Although this has been previously discussed and known to work under the right conditions the whole concept has always been considered something of a black art as none of the usual sources give any real details as to why and how things should work. At our level I suspect that most of the original sources are third or forth hand anyway and mostly of the "this recipe worked for me" type.

I found this statement from the second patent in the first link interesting:-

"Applicant discovered that with a certain composition of electrolyte bath, and if the electric potential was used at low levels, that even erosion would occur across the side surfaces of a cutting edge. No concentration of the erosion would occur at any given point. The net effect is that with a dull edge, which is where the sharpened edge has assumed a slightly rounded configuration, the side surfaces of the edge will be eroded away evenly substantially eliminating the rounded cutting edge and reproducing the initially established sharp edge. The initial geometry of the side surfaces of the cutting edges are maintained."

Which neatly summarises the whole problem. Left to its own devices erosion preferentially occurs at the sharp(er) points. Which is why chemical polishing works so well, the rough bits are sharper than the smooth ones and so get eroded first. The files dipped in acid version works because the shape of the teeth shields the back side and point so the flat erodes preferentially. Neither case applies to the general run of lathe and milling tools. Unfortunately sharp is not purely defined in geometrical terms. There are a also electro-chemical aspects so impurities, cracks or even stress can be "seen" as being electro-chemically sharp so erosion can occur in the middle of a flat surface. Think rust pitting and the various chemical methods to make material structures visible under a lab microscope.

Assuming a suitable composition of electrode bath is able to effectively alleviate such geometric effects the big question is how reliable is it over the normal range of material compositions. My guess is that proper operation would be pretty material specific needing, hopefully small, adjustments when the material composition varies outside a fairly narrow range. Makes the concept impractical unless you know exactly what the material is. Shame really as it would be so nice to just dunk your tools in the sharpener bath when you knock off for the night and come back to razor sharp cutters next day.

If it really did work reliably the toolmakers would surely have latched on to it as an effective way of locking in their customers by supplying a bath unit and electrolyte suitable for their special materials but no good for brand X, Y & Z. Naturally brands X, Y and Z would have their own specials to lock in their customers. In pre-carbide and pre-insert days sharpening HSS tooling was a major issue for larger companies who would have specialist departments with lots of skilled operatives and T&C grinders. Samller companies would rely on skilled men (& women) doing their own. Either way a just dunk it device would have been pretty attractive. Lots less messy too.

Clive