Friday, October 01, 2010
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Q&A: The forces produced during EDMing

Q - What kind of force(s) are put out during the process of EDM? What is required to hold the specimens? I would presume a magnetic table, but then you’re severely limiting what you can cut to ferrous materials only, and my understanding with EDM is that any conductive material can be cut.  So is there a complex clamping system involved?  A standard T-table, with bolts/nuts/clamps, etc?  Or is the force not significant enough to move the specimen? I understand that there is no direct contact between the wire and the specimen, but I would imagine there’s still significant pushing power involved.

A - Even though I have heard many EDM old timers say that there are no forces in play during EDM, they are wrong.  When I was a sinker EDM instructor for another EDM builder, I tested this. I placed a block of steel on a magnetic chuck, but did not activate the magnet. I then used a 1” round graphite electrode to do a side burn across the block, about 0.25” deep. In essence burning a 1” channel across the top of the block. As the burn progressed, I watched the block slowly move in the direction of the burn. This indicated to me that there were electromagnetic forces in play that had enough energy to move the work piece if it was not held down. It also seems that this effect is different from one machine make to another, as each manufacturer designs and builds their spark generator to different specs.

One thing I discovered in using magnetic chucks was that the closer the actual burn area was to the chuck (and the magnetic flux field), the more it affected the spark activity, resulting in a shallow burn. The flux field can actually change the spark activity in the gap, and in the cases I saw, seemed to shorten the sparks, resulting in a shallow cavity. So my extrapolation was that the stronger the magnet, coupled with a deeper burn (closer to the flux field), resulted in inaccurate burn depths. After all, any electrical activity will produce some kind of electromagnetic field. When you add another electromagnetic or mechanical magnetic field to the equation, it will affect the field produced by the first electrical source. During WEDM processing, the electromagnetic field around the traveling wire can actually cause the wire to push away from the work piece, resulting in a bowing of the cut area.  The thicker the material, the more pronounced the bow.

As for work holding, I have seen the use of mechanical magnets, electromechanical magnets, mechanical chucking systems (Erowa, System3R, Hirschman), vacuum chucks (especially good for thin work, like typical test coupons), T-slot clamps, toe clamps, V-blocks with clamps, fixtures, etc.  The size and shape of the work piece can help determine the best kind of work holding to use for a particular cut.

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