Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – Gap maintenance or defined tool-workpiece gap
Reexamination Certificate
1999-04-02
2001-04-10
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
Gap maintenance or defined tool-workpiece gap
C205S654000, C205S685000, C205S686000, C204S22400M, C204S225000
Reexamination Certificate
active
06214200
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of electrochemically machining a workpiece by means of an electrode, comprising the steps of
bringing the workpiece and the electrode into contact with one another,
setting a working distance between the electrode and the workpiece while keeping the gap between the workpiece and the electrode filled with an electrolyte, and
passing an operating current through the electrolyte in the gap via the electrode and the workpiece in order to machine the workpiece.
The invention further relates to an arrangement for electrochemically machining a workpiece by means of an electrode, in which a working distance is set between the electrode and the workpiece while the gap between the workpiece and the electrode is filled with an electrolyte, and in which the workpiece is machined by passing an operating current through the electrolyte in the gap via the electrode and the workpiece, which arrangement includes
a base for positioning a workpiece,
a holder for positioning an electrode,
an actuator for moving the holder and the base with respect to one another,
a detection device for detecting contact between the electrode and the workpiece, and
a control unit for controlling the actuator.
Moreover, the invention relates to a control unit for controlling an actuator in an arrangement for electrochemically machining a workpiece by means of an electrode.
Such a method is known from the publication “Modelling and Monitoring Interelectrode Gap In Pulse Electrochemical Machining” in Annals of the CIRP Vol. 44/1/1995. In the known method a workpiece is machined by means of an electrode in an electrolyte consisting of a 15% solution of NaCl in water. Said publication states that the machining accuracy is higher when a small working distance is provided between the electrode and the workpiece. In a test mode a working distance of at least 0.1 mm is set, after which an operating current is passed through the electrolyte in the gap between the workpiece and the electrode. According to the publication this working distance is usually set by bringing the electrode and the workpiece into contact with one another and subsequently positioning the electrode at a desired distance from the workpiece. A disadvantage of this method is that in the case of a small working distance being set it is not unlikely that a flash-over or a short-circuit occurs between the electrode and the workpiece when the operating current is applied, as a result of which the electrode and/or the workpiece are damaged by the heat produced by the operating current at the location of the flash-over or the short-circuit.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce the likelihood of a flash-over or a short-circuit. To this end, the method in accordance with the invention is characterized in that the working distance is set by
increasing the distance between the electrode and the workpiece by a first distance starting from a situation of contact between the electrode and the workpiece, followed by
detecting whether the contact between the electrode and the workpiece is broken, and
increasing the distance between the workpiece and the electrode by a second distance if the detection shows that the contact between the electrode and the workpiece has been broken.
The invention is based on the recognition of the fact that moving the electrode and the workpiece away from one another over a desired distance after they have brought into contact does not always result in the desired working distance being set. Owing to a resilient element the resulting working distance may be smaller than the desired working distance and it is even possible that electrical contact persists. The likelihood of this is substantial if the desired working distance is of the order of micrometers. Said resilient element may form part of the arrangement used for positioning the electrode with respect to the workpiece and/or may be formed by a burr or contaminant in the gap. If the working distance is smaller than desired, the current density is higher than intended and there is a large risk of gassing in the gap. As a result of this, there is a large risk of flash-over. If electrical contact persists after the working distance has been set the current density at the location of this electrical contact owing to the operating current will be so high that the electrode and/or the workpiece may be damaged. The measures in accordance with the invention ensure that the actual gap has at least the magnitude of the second distance. This precludes short-circuiting or the generation of a flash-over. As a result of these measures the risk of damaging of the workpiece and/or the electrode is reduced substantially and a working distance of an order of magnitude of micrometers can be set in a reliable manner.
The measure of setting working distance by a method wherein the first distance is a predetermined distance and the first distance is comparatively small with respect to the second distance has the advantage that the uncertainty as regards the magnitude of the working distance set is small with respect to the average magnitude of the working distance set. This results in a uniform dissolution rate and an accurate reproduction of the electrode.
The measures of setting working distance by a method wherein the distance between the workpiece and the electrode is increased by a third distance, after which the electrolyte in the gap is renewed by flushing, upon which the workpiece and the electrode are again brought into contact with one another if the contact between the electrode and the workpiece is found not to be broken after the predetermined distance has been set have the advantage that if the electrical contact is not interrupted by the presence of a burr or a contaminant the burr or the contaminant can be detached by increasing the distance between the electrode and the workpiece and can, for example, be flushed away with the electrolyte. The optimum value for the third distance depends on the geometry and will be approximately 0.1 mm in practice.
The measure of setting working distance by a method wherein the steps of bringing the workpiece and the electrode into contact with one another and setting the working distance (dw) between the electrode and the workpiece are repeated periodically has the advantage that a possible drift in working distance is corrected periodically and that the electrolyte can better flow through and, as a consequence, can better renew when the gap is enlarged.
The measure of setting working distance by a method wherein as a result of the operating current material of the workpiece is dissolved and during the passage of an operating current through the electrolyte the electrode and the workpiece are moved towards one another in accordance with an estimate of the rate (Vf) at which the material is dissolved has the advantage that the magnitude of the working distance remains substantially constant during the dissolution of the workpiece.
The measure of estimating the dissolution rate (Vf) from the relative displacement between the electrode and the workpiece between two contacting phases is a simple method for estimating the dissolution rate.
The measure of detecting contact between the electrode and the workpiece by connecting a current-limited voltage source between the electrode and the workpiece and monitoring the voltage difference between the electrode and the workpiece has the advantage that the presence of contact between the electrode and the workpiece can be detected by simple means.
The measure of employing a voltage source that supplies a voltage (Um) between 1 and 3 V has the advantage that, on the one hand, the voltage is high enough to minimize noise and battery effects between the workpiece and the electrode and, on the other hand, the voltage is low enough to preclude dissolving of the electrode or the workpiece.
The measure of using an electrolyte comprised of NaNO
3
has the advantage that the workpiece dissolves at a high rate when the current density
Altena Hermanus S. J.
Boorsma Anton M.
Brussee Maarten
Kramer Foppe
Bartlett Ernestine C.
U.S. Philips Corporation
Valentine Donald R.
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