Abstract:
An electrochemical machining tool assembly includes first and second tools that are spaced apart from one another so that a workpiece can be located therebetween. The assembly also includes a first ultrasonic transducer mounted in the first tool and a second ultrasonic transducer mounted in the second tool. The electrochemical machining process is monitored by generating ultrasonic waves with the first and second ultrasonic transducers, and then detecting the arrival times, at the first and second ultrasonic transducers, of certain reflections of the ultrasonic waves. The detected arrival times are then used to calculate parameters such as the gap sizes between the first and second tools and a workpiece situated therebetween and the thickness of the workpiece.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to electrochemical machining and more particularly to monitoring gap sizes and workpiece thicknesses during electrochemical machining operations. 
   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 sodium nitrate. 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. 
   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 non-uniform machining as well as a reduction in machining rate. Monitoring and controlling the gap size between the tool and the workpiece is thus important for ECM tolerance control. In addition, in process measurement of workpiece dimensions is important in many operations. For example, in machining a rotor blade for a gas turbine engine, the blade thickness should be directly measured during machining so that the desired blade thickness is obtained. 
   Lack of suitable means for sensing gap sizes and workpiece dimensions 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 the conductivity of the electrolyte may change in the gap due to Joule 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 detection of gap sizes and workpiece dimensions is thus needed to improve ECM process control. 
   Several types of ECM sensors have been developed over the years. An ECM control method using ultrasonic sensors is described in U.S. Pat. No. 5,672,263 issued Sep. 30, 1997 to David A. Raulerson et al. This control method is used in connection with electrochemical machining of large cylindrical workpieces, where a machining head is located outside of the workpiece for machining its outer surface. One or more ultrasonic sensors are located within the cylindrical workpiece for monitoring workpiece wall thickness during the machining operation. Movement of the ultrasonic sensors relative to the workpiece results in significant signal noise and inaccuracy because of local workpiece metallurgical inhomogeneity. Furthermore, the Raulerson et al approach is limited to workpieces having a large, inner opening for containing the sensors and storing the fluid through which the ultrasonic waves propagate. By way of example, the Raulerson et al approach cannot be used while electrochemical machining small, compact workpieces, such as rotor blades used in gas turbine engines, because ultrasonic sensors cannot be placed inside such workpieces. In addition, Raulerson et al does not measure gap size. It is intended to only measure a workpiece wall thickness near a wide open space. 
   Recently, an approach to in-situ measurement of gap size and workpiece thickness has been proposed for ECM process control. In this approach, a single 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. 
   In most situations, this approach works quite well. However, with some workpiece materials, material impedance mismatches between the tool, solution and workpiece can be extremely large. Due to such large impedance mismatches, and signal attenuation in the materials, the ultrasonic signals transmitted through the workpiece and then reflected from the back side of the workpiece can become extremely small and difficult to detect. Furthermore, due to the inhomogeneity of some workpiece materials, the acoustic velocity of these materials could vary from location to location, thereby reducing the accuracy of the thickness measurements. Accordingly, it would be desirable to have an approach for the in-situ measurement of gap size and workpiece thickness that is independent of workpiece material. 
   BRIEF SUMMARY OF THE INVENTION 
   The above-mentioned need is met by the present invention, which provides an ECM tool assembly having first and second tools spaced apart from one another so that a workpiece can be located therebetween. The assembly also includes a first ultrasonic transducer mounted in the first tool and a second ultrasonic transducer mounted in the second tool. The ECM process is monitored by generating ultrasonic waves with the first and second ultrasonic transducers, and then detecting the arrival times, at the first and second ultrasonic transducers, of certain reflections of the ultrasonic waves. The detected arrival times are then used to calculate parameters such as the gap sizes between the first and second tools and a workpiece situated therebetween and the thickness of the workpiece. 
   The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is a perspective view of an electrochemical machining tool assembly having means for in-situ measurement of gap size and workpiece thickness. 
       FIG. 2  is a sectional view of the electrochemical machining tool assembly taken along line  2 — 2  of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  show an electrochemical machining (ECM) tool assembly  10  used for machining a workpiece  12 . As shown in the Figures, the workpiece  12  is a rotor blade of the type used in a gas turbine engine (such as a compressor blade or a turbine blade). The rotor blade  12  includes a shank portion  14  for mounting the blade to a rotor disk and an airfoil  16  that either adds work (in the case of a compressor blade) to or extracts work (in the case of a turbine blade) from the gas flow within the gas turbine engine. 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 . The airfoil  16  extends chordwise from the leading edge  22  to the trailing edge  24  and spanwise from the shank portion  12  to its outer tip. It should be noted that a rotor blade workpiece is used here only as one example to facilitate description of the present invention. The present invention is also applicable to other types of workpieces. 
   The ECM tool assembly  10  includes first and second tools  26  and  28  arranged on opposite sides of the workpiece  12 . Each tool  26 ,  28  has a suitable configuration to electrochemically machine the workpiece  12  into the desired shape. The first tool  26  has a convex cutting surface  30  formed therein and facing toward the workpiece  12  for machining the concave side  18  thereof. The second tool  28  has a concave cutting surface  32  formed thereon and facing toward the workpiece  12  for machining the convex side  20  thereof. The two tools  26 ,  28  are mounted on opposite sides of the workpiece  12  so as to be movable toward the workpiece  12  as indicated by arrows F. (The tools  26 ,  28  can also move in the directions opposite the arrows F for retraction away from the workpiece  12 .) Movement of the tools  26 ,  28  is accomplished by any suitable means (not shown), many of which are well known in the art, under the control of a motion controller  33 . The controller  33  is programmed to control the advancement and feed rate of the tools  26 ,  28 . A first position sensor  27  is associated with the first tool  26 , and a second position sensor  29  is associated with the second tool  28 . The position sensors  27 ,  29 , which can be any suitable device, provide tool position feedback data. The feedback data can be fed to the controller  33  as shown in  FIG. 2 . Alternatively, the feedback data can be displayed on a readout for use by the operator of the ECM tool assembly  10 . 
   During operation, the tools  26 ,  28  are positioned in a desired spatial relationship with respect to the workpiece  12 . Specifically, the first tool  26  is positioned relative to the workpiece  12  so as to define a first gap  34  between the first cutting surface  30  and the concave side  18  of the workpiece  12 . The second tool  28  is positioned so as to define a second gap  36  between the second cutting surface  32  and the convex side  20  of the workpiece  12 . 
   The first and second tools  26 ,  28  and workpiece  12  are disposed in a receptacle (not shown) filled with an electrolytic fluid  38  such that the gaps  34 ,  36  are filled with the electrolytic fluid  38  as shown in  FIG. 2 . Suitable known means, such as a pump system (not shown), are provided for circulating the electrolytic fluid  38  such that it flows through the gaps  34 ,  36  in the direction of the arrows A shown in  FIG. 1 . It should be noted that the fluid flow direction represented by arrows A is only one possible direction. The electrolytic fluid can flow in many other directions including spanwise from shank-to-tip or from tip-to-shank. 
   The ECM tool assembly  10  further includes a DC power supply  40 . Both tools  26 ,  28  are connected to the negative terminal of the DC power supply  40  so as to function as cathodes, and the workpiece  12  is connected to the positive terminal of the DC power supply  40  so as to function as an anode. Thus, a DC electrical potential will be established between the workpiece concave side  18  and the first cutting surface  30  and between the workpiece convex side  20  and the second cutting surface  32 . This will cause controlled deplating of the workpiece sides  18 ,  20  so as to machine the workpiece  12  to its desired shape. The flow of electrolytic fluid  38  through the gaps  34 ,  36  will remove the deplated material and prevent it from being deposited on the tools  26 ,  28 . 
   A first ultrasonic transducer  42  is embedded or mounted in a cavity  44  formed in the back side of the first tool  26  so as to be directly behind the first cutting surface  30 . An acoustic couplant  46 , such as a gel or oil, is applied between the transducer  42  and the tool  26  for assisting sound emitted from the transducer  42  to penetrate the tool material. A second ultrasonic transducer  48  is similarly embedded or mounted in a cavity  50  formed in the back side of the second tool  28  so as to be directly behind the second cutting surface  32 . An acoustic couplant  52  is applied between the second transducer  48  and the second tool  28 . 
   Both ultrasonic transducers  42 ,  48  are capable of generating and sensing ultrasonic waves that are used to measure gap size and workpiece thickness. The transducers  42 ,  48  can be any suitable type, such as contact or immersion transducers. Oil is preferably used as the acoustic couplant  46 ,  52  if contact transducers are used. Oil having a low viscosity is used for measurements through smooth surfaces, while oil having a higher viscosity is used for measurements through rough surfaces. An aqueous couplant is used with immersion transducers. 
   Preferably, but not necessarily, the ultrasonic transducers  42 ,  48  are both located on an axis B that extends normal to both the first and second cutting surfaces  30 ,  32 . The transducer axis B is also preferably either parallel to, or at a small angle to, the tool feed direction represented by arrow F. This arrangement facilitates using ultrasonic time-of-flight (TOF) measurements to calculate the gap sizes and workpiece thickness, as will be described below. Although the transducer pair  42 ,  48  can be mounted at any location on the first and second tools  26 ,  28 , they are generally placed in a location where it is most useful to closely monitor and control the gap sizes and workpiece thickness. It is also within the scope of the present invention to provide multiple pairs of such transducers at various locations in the first and second tools  26 ,  28 . 
   The first and second ultrasonic transducers  42 ,  48  are each connected to a corresponding pulser/receiver  54 ,  56 , respectively. Each pulser/receiver  54 ,  56  transmits electrical energy to its associated transducer, causing the transducer to emit ultrasonic energy. Each pulser/receiver  54 ,  56  also receives the electrical signal generated by its associated transducer when the transducer senses a reflected ultrasonic pulse. The pulser/receivers  54 ,  56  are both connected to a data acquisition and analysis system  58 , which controls the operation of the transducers  42 ,  48  and makes the necessary calculations for determining the gap width and workpiece thickness measurements. 
   In operation, the ECM tool assembly  10  uses ultrasonic TOF measurements for gap sizing and workpiece thickness evaluation. The first ultrasonic transducer  42  mounted in the first tool  26  generates an ultrasonic wave that propagates through the acoustic couplant  46  and into the tool material. Part of the ultrasonic wave transmitted through the first tool  26  is reflected by the interface between the first cutting surface  30  and the electrolytic fluid  38 . The reflected portion of the wave returns to the transducer  42  (which now functions as a receiver) with an arrival time t a1 , while the other part of the ultrasonic wave is transmitted through the electrolytic fluid  38 . This portion of the wave is reflected by the interface between the workpiece concave side  18  and the electrolytic fluid  38  and returns to the transducer  42  with an arrival time t a2 . These arrival times are sent to the data acquisition and analysis system  58  via the pulser/receiver  54 . The data acquisition and analysis system  58  calculates the ultrasonic time-of-flight through the first gap  34  as Δt a =t a2 −t a1 . 
   The second ultrasonic transducer  48  operates in a similar pulse-echo mode at the same time. That is, the second ultrasonic transducer  48  mounted in the second tool  28  generates an ultrasonic wave that propagates through the acoustic couplant  52  and into the tool material. Part of the ultrasonic wave transmitted through the second tool  28  is reflected by the interface between the second cutting surface  32  and the electrolytic fluid  38 . The reflected portion of the wave returns to the second transducer  48  (now functioning as a receiver) with an arrival time t b1 , while the other part of the ultrasonic wave is transmitted through the electrolytic fluid  38 . This portion of the wave is reflected by the interface between the workpiece convex side  20  and the electrolytic fluid  38  and returns to the transducer  48  with an arrival time t b2 . These arrival times are sent to the data acquisition and analysis system  58  via the pulser/receiver  56 . The data acquisition and analysis system  58  calculates the ultrasonic time-of-flight through the second gap  36  as Δt b =t b2 −t b1 . 
   The gap sizes of the first and second gaps  34 ,  36 , denoted as W a  and W b  respectively, are calculated by the data acquisition and analysis system  58  using the following equations:
 
 W   a =( V   S   ×Δt   a )/2
 
 W   b =( V   S   ×Δt   b )/2
 
where V S  is the ultrasound velocity in the electrolytic fluid  38 . The ultrasound velocity in the electrolytic fluid  38  can vary during electrochemical machining due to changes in the fluid density. However, because the density of the electrolytic fluid  38  typically reaches a constant value after a certain amount of machining time, the ultrasound velocity can be considered to be constant at this point. The ultrasound velocity can be calibrated using a known workpiece thickness or known gap size. An additional probe for updating the ultrasound velocity could be used as an alternative to using a constant velocity value.
 
   Gas bubbles can be generated at the cutting surfaces of the first and second tools  26 ,  28  during electrochemical machining. In some instances, such gas bubbles can attenuate the ultrasonic waves and hinder the TOF measurements. Various measures optionally can be employed to counter this effect. For example, the TOF measurements can be made during a planned shut down of the DC power supply  40 , such as the time interval used in pulsed electrochemical machining. Alternatively, the voltage DC power supply  40  may be reduced or regulated to minimize gas bubble generation. Insulation of the ultrasonic transducers  42 ,  48  may also be used. 
   The thickness of the workpiece  12  along the line of the transducer axis B can be determined from the measured gap sizes and the distance, D, between the first and second tools  26 ,  28 . Specifically, the workpiece thickness, T, is given by:
 
 T=D−W   a   −W   b 
 
where the distance D, which is determined from the data provided by the position sensors  27 ,  29 , is fed to the data acquisition and analysis system  58  from the controller  33 .
 
   In the case of the rotor blade workpiece  12  illustrated in the Figures, the ECM tool assembly  10  machines the rotor blade&#39;s airfoil  16 . As the airfoil  16  is being machined, the data acquisition and analysis system  58  can also detect the airfoil position relative to a workpiece datum (such as the shank portion  14 ). That is, during machining of the airfoil  16 , the shank portion  14  is secured by a fixture (not shown) as is known in the art. By monitoring the position data from the position sensors  27 ,  29  relative the fixed position of the shank portion  14 , the data acquisition and analysis system  58  can detect the position of the airfoil  16  relative to the shank portion  14 . This means that the ECM tool assembly  10  will not only produce an rotor blade  12  with an airfoil  16  having the desired shape and dimensions, but will also ensure that the airfoil  16  is properly oriented with respect to the shank portion  14 . 
   The gap sizes, workpiece thickness and airfoil position values calculated by the data acquisition and analysis system  58  are fed to the controller  33 . The controller  33  uses these values in a feedback loop to control the advancement and feed rate of the first and second tools  26 ,  28 . 
   The foregoing has described an approach for the in-situ measurement of gap size and workpiece thickness that is independent of workpiece material. Generally, the present invention is capable of resolving measurements of 0.1 millimeter. The applicable spatial resolution of the ultrasonic measurements can range from 1.0 to 20 millimeters or can have an even broader range depending on the type of transducers chosen and upon factors such as frequency, size, focal length, etc. Local information on the gap sizes and workpiece thickness can be obtained if focus-type transducers are used. The measured quantities reflect the averaged properties over the sensor surface areas if planar-type transducers are used. 
   While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.