RE: stepper edm

Graham,
IMO the advantage of orbiting is simplification of electrode manufacture.
IMO orbiting is NOT faster/better than plunging.

Having 2 exactly the same electrodes undersized exactly the same and on the same centers is very difficult.
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 )

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.

Semi-modern thinking (70's 80's) was that it was cheaper to orbit,
because the neccesary 'sameness' was better with good tooling
( Erowa Mecca 3R Istema etc )
And using the same setup to make each electrode, reduced manufacturing error..

Current production methods, fixturing and machines allow different undersize tools that are accurate and cheap.
I'll go further and say that simpler shapes ( considering path time as money ) will show that old timey negative polarity
cutting is going to be the fastest.
Neg Pol also has the benefit of being a bit safer with larger 'gaps'.
30% wear is not bad if you make up the money in time, and ease of operation.
(Volumetric removal rates are higher Graphite to Steel with negative polarity in the rough & finish ranges,
Volumetric wear is higher tho in the rough range, not in the fine range )

regards,
TomP

> I remember seeing this myself a while back, interestingly he seems to
> have set up his software for profile cutting on a CNC sinker machine.
>
> I'm working away with the propeller starting to get into the motion
> control side of things to allow basic moves in three axis for rapids
> and hopefully what I learn will be applicable to servo type controls.
> I think I can control 3-axis with one processor as that is all it
> has to do but time will tell as I work out the details.
>

Please read the ISEM and JSEM journals, Uni Leuvan, Prof Crookhall etc,
for such information. Old issues of EDM digest were also good.
there is really quite a bit of good info on EDM one the web,
even more for someone like you who has Uni access to the engineering journals.

> I've been reading The EDM Handbook by E. Bud Guitrau and learning
> about the benifits of CNC orbiting. I knew that conventional orbiting
> helps with flushing as your electrode enhabits more space in the
> workpiece than its own dimensions helping with flushing nut I hadn't
> realized that CNC versions of this can in some cases allow roughing
> and finishing with a single electrode.
>

EDM is slow,
and when you orbit, the cutting area is small,
( it's like rolling a ball in a bowl, the contact area is small)
and therefore a fraction of the cutting area that it could be.
This is a bit slower and a bit safer...
(like drive by shooting... but it's drive by edm )

Thus the correctly undersized ( tho diff undersize ) is faster
( do the hokey pokey and put your BIG foot out ! )

> You rough as normal with the orbiting action for good flushing, high
> power/low frequency for low electrode wear, then after flushing you
> lower the electrode into the cavity and erode sideways into the walls
> of the cavity with finishing settings.

The 'improved' flushing is a biased statement.
(based on a view that neglects some simple basics)...
when you finish, the flushing gets bad, becuz the gap is smaller.
so, the orbiting motion allows space , this is true,
but it is space where you're NOT cutting ,
it is space where the dirt is NOT being generated,
it is a cleaning in the area where there is no cutting.
The cutting interface is still 'crowded'.
So orbiting is a TEMPORAL advantage,
like jumping, it simply doesnt let bad things happen too long in one place.

Orbiting's nature is to use a small cutting interface.
Orbiting's efficicency is less than full interfacial cutting.

Well, the asians just jump a lot, and that works very well.
In the US we prided ourselves on never jumping and 'just cutting'
but for fine finishes & difficult flushing conditions,
a good jump is hard to beat.

I'd vote for sized tools and good jump.
but YMMV, it depends on the job ( it depends on the $$$)

Orbiting vs Flushing Holes vs Jumping...
Putting flush holes in copper electrodes isnt an option.
Putting flush holes in subgates, and skinny ribs isnt an option,
So jumping is often used, and hi speed jumping is good
(untill you overcome the stiffness of your machine and servo )

> You still have improved
> flushing and the larger contact area allows higher powers to be used,
> the orbiting action also means that the

Orbititng does have a 'wiping action',
the surface of the tool is not replicated (as much) into the workpiece.
Tho you could change the tool to one that wasnt 'work-textured'

> surface finish doesn't leave a
> copy of itself in the side walls but is "sawn" off by the movement
> improving finish.

There is some attrition in any approach where the tool wears.
More so when the working area is small
( ie: engraving tools wear thier tips off, electordes loose corners )

The winner is the approach that gets more money than it looses.
That depends on your tooling, machine resources.....
If you can make electrodes accurately and cheaply, then size 'em and plunge.
If the tool is a real work of art, then orbit it

> There is also no/less rounding of the leading edge
> of the electrode as unlike a conventional finishing electrode it does
> not have to sink through the whole height of the cavity, the large
> side surfaces share the wear.
>

Yep, theory doesnt pay the bills.
The work will tell you,
you dont tell the work :-)

> This could all get quite interesting in theory.
>
> Cheers,
>
> Graham
>