Wire EDM machine

[Neale]

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]

Quick video showing the main features of the wire EDM machine. Video clip of operation currently in the editing suite!

[magicniner]

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]

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]

If anyone wants to see metal being cut with a bit of sparking from a bit of wire, there's a clip here. 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]

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]

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.

[magicniner]

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 -



?

[Neale]

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

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]

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