You cam look at this reference about a company that EDM's chambers
http://www.6mmbr.com/gunweek055.html
and the following newsgroup discussion that I copied here. If you want the entire thread:
http://groups.google.com/group/rec....ce0bd0676867?hl=en&lnk=st&q=#8419ce0bd0676867
I would guess the suitability of using EDM for burning parts would be based on the sophistication of your EDM power supply/generator, and it's ability to minimize recast layer,, and then you would need to lap/grind out the layer to remove it and any microcracks.
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Here's something that isn't in the article, but it should be. They talk
about the recast layer of 0.0001", and about lapping it off. That's all well and good. If the barrel is made of, say, 4140, the recast layer is going to be harder than a witch's heart, and just as brittle. Or, maybe not, if some of the latest fine-finishing circuitry is used. But it probably will be. And it probably will have microcracks in it. Possibly not, but probably so. If the EDM is more than a decade old, most likely so.
The problem is that the microcracks may extend well below the recast layer, into the heat-affected zone (HAZ) and even into the parent metal. Avoiding that is a matter of having good knowledge of the process and good knowledge of the way to avoid it -- if you can -- with your particular EDM. This subject is a critical one with guns and it requires some long talks with the top engineers at your EDM company, and probably some further research.
As of 25 years ago, the mil spec for military gun manufacture specifically disallowed any use of EDM, for that reason. I researched the subject myself. A gunsmith who wanted to use EDM for custom chambering was corresponding with me about it (I was EDM editor at _American Machinist_ at the time), and we both got scared off by what we learned. Later, in the early '90s, I had some discussion with Greg Langenhorst at Mitsubishi about their new ultra-fine-finish circuitry and it sounded to me like they had the problem
cured.
All this being said, the 'smith who's doing this work sounds like he knows his stuff, whether or not he knows the underlying physics. The T/C is a pretty strong affair; 4140 (if that's what it is) is pretty forgiving; and the whole thing is a matter of being very cautious, to begin with.
I was very interested at the time in using wire EDM to hake the rectangular hole in a falling-block receiver, and I had worked out what I considered to be a safe procedure, lapping out 0.004" per side after EDMing. But I never had one made, so that was the end of it for me.
It does sound like he has something very good going here, and EDM is going to find its way into some kind of precision gun manufacture, if it hasn't already. You just have to be aware of what's going on (melting versus "burning," for example), and you have to dig into it pretty deeply before taking the leap. In other words, don't try this at home until you really know what's what.
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Ed Huntress