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Neale
10-08-2017, 11:49 PM
This thread is under the "Other Machines..." heading as it doesn't have its own category - yet?

I know that some forum members will be aware of the Society of Model and Experimental Engineers (SMEE), a old-established society with mainly UK but also international membership. A small group of members of the SMEE Digital Group have been collaborating on a wire EDM(*) machine for the last couple of years. For those who haven't come across wire EDM, think of a hot-wire cutter as used for foam, but this time using a spark between the wire and the (metal) workpiece. Enough sparks for a long enough time, and the workpiece gets cut away, leaving a cut around 0.1mm wider than the wire used for "cutting" - and this machine uses 0.25mm wire, so that's a pretty fine cut. Move the wire under CNC control, and you can cut intricate and accurate shapes out of difficult-to-cut materials, including hardened metals that are next to impossible to cut otherwise. Wire EDM sits alongside laser and water-jet cutting as a go-to technology for tricky jobs. If the principle sounds easy (which it is), the practice is a bit more tricky. The wire has to keep moving as it gets eaten away as well, while maintaining tension; the spark voltage has to be accurately controlled by adjusting the position of the wire and thus the spark gap - too close and you need to back off, too far away and there's no spark; the whole thing is working in an incredibly noisy electrical environment as all the electronics is running a few centimetres from this spark transmitter.

If you do a bit of googling on "wire EDM", you will find plenty of big - and I mean pretty big - commercial machines and companies offering wire EDM services. Look hard and you will find a few, a very few, "home-built" wire EDM machines. We now have a working wire EDM machine that is totally self-contained and small enough to go into the back of a car, and as a small demonstration of its capabilities, here are a couple of sample pieces I cut on it earlier this evening:
22513
The letter E is about 8mm high, the star about 4-5mm across. In fact, I had difficulty in finding the star in the tank (the cutting operation happens submerged in distilled water with a jet directed at the cutting area to flush away cutting debris).

I know that there are people who have made claims to fabulous machines on this forum but have offered little proof of their existence. Well, you will be able to see this machine in the flesh at the Bristol Model Engineering Exhibition (http://www.bristolmodelengineers.co.uk/Exhibition/exhibition.html) next week, where with a bit of luck and a following wind, I shall have it running on the SMEE stand for all three days of the show. Forum members are cordially invited to come along, and if you can't find me, just ask anyone on the stand. If you are also a wire EDM user, then you are even more welcome to come along and compare our efforts with your commercial machine!


(*) Electrical Discharge Machining

m_c
10-08-2017, 11:58 PM
WEDM is on my desired toy list.

Are you aware of the plasmaboog yahoo group, or http://www.mechatronicprojects.com/blog/ ?

Neale
12-08-2017, 10:47 AM
Thanks - I had found EDM Yahoo group, but mostly seemed to be concerned with building from a kit of parts available in US. Hadn't seen that blog before - looks as if it started relatively recently.

I'm currently custodian of our team's machine as I'm taking it to Bristol next week so I shall try to get some pictures, and if the technology isn't too much for me, a clip or two of it cutting.

m_c
12-08-2017, 08:33 PM
The mechatronics site is fairly new, but I think the author has been working on the machine for a couple years.

Benny Croonen (IIRC) who started/runs the plasmaboog group has been working on one for a good few years, with the group being a spillover from the EDM group run by Ben Fleming (I've got his original EDM book/plans).

I've liked reading the periodic posts regarding the problems faced. They're quite a complex machine that needs everything to work nicely together to get a good end result.

Is there any more info about your machine published anywhere?

Neale
12-08-2017, 11:49 PM
No, nothing published as yet. You are also right about the "complex machine that needs everything to work nicely together to get a good end result" - or even to work at all. The wire feed and tensioner (DC motor with PWM control for wire feed plus stepper for tensioning) is one sub-assembly and includes a solenoid-operated guillotine mechanism for chopping used wire into short lengths as it comes out the end. Then there's the water flow and filtration, the XY movement with ballscrews and profile rails (tank moves on one axis and wire guide assembly on the other) plus stepper motors, and then the electronics. This machine has three PIC embedded microcontrollers in it - one for the keypad and screen, one for wire control, and one for spark generation which also controls the XY motion as these are inter-related. If anything is below par, then nothing works. In testing, a few weeks were wasted chasing electronic faults when it turned out that the deionised water wasn't quite pure enough. It's that bad. Electrical noise has been a perennial problem which is why the main control board is now up to version 9b, there are three separate power supplies, and it's wall-to-wall opto-isolators and so on.

Cutting area is something like 100mm square (although I need to check this). It uses a variable spark voltage of something like 35-60V although it generally seems happy at about 50V. Sparks are generated from a bank of capacitors switchable from 10uF to 100uF or so (again, need to check but it's something like that). It uses MOSFETs for switching - turn on one to charge capacitor bank, when charged that MOSFET turns off and another turns on to cause the spark. When the capacitor voltage has dropped below some threshold, the spark is turned off and the cycle restarts. The motion control is linked to this; if no spark occurs the wire is advanced and if the voltage drops too low the wire is stopped or even backed up a little. It's a very simple idea but has taken a lot of work to make it function. Wire speed is separately controllable. All the motion control software was custom-written; no use of modified Mach3, LCNC, Kmotion, grbl, or anything else. This does mean that the software has complete control of wire motion so it is possible to backtrack the entire cutting path if needed, for example if a cut ends in the middle of a workpiece rather than cutting its way out again.

We do not use any fancy commercial EDM components apart from the reel of EDM wire. So, no commercial wire guides, wire contacts, or things like that. Keeps the price down but whether this is going to hang together in the long term remains to be seen. It certainly works at the moment. There is already a long list of "wouldn't it be great if we could..." ideas!

Neale
30-08-2017, 07:40 PM
Quick video (https://www.youtube.com/watch?v=i2VluAqAblI)showing the main features of the wire EDM machine. Video clip of operation currently in the editing suite!

magicniner
31-08-2017, 09:17 PM
If it can't take G-Code from a CAM package it will remain happily niche.
I suppose it could be seen as an advantage that it will have no mass appeal to the new "maker" generation :D

Neale
31-08-2017, 10:40 PM
If it can't take G-Code from a CAM package it will remain happily niche.
I suppose it could be seen as an advantage that it will have no mass appeal to the new "maker" generation :D

I may have misunderstood, but I'm not quite sure what you mean by this.

Wire EDM is certainly not niche as a technology. Even though to date it has been relatively little used commercially because of cost and complexity compared to more traditional methods of machining, this is starting to change. For 12K or so delivered, you could order a wire EDM machine from suppliers on AliExpress. I was talking to someone recently from a university department where they have recently installed a wire EDM machine and it is starting to take over from more conventional milling machines as the "go-to" choice.

The machine I'm talking about here is certainly niche. The fact that you probably won't find descriptions of more than, say, a half-dozen or so home-built/DIY machines like it world-wide rather proves that fact. However, it is precisely because it is a bit unusual that I am describing it here. I thought that readers of this forum might have at least a passing interest in something just a little bit different. This is not a build log, and I'm not expecting anyone to copy it. It's not a machine intended for the commercial market. The work needed to commercialise something like this is beyond our little team, and really doesn't interest us anyway.

It is also niche in the sense that it was not intended for "production" use, although people seeing it are not slow to come up with suggestions for how it could do some useful little jobs, certainly on the "model engineering" scale. It was intended to be relatively portable and, at a pinch, can be loaded and unloaded into the back of a car by one man so that it can be taken to exhibitions and so on for demonstration purposes. It spent three days at the recent Bristol exhibition and the plan is to take it to the Midlands Model Engineering exhibition in October. Of course, these exhibitions themselves are rather niche (although a surprising number of the general public do attend) so that just underlines the niche-ness, perhaps.

This machine fits into the CAD-to-cutting cycle in the same way as the ordinary CNC router or mill. We have generated our cutting files from Vectric software, producing gcode using a standard CAM post-processor. We take step-by-step instructions to drive the motion of the machine. We differ only slightly from the usual Mach3/LinuxCNC type gcode processing and step generation in that we have chosen for practical reasons to use what in computing terms would be a compiler, converting gcode into steps as a separate distinct phase, rather than the more common approach of using an interpreter running in more-or-less real-time to do exactly the same thing.

It is very easy to look at something like this and say, "well, all you do is connect a pulse generator to a bit of moving wire and shove a workpiece around with a bit of gcode and some steppers. How difficult can that be?" Well, we do just connect and shove. It's just that it's not quite as easy as it seems.

The controls on this machine - the first successful version of a number of prototypes - are simple and limited in scope and function. Too limited at the moment. However, they are the minimum needed to get the thing to cut. With a bit more user interface development and what my computing industry colleagues always referred to as SMOP - a simple matter of programming - this can and probably will be enhanced. The really difficult bits have been done.

Interestingly, there were two broad classes of visitor/spectator that I spoke to at Bristol. There were members of the public as well as model engineers with no knowledge of EDM at all, and I think that most of them went away with at least the idea of how it worked without ever wanting to replicate the technology or even have it in their workshop. And there were the guys who really know EDM in all its forms (which is not surprising being a stone's throw from Rolls-Royce who use this stuff extensively). They were, I think, both interested and impressed - a typical comment was, "I wouldn't have believed you could build a working wire EDM machine that small!" and they were generally kind enough to pass on some of their own knowledge. Well, to be honest, there was a third class as well. That was the guy who took one look (after his small son pointed it out) who said, "I've just bought a real one of those for my workshop" and walked off. Pillock.

Well, after all that, I'm prepared to make a bet with any forum member here. What's the chances that Boyan will have one of these things up and running by Christmas?

Only joking, Boyan - I think that you have rather more real work to do than play with toys like this!

But if anyone is interested to know a bit more, just ask.

Neale
31-08-2017, 10:51 PM
If anyone wants to see metal being cut with a bit of sparking from a bit of wire, there's a clip here (https://youtu.be/lzep7b4gLYU). That shows the machine that I've been talking about, although there are plenty of Youtube videos of "real" machines in operation. I have to say that some of those are pretty damn impressive. All the same, to have something using the same underlying technology actually cutting in your own workshop is quite something. YMMV.

Clive S
31-08-2017, 11:11 PM
Neale What a fantastic project. About 12 - 15 years ago I happened to be in a small engineering place and I saw an EDM machine cutting a gear cog about 250mm thick and about 400mm dia. out of bronze or similar, I was mesmerised by it I think it took about 28 hours to cut it. There were three EDM machines in there.

Neale
31-08-2017, 11:17 PM
For testing purposes (and because I wanted one) I tried cutting a tiny screw-cutting tool for my lathe on this machine. It cut a 3/16" high-speed steel toolbit with a 1mm long 47.5deg tip (which some people might recognise as the thread angle of a BA thread) basically by cutting straight across with a deviation to form the "point" of the tool. Took around 5-10mins to cut - wish I'd timed it but I wasn't really expecting it to work at all! I can see a use for this machine for myself, cutting form tools for lathe work directly into HSS. According to one guy I spoke to, EDM works well with tungsten carbide as well.

m_c
01-09-2017, 12:28 AM
That cuts far quicker than I thought it would.

For video purposes, would slowing the pump flow make the work area more visible?

Neale
01-09-2017, 08:40 AM
Cuts a lot faster than we expected! Faster than a blunt junior hacksaw blade...

Pump flow is something we might look at. If you start without it running (which is what I normally do for demonstrations) you hear the spark quality go down after a few seconds and pick up once the pump is started. So, some flow is needed. At the moment, we probably have too much flow for this thin material, but I'm not sure that there is enough for the thicker cut like the HSS toolbit. The debris can build up in the cut behind the wire and almost weld the waste material back to the bulk. A nozzle to direct the flow would help. Commercial machines tend to blast liquid at to and bottom, I believe, but that's for much thicker cuts than we anticipate.

Neale
01-09-2017, 08:42 AM
Double post...

magicniner
01-09-2017, 09:24 AM
I may have misunderstood, but I'm not quite sure what you mean by this.

How would you cut any of the more complex of these shapes, the four largest generally round ones for instance -

https://2.imimg.com/data2/AA/AF/MY-2727238/cnc-wire-edm-services-250x250.jpg

?

m_c
01-09-2017, 12:11 PM
Cuts a lot faster than we expected! Faster than a blunt junior hacksaw blade...

Pump flow is something we might look at. If you start without it running (which is what I normally do for demonstrations) you hear the spark quality go down after a few seconds and pick up once the pump is started. So, some flow is needed. At the moment, we probably have too much flow for this thin material, but I'm not sure that there is enough for the thicker cut like the HSS toolbit. The debris can build up in the cut behind the wire and almost weld the waste material back to the bulk. A nozzle to direct the flow would help. Commercial machines tend to blast liquid at to and bottom, I believe, but that's for much thicker cuts than we anticipate.

If you're just using a basic impeller pump, can you add a flow control valve? A pair of mole grips/small g-clamp/pipe clamping pliers would also work for testing purposes.
Or would increasing the depth of water work, so there's not quite as much disturbance from the flow?

Neale
01-09-2017, 01:54 PM
If you're just using a basic impeller pump, can you add a flow control valve? A pair of mole grips/small g-clamp/pipe clamping pliers would also work for testing purposes.
Or would increasing the depth of water work, so there's not quite as much disturbance from the flow?

It's actually a diaphragm pump intended for use in caravans with a pressurised accumulator to damp out pressure pulses. We do have a control valve - a little clamp with an adjustable screw! The pump output splits; part goes through a deionising filter to keep the conductivity of the water down and the rest goes through the jet playing on the cutting area. It's possible to juggle the relative flows but it's an adjustment that I have not played with. Depth of water is a bit critical - too low and it doesn't flood the work (sparking is much more effective under water or the right kind of oil, believe it or not), too high and the splashing ends up in the electronics. Bad news - the machine's first outing came to an end during the first cut when spillage on to the control panel shorted out something critical which meant a new PC board was needed. A good question to which I do not have the answer is, "but why did non-conductive water short something out?" It just did!

Neale
01-09-2017, 02:24 PM
How would you cut any of the more complex of these shapes, the four largest generally round ones for instance -



Assuming the question is a serious one and not "if your CNC router can't handle 8x4 25mm ply, why are you bothering to talk about it at all - it's a toy", then there are a number of answers.

First is that we don't know if it can cut this kind of depth. Unexplored territory. There are issues of wire tension that become more significant the deeper the workpiece, which affect surface finish as well as accuracy. The current control panel does not give the kind of accuracy, rehoming, etc, that the usual motion control software provides and as a result it is more difficult for us to do what some of the big boys do which is to take a roughing cut somewhat oversize - say, 0.05mm - which is aimed more at speed of cut than anything. The narrow kerf means poor debris clearance which combined with a more intense spark leads to more surface pitting leading to poor surface finish. However, you then take a second, sizing, cut with a lower-power spark which is now moving along an open face. Better surface finish and accuracy. Again, the big boys talk of micron accuracy with this kind of technology. If and when we upgrade our controls, this is an area to explore.

Second answer relates to work-holding. Something else we haven't figured out. The workpiece clamp we have at the moment is, again, a quick and simple solution for testing purposes. I would not trust it to hold the kind of blank (presumably by the edge to allow uninterrupted cutting) of the depth used for some of those examples.

Your picture shows some workpieces with a feature that we can only dream about (along with auto-wire threading, wire break detection, etc). That is the ability to separately move top and bottom of the wire. This allows sloping cuts, bevelled edges, and all sorts of features. It's a kind of wire EDM version of a 5-axis VMC. In principle the hardware is do-able, but the software sounds like fun. For someone else...

On the other hand, coming back to the bit of the real world that the team inhabits, we have already had a request to cut out custom brass letters to make nameplates for model locomotives and similar. Rolls Royce use EDM to cut 2mm curved holes through the length of nimonic alloy turbine blades (something to do with running fuel through for cooling, I understand). We could cut out little brass letters. Horses for courses:smile:

A final point is that anything our machine could do, a laser could probably do as well, maybe better. At the high end, lasers are sometimes unacceptable due to metallurgical changes at the cut surface that can lead to micro-cracking which is a reason that EDM is used instead. I doubt somehow that we are likely to be working with materials where this is going to be a problem. However, I suspect that despite the work that has gone into it, wire EDM is probably a better cutting technology for the home workshop than high-power laser; I know already that we can cut materials that are just not possible with any laser that is reasonably available to the amateur. If you are a commercial workshop, justify the cost of a commercial machine based on your own workload, or outsource/subcontract to a service company. As a bunch of amateurs, we need no cost justifications to show to shareholders, we do it for the fun of it. I would argue that our machine is a little more useful than a steam model locomotive or a matchstick model of Salisbury Cathedral, but I would not deny the right of anyone to build those if it takes their fancy.

magicniner
01-09-2017, 06:23 PM
Sorry,
I thought you might be developing a useful, functional machine with wider applications, Kudos for making it work at all but it's so disappointing that it's been intentionally developed down a remarkably limiting dead-end :-(

- Nick

Neale
01-09-2017, 10:43 PM
Nope, if you're looking for a commercial scale, production-quality machine, you're right out of luck in this thread.

Life's a bitch sometimes...

Let's be realistic here. A bunch of amateurs wanted a challenging technical cross-discipline project that was feasible in a home workshop (or three). The team has built a prototype that met and actually exceeds its modest ambitions and design goals. It would scale fairly easily - the mechanical bits are straightforward application of ballscrews and profile rails, cutting and inertia loads are low, and a bigger, heavier machine would be easy. The wire support arm needs beefing up anyway, but that's not rocket science at this level of sophistication. The control electronics and software are not a problem - probably need to move away from the PIC to an Arduino or something like that. SMOP, as I said earlier. The really difficult bits, the spark and motion control electronics, don't really need to scale at all within reason. Could up the spark energy a bit with some bigger capacitors, but my guess is that we could do a reasonable size (for some definition of "reasonable") job with what we have.

My bet is that we could cut things like die blocks for model steam loco valve gears, ratchet wheels and pawls directly into hardened carbon steel rather than cutting first and risking shape change on heat treatment; one suggestion received was cutting combs for musical boxes. So, our toy machine could do a useful job making bits for other toys. Not really in the business of cutting keyways in bevel gears for wind generators or any of the jobs the wire EDM service companies provide. I'm quite interested in the idea of being able to cut profile tools for use in a lathe; although my lathe would take something like 16mm shank tooling, for many purposes something much smaller is perfectly OK and within the scope of this machine. Quick search on AliExpress, wave a credit card, and you could have your own machine delivered with much more capacity and sophistication with a lot less effort.

Happy to discuss what we did and how we did it, if there's any interest, but I'm not putting this forward as "the only way to do it" or even "the best way to do it." Whether our original aims were challenging enough is a separate question but even George Stephenson didn't start out by building Mallard!

Apologies to anyone who thought that this thread was advertising machines for sale, or even a proven design and source of PC boards and components to build one. Mike Bax in the Netherlands is, I believe, working towards doing something like that; the plasmaboog Yahoo group would give more information on that although Mike seems a little reticent to say too much about how his development works for commercial reasons.

m_c
02-09-2017, 12:34 AM
Getting water on electronics is rarely good. Perhaps an additional cover, or maybe some tank extensions that slope inwards to try and help contain splashes?

What I suspect Nick is getting at, is how are you going to generate the code to machine complex parts, if you can't use some kind of standard/common code that most CAM packages can produce?

While I remember, a KFlop running KMotionCNC can be run in reverse. Simply command a negative FRO, and it'll run as fast in reverse at it will forward. If it only it could also put the material back while running in reverse... :-)

Neale
02-09-2017, 09:15 AM
Ah, design in retrospect - always the easiest approach! You are quite right, though, and one simple change has been to put a clear waterproof cover over the front panel, which should help. There are also covers over the rest of the electronics, partly to keep out fingers as well as water. No-one but an idiot would put a large open container of water directly over a heap of sensitive electronics... I remember once visiting a hotel that had just been closed - the swimming pool had developed a leak into the electrical plant room below, so we aren't the only ones. The real point, though, is that this was built as a proof of concept, something that would, if it worked, be able to do something demonstrably useful albeit on a small scale. If I admitted that the current controller can only accept designs that are something like 16K steps long, which equates to about 200mm cutting length, am I going to be nailed to the wall again? Extending that part is, again, just a matter of upgrading the front-end software and hardware. SMOP.

As for "standard gcode", I possibly did not explain myself very well. Normally, for example, you would design in, say, Vectric vCarve and use its CAM module to produce gcode aimed at specific motion control software - Mach3, say. Mach3 translates that gcode on the fly into steps which are sent to the stepper drivers. We design in Vectric vCarve and use its CAM to produce gcode. We use the grbl post-processor. To simplify the software running in the EDM machine micro-controller, we do the gcode to step translation in a separate application which runs on a PC. The step file is then downloaded to the machine. The work flow is very little different from, and the design stage is identical to, that used with the kind of machine that most people on this forum use. There is one extra step that we make explicit that most people do not see as it happens in the motion control software.

Use of Kflop or similar is a valid point, but we took a simpler approach. Our approach means tight coupling between spark generator and motion control at a spark-by-spark level. I suspect that this is one reason for the surprising rate of cut but this is a guess.

Robin Hewitt
02-09-2017, 11:05 AM
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
02-09-2017, 12:50 PM
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
02-09-2017, 04:59 PM
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
03-09-2017, 07:14 PM
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
03-09-2017, 08:47 PM
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
03-09-2017, 09:25 PM
power it from both ends of the wire which sounds tricky

Thanks Robin, I needed a laugh today! :D

Neale
18-09-2017, 01:21 PM
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 (http://www.midlandsmodelengineering.co.uk/) 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
18-09-2017, 03:09 PM
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.

Neale
18-09-2017, 03:58 PM
Thanks, George - good to have corroboration that it was there and working! I'm not suggesting that this might be your next commercial offering, though :smile: After all, where's the market for a machine that cuts out small aluminium letters?

Anyway, good luck with the CNC conversion kits - hope there are folk out there that recognise that C5 costs just a bit more than Chinese C7...