Wire EDM machine

[Robin Hewitt]

So if someone wanted to rip off all your good work and build one of these things to play with, what do they have to do? I am a long way from Bristol.

[Neale]

So if someone wanted to rip off all your good work and build one of these things to play with, what do they have to do? I am a long way from Bristol.

Interesting question, but probably not a relevant one. As has already been suggested and discussed, what we have is by way of a working prototype. Yes, it does work. However, if any of the team felt motivated enough to build another, it would use some of the same principles and key bits of technology, but probably wouldn't look that much like the one we have. By the same token, no-one in their right mind would want to make an exact copy of what we have built. That's exactly the same reason that I, or many other members of this forum, wouldn't bother to publish plans of their CNC router or whatever because the majority of people don't want to build a copy, they have their own ideas that they want to contribute. There are common basic principles but the application of them can and probably will differ for each machine builder. Despite many pleas for "publish a design that anyone can build!", it ain't happened yet and probably won't. Same for our wire EDM machine. No point in publishing details as someone skilled and experienced enough to build one of these would want to change a lot of it. Look at the number of threads on this forum that deal in some way or other with noise and grounding problems with a router. That is dealing with a bit of EMI from a spindle or motor drive pulses, probably an order of magnitude or two below the kinds of EMI when you have digital electronics a few inches away from a spark transmitter and trying to reproduce someone else's design won't necessarily help much with that.

However, there is nothing secret about the working principles here, it's just that there is little point in detailed circuit diagrams designed around what was to hand as much as the ideal components, and the electronics is also tightly coupled to code running in the PIC microcontrollers. Again, nothing secret about the code, it's just that it would be difficult to reproduce in a different environment.

Distance is not an issue as the original design was put together by a team based variously in Aberdeen, South Devon, and various locations in the home counties, often via video-conferencing. I'm happy to discuss any technical points here on the forum or by PM, although the topic is really not mainstream for this forum (the difficult and interesting bits of the project are not directly the CNC stuff, the mechanics of which are trivial) and the wire EDM Yahoo group (plasmaboog) is the place to go for more technical and specialist discussion. That has a number of contributors with strong backgrounds in building and developing wire EDM technology on this kind of small scale. I'm not ducking the issue; I'm just feeling that this isn't the appropriate place to have brought up the topic. But it's here now, it is what it is, warts and all.

[phill05]

Neale,

You have made some interesting reading so far, I cleared the workshop of an old friend when he passed on and was left with a lot of old drawings to good to throw out among them dated 1977 there are A1 size layout and construction drawings of a dry electric discharge machine, if they are of any interest to your group feel free to PM me.

Phill

[magicniner]

Neale,

You have made some interesting reading so far

All I've gleaned from this so far is that something wonderful has been achieved but it's all too complicated for it to be worth any of us seeing anything beyond a vague overview!

[Robin Hewitt]

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.

[magicniner]

power it from both ends of the wire which sounds tricky

Thanks Robin, I needed a laugh today! :D

[Neale]

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.

[komatias]

Good to meet you again at the BSMEE Neale.

Saw it in action, really fun to watch. Cut out aluminium letters about the size of a 5p within a minute or so.