Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
Tl~is invention is in the field of a power supply for
elecl-rochemical machining of the type employing a nozzle composed
of insulating matexial which directs a stream of cathodically
charged electrolyte at the workpiece to drill holes of controlled
S diameter through the workpiece.
There have been a number of electrochemical machining
methods described and developed in ~he past. Electrochemical
machining involves the use of a tool in the form of a nozzle which
is made cathodic and which cooperates with an anodic workpiece
through an electrolyte which contacts both the tool and the work-
piece. Upon flow of electric current through the electrolyte,
material of the workpiece is removed or deplated, These pro-
cesses have been used to a large extent in drilling small diameter
holes in metal which are difficult to drill mechanically.
One of the more recently developed processes
employing electrochemical machining is the impingement process
which is sometimes known as "Electrostream" (a trademark of
General Electric Corp. ) drilling. This process has been used to
drill extremely small holes in tough metals such as nickel base
superalloys. With this process, it is possible to drill holes
having a diameter on the order of 0. 005 to 0. 020 inch. The
impingement process normally makes use of a glass tube nozzle
with an elec~rode behind the tip of the nozzle. The impingement
process is operated under conditions of relatively high resistance
and high voltage, typically on the order of 300 to 600 volts DC.
Either a salt or an acid electrolyte is usea Electrolyte pres-
sures in this ~ype of process are typically on the order of 20 to
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100 pounds per square inch.
It has been found difficult, however, in conventional
electromachining operations of the impingement type to drill holes
whicll have a uniform diameter throughou~ their axial length.
Experience has indicated that the holes produced generally have
tapered sides in the portion of ~he hole nearest to the electrode
nozzle, and the desired diameter of the hole appears only at the
vicinity of the hole farthest from the electrode. This condition,
known as "washout" is particularly troublesome where a plurality
of holes are to be drilled in closely spaced relation since the
washout from adjoining holes can overlap resulting in an undesired
depletion of metal in the surface of the workpiece between the
holes. - -
The impingement process as it is presently practiced
also suffers from the disadvan~age that there is excessive wear on
the tip of the insulating nozzle, requiring replacement of nozzles
at relatively frequent intervals. While the cause for such wear
has not been precisely determined, it is believed that superheated
steam is generated under the conditions of sustained voltage
application and that this steam attacks the glass electrode resulting
in erosion by leaching of the glass composition.
The present invention provides a method for
electrochemical drilling which makes it possible to produce holes
of more uniform diameter than heretofore, and at the same time
reduces the wear on the tip of the electrode,
The present invention is directed to an improved
electrochemical drilling process in which the elech-olyte is
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charged with a negative voltage having a substantial ncgative DC
component and negative pulses superimposed thereon. The ripple
used in the power supply constitutes from about 75 to 600% of the
DC component, that is, the maximum swing of negative voltage in
S the pulses ranges from about 75 to 600~7o f the value of the DC
component, but insufficient to cause a spark discharge to occur
between the electrode and the workpiece. Best results are
obtained when the pulses have a pulse repetition frequency of from
60 to 720 Hertz. The wavcfoxm of the pulses is generally of the
saw tooth type although other wave shapes can be employed. The
pulses constitute the sole source of electrochemical drilling cur-
rent. Apparently, this periodic variation in the negative voltage
allows the electrolyte liquid to remain cooler so that less super-
heated steam is produced during drilling.
Particularly good results are obtained by varying the
voltages and currents as the drilling operation progresses. It is
particularly important in this regard that the voltage is increased
when "breakthrough" is accomplished, this being the condition
when the electrolyte emer~,es through the back face of the surface
being drilled. By suitable regulation of the voltage and current,
particularly at breakthrough, the method of the present invention
can provide little or no washout, resulting in the production of
holes of uniform diameter througllout their axial length
ON TI~E DRAWINGS
Other objects, features and advantages of the invention
will be readily apparen~ from the following description of certain
preferred embodiments thereof, t~ken in conjunction witll the
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accompanying drawings, although va.rial:ions and modifications may
be effected without departing from the spirit and scope of the
novel concepts of the disclosure, and in which:
Figure 1 is a somewhat schematic view partly in cross-
section illustrating the configuration of a typical electrode and
nozzle assembly;
Figure 2 is a schematic view of the nozzle and
workpiece just at the commencement of hole drilling;
Figure 3 is a view similar to Figure 2 but illustrating
the nozzle and workpiece just prior to breakthrough;
Figure 4 is a view similar to Figures 2 and 3 and
illustrating the nozzle and workpiece after the voltage has been
stepped up subsequent to breakthrough to complete the drilling
operation;
Figure 5 is a fragmentary cross-sectional view of a
workpiece in which a hole has been electrochemically drilled by an
impingement method typical of the prior art; and
~igure 6 is a plot of a typical voltage waveform which
can be used for the purposes of the present invention.
In Figure 1, reference numeral 10 indicates generally
an electrode assembly of the type used in accordance with the
present invention and including a tube 11 composed of insulating
material such as glass, the tube having a necked down portion 12
terminating in a relatively thin nozzle portion 13, Typically, the
2~ ilmer diameter of the nozzle may be on the order of 0. OOS to
0. 010 inch. Disposed rearwardly of the nozzle 13, and preferably
within a manifold (not shown~ which feeds a plurality of tubes 11
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is a metallic electrode l~ which i6 connectcd to a source of
pulsating nega~ive voltage of the type to be described.
Figure 6 illustrates a preferred wave shape for the
negative waveform applied to the electrode 14. It consists of a
generally saw tooth wave impressed over a predetermined negative
DC potential which in the waveform shown in Figure 6 is a nega-
tive ~00 volts. As illustrated, the peak negative value reached is
a negative 1100 volts. The root mean square (rms) voltage of the
waveform shown in l~igure 6, as read on a conventional direct
current meter would be about 750 volts. For the purposes of the
present invention, the rms value of the voltage applied to the elec-
trode is in the range from about 100 to 800 volts. Depending on
the configuration of the waveform, peaks up to about 1200 volts or
so can be achieved.
The particular waveform shown in Figure 6 of the
drawings has a 700 volt excursion from the constant DC value of
400 volts so that the ripple factor of the waveform illustrated in
l~igure 6 is 175~ For the purposes of this invention, the ripple
may range from about 75 to 600æ of the predetermined DC
component, and preferably from 100 to 300~Y0 of the DC component.
The frequency of the pulsations is also important to
secure the best results. The waveform illustrated in Figure 6
has a pulse duration cf 2 milliseconds, so that the pulse repetition
frequency is 500 Hertz. Generally, pulse repetition frequencies of ~ -
from 60 to 720 I-lertz have been found to be satisfactory.
A suitable electrolyte eitller of the salt or acid type is
passed through the nozzle 13 a~ the object to be drillcd.
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sulfuric acid solution containing 15 to 30970 by volume sulfuric acid
and preferably about 20~'0 by volume sulfuric acid is appropriate
with electl-olyte pressures ranging from 20 to about 100 pounds
per square inch
The spacing between the nozzle and the workpiece for
drilling purposes should be controlled either manually or automat-
ically at a distance dependent essentially upon the nature of the
material being drilled, Por most materials, a gap of about 0. 030
to 0, 060 inch will be satisfactory. It is important not to reduce
the spacing below about 0. 010 inch as excessive material removal
might be encountered, resulting in the washout condition previously
described.
In Figure 2, the nozzle 10 is shown delivering an
impinging stream o~ electrolyte 15 at a fixed distance from an
anodically charged workpiece 16. As metal removal occurs, the
current drops and it is then desirable to increase the voltage and
current to compensate for this increased gap.
As dri]ling proceeds, a generally conical depression 17
is formed in the workpiece and ultimately, the electrolyte breaks
through the rear surface of the workpiece 16. At this point, the
effective gap rises substantially and it is particularly important to
increase the voltage after breakthrough and maintain the increased
voltage on the electrode 10 until a substantially uniform diameter
hole 1~ results. The extent of time xequired for this dwell
procedure can be readily ascertained by a few preliminary tests.
The method of the present invention is capable of
producing straigilt sided holes in a relatively short periocl of time
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In contrast, impingement processes of the prior art utilizing a non-
varying DC potential frequently result in the production of apertures
of the type shown in Figure 5 wherein a workpiece 19 is shown
with a drilled hole having a substantially flared surface 20 and
having the desired diameter D only in the region in which break-
through was accomplished. This flared configuration, or washout
as it is called, can be very undesirable, particularly where close-
ly spaced holes are being drilled.
A particular schedule for increasing current and
voltage can be derived after a few experimental tests. The follow-
ing examples set forth treating schedules which have been found
applicable in many cases
EX~MP~E 1
A workpiece having a thickness of 0.100 inch was
subjected to electrochemical drilling using an electroly~e pressure
of 20 pounds per square inch, and an electrolyte of 2077o sulfuric
acid by volume. The gap between the electrode and the piece
being drilled was maintained at between 0. 030 and 0. 060 inch.
The drilling commenced wi~;h an applied voltage of 100 volts (rms)
and a current draw of 100 milliamperes. When the current
dropped to 90 milliamperes as drilling progressed, the voltage
was increased to 300 volts, increasing the current to 120 milli-
amperes. The impressed voltage was a saw tooth voltage having
generally the configuration shown in Figure 6. When the current
had subsequen~ly dropped to 110 milliamperes, the voltage was
increased to 600 volts, raising t:he curren~ to ]~0 milliamperes.
At breakthrough, the voltage was raised to 750 volts and the
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current draw was ~0 milliamperes. The entire process for
drilling througll the worl~piece to provide a hole of uniform
diameter took 3-1/2 minlltes.
EXAMPLJ~ 2
Another workpiece was drilled using the electrolyte,
electrolyte pressure and gap of Example 1 except the starting
voltage was 500 volts (rms). Breakthrough resulted in two
minutes, whereupon the voltage was raised to 750 volts and main-
tained at that value for thirty seconds. The workpiece was found
to have a uniform diameter hole with no significant washout.
The process of the present invention is believed to be
more effective than conventional impingement processes because
it allows pulses of higher voltages to be applied safely to the
electrode, thereby supplying more power for hole drilling.
Furthermore, the pulsing of the negative DC source allows the
electrolyte liquid to remain cooler and less possibility exists for
providing superheated steam at the working tip of nozzle~ With
the system of the present invention, many closely spaced holes
can be drilleA simultaneous].y, a capability not ordinarily possessed
by impingement processes of the prior art.
The wear on glass electrodes when using a convention-
al DC power supply having a ripple voltage of 15Yo or less was
compared with the wear on identical electrodes using the pulsating
DC power supply of the type described in this application. The
following wear life figures compare the capillary length change of
the electrode after vc~-ious hours of operation.
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Cl~PILLARY LENGTII CIIANGE
Ho rsFil~ercd D. C.__ Pulsatill~, D. C.
. 003 0
. 005 0
0075 o
. 0092 0
100 .011 0
120 . 0132 0
140 . 0153 0
160 . 0171 . 001
180 . 0193 . 0022
200 . 022 . 0025 :
The drawings illustrate a process wherein the
positioning of the workpiece and the nozzle are fixed. The power
supply of this invention is also applicable to the standard feed
drill p~ocess wherein the nozzle is moved into the hole as drilling
progresses and the movement of the nozzle is terrninated when
breakthrough is achieved.
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