Re: [CAD_CAM_EDM_DRO] Re: Simple EDM schematics...

Lee Studley wrote:

> Hi,
> I got the EDM working somewhat..., but its taking way too long. I'm
> going to find some of that Aluminum Sulfate and give that a try.
>
> On the EDM I ended up getting best results using a carbide PCB drill
> bit shank ~1/8 inch and using that as the "sinker". I put it in
> a 'pin' vise and used heat shrink tube to insulate the handle from
> the mill's jacob chuck. It looked really cool, but didnt spin
> very true. It eat away about 3/16" in about 2.5 hours of trial and
> error. Very painful!!! The tap was a 10-24.

Well, I also found it to be real slow. I have used it for burning out
broken taps without toucking the threads in the part. This is in 6-32
holes, so the electrode has to be pretty small. If you can get everything
going just right, the EDM process will be self-driving, and you get a
strong buzzing sound with the electrode just stationary. Without a
flow of EDM fluid, it is pretty hard to sustain this. The fluid I've
been using, Alum-Tap, seems to really work well for this. (It must
have the right dielectric strength.) I have used .060" brazing rod
ends, which I have a modest supply of.

I'm waiting for the Robert Langlois article in Machinist's Workshop
(I think) starting this month or next. I have also come up with a
circuit for an EDM pulse control, if I don't like his circuit. I suspect
that with a pulse controller, you can really let the current fly, as
the transistors will cut off the discharge after so many micro-seconds,
allowing a new discharge to form at another spot. Instead of a few
hundred sparks a second, you should be able to go to tens or
hundreds of thousands, vastly increasing the burn rate.

If you need to remove taps, one other trick that works in some cases
is to use a fairly hefty solid carbide circuit board drill, about .040"
or so. If there is a relatively flat spot on the top of the broken tap,
let the drill bit down until it just barely touches the tap, and let it
chatter around there for a while, lowering the drill just a hair at a
time. Eventually, it will make a fairly good dimple in the tap, and then
you can apply a little more pressure and start drilling into the tap.
Once you have a hole .050" deep in a small tap, or maybe .1"
deep in a larger one, you can put a center punch in the hole and
shatter the tap with one whack. Then, you can usually pick the
fragments out without excessive damage to the hole.

Jon