In this paper, the mechanism of single discharged crater shape, repeated discharging crater distribution, and surface roughness formation on electrical discharge machining is described.
1. On the single discharged crater shape, the crater diameter is shown as follows.
d=a·V^ξ_d·C^η
where d: the crater diameter (mm)
V_d: initial discharge voltage (V)
C: condenser capacity (μF)
a, ξ, η: constant
The crater depth is analysed on the view point of thermal conduction theory based on the assumption that the anode surface temperature is limited by the boiling point of the materials during the discharge duration, as shown in the following.
θ_m=θ_b[1-erfh₀/2√k·t_s]
Where h₀: calculated depth
θ_m: melting point of electrode material
θ_b: boiling point of electrode material
k: thermal diffusivity
t_s: discharge duration
The crater depth of experimental results is nearly equal to the calculated depth in the range of 100 μF>C>0.01 μF condenser capacity. However, when the condenser capacity range is 800μF>C>100 μF, they are not agreed. In the latter case, there remained rapidly cooled molten metal at the bottom, which is called columner crystal, and the crater is shallower.
2. The swollen upheaval of discharged crater makes short circuit between both electrodes after discharge, and its swelling up time is found very long as compared with a discharge duration.
3. In case that several discharge strikes are made with an ample time interval on plain electrodes, the craters are placed closely. Namely one crater is superposed on the upheaval of another and it is considered that the upheavals of the crater introduce next discharge.
If, by selecting a charging resistance property, the next discharge is made before the upheaval makes sufficient growth, the discharges scatter. If the charging resistance is reduced further, the discharge concentrates on one point to become a continual arc instead of intermittent discharges, making only one large cavity.
If smaller condenser capacity is used, the discharges scatter easily. High frequency discharges also scatter. So, in actual discharge machining, scattering condition is desirable to be selected.
4. The relationship between surface roughness and single discharged crater shape is found out as follows,
H_max=2h₁+h₂
where H_max: surface roughness shown by peak to valley height (μ)
h₁: depth of single discharged crater
H₂: maximum height of crater upheavals (μ)
μ: micron

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The results of some consideration and experiments about the observational distortions of a wave-form of the discharge current or the voltage across electrodes in a condenser discharging circuit used mainly in the electric discharge machining are described.
1. Concerning the damped oscillating current,
(i) the amplitudes of high frequency components of the current,
(ii) the skin-effect error of shunt with a straight resistance wire (circular crosssection) and
(iii) the effects of the residual inductance of shunt are given in equations or graphs.
These equations and graphs offer us convenience on designing of a current measuring circuit or on examining of an observed current wave-form.
2. Even at the voltage and current comparable with those in the electric discharge machining, the inductive interference exerted on the part which picks up a signal voltage may not be negligible unless the part is suitably shielded.
3. The disturbing voltage by electromagnetic induction, which is induced by the residual inductance of shunt also, is originally to bring about jumpings or projections in the observed wave-form, but practically the projection may not appear at high discharge voltage.
The appearance of the jumping or the projection may reqire us to examine whether it has been due to such the essential factors belonging to the discharging gap as the residual ions or not.

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The melting and vaporizing of metals at an electrical discharged point is one of the most important matters for the theoretical studies of electro-discharge machining. In this report, some models of temperature distribution due to the heat conduction at the electrical discharged point are discussed, and quantities of heat necessary to melt or vaporize an unit volume of metals are calculated. A reciprocal of the quantity of heat is estimated as a heat efficiency of machinability, and efficiency diagrams of machinability are discussed for iron and other metals.

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Discharge machining which makes use of the small gap electric discharge in oil is occupying an important position in the machining of punch and die of press. In this case, howmachining clearance should be adjusted becomes a problem to be solved.
In order to adjust the discharge machining clearance, the author used the double spark circuit which consists of subordinative and main electric power sources.
The paper describs the principle and method by which discharge machining clearance is adjusted and also presents some results of experimental investigations as follows:
(1) The discharge machining clearance is adjustable by means of controling the firing voltage.
(2) The roughness of machining surface is independent of the machining clearance.
(3) The discharge machining using high firing valtage can reduce the decreace of machining speed in deep hole.

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With the aim of obtaining some fundamental data for clarifying the mechanism of the electrode-erosion in the electro-discharge machining, the influence of the inductance in the fundamental R-C circuit on the erosion-characteristics of the electrode-metals was studied.
The experiments were carried out with both the continuous repetition of discharge method and the method of the controlled repetition of single discharge.
It was recognized that a decided relatition exists between the wave-form of the discharge and the erosion-characteristics of the metals.
The results of the experiments are : (1) In machining steels withe brass, the rate of machining and the rate of the electrode-erosion are improved by increasing the inductance of the discharge circuit. (2) In machining hard alloys with copper, by the inversion of the polarities of the electrodes and the increase of inductance, the value of 1112% is obtained as the rate of the electrode-erosion. This is the most interesting result the authors have ever obtained.

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In the electrical discharge erosion circuit which consists of capacitive reactances in the a-c side and discharge condensers in the rectified d-c side, some experiments were carried out as to its erosion speed for a hardened steel and the ratio of electrode-wear, and obtained some fundamental informations on the singularity of this circuit.
This paper describes some experimental results obtained and a simple analysis of the circuit.

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In order to keep efficient electrospark machining the discharge gap between electrode and work should be controlled by automatic electrode feeding apparatus.
For the automatic electrode feeding apparatus, the continuous servomechanism is commonly used. The authors applied the on-off servomechanism utilizing stepping moters to the apparatus.
The features of stepping motor that the overshoot at stopping is very small and the response is very high enable the realization of on-off servomechanism without hunting. The applied stepping motor is the two phase type with mechanical one-way brake.
For driving stepping motor, flip-flop method, AC method and DC superposed AC method were developed.
To investigate the performances of stepping motor driven feeding apparatus, working tests by means of RC discharge circuit were done. The test results expressed by relationship between working speed and work surface roughness indicate that stepping motor driven feeding servomechanism with flip-flop driving method or AC driving method is effective.

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To remove sludge produced in a dielectric fluid during electrical discharge machining, it is a usual practice to use a mechanical filter. The mechanical filter, however, is poor in filtering capacity and tends to be clogged.
This paper is concerned with performance of an electrostatic filter utilizing electrophoretic phenomena, illustration of which is made by YASPARK FILTER F-60. This filter, (tank capacity of 60 l and filtering capacity of 20 l/min), removes sludge completely from the fluid in electrical discharge machining on 1.5 kW power supply, making possible continuation of machining with the fluid free from sludge.

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