Abstract:
An electrochemical machining process is monitored by embedding an ultrasonic sensor in an electrochemical machining tool to provide a tool assembly, placing the tool assembly in a spatial relationship with a workpiece, disposing an electrolytic fluid at least in a gap between the tool and the workpiece, connecting the tool and the workpiece to an electrical power source, generating an acoustic wave from the ultrasonic sensor to propagate through the electrolytic fluid to the workpiece and reflect back from the workpiece, and, based on the propagation and reception of the acoustic wave, calculating measurement of at least the size of the gap or the thickness of the workpiece.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the process of electrochemical machining and, more particularly, concerns a method of monitoring an electrochemical machining process and a tool assembly therefor. 
     Electrochemical machining (ECM) is a non-mechanical process in which the tool never comes in contact with the workpiece during the machining process. The tool as a cathode and the workpiece as an anode are connected to an electrical power source. A gap that exists between the tool and the workpiece is filled with a pressurized, flowing, aqueous electrolyte. ECM is generally the reverse of electroplating. The flowing electrolyte, acting as an electrical current carrier, removes metal ions from the anodic workpiece and carries them away via the gap. The gap ranges in size from 0.1 millimeters to several millimeters. The tool is typically made of brass, bronze or stainless steel. The electrolyte is a highly conductive inorganic salt solution, such as sodium nitrate. A cavity which is produced in the anodic workpiece is a female mating image of the cathodic tool. 
     Given a 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, would lead to arcing or short-circuiting between the tool and the workpiece. Too large a gap would 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 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. 
     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 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 gap detection or workpiece thickness detection is thus important for improving ECM process control. 
     Several types of ECM sensors have been developed over the years since ECM came to industrial uses four decades ago. An eddy current ECM gap sensor was reported in  Annuals of the CIRP  (1982, Vol. 37/1, pp.115-118, by C. Bignon). An ECM control method using an ultrasound sensor is described in U.S. Pat. No. 5,672,263 to Raulerson et al. and is used for ECM of a large casing. However, the Raulerson et al. method is limited to applications which have a large space for housing the sensor and storing the fluid through which the ultrasonic wave propagates. By way of example, the Raulerson et al. method cannot be applied to the ECM of turbine compressor airfoils because space is limited in the machining area and also because the airfoil is surrounded by cathodes that make it impossible to directly measure airfoil thickness. The Raulerson et al. method also does not measure the gap size and is intended only to measure the workpiece thickness that is near a wide open space. Consequently, need remains for a method of monitoring an ECM process which overcomes the aforementioned limitations of the prior art without introducing any new problems. 
     BRIEF SUMMARY OF THE INVENTION 
     Monitoring an electrochemical machining process and a tool assembly therefor is achieved by embedding an ultrasonic sensor in the ECM tooling assembly. Measurement of both the gap size and workpiece thickness is performed using ultrasonic signals and is not limited by the amount of space in the machining area and is particularly applicable to the ECM of turbine compressor airfoils. 
     In a preferred embodiment of the invention, a method of monitoring an electrochemical machining process comprises the steps of: embedding an ultrasonic sensor in an electrochemical machining tool to provide a tool assembly; providing the tool assembly in a spatial relationship with a workpiece; flowing an electrolytic fluid at least between the tool and the workpiece; connecting the tool and the workpiece to an electrical power source; generating an acoustic wave from the ultrasonic sensor so as to propagate from the tool through the electrolytic fluid to the workpiece; receiving reflections of the acoustic wave from the workpiece; and, based on the propagated acoustic wave and the reflections thereof, calculating measurement of at least one of (a) the size of a gap between a cutting surface of the tool and a first working surface of the workpiece facing the cutting surface of the tool and (b) the thickness of the workpiece between the first working surface of the workpiece and a second working surface thereof facing away from the first working surface. The method also comprises the step of applying an acoustic couplant between the ultrasonic sensor and the tool. 
     More particularly, the receiving step includes reflecting a first part of the acoustic wave at the cutting surface of the tool and returning it to the ultrasonic sensor at a first arrival time, and reflecting a second part of the acoustic wave at the first working surface of the workpiece and returning it to the ultrasonic sensor at a second arrival time. The calculating step includes subtracting the first arrival time from the second arrival time, multiplying the difference by the velocity of the acoustic wave in the electrolytic fluid, and dividing the product by a factor of 2 to obtain the gap size between the cutting surface of the tool and the first working surface of the workpiece. 
     The receiving step also includes reflecting a first part of the acoustic wave at the first working surface of the workpiece and returning it to the ultrasonic sensor at a third arrival time, and reflecting a second part of the acoustic wave at the second working surface of the workpiece and returning it to the ultrasonic sensor at a fourth arrival time. The calculating step includes subtracting the third arrival time from the fourth arrival time, multiplying the difference by the velocity of the acoustic wave in the electrolytic fluid, and dividing the product by a factor of 2 to obtain the thickness of the workpiece between the first and second working surfaces of the workpiece. 
     In another exemplary embodiment of the invention, an electrochemical machining tool assembly is provided which comprises: an electrochemical machining tool positionable in a spatial relationship with respect to a workpiece and positionable in contact with an electrolytic fluid disposed at least in a gap between the tool and the workpiece, the tool having a cutting surface facing the workpiece; and an ultrasonic sensor embedded in the tool for generating an acoustic wave that propagates from the tool through the electrolytic fluid to the workpiece and is reflected back to the ultrasonic sensor for use in calculating a measurement of at least one of (a) the size of the gap between the cutting surface of the tool and a first working surface of the workpiece facing the cutting surface of the tool and (b) the thickness of the workpiece between the first working surface of the workpiece and a second working surface of the workpiece facing away from the first working surface of the workpiece. The assembly also comprises an acoustic couplant applied between the ultrasonic sensor and the tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an electrochemical machining tool assembly of the invention, used in monitoring an electrochemical machining process. 
     FIG. 2 is a sectional view taken along line  2 — 2  of the electrochemical machining tool assembly of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate an electrochemical machining tool assembly  10  of the invention, as used in a conventional electrochemical machining process and in a method of the invention for monitoring the electrochemical machining process. The tool assembly  10  basically includes an electrochemical machining tool  12  and an ultrasonic sensor  14 . Tool  12  and sensor  14  are used in conjunction with a workpiece  16 , an electrolytic fluid  18  and a DC power supply  20 . 
     Electrochemical machining tool  12  has a suitable configuration to electrochemically machine workpiece  16  into the desired configuration. Tool  12  has at least a first cutting surface  22   a  and, more particularly, where workpiece  16  is to be machined on opposite sides, tool  12  has two parts  12   a ,  12   b  with first and second cutting surfaces  22   a ,  22   b  of the desired configuration thereon facing toward workpiece  16  for machining first and second working surfaces  24   a ,  24   b  of complementary shapes to first and second cutting surfaces  22   a ,  22   b . The two parts  12   a ,  12   b  of tool  12  are mounted in any suitable manner on opposite sides of workpiece  16  so as to be movable toward and away from workpiece  16  in setting up tool  12  for, and adjusting tool  12  during, the machining process. By way of example, where workpiece  16  is a turbine blade, the first cutting surface  22   a  has a substantially convex configuration for machining a first working surface  24   a  of a substantially concave configuration and the second cutting surface  22   b  has a substantially concave configuration for machining a second working surface  24   b  of a substantially convex configuration. 
     Electrochemical machining tool  12  is positionable in a desired spatial relationship with respect to workpiece  16 . Workpiece  16  may be disposed adjacent to and spaced from first cutting surface  22   a  or between first and second cutting surfaces  22   a ,  22   b  of tool  12  such that a gap  26  is provided therebetween. Each of cutting surfaces  22   a ,  22   b  faces toward one of the first and second working surfaces  24   a ,  24   b  of workpiece  16  across gap  26 . 
     Tool assembly  10  and workpiece  16  are disposed in a receptacle (not shown) which also contains electrolytic fluid  18  disposed at least in gap  26  between tool  12  and workpiece  16 . Electrolytic fluid  18  may immerse portions, or all, of each of electrochemical machining tool  12  and workpiece  16 . Suitable known means, such as a pump system (not shown), is connected to the receptacle to cause electrolytic fluid  18  to flow in the direction of the arrows A and recirculate through gap  26  past tool  12  and workpiece  16 . Tool  12  is connected to a negative (−) terminal of D.C. power source  20  so as to function as a cathode and workpiece  16  is connected to a positive (+) terminal of D.C. power source  20  so as to function as an anode. Consequently, conventional electrical current flows from first and second working surfaces  24   a ,  24   b  of workpiece  16  through electrolytic fluid  18  to first and second cutting surfaces  22   a ,  22   b  of tool  12 . The flow of electrolytic fluid  18  prevents material removed from workpiece  16  from being deposited on tool  12 . 
     Ultrasonic sensor  14  is embedded in a recess  28  in the one part  12   a  of electrochemical machining tool  12 . Electrical cable  30  is connected to sensor  14  and extends from recess  28  for connection to a pulser-receiver device  32 , and electrical cable  31  connects pulser-receiver device  32  to a data acquisition system  34 , for controlling operation of sensor  14  and making the necessary calculations for providing the measurements of the width W of gap  26  and the thickness T of workpiece  16 . Sensor  14  generates an ultrasonic wave that is used to measure at least one of the width W of gap  26  or the thickness T of workpiece  16 . Recess  28  and thus ultrasonic sensor  14  can be disposed at any suitable location on tool  12 . As one example, sensor  14  is disposed adjacent to first cutting surface  22   a  of the one part  12 A of tool  12  such that the axis of sensor  14  is substantially normal to first cutting surfaces  22   a  and coincident with the feed of the tool parts  12   a ,  12   b  in the direction of arrows F or forms the smallest angle therewith as compared to angles from other locations. Sensor  14  can be any suitable type, such as a contact or an immersion transducer. 
     Sensor  14  generates the acoustic wave so as to propagate from tool  12  through electrolytic fluid  18  to workpiece  16  and reflect back to sensor  14  where it is received and used to calculate the measurement of the at least one of the width W of gap  26  between first cutting surface  22   a  of tool  12  and first working surface  24   a  of workpiece  16  facing first cutting surface  22   a  of tool  12  and of the thickness T of workpiece  16  between first and second working surfaces  24   a ,  24   b  thereof. 
     If the width W of gap  26  is to be determined, a first part of the acoustic wave of ultrasonic sensor  14  is reflected at first cutting surface  22   a  of part  12   a  of tool  12  and returns to ultrasonic sensor  14  at a first arrival time, and a second part of the acoustic wave of ultrasonic sensor is reflected at first working surface  24   a  of workpiece  16  and returns to ultrasonic sensor  14  at a second arrival time. These arrival times are calculated by data acquisition system  34 , and a measurement of the distance across, or the width, of gap  26  between first cutting surface  22   a  of tool  12  and first working surface  24   a  of workpiece  16  is calculated by data acquisition system  34  by subtracting the first arrival time from the second arrival time and multiplying the difference by the velocity of the acoustic wave in electrolytic fluid  18  and div dividing the product by a factor of 2. 
     If the thickness T of workpiece  16  is to be determined, a first part of the acoustic wave of ultrasonic sensor  14  is reflected at first working surface  24   a  of workpiece  16  and returns to ultrasonic sensor  14  at a first arrival time and a second part of the acoustic wave of ultrasonic sensor  14  is reflected at second working surface  24   b  of workpiece  16  and returns to ultrasonic sensor  14  at a second arrival time. These arrival times are calculated by data acquisition system  34  and a measurement of the thickness T of workpiece  16  between first and second working surfaces  24   a ,  24   b  of workpiece  16  is calculated by subtracting the first arrival time from the second arrival time and multiplying the difference by the velocity of the acoustic wave in the electrolytic fluid  18  and dividing the product by a factor of 2. 
     Tool assembly  10  also includes an acoustic couplant  36  which is applied in a recess  28  between ultrasonic sensor  14  and the one part  12   a  of tool  12 . The acoustic wave of ultrasonic sensor  14  passes through acoustic couplant  36  and transmits through tool  12  before passing into electrolytic fluid  18  and through workpiece  16 . Oil is used as acoustic couplant  36  if ultrasonic sensor  14  is a contact transducer. For measurements through smooth surfaces, oil having a lower viscosity is used. For measurements through rough surfaces, oil having a higher viscosity is used. Acoustic couplant  36  is aqueous if ultrasonic sensor  14  is an immersion transducer. 
     A large variety of contact transducers and immersion transducers can be used as ultrasonic sensor  14 , depending upon the specific part surface condition and applications, such as the ranges of the gap sizes and the workpiece thicknesses to be measured. Generally, ultrasonic measurement is capable of resolving a gap  26  size of 0.1 millimeter. The applicable spatial resolution of the ultrasonic measurements can range from 1.0 millimeter to 20 millimeters or can have an even broader range depending upon the type of transducer chosen or depending upon the factors of frequency, size, focal length, etc. Local information on the gap size and workpiece thickness can be obtained if a focus-type transducer is used. The measured quantities reflect the averaged properties over the sensor surface area if a planar-type transducer is used. The acoustic wave velocity in electrolytic fluid  18  can vary due to changes in density of the electrolytic fluid. The density of electrolytic fluid  18 , however, reaches a constant value after a certain duration of machining and so the acoustic wave velocity can be considered constant. The acoustic wave velocity can be calibrated using a known gap size or a known workpiece thickness. During the electrochemical machining process, gas bubbles are usually generated at cutting surfaces  22  of electrochemical machining tool  12 . The gas bubbles may cause ultrasonic acoustic wave attenuation. D.C. power supply  20  may be turned off for a brief period of time, such as for the time interval used in pulsed electrochemical machining, or the voltage of D.C. power supply  20  may be reduced so as to minimize the generation of gas bubbles in order for a more accurate measurement to be made. Insulation of the ultrasonic sensor  14  casing might also be used. 
     While only certain preferred features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.