I published the plans for the 12 VDC EDM you've seen at Cabin Fever and NAMES in the last issue of Model Engine Builder magazine. I'll also be placing the article in our free newsletter which will be out this month.
The existing design leaves a lot to be desired starting with a lack of precision when positioning the electrode. Could not really use it to burn out a 0-80 screw or tap with any degree of certainty. But it will burn a hole in steel in a reasonable amount of time. Soft steel or HSS doesn't make much difference to the burn time.
I'm asking anyone with a better idea to contribute it to another article.