Re: stepper edm

On Wednesday 09 August 2006 04:48 am, CAD_CAM_EDM_DRO@yahoogroups.com wrote:
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Thomas J Powderly
>
> <tomp-tag@...> wrote:
> > IMO the advantage of orbiting is simplification of electrode >
> >manufacture.IMO orbiting is NOT faster/better than plunging.
>
> Even despite improved flushing?
Yes, cutting all the time and over a larger surface is faster and more
economical. If the part doesnt flush well, orbiting can help but will slow
you down. Reducing the swearf-per-minute rate can be just as effective.
Simply increase the off time
( the distnce between the teeth of the cutter )
so you dont over burden the available swarf-removal-rate.
There are alternates of flushing like jump and thru-flush that beat orbiting.
>
> > But having 2 differently undersized electrodes at exactly the same >
> >center is even more difficult. But, the 2 differently sized tools
> >will cut faster than orbiting because the facial interface  is larger
> > ( the big tool cuts faster, hands down )
>
The slightly smaller size is still like a 2.002" dia ball in a 2.004" cavity,
the interfacial area ( cutting area ) is small (in theory a point ), and
whatever that area is, that is the 'leading edge' you speak of.
You always have a 'leading edge'.
If you make the leading thingy a surface, it wear less.
The form of the leading edge is very important,
if the interface a plane there is very little wear.
If the interface is an edge , it is higher.
If the interface is a point it is great.
All these are relative and influenced by the parameters chosen and the pairing
( paarung) of the materials and the generator.
A deep straight walled cavity (die opening)
is best sunk by a plate electrode
(or cut laterally by the flanks of a cylinder )
of < 3mm thickness ( or wedm).
The reason is that the walls of the electrode
and the workpiece will inhibit the escape of the swarf and
cause 2ndry erosion that is perceived as 'taper'.
In general the wear occurs where the work is concentrated,
and the wear occurs where the dirt has to turn a corner.

My suggestion was that cheap plentiful electrodes are fastest.
It is not the hobbiest approach.

> The electrode in orbiting only needs to be slightly smaller than it
> would be for a normal plunge.  The improved flushing should also help.
>  Then there is the fact that with a straight plunge you will probably
> need several finishing electrodes for a blind cavity as the leading
> edge of the electrode has to cut the entire z-depth of the cavity so
> will be eroded more quickly leaving a taper at the bottom.
>
> > So, orbiting is an attempt to reduce error, not to increase speed.
> > Believe what you see, not what you read.
> > Dont even believe me, try it and see for yourself.
>
I suggest the accuracy of 1 electrode is high.
There is no repeatability issue.
That was teh initial argument in favor of orbiting.
But if that can be resolved,
and electrodes ( tools) made inexpensively,
then multiple electrode are faster due to a larger cutting area
( shell mills cut faster than engraving tools )
> What I have read makes sense to me, the roughing cut might not be
> faster than a very well flushed straight sink but the total job time
> for a given accuracy should be much less given the reduced number of
> electrodes required, the fact that it allows some jobs to be done with
5 electrodes can be regarded as the norm for very fine finishes even if
orbiting!
The number is relative, and the problem of multiple electrode
accuracy is a given element in the work equation.
Standard technologies from manufacturers will suggest up to 7.
( ie: Mitsubishi & Agie, carbide < 1.2 um finishes )
> a single electrode that may have required 5 otherwise (a practical
> real example given in the book) suggests to me it is pretty useful.  
>
Use orbiting, it will be useful for you,
but include fast jumping and neg pol also,
they are cheap and handy.
( you get the 'better flushing' you mentioned )
They will add very little expense to the hobbyist edm machine.

I dont know if we ever discussed the retract in Senkung EDM but there's
only 2 geometric points to remember ( to run away to ).
The beginning of the cut, and
the beginning of any orbit.
If you dont orbit, there's only 1 point to retract to.
The retract always almost linear.

So these 2 points and linear interp allow retract and jump and inspection
during execution.

When cutting threads by orbital expansion,
the retract is just to center of bore, then toward program start.

And there are some shapes that cannot be orbited easily.
I've done washing machine agitors and other 'impossible' forms.
Often the problem is some 'washing out' of geometry.

> More importantly for the hobbiest (which I am) it may well allow me to
> achieve better flushing than I might otherwise.
>
> Graham
TomP