Quote Originally Posted by Robin Hewitt View Post
I'd guess the clever bit must be in how much power you can feed down that tiny wire without over egging the pudding with Volts and lengthening the spark or over ramping the Amps and melting it. Lots of sparks per second seems to be better than a few fat ones, so you need low inductance and presumably power it from both ends of the wire which sounds tricky 'cause one end is under water and has to reach around the part being cut.
Not quite, Robin. The wire is earthed (it runs over various metal components in the frame that carries it). The workpiece, under water, is connected to the "live" side of the spark generator so that you get a spark between wire and workpiece. The water is non-conductive (there is a deioniser filter and a recirculating water pump to try to keep it clean) so that it does not short out the spark. The actual current during the spark we estimate to be an amp or so, which is not very much and certainly much less than the wire could carry. It's not really practical to try to measure the current so this is a "best guess" based on voltages, capacitor values, spark duration, and similar. The spark duration is also quite short - maybe 100microsec or so - so each individual spark does not have a lot of energy. The tricky bit is monitoring the spark voltage so that we know whether to advance the wire by a step (no spark so must be too far away), leave it where it is (spark happened) or move it away (shorted out - maybe wire touched workpiece or gap filled with debris from cut). On paper that's not too difficult, but actually implementing a system which reliably measures the relevant voltages in the presence of high levels of interference from the sparking isn't so easy.

If anyone is interested (and I get the feeling that this isn't very interesting to members of this forum but I'll say it anyway), then the machine should be on display and working at the Midlands Model Engineering Exhibition next month. One of the build team should be there with it to answer questions. Its next public outing will probably be at the Alexandra Palace model engineering exhibition in January.