Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With control responsive to sensed condition
Reexamination Certificate
2002-05-06
2003-09-16
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With control responsive to sensed condition
C205S645000, C205S652000, C204S228700
Reexamination Certificate
active
06620307
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of electrochemically machining an electrically conductive work piece in an electrolyte by applying bipolar electric pulses between the work piece and an electrically conductive electrode, one or more voltage pulses of unipolar machining polarity being alternated with voltage pulses of opposite polarity, while a gap between the work piece and the electrode is maintained, which gap is filled by the electrolyte.
The invention further relates to an arrangement for electrochemically machining of an electrically conductive work piece in an electrolyte by applying bipolar electric pulses between the work piece and an electrode, one or more voltage pulses of unipolar machining polarity being alternated with voltage pulses of an opposite polarity, while a gap between the work piece and the electrode is maintained, which gap is filled by the electrolyte.
2. Description of the Related Art
Electrochemical machining is a process in which an electrically conducting work piece is dissolved at the location of an electrode while electrolyte and electric current is supplied. For this purpose, the electrode is brought in the proximity of the work piece and, while electrolyte is fed into the gap between the work piece and the electrolyte a powerful current is passed through the work piece and the electrode via the electrolyte, the work piece being positive with respect to the electrode. The current is applied in the form of machining pulses having a given amplitude and duration. In the intervals between the machining pulses the electrolyte is renewed. Under the working conditions the work piece is being dissolved, thus increasing the value of the gap between the work piece and the electrode. To compensate for this, the electrode and the work piece are moved towards one another with a given feed rate, as a result of which the electrode forms a cavity or eventually a hole in the surface of the work piece, the shape of the cavity or hole having the shape corresponding to the shape of the electrode. This process can be used, for example, for making intricate cavities or holes in or for shaping hard metals or alloys. The copying precision with which the shape of the cavity or the hole in the work piece corresponds to the shape of the electrode is important for the quality of the result.
A method to perform electrochemical machining where bipolar voltage pulses are utilized is known from U.S. Pat. No. 5,833,835. A pulse component of opposite polarity to that of the machining pulses is used to remove deposits on the front surface of the electrode. The amplitude of the pulses of the opposite polarity is limited by the condition of a wear of the surface of the electrode. This condition is checked by performing a test based on a value of the polarization voltage between the work piece and the electrode after a termination of machining pulses.
The disadvantage of the known method is the fact that the process of the removal of cathode depositions is not controlled, as there is no information available in the system upon the extent of the cathode depositions. Therefore, it is possible that the surface of the cathode is not completely cleaned, leading to deviations in the effective geometrical shape of the cathode. This results in an inferior accuracy of the electrochemical machining.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method to improve the accuracy and the efficiency of the electrochemical machining, due to an improved process of determination of cathode depositions followed by a controlled removal of the depositions from the front surface of the electrode.
The method according to the invention is characterized in the steps of:
performing a measurement of a first value of an operational parameter that depends on the cleanness of the front surface of the electrode, the first value corresponding to a clean front surface of the electrode;
performing a measurement of a second value of the operational parameter in an interval between the unipolar machining voltage pulses after at least one unipolar machining voltage pulse is applied;
performing a computing of a deviation between the first value and the second value of the operational parameter; and
application of at least one voltage pulse of the opposite polarity only after the computed deviation is non-zero.
According to the technical feature of the invention a measurement of the first value of the operational parameter corresponding to a condition of a clean electrode surface is performed and is further used as a reference value in order to derive the extent of the cathode depositions. It is understood that after application of machining pulses depositions occur on the front surface of the electrode. This phenomenon is pronounced especially under conditions of difficult evacuation of the products of chemical reactions taking place in the gap. The depositions thus formed comprise mainly hydrates and oxides of chemical elements present in the work piece. The mechanism of a deposition formation and its removal under bipolar electrochemical machining is as follows. It is understood that the metals, for example Fe, Ni, Al, Ti, Cr are ionized in water solutions of salts under electrochemical machining of different types of metals. These ionized metals are transported by the electrolyte flow in the vicinity of the cathode, where they form oxides, hydroxides and salts, for example Fe(OH)
3
, Cr(OH)
3
, Ni(OH)
2
, Al(OH)
3
, FeOH(NO
3
)
2
, Fe(OH)
2
NO
3
. These compositions further lead to a formation of positively charged colloids, like [mFe(OH)
3
nFe
3+
(n-x)OH
−
]
2+
. The underlying chemical reactions form a basic knowledge for a person skilled in the art. When these colloids reach the surface of the cathode, they are deposited there in a form of cathode depositions. The main physical processes for the formation of the cathode depositions are electrophoretic transport of positively charged particles and their further adsorption at the cathode surface. In the bipolar mode, the anode processes occur at the cathode surface, when the voltage pulses of the opposite polarity are applied to the gap. This process is characterized by an intense oxygen formation, according to a reaction:
2H
2
O-4e→O
2
+4H
+
The oxygen formation leads to the removal of the depositions from the cathode surface first by means of mechanical rupture of the layer of the depositions. Secondly, an acid is formed in a vicinity of the cathode, with a pH-value in the order of 1-2. Several chemical reactions occur at the surface of the cathode leading to a further removal of the depositions under the influence of the acid thus formed. Thus, the front surface of the cathode is cleaned, as a result of a mechanical influence of the oxygen formation accompanied by a chemical dissolution of the depositions in the formed acid layer.
The chemical and phase composition of cathode depositions is determined by the material of the work piece (anode) and can differ from the material of the electrode (cathode). Thus, properties of the cathode are varied, if the depositions of a different element composition occur on the surface of the cathode. By selecting an appropriate property of the cathode as the operational parameter, the generation of the depositions can be detected. This provides the possibility to obtain the information about the extent of the formation of the depositions on the front surface of the electrode. Thus, one obtains a quantitative information about the extent of the cathode depositions by performing on-line measurements of the operational parameter and by comparing the measured value with a reference value corresponding to a clean surface of the cathode. This information is used in the method according to the invention in order to perform a removal of the cathode depositions in an accurate and controlled way by means of an application of voltage pulses of the opposite polarity if the deviation between the first value and the second
Amirchanova Nailya A.
Belogorsky Aleksandr Leonidovich
Brussee Maarten
Gimaev Nasich Zijatdinovich
Kutsenko Voctor
King Roy
Koninklijke Philips Electronics , N.V.
Nicolas Wesley A.
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