1, for example, the ECM tool assembly 10 also includes a delay generator 110, which is adapted to delay the excitation of ultrasonic sensor a dwell time T.sub.d after a reduction of the potential difference .DELTA.V across the electrode 26 and the workpiece 12 occurs. An exemplary dwell time T.sub.d is in a range of about seven milliseconds (7 ms) to about 15 milliseconds (15 ms). For one embodiment, the delay generator 110 is adapted to adjust the dwell time T.sub.d, for example to shorten or lengthen the dwell time T.sub.d. Beneficially, by delaying the excitation of the ultrasonic sensor 42 by a dwell time T.sub.d, excitation of the ultrasonic sensor 42 may be synchronized to the machining cycle, such that the ultrasonic sensor is used during machining off-times, that is during portions of the machining cycle in which the machining potential across the electrode 26 and workpiece 12 is either off or reduced. This helps clear the bubbles and reduce electromagnetic interference with the measurement. Continue reading about Electrochemical machining tool assembly... 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