Methods and apparatus for monitoring the condition of dielectric liquid in electric discharge machining

• US 4,392,110 A
• Filed: 06/12/1980
• Issued: 07/05/1983
• Est. Priority Date: 06/12/1979
• Status: Expired due to Term

• Current Assignee: National Research Development Corporation
• Original Assignee: National Research Development Corporation
• Inventors: El-Menshawy, Mohamed F., Bhattacharyya, Sushantha K., Woodrow, Peter A.
• Primary Examiner(s): Tokar, Michael J.
• Application Number:
  US06/158,813
• Time in Patent Office: 1,118 Days
• Field of Search: 324/446, 324/448, 324/204, 324/71 CP, 204/129.2, 204/129.25
• US Class Current: 324/453
• CPC Class Codes: G01N 27/221 by investigating the dielectric properties using microwaves G01N22/00; measuring loss factors or dielectric constants per se G01R27/26
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G01R 31/1281 of liquids or gases

Abstract

Methods and apparatus for monitoring the condition of dielectric fluid used in EDM are described. A continuous sample of dielectric liquid is withdrawn from a tank containing the EDM electrode and workpiece and is passed between electrodes of a capacitance cell before being returned to the tank. The capacitance between the electrodes gives an indication of the amount of metal particles resulting from EDM in the dielectric liquid. In another arrangement a magnetic field is used to direct most ferrous particles through a capacitance cell or part of a capacitance cell while most non-ferrous particles pass through another cell or another part of the cell. Measurement of the capacities of the cells or parts of cells provides an indication of the relative amounts of ferrous and non-ferrous particles in the dielectric liquid. The relative concentrations of particles of different sizes can be measured by passing the sample dielectric through a number of cells with each cell having a different pore-size filter at its inlet. The capacitance of each cell then indicates the amount of particles within a predetermined range in the dielectric liquid.

Claims

1. A method of monitoring the condition of dielectric liquid being used in an electrical discharge machining process, comprising:

causing at least a portion of the dielectric liquid used in an electrical discharge machining process to flow in contact with two adjacent but separated electrodes, and

at least from time to time, deriving a signal representative of change in at least one component of the impedance across the electrodes due to change in condition of said liquid.

10. A method according to claim 1 wherein change in one component of the impedance across the electrodes is indicated by a change in a signal representing one component of the impedance across the electrodes.

2. Electrical discharge machining apparatus including means for monitoring the condition of dielectric liquid used in electrical discharge machining apparatus, comprising:

a cell defining a passage for liquid to flow in contact with two adjacent but separated electrodes,

circulating means for circulating at least a portion of dielectric liquid being used in an electrical discharge machining process through the cell, and

means for deriving, while machining is taking place, signals representative of change in at least one component of the impedance across the electrodes due to change in condition of said liquid.

3. Apparatus according to claim 2 wherein the cell comprises a hollow cylindrical chamber forming one electrode and containing a hollow cylindrical intermediate electrode which surrounds a cylindrical central electrode, the three cylinders being coaxial with one another, and the said two electrodes being selected from the following arrangements:

any two of the said electrodes,

any two of the said electrodes connected to form a single electrode, and

the other said electrode.

4. Apparatus according to claim 2 including means for providing a magnetic field such that ferrous particles tend to pass along one of two paths, said one path being between the said electrodes and the other said path avoiding passage between said electrodes, non-ferrous particles tending, in operation, to pass along the said other path.

5. Apparatus according to claim 2 including a further pair of separated electrodes in the said cell, the spacings between the electrode pairs providing parallel paths for the dielectric liquid, and means for providing a magnetic field such that most ferrous particles in the dielectric pass between one pair of electrodes and most non-ferrous particles pass between the other pair of electrodes.

6. Apparatus according to claim 2 including at least one further cell, the cells being connected to provide respective parallel paths for the dielectric liquid sample, and a number of filters, one for each cell and located at the inlet thereto, each filter passing particles in a size range which is different from that of the other filter or filters, and an electrode arrangement which allows said signal deriving means to derive respective signals for each cell.

7. Apparatus according to claim 2 wherein the cell contains at least one further pair of separated electrodes, the electrodes of each pair are positioned opposite one another with a passage for dielectric liquid between the electrodes of that pair, and the spacings between the electrodes of each pair differ from pair to pair.

8. Apparatus according to claim 2 wherein signals representative of change in capacity, between the electrodes are derived by the means for deriving signals.

9. Apparatus according to claim 8 wherein the means for deriving signals comprises an oscillator which includes the capacity between the electrodes and which has an oscillation frequency dependent on the said capacity, and a phase-locked-loop circuit with an input terminal connected to receive signals from the oscillator and an output terminal at which a voltage representative of the said oscillation frequency appears.

11. Electrical discharge machining apparatus according to claim 2 wherein the means for deriving signals representative of change in at least one component of the impedance across the electrodes is constructed to provide an indication of successive values of the said impedance.

12. A method of monitoring the condition of dielectric liquid being used in an electrical discharge machining process, comprising:

causing at least a portion of the dielectric liquid used in an electrical discharge machining process to flow in contact with two adjacent but separated electrodes, and,

at least from time to time, deriving a signal representative of at least one component of the impedance across the electrodes, the said component being dependent on the condition of the said liquid.

13. Electrical discharge machining apparatus including means for monitoring the condition of dielectric liquid used in electrical discharge machining apparatus, comprising:

a cell defining a passage for liquid to flow in contact with two adjacent but separated electrodes,

circulating means for circulating at least a portion of dielectric liquid being used in an electrical discharge machining process through the cell, and

means for deriving, while machining is taking place, signals representative of at least one component of the impedance across the electrodes, the said component being dependent on the condition of the said liquid.

14. Apparatus according to claim 2 or 13 including a further cell, the cells providing parallel paths for the dielectric liquid, and means for providing a magnetic field such that most ferrous particles pass to one cell and most non-ferrous particles pass to the other cell.

15. Electrical discharge machining apparatus comprising:

a tank which, in operation, contains a workpiece,

a primary electrode,

a first electrical supply for applying voltage between a workpiece and the primary electrode such that electrical discharge machining of the workpiece by discharges from the electrode takes place,

a pair of secondary electrodes located, at least in operation, in the tank,

a second electrical supply for applying a voltage between the secondary electrodes which is just smaller than the voltage which causes breakdown between the electrodes when the dielectric liquid is not significantly contaminated,

discharge-detection means for detecting discharges between the secondary electrodes, and

voltage control means, coupled to the discharge-detection means, for reducing the voltage applied between the secondary electrodes by a predetermined decrement when a discharge is detected as indicated by the discharge-detection means, the voltage applied between the secondary electrodes providing an indication of contamination of the dielectric liquid.