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

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 A continuous sample of dielectric liquid is withdrawn from a tank containing the EDM electrode and workpiece and is passed between electrodes of a capacitance cell before being returned to the tank.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.