I thought this may be of interest to the group as it's been a topic a few times inthe recent past. I recently discovered a model steam engine casting I'd buggered as a teenager some 35 odd years ago by breaking a 7BA tap in a hole. Given the prices Stuarts are charging these days I thought it might be worth trying to recover the situation.
I don't know anyone near me who might do spark erosion and briefly considered a home-brew attempt, but some research indicated it would need a fair investment in time and basic equipment just to 'have a go'. However, along the way I also noted some of the details for electro chemical machining, which seemed to offer an alternative with potentially less overhead to start with.
So armed with a battery charger and a couple of pints of salt water (no idea what concentration, it was sea-water) I set to work. The tap was broken off just 'under flush' as one would expect. Initially I used a piece of 1.5mm2 core from some lighting wire, cut off flush at the end ie only the end of the wire exposed. I held it against the end of the relevant hole. Switched on the charger and there were immediately some bubbles from the hole and then nothing. I concluded that the bubbles effectively drove the electrolyte out and then the current stopped so I shaved the sides of the insulation to allow the brine to flow. Continued bubbles - promising. I left it for half an hour and came back 'just to see if anything had happened' and was staggered to find the hole had been opened up to the diameter of the insulation an down to the top of the tap. So I swapped the 1.5mm2 for a piece of single filament from some telephone wire and re-started the charger.
Having seen the effect I made sure I came back every 15~20 mins to stir the sludge in the hole and push the tip of the wire further down and ensure it was in the centre of the hole and not resting against the side. In this way I disolved the entire tap in under 2 hours.
Given the extremely rudimetary setup I had and the complete lack of accurate control of either the current or the electrode position I consider this an amazing success. I may just have been very lucky, and I would make a somewhat more engineered rig for use if I need it again. I don't often break taps, but such a simple in-house means of sorting the problem seems much better than having to hunt round for tool makers with spark eroders or the restrictions of sulphuric acid provided it's in brass etc. Just make sure the work-piece is the anode or you'll build-up the broken end, not disolve it!