This paper deals with the micro-cracks which were generated by EDM of carbon steel (S45C) and tool steels (SK3 and SKD11). The cracks were studied experimentally under various machining conditions. The results are as follows:
(1) The critical pulse duration which does not generate the cracks is from 20 to 40μs for these steels and it becomes shorter as the peak current decreases.
(2) It is desirable to consider both the crack depth and the crack density when the surface damage occurs by the cracks. The crack depth becomes deeper with increasing pulse duration or peak current and the maximum depth of cracks is about 20μm.
(3) The cracks in S45C and SK3 are confined to the solidifying white layer and often the ones in SKD11 extend into the parent metal below the white layer.
(4) In order to reduce the surface damage based on the cracks, it is necessary to machine with he pulse duration which is as short as possible. And for this purpose the condition is suitable under which the spark machining is stable without localized arcing.