Graphite Electrodes in Electric Discharge Machining
Effects of Properties of Electrodes on Machining Ability
1974 Volume 23 Issue 249 Pages 464-469
Details
1974 Volume 23 Issue 249 Pages 464-469
Machining ability in electric discharge machining is judged by means of four factors, “machining speed”, “electrode wear ratio”, “surface roughness of worked pieces”and“isotropism of working clearance”. Since it is generally believed that the machining ability depends largely on the properties of graphite materials used as the tool-electrodes, it is obviously useful for the development of the most suitable graphite materials to make clear the relationship between the properties of graphite electrodes and the factor influencing machining ability.
This report describes some of the test results examined on a series of graphite materials of various physical characteristics as well as of various grain constitution (both in shape and size) in relation to the effects of their properties on those judging factors. Experiments were carried out on an electric discharge machine currently in use, with graphite as an anodic tool-electrode under the common discharge conditions that have been proven to be optimum for graphite electrodes.
The results obtained are:
(1) “Machining speed”increases apparently with lowering electric resistivity of graphite electrodes as being generally considered, but since the resistivity is influenced decisively by the grain size distribution of raw materials, “machining speed”would rather be regarded to depend on the grain size.
(2) “Electrode wear ratio”and“surface roughness”are, in contrast to the general belief, independent of any of the physical properties of the electrodes but depend significantly on the grain size distribution.
(3) It should be pointed out that the above mentioned two factors tend to change sigmoidally in the range of mean grain size of 5 to 10μm.
(4) “Anisotropy ratio”of working clearance decreases with decreasing anisotropy ratio of graphite materials being represented by the ratio of CTE's (coefficient of thermal expansion) measured in different directions of“across grain”and“with grain”.
