Electric discharge machine using a ceramic filter to filter the working fluid
BACKGROUND OF THE INVENTION 
     This invention relates to an improved electric discharge machine. 
     An electric discharge machine has numerous applications, such as machining of materials with poor machinability, and can be used to generate almost any complicated geometry. Electric-discharge machining is based on the principle of erosion of metals by spark discharges. The discharge occurs when a potential difference between a tool and a workpiece is large enough to cause a breakdown in a dielectric fluid or work fluid such as an oil. The discharge is rapidly repeated, and each time a small amount of workpiece material is removed so as to produce particles. 
     The cleanliness of the work fluid has a significant effect on occuracy of the machining. If many particles removed from workpieces are suspended between a tool and a workpiece in the work fluid, the discharge easily occurs, so that rates of metal removal can not be precisely controlled. 
     To ensure that the work fluid is clean, the work fluid containing the removed particles has been filtered with a filter paper or a filter cloth coated with clay or diatomaceous earth. 
     However, in the case of an electric discharge machine equipped with such a conventional filter, the replacement of clogged up filters is often required. Thus, the workability is decreased. Also, nonuniformity of a coating formed on a filter cloth allows some removed particles to pass through the filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic explanation view showing an electric discharge machine according to an embodiment of this invention; 
     FIG. 2 is a schematic explanation view showing a modified portion of the electric discharge machine; 
     FIG. 3 is a graph showing a relationship between pore diameter and cumulative intrusion volume of a ceramic filter for use in the electric discharge machine; and 
     FIG. 4 is a graph showing a relationship between machining number and accuracy of a ceramic filter according to this invention in comparison with a prior art filter. 
    
    
     DESCRIPTION OF THE EMBODIMENT 
     Referring to FIG. 1, an electric discharge machine includes a clean work fluid tank 1 for storing clean work fluid such as oil or pure water, a work tank 4 in which a workpiece or workpieces (not shown) are set in a conventional manner, a settling tank 6 for settling particles produced by the electric discharge in the work tank 4, a pump 2, a pipe 3 for supplying the clean work fluid into the work tank 4 by means of the pump 2, and a pipe 5 connected to the settling tank 6. In the work tank 4, a transient electric discharge repeatedly occurs through a small space between two electrodes which are the workpiece and a tool immersed in the work fluid. As the particles are suspended in the work fluid, the work fluid functions to carry away the particles produced by the electric discharge. The work fluid containing the particles is sent through the pipe 5 to the settling tank 6. 
     A ceramic filter 10 is arranged in a filter container 9. The settling tank 6 is connected to one end of the ceramic filter 10 through a pipe 8 and a pump 7. The other end of the ceramic filter 10 is connected via a pipe 11 to the settling tank 6. When the work fluid is fed by the pump 7, it is forced to pass through the ceramic filter 10 in such a way that a pressure in the ceramic filter 10 is larger than a pressure outside of the ceramic filter 10. The filtrate or work fluid which has been filtered by the ceramic filter 10 is discharged through a port 9a of the filter container 9 into the clean work fluid tank 1. The particles are caught by the ceramic filter 10. The work fluid which has not been filtered by the ceramic filter 10 is sent back through the pipe 11 into the settling tank 6. 
     It is preferable that the ceramic filter 10 is back-washed in the filter container 9. For example, as shown in FIG. 2, a piston-cylinder type back washing apparatus 12 is attached to the filter container 9 so as to back wash the ceramic filter 10 with the filtrate remaining in the filter container 9. A piston 13 can be actuated by an air cylinder means (not shown). Reference numerals 14 to 17 denote valves for controlling liquid flows. Reference numerals 18 and 19 denote means for interrupting the liquid flow. The valves 14, 16 and 17 are closed when the ceramic filter 10 is back washed. Reference 20 denotes a flow meter. 
     Preferably, the ceramic filter 10 includes a filter layer having a thickness of 10 and 40 micron meters (hereinafter called merely microns),from a filter surface. When it is measured by a mercury porosity meter, a cumulative intrusion volume of all pores in the filter layer is 0.2 cc/g or less. The cumulative intrusion volume of the pores having pore diameters of 0.1 to 3.0 microns is 0.1 cc/g or more. The cumulative intrusion volume (H) of the pores ranging within the pore diameter width (w) of 0.1 microns around a center pore diameter (PD), which is the pore diameter for a half of a cumulative intrusion volume (IV) at the pore diameter of 0.1 microns, is 50% or more of the cumulative intrusion volume of all pores. 
     If the thickness of the filter layer is less than 10 microns, high strength can not be obtained, and nonuniformity can not be avoided. If it is more than 40 microns, the filtration performance is decreased, and the filter layer is sometimes broken away due to heat expansion. 
     If the cumulative intrusion volume of all pores is more than 0.2 cc/g, a desired strength can not be easily obtained. If the cumulative intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns is less than 0.1 cc/g, the filtration rate is decreased. 
     Referring to FIG. 3 showing an example of the ceramic filter (10), the cumulative intrusion volume of all pores is 0.18 cc/g. The cumulative intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns is 0.17 cc/g. The center pore diameter (PD) is 0.2 microns. The cumulative intrusion volume (H) of the pores ranging within a pore diameter width (W) of 0.1 microns is 0.12 cc/g, the percentage of which is 66.7%. This cumulative intrusion volume (H) is larger than a half of the cumulative intrusion volume (IV) at a pore diameter of 0.1 microns. 
     The filter layer preferably consists of plural layers, for example, such as an intermediate layer coated on the inner surface of a support layer and an inner layer coated on the intermediate layer. The intermediate layer is made of alumina powders having a article size of 2 to 10 microns, and the inner layer is made of alumina powders having a particle size of 0.4 to 1 microns. 
     FIG. 4 shows relationships between the number of electric discharge machining and the accuracies of machining with respect to a ceramic filter of this invention and a conventional filter including a filter cloth coated with clay. The filters are set in the electric discharge machine of FIG. 1. The tool is a wire. The workpieces are for automobile parts having an accuracy of ±0.01 mm in size. When the ceramic filter 10 of this invention is used, the flow rate of the work fluid is 2.5 m/sec and the pressure difference is 1 kg. In case of the conventional filter, a filter is replaced whenever about 100 parts are machined. When 10,000 parts are machined, the number of inaccurate parts is from 100 to 120 according to the conventional filter while that is only from 3 to 10 according to this invention. 
     This application incorporates by reference the disclosure of co-pending U.S. application Ser. No. 087,290, filed Aug. 20, 1987, which disclosed construction details of a filter unit using the ceramic filter of this invention, and co-pending U.S. application Ser. No. 087,352, filed Aug. 20, 1987, which is directed to the ceramic filter used in this invention. This application also incorporates by reference the disclosure of co-pending U.S. application Ser. No. 087,357, filed Aug. 20, 1987, which discloses a further specific use for the ceramic filter used in this invention to purify a reaction mixture used in the manufacture of esters. 
     According to this invention, the workability can be improved. The number of used filters to be discharged is decreased because each ceramic filter has a long life time. A lot of accurate parts can be machined at a high efficiency.