U.S. patent application number 11/733477 was filed with the patent office on 2007-08-02 for electrochemical machining tool assembly.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Thomas James Batzinger, Robert John Filkins, Michael Scott Lamphere, Carl Stephen Lester, Wei Grace Li, Thomas Walter Rogenski, Bin Wei.
Application Number | 20070175751 11/733477 |
Document ID | / |
Family ID | 34435629 |
Filed Date | 2007-08-02 |
United States Patent
Application |
20070175751 |
Kind Code |
A1 |
Batzinger; Thomas James ; et
al. |
August 2, 2007 |
ELECTROCHEMICAL MACHINING TOOL ASSEMBLY
Abstract
In an electrochemical machining tool assembly having at least
one electrode arranged across a gap from a workpiece, the electrode
being energized by application of a potential difference .DELTA.V
between the electrode and the workpiece, a method of monitoring
machining 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 a reduction of the potential difference .DELTA.V
across the electrode and the workpiece occurs.
Inventors: |
Batzinger; Thomas James;
(Burnt Hills, NY) ; Li; Wei Grace; (Mill Creek,
WA) ; Lamphere; Michael Scott; (Hookset, NH) ;
Rogenski; Thomas Walter; (Rutland, VT) ; Wei;
Bin; (Mechanicville, NY) ; Lester; Carl Stephen;
(Porters Corners, NY) ; Filkins; Robert John;
(Niskayuna, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
1 River Road
Schenectady
NY
12345
|
Family ID: |
34435629 |
Appl. No.: |
11/733477 |
Filed: |
April 10, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10706472 |
Nov 10, 2003 |
|
|
|
11733477 |
Apr 10, 2007 |
|
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|
Current U.S.
Class: |
204/218 |
Current CPC
Class: |
B23H 3/02 20130101 |
Class at
Publication: |
204/218 |
International
Class: |
C25F 7/00 20060101
C25F007/00 |
Claims
1-21. (canceled)
22. An electrochemical machining tool assembly comprising: at least
one electrode adapted to machine a workpiece across a gap upon
application of a potential difference .DELTA.V across said
electrode and the workpiece; means for flowing an electrolyte
through the gap and for flushing the electrolyte from the gap;
means for feeding said at least one electrode toward the workpiece;
at least one ultrasonic sensor adapted to direct an ultrasonic wave
toward a surface of said electrode and to receive a reflected
ultrasonic wave from the surface of said electrode, the reflected
ultrasonic wave comprising a plurality of reflected waves from the
surface of said electrode and from a surface of the workpiece; and
a delay generator adapted to delay the excitation of said
ultrasonic sensor a dwell time T.sub.d after a reduction of the
potential difference .DELTA.V across said electrode and the
workpiece occurs.
23. The electrochemical machining tool assembly of claim 22,
further comprising: a power supply adapted to energize said at
least one electrode for machining by applying the potential
difference .DELTA.V across said at least one electrode and the
workpiece; and at least one pulser-receiver connected to a
respective one of said at least one ultrasonic sensors, each of
said at least one pulser-receivers being adapted to excite the
respective ultrasonic sensor and to receive the respective
reflected ultrasonic wave, each of said at least one
pulser-receivers being further adapted to be triggered by said
delay generator to excite the respective ultrasonic sensor after
the dwell time T.sub.d after a reduction of the potential
difference .DELTA.V across said electrode and the workpiece
occurs.
24. The electrochemical machining tool assembly of claim 23,
wherein said delay generator is adapted to monitor the output from
said power supply.
25. The electrochemical machining tool assembly of claim 23,
wherein said power supply is adapted to supply a plurality of
pulses to generate the potential difference .DELTA.V between said
at least one electrode and the workpiece during a plurality of
pulse-on periods, and wherein said delay generator is adapted to
delay the excitation of said at least one ultrasonic sensor the
dwell time T.sub.d after a transition from the pulse-on state to a
pulse-off state.
26. The electrochemical machining tool assembly of claim 23,
wherein said power supply is a DC power supply adapted to apply the
potential difference .DELTA.V across said at least one electrode
and the workpiece, said electrochemical machining tool assembly
further comprising a controller adapted to repeatedly reduce the
potential difference .DELTA.V applied across said at least one
electrode and the workpiece to generate a series of measurement
periods .DELTA.t.sub.M, wherein said delay generator is adapted to
delay the excitation of said ultrasonic sensor the dwell time
T.sub.d after a start of one of the measurement periods
.DELTA.t.sub.M.
27. The electrochemical machining tool assembly of claim 23,
wherein the dwell time T.sub.d is in a range of about seven
milliseconds (7 ms) to about 15 milliseconds (15 ms).
28. The electrochemical machining tool assembly of claim 23,
wherein said delay generator is adapted to adjust the dwell time
T.sub.d.
29. The electrochemical machining tool assembly of claim 23,
further comprising a controller, said controller being adapted to
generate a plurality of monitoring data by analyzing the reflected
ultrasonic wave to determine at least one of (a) a size of the gap
between said electrode and the workpiece and (b) a thickness of the
workpiece.
30. The electrochemical machining tool assembly of claim 29,
wherein said controller is further adapted to control at least one
of (a) said means for feeding said at least one electrode toward
the workpiece and (b) said power supply, in response to the
monitoring data.
31. The electrochemical machining tool assembly of claim 23,
wherein each of said at least one ultrasonic sensors comprises an
ultrasonic transducer.
Description
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.
[0021] As noted above, reducing the ECM voltage may impair ECM
machining quality. Accordingly, it is desirable to complete the
voltage adjustment in a short time period, to avoid compromising
ECM machining quality. Under typical ECM conditions, the gas
bubbles are flushed away in less than about fifteen milliseconds
(15 ms). More particularly, the gas bubbles are flushed away in
about seven to fifteen milliseconds (7-15 ms). Generally, the
higher the electrolyte rate flow, the faster the bubbles are
flushed. Moreover, the ultrasonic measurement itself takes only a
short time, typically on the order of less than about fifty
microseconds (50 .mu.s). Under these conditions, the ultrasonic
measurement cycle, which includes the above-noted delay for the
electrolyte to wash away the gas bubbles, as well as the actual
ultrasonic measurement time window, may be relatively short, for
example less than about twenty milliseconds (20 ms), during which
time the voltage level is reduced, such that the ultrasonic signals
are not significantly attenuated. Beneficially, because this period
is relatively short, ECM machining quality is not compromised.
Moreover, because of the delay, adequate flushing of the bubbles
occurs, permitting relatively clean ultrasonic measurements.
[0022] According to a more particular embodiment, the ECM tool
assembly 10 also includes a power supply 40, which is adapted to
energize the electrode 26 for machining by applying a potential
difference .DELTA.V across the electrode 26 and the workpiece 12.
For the example of FIG. 1, the electrodes 26, 28 are connected to
the negative terminal of the power supply 40 to function as
cathodes, and the workpiece 12 is connected to the positive
terminal of the power supply 40, to function as an anode. In this
manner, a potential difference .DELTA.V is established between the
workpiece 12 and the electrodes 26, 28, thereby causing controlled
deplating of the workpiece sides 18, 20, to machine the workpiece
12 to its desired shape. The flow of electrolyte 38 through the
gaps 34, 36 removes the depleted material, thereby preventing it
from being deposited on the electrodes 26, 28. For the particular
embodiment of FIG. 2, at least one pulser-receiver 54 is connected
to a respective one of the ultrasonic sensors 42. Each of the
pulser-receivers 54 is adapted to excite the respective ultrasonic
sensor 42 and to receive the respective reflected ultrasonic wave.
Each of the pulser-receivers is further adapted to be triggered by
the delay generator 110 to excite the respective ultrasonic sensor
42 after a dwell time T.sub.d after the occurrence of a reduction
of the potential difference .DELTA.V across the electrode 26, 28
and the workpiece 12. The timing is discussed in greater detail
below.
[0023] For the particular embodiment of FIG. 1, the delay generator
110 is adapted to monitor the output from power supply 40. The
power supply may be configured to supply a number of pulses to
generate the potential difference .DELTA.V between the electrode 26
and the workpiece 12. Alternatively, the power supply 40 may be a
DC power supply. For the pulsed power supply 40 embodiment, the
power supply 40 is adapted to supply pulses during a number of
pulse-on periods, and the delay generator 110 is adapted to delay
the excitation of the ultrasonic sensor 42 for the dwell time
T.sub.d after a transition from the pulse-on state to a pulse-off
state, as shown for example in FIG. 3. For the DC power supply 40
embodiment, the electrochemical machining tool assembly further
includes a controller 120 (see FIG. 1) that is adapted to
repeatedly reduce the potential difference .DELTA.V applied across
the electrode 26 and the workpiece 12 to generate a series of
measurement periods .DELTA.t.sub.M, as indicated, for example, in
FIG. 3. For this latter DC power supply 40 embodiment, the delay
generator 110 is adapted to delay the excitation of the ultrasonic
sensor 42 for the dwell time T.sub.d after a start of one of the
measurement periods .DELTA.t.sub.M, as indicated in FIG. 3.
[0024] For the particular embodiment of FIG. 2, the electrochemical
machining tool assembly 10 includes a controller 120 that is
adapted to generate a set of monitoring data by analyzing the
reflected ultrasonic wave to determine at least one of (a) a size
of the gap 34 between the electrode 26 and the workpiece 12 and (b)
a thickness of the workpiece 12. More particularly, the controller
120 is further adapted to control at least one of (a) the means for
feeding the electrode 26 toward the workpiece 12 and (b) the power
supply 40, in response to the monitoring data. In other words, the
controller is adapted to use the monitoring data in a feedback loop
to control the advancement and feed-rate of the electrode 26
relative to the workpiece 12. The term "controller," as that term
is used herein, is intended to denote any machine capable of
performing the calculations or computations and control operations
necessary to perform the tasks of the invention. The phrase
"adapted to" as used herein means that the controller is equipped
with a combination of hardware and software for performing the
tasks of the invention, as will be understood by those skilled in
the art.
[0025] A method of monitoring machining in the electrochemical
machining tool assembly 10 is also described with reference to
FIGS. 1-4. The monitoring method includes exciting at least one
ultrasonic sensor 42 to direct an ultrasonic wave toward a surface
102 of the electrode. As indicated, for example in FIG. 3, the
ultrasonic sensor 42 may be excited by pulsing the sensor 42, for
example using a pulser/receiver 54. The monitoring method further
includes receiving a reflected ultrasonic wave from the surface 102
of the electrode 26 using the ultrasonic sensor 26. The reflected
ultrasonic wave comprises a number of reflected waves from the
surface of the electrode 26 and from the surface 104 of the
workpiece 12. The monitoring method further includes delaying the
excitation of the ultrasonic sensor 42 a dwell time T.sub.d after a
reduction of the potential difference .DELTA.V across the electrode
26 and the workpiece 12 occurs, as indicated in FIG. 3, for
example.
[0026] According to a more particular embodiment, the monitoring
method further includes analyzing the reflected ultrasonic wave to
determine at least one of (a) a size of the gap 34 between the
electrode 26 and the workpiece 12 and (b) the thickness of the
workpiece 12. Because the acoustic velocities of the two materials
are known, the gap 34 and workpiece thickness can be calculated. As
noted above, by monitoring the size of the gap 34 and/or the
thickness of the workpiece 12 during the machining process, this
data can be used in a feedback loop to control the advancement
and/or feed-rate of the electrode 26 relative to the workpiece
12.
[0027] According to one embodiment, the electrochemical machining
tool assembly 10 is a pulsed electrochemical machining tool
assembly, and the electrode 26 is energized by a periodic
application of a potential difference .DELTA.V between the
electrode and the workpiece 12 during a number of pulse-on periods.
For this embodiment, the excitation of the ultrasonic sensor 42 is
delayed for the dwell time T.sub.d after a transition from the
pulse-on state to a pulse-off state, as indicated in FIG. 3. For
another embodiment, the electrochemical machining tool assembly 10
is a continuous electrochemical machining tool assembly, for
example employing a DC power supply 40. For this latter embodiment,
the monitoring method further includes repeatedly reducing the
potential difference .DELTA.V across the electrode 26 and the
workpiece 12 to generate a series of measurement periods
.DELTA.t.sub.M, as is also shown in FIG. 3. For this latter
continuous embodiment, the excitation of the ultrasonic sensor 42
is delayed a dwell time T.sub.d after a start of one of the
measurement periods .DELTA.t.sub.M, as indicated in FIG. 3.
[0028] According to a particular embodiment, the monitoring method
further includes adjusting the dwell time T.sub.d. For example, the
dwell time T.sub.d may be decreased, in order to accommodate a
shorter pulse off-time (or shorter measurement period
.DELTA.t.sub.M) to facilitate higher frequency ECM pulse
excitation. The dwell time T.sub.d may also be increased, in order
to lengthen the deactivation/flush time. By increasing the delay,
the bubbles generated during machining can be more completely
flushed away, in order to reduce attenuation of the ultrasonic
signals.
[0029] As noted above with respect to FIG. 2, for certain
embodiments the electrochemical machining tool assembly 10 includes
at least two electrodes 26, 28, each of the electrodes being
arranged across a respective gap 34, 36 from the workpiece 12. For
the embodiment of FIG. 2, a first ultrasonic sensor 42 is excited
to direct an ultrasonic wave toward a surface 102 of one of the
electrodes 26, and a second ultrasonic sensor 44 is excited to
direct an ultrasonic wave toward a surface 106 of another of the
electrodes 28. For this two-electrode embodiment, reflected
ultrasonic waves are received from the surface 102, 106 of each of
the respective electrodes 26, 28 using the respective ultrasonic
sensors 42, 44, and the excitation of each of the ultrasonic
sensors 42, 44 is delayed for at least the dwell time T.sub.d after
the occurrence of a reduction of the potential difference .DELTA.V
across the electrodes 26, 28 and the workpiece 12. More
particularly, for colinear ultrasonic sensors 42, 44, excitation of
one of the ultrasonic sensors 42, 44 may be delayed by the dwell
time T.sub.d after the occurrence of a reduction of the potential
difference .DELTA.V across the electrodes 26, 28 and the workpiece
12, while excitation of the other of the ultrasonic sensors 42, 44
may be delayed by the dwell time plus an offset (T.sub.d+.delta.)
after the occurrence of a reduction of the potential difference
.DELTA.V across the electrodes 26, 28 and the workpiece 12. The
offset .delta. is greater than or equal to the time required to
attenuate the ultrasound from the first excited ultrasonic sensor
42, 44.
[0030] A method of monitoring machining in a pulsed electrochemical
machining (ECM) tool assembly 10 is also described with reference
to FIGS. 1-4. As noted above, for a pulsed ECM tool assembly 10,
the electrode 26 is periodically energized by application of a
number of pulses, as indicated for example in FIG. 3. For this
embodiment, the method includes exciting (for example, pulsing) at
least one ultrasonic sensor 42 to direct an ultrasonic wave toward
a surface 102 of the electrode, receiving a reflected ultrasonic
wave from the surface of the electrode using the ultrasonic sensor,
the reflected ultrasonic wave comprising a number of reflected
waves from the surface of the electrode and from the surface 104 of
the workpiece, and delaying the excitation of the ultrasonic sensor
42 a dwell time T.sub.d after a transition from a pulse-on state to
a pulse-off state. The method may further include adjusting the
dwell time T.sub.d.
[0031] An electrochemical machining (ECM) method for machining a
workpiece 12 is described with reference to FIGS. 1-4. This ECM
method is equally applicable to ECM tool assemblies 10 having one
or multiple electrodes 26, 28. The ECM method includes energizing
at least one electrode 26 positioned in proximity to the workpiece
12, the electrode 26 and the workpiece 12 being separated by a gap
34, for example by a gap 34 of about one hundred microns (100
.mu.m) to about two millimeters (2 mm) but not touching. The ECM
method further includes flowing an electrolyte 38 through the gap.
The electrolyte 38 may be continuously pressurized at about twenty
to about two hundred (20-200) psi and flowed using a pump 130, as
indicated in FIG. 1, for example. The ECM method further includes
flushing the electrolyte from the gap 34. In this manner, the
dissolved metal, heat and gas bubbles are removed from the gap 34.
The ECM method further includes feeding the electrode 26 toward the
workpiece 12, to maintain a desired gap, and monitoring at least
one of the gap 34 and the workpiece 12 using the ultrasonic sensor
42. The monitoring includes exciting the ultrasonic sensor 42 to
direct an ultrasonic wave toward a surface 102 of the electrode 26,
receiving a reflected ultrasonic wave from the surface 102 of the
electrode 26 using the ultrasonic sensor 42. As noted above, the
reflected ultrasonic wave comprises a number of reflected waves
from the surface of the electrode and from the surface 104 of the
workpiece 12. The monitoring further includes delaying the
excitation of the ultrasonic sensor 42 a dwell time T.sub.d after a
reduction of the potential difference .DELTA.V across the electrode
26 and the workpiece 12 occurs. Beneficially, by delaying the
excitation of the ultrasonic sensor 42 by a dwell time T.sub.d, the
monitoring may be synchronized such that the monitoring is
performed 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. According to a more particular embodiment, the
monitoring further includes adjusting the dwell time T.sub.d, for
example shortening or lengthening the dwell time T.sub.d.
[0032] According to a particular embodiment, the monitoring further
includes generating monitoring data by analyzing the reflected
ultrasonic wave to determine at least one of (a) a size of the gap
34 between the electrode 26 and the workpiece 12 and (b) a
thickness of the workpiece 12. More particularly, the method
further includes controlling at least one of (a) energizing and (b)
feeding the electrode in response to the monitoring data. As
discussed above, the monitoring data may be used in a feedback loop
to control the advancement and/or feed-rate of the electrode
26.
[0033] For one embodiment, the ECM tool assembly 10 is a pulsed ECM
tool assembly 10. For this embodiment, a potential difference
.DELTA.V is periodically applied between the electrode 26 and the
workpiece 12 during a number of pulse-on periods, and the
excitation of the ultrasonic sensor 42 is delayed by the dwell time
T.sub.d after a transition from the pulse-on state to a pulse-off
state.
[0034] For another embodiment, the ECM tool assembly 10 is a
continuous ECM tool assembly 10. For this embodiment, the method
further includes repeatedly reducing the potential difference
.DELTA.V across the electrode 26 and the workpiece 12 to generate a
series of measurement periods .DELTA.t.sub.M, and the excitation of
the ultrasonic sensor 42 is delayed by the dwell time T.sub.d after
a start of one of the measurement periods .DELTA.t.sub.M.
[0035] Although only certain features of the invention have been
illustrated and described herein, 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.
* * * * *