"Clark Magnuson" wrote in message
...


Using EDM to cut rifling is at least 30 years old, although I don't know
if
it was ever used in any kind of production (ECM has been used to rifle
barrels for longer than EDM, and it's been used in production). However,
I
hadn't heard of using it to cut chambers. I'd like to know what the
"patent"
is about.

The description of EDM in that article is about half right.

--
Ed Huntress




Ed,
What is right and what is not right in that article?
Clark

The general description is pretty good, but there are some important things
missing, and some errors. The errors:

"The spark heats the grains and structure of the chamber wall beyond their
critical temp in the gap..."

Heating beyond the critical temperature is not the point. Heating beyond the
MELTING temperature is the point. EDM melts the surface of the metal.

"The removed metal is then carried away as smoke within a dielectric
insulating fluid..."

The smoke is not from the metal. It's from burned dielectric fluid. The
metal is melted and expulsed from the parent metal by vaporization and
cavitation. The molten metal forms little balls (interestingly, most of them
are hollow) that are then carried away by the dielectric.

"Coolant is provided to insulate the cut from the muzzle end of the barrel
as the EDM..."

I don't know what he's talking about here, and the sentence was
inadvertently truncated. He may mean that the dielectric is pumped in from
the muzzle end, which would be the best way to do it. In any case, if he's
still talking about EDM, it isn't "coolant." It's dielectric fluid. It does
cool, but, again, that's not the reason it's there.

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.

--
Ed Huntress