Newsgroups: rec.crafts.metalworking
Subject: Re: FAQ on EDM (additions re: CNC and wire EDM)
From: jmorton@euler.Berkeley.EDU (John Morton)
Date: 23 Sep 1994 17:16:51 GMT

I have some additional information to add (from memory):

CNC vs MANUAL CONTROL

EDM is particularly apt for automated control, because there are
electrical values that can be monitored and corrected on the fly more
easily by a feedback system than by a human watching meters.    
A CNC EDM machine leaves the operator free to tweak the current
and the proportions of the on-time/off-time cycle, while the
control watches the gap and handles movement.

A simple tap-removal rig is easily operated manually, probably
simple to build, and worth it's weight in gold if you're facing
a spoiled $5000 part.  An old Elox "Electron Drill" like you see
in a lot of job shops is typical (and not worth much, BTW).
However the revolution that has put EDM on everybody's mind is
all about the new invocations of the same process, especially
wire EDM.


WIRE EDM

A wire EDM machine uses a wire (usually brass) as the electrode,
which passes between guides like a bandsaw blade.  The wire is used only 
once.  As it runs from a spool through the job, it is eroded and 
reduced in diameter by as much as 1/3.  The old immersion system
has now been largely replaced by flushing nozzles which surround
the upper and lower wire guides.  These blast the dielectric (which
is just deionized water) through the cut.  This system works well,
and has the tremendous advantage of being quick to shut off to
remove parts, rethread broken wire, etc.

As to the functionality of the wire EDM system, just imagine what
you can do with a wire that leaves a .010" kerf through 6" thick
stainless steel without heating it up.  With the CNC control 
a taper can be imparted to the workpiece, because even the low-end
machines have independently controlled upper and lower guide 
movement.  High-end machines are mostly a way of getting more
power (and therefore faster operation); more capacity; and 
convenience features, like automatic wire threading and a 
sinker-EDM head to create internal starting holes in hard material.

John Morton					University of California
jmorton@euler.berkeley.edu			Mechanical Engineering
{decvax,cbosgd}!ucbvax!euler!jmorton		Machine Shop