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EDM, a process actually invented by Russian Engineers, requires some rather specialized and expensive equipment. Basically, an electrical potential difference is applied across two electrical conductors comprising a shaped electrode and the work piece which are immersed in a non-conducting, fluid electrolyte. When the two electrodes are close enough together, a plasma forms (think spark, or arc) and the high temperature produced vaporises a miniscule amount of material. By itself, this is not very effective, but if we repeat it quickly and keep it up long enough, metal in the shape of the electrode cross-section with a narrow kerf is eroded away from the target. This is a relatively slow process and control of the electrode (which is being eaten away too) is critical in maintaining the spark. The process produces a sizzling sound not unlike bacon frying.

Commercial units are complicated and their internals are carefully guarded industrial secrets, but the basics of the technique are simple and the effect is so useful that there have been quite a number of designs published for home construction.

We can classify the machines into sinker, and wire eroders. The Sinker type uses a rigid electrode in the shape of the desired cut that is positioned by a servo controlled ram. The DC (direct current) spark generator is connected to the insulated electrode and the workpiece. Generally, the positive side of the supply is connected to the electrode and the negative to the work. Movement of the ram occurs in the Z axis only, gradually sinking into the work.

Wire eroders are much more sophisticated, being a marriage of numeric controlled (NC) machine center and EDM sinker. They use a fine wire as the electrode that is continually fed through the work from supply to take-up reels. In most designs, the wire guides which are positioned either side of the work are moved in the X-Y plane like a large plotter table. Other designs use the wire like a saw simply lowering supply and take-up reels in the vertical plane. These have been used to slice off very thin sections of samples for electron microscopy and x-ray topography [2]. The wire cannot be reused as its surface and diameter will be irregular after emerging from the work.

A researcher at the University of Wyoming compiled a reasonably complete list of published EDM sinker designs. Full construction details for a wire eroder appeared in Model Engineers' Workshop [3]. This magazine has also published two very simple sinker designs specifically for broken tap removal that use a solenoid to extend and retract the ram [4][5]. My experience, although limited, strongly suggests that these may be effective on large jobs, say 3/16" up, but not fine work that requires the broken tap or screw be removed without damage to the thread.

Initially, I considered building the quickest and dirtiest unit that could conceivably do the job. The two solenoid based designs require a large amount of interpolation in their construction and the more I planned the detail design, the less I liked them. Then a Model Engine News reader suggested I look at a slightly more complex design detailed in a book by Ben Fleming [1]. That very kind reader was ETW Kiwi builder, Nick Jones. Nick had constructed one and offered to send me some spare chips and a circuit board he'd made. An email to the designer obtained a copy of his book for review and evaluation, but even before the book arrived, it seem clearly evident from photos of completed machined on the Yahoo EDM group dedicated to this machine that it had the edge over all the others I'd seen published for a number of reasons:

I've always wanted to build one of these gadgets and now had an urgent need to push the desire. So Nick sent a Care Package of spare chips and other bits and I started leafing through suppliers catalogues for all the other parts. The EDM Book gives domestic USA part numbers and suppliers for the components. Builders in the UK, Australia, and New Zealand may like to consider the alternates Nick and I have identified from the RS Components Catalogue. These appear in the last section of this series of pages.

 

 

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