Re: [CAD_CAM_EDM_DRO] Re: EDM Rotation speed

Dave,

There's a lot to edm'ing holes.

The straighter the tool, the less stock is cut, so it's faster.

The deeper the cut , the gap gets polluted faster so you have
to jump more and therefore cut less, so it's slower. Water based
hole drilling edms don't jump at all. Oil based sinking edm's do
jump, all the time, while cutting small holes.

Deep means greater than 4 units of depth per 1 unit of diameter.
(anyone can cut 1mm to 8mm depth, but 2" is difficult)

Your speed changes significantly with depth in a simple edm w/o forced
flush.

Hole drilling edms often use air powered water pumps with
a 8 inch double acting piston, which has maybe a 1" dia. rod. This
rod ends fit into tiny cavities. say 1 inch deep. Water flows thru
the tiny cavities, air flows thru the big bore. The 8 bar of the large
bore creates over 1500 psi in the small bore. A small volume but
a huge pressure. This water is sent thru the edm drilling electrode,
and works well up to over 1" of depth with 0.020" dia tubing.
These machine never jump (never peck drill). They rotate at up to
200rpm, but in use, much lower rotational speeds.The leading
edge Mikuni uses a constant pump.

These are generalities, some manufactureres do better, some worse.
But these ideas of 'straight' and 'hi-pressure' are true and tested.

For a great set of pictures (cartoons) of the life of an EDM spark,
look on the AGIE site for the famous drawings of Dr. Bernard
Schumacher. These have been copied by every manufacturer I know
in thier attempts to explain 'what is this EDM anyways?'
(and expect to find a lot of hockey info if you search for EDM,
as that is acronym for Edmundton CA, better you should search
for 'funk erodieren' and get all the cool German EDM sites).

The power is often applied to a brush at the top of the rotated chuck.
The chuck uses a rubber in a bore above the jaws to seal water.
Here's another tiny thing that makes a huge difference. The length of
the tool, usually brass (not copper usually) makes a resistor to the
process, and the wear is greater when the tool is 18" long ( typical)
and low when it is 3". The difference in resistance/length is the only
factor that changed. but the wear will be up to 4x different!

Jon's method of applying current lower is much better. I often drop
the contact to the guide ( just above the workpiece).

Tungsten wire? often in tool &die & mold shops, the operators knew
that tungsten rod/ was the way to go when you didnt have a good
rotater. It's very straight ( breaks if you try to bend it ). And if it is
alligned with the Z travel, it cuts a round hole. Experience of many
operators say that copper and brass rods or tunes wont yield a
round hole without rotating. dunno why, i guess it deforms during the
cut.

A good rotating device is a RotoBore (tm). It was hard to learn,
but made beautiful cuts. It had a fine chuck mounted on an tiltable
and centrerable plate. thumbs screws allowed you to 'true' the tool
and center it. Pretty much all done by eye, with maybe a sheet of white
paper and a machinists angle behind the tool as reference. There was a
really cool trick for when the tool got bent... you sped the motor up,
dipped your hand in the oil, grabbed the tool near the chuck and pulled
down, sliding down and off the tool... it'd snap straight as an arrow if
done correctly! (uh, power was off)

why rotate anything that's not round? well, if you cant use a tube for
your electrode, and you need a round hole. Try a round with a
flat on one side. This is the 'gun-drilling' trick, and works in a pinch.

Why rotate at all? It's an effort to reduce the thermal concentration on
the tool. When a large surface is eroded the heat is spread over the
entire surface. Spark N ( of a zillion ) happened on this corner, and
spark N+1 happened just a bit further away, and spark N+99
happened in a whole different country (considering the teeny scale
of sparks on a 1x1" tool ).
So the time between sparks >at the same point on the tool<
is way more than what is called OffTime. That means any single point
on the tool runs cooler. But, when the tool becomes a tube or more
like a point, all the sparks happen right >here<, nearly all in the same
place. So, by rotating the tool, the sparks are more likely distributed
over the tiny surface that you do have. in general you still need to
increase the OffTime to accomodate the 'spark density' ( i called it
spark density, Dr. Yu Feng Luo called it the 'space effect' at same
time, he in China & I in US, we met 10 yrs later, but spoke the same
language... edm! ).

TomP

----- Original Message -----
From: turbulatordude <davemucha@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, August 13, 2003 2:50 PM
Subject: [CAD_CAM_EDM_DRO] Re: EDM Rotation speed


> Hi Richard,
>
> I plunged a hole using a 12 ga copper wire into a steel part as a
> test and one of the results was that the wire was not perfectly
> aligned. as the cut progressed, the side of the wire started to
> expand the hole and the progress slowed as the surface area increased.
>
> by rotating, it seems that a more perfectly round hole would be
> created and possibly, the time would decrease?
>
> you are correct in that any shaped hole would need not only a
> straight plunge, but a perfectly straight plunge.
>
> of course, electrode style enters into the equation.
>
> Jon mentioned that he used a titanium electrode wire with a slight
> bend in the end. that would have the effect of directing the
> sparking action to a narrow section under the rotating wire.
>
> since this is all new technology to me it seems there are some gaps
> in my learning curve.
>
> Dave
>
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Richard" <phrh@w...> wrote:
> > Dave,It seems that adding a spinning motion just complicates the
> > process since one must also move the electrode up and down. When
> > things are adjusted properly my stepper output shaft rotates back
> and
> > forth about 1/8th turn which results in the electrode moving up and
> > down about 4 to 5 mils (like a sewing machine, at a pretty good
> > clip). Wouldn't this have a similar effect to rotating the
> electrode?
> > And one would not want to rotate a square electrode<G>.
> >
> > The distance moved can be adjusted by setting the comparator
> > reference voltages. I find that the method works really well.
> >
> > Richard
>
>
>
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