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Going for an EDM toolheadSunday, August 10. 2008In which the narrator decides that if an EDM toolhead is going to get built he'd better get on with it... For the past year or so, I've been mouthing off about how it would be nice to have an Electric Discharge Machining (EDM) toolhead for Reprap. This weekend, after I overcame what I think is the last major technical hurdle in designing and building Tommelise 2.0, I began to wonder what I ought to be doing to keep my mind challenged for the next six months or so. I already committed to seeing if I could move tin-can linear stepper technology a couple of notches closer to something that a Reprap machine could print major parts of. I've got most of the parts for that and am starting to cobble together a first try at making a crude tin-can stepper using ring magnets. The problem with that project, which I will continue to work on, is that it doesn't benefit the wider Reprap community that is building belt-driven Darwins and not linear stepper-driven Tommelises. That's when I got to thinking about Electrical Discharge Machining (EDM). A lot of people in the Reprap community have been talking about wanting their Reprap machines to be able to do milling. The problem with that is that you need a much more robust positioning system and very different software to develop the tool trajectories for ordinary milling and shaping toolheads. Milling puts loads on the positioning system that printing plastic doesn't. EDM, on the other hand, doesn't load the positioning system either, aside from its having to support a dielectric bath to contain the piece being milled. That's a little tricky, but if you keep it shallow enough and don't run your positioning system (in Tommelise's case) fast enough to cause sloshing, you're good to go. A few months ago, I started to get read up on EDM by getting a free subscription (sorry guys, Americans only for some reason having to do with postage, I think) to EDM Today. Yesterday, though, I went ahead and bought two books on how to build a desktop-sized sink EDM machine. I also joined a chat group of people who are building such machines, the EDM Home Builders run by the author of one of the books I bought, Ben Fleming. Ben even sells a ready-made PCB for his machine.  A sink EDM machine basically lowers an electrode cut into the shape of what you want removed from the billet. If you use a thin, stiff electrode, according to what I've been reading, you can shape surfaces if you mount the tool head onto an xyz positioning system.  If you want very fine cuts, however, you have to go to a wire EDM machine. It's works a bit like a scroll saw.  Camtronics sells what I am told are a great set of plans for this sort of machine. While a wire EDM machine can make very fine cuts with great accuracy, I am told, it has a very limited ability to shape in 3 dimensions. In any case, I've felt for a long time that we are going to have to be able to deal with structural materials other than plastic if we are to reduce the number of parts that our 3D printers can't make. I am going to see if it is practical to use a EDM toolhead to achieve this end. Trackbacks
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Here is a book from Lindsay's Books on how t build an EDM machine. Costs 19.95
http://www.lindsaybks.com/bks9/edm/index.html
It's one of the two I've bought so far. Hasn't arrived yet, though.
I looked into this myself when you first mentioned it. It does become somewhat messy though, with the need for either refreshed or constantly flowing dielectric fluid. No worse than, say, Fernando's research into liquid calalyst "tank" systems.
I take it you looked at homemade wire EDM "drills" then. This would almost work but there's no real depth control as the wire erodes, although I've seen notes that this CAN be realistically calculated but not the equations for doing so. It would definitely be worth it for making size controlled acorn nozzles, but we'd need a high step count threaded rod positioner for the small, slow movements. We're talking inches per HOUR of movement after all.
The Home EDM chat group says that the Fleming system burns about 1.5 mm^3/min. I think that the resolution of the positioning system is probably dependent on the distance which the system is able to throw an arc. Looking at Fleming's design he is using a tin-can stepper and a fine threaded rod of some sort to control the sink rate. Tin can steppers rarely have a step size of less than 7.5 degrees and it is very hard to get a thread pitch at any reasonable diameter of less than about 1 mm/turn. These little Haydon 26000's that I have can do 0.05 mm/step. If I really push I can get them to half step. Easier would be to just buy another one with a shorter step distance.
I keep track of the homebrew EDM group as well. Ben does not use a tin can stepper. The design calls for a geared down motor or a trim tab server. A controller monitors the voltage across the electrode. If it gets too low, the bit is retracted and too high the bit is lowered.
Why not do electrochemical machining instead(ECM), it's much simpler than EDM. It also doesn't require the machining tip to be replaced as often.
http://www.eod.gvsu.edu/eod/manufact/manufact-281.html
http://www.indoor.flyer.co.uk/ecm.htm
A 24 gauge blunt tip stainless steel dispensing needle coated with high temperature lacquer up to just the tip should make a good machining head. You could use a smaller diameter needle for better accuracy, but smaller diameter needles tend to require higher pressures to push water through them(~20-50 psi).
http://www.zeph.com/applicator_tips.htm
Couple of reasons. First is there doesn't seem to be a lot of information on DIY ECM out there. Second is that it seems to be a real pig, insofar as energy usage (3 kwhr/in^3 - http://www.eod.gvsu.edu/eod/manufact/manufact-281.html).
Three kWhr/in^3 is actually affordable. When machining iron, with a density of 7,870 kg/m^3 (7.87 g/cm^3):
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http://hypertextbook.com/physics/matter/density/
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that works out to: 3 kWhr/in^3 / (2.54cm/in)^3 / 7.87g/cm^3 * 1,000g/kg =~ 23 kWhr/kg
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At a high electricity price of 0.3$/kWhr, that works out to: 0.3$/kWhr * 23 kWhr/kg =~ 7$/kg
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7$/kg is in the ballpark of plastic filament, assuming that you remove a similar amount of metal as you keep, which would be the case for a sphere for instance.
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