Hi again HSM_CHIEF:
This can be a bit tricky to visualize, but here's how it works.
The first fundamental thing to remember is that the amount the electrode will orbit is NOT the same as the amount the electrode must be undersized; that is because what we usually call the "Orbit" is actually the orbit plus the overburn.
The overburn of course varies with the power we put into the electrode, so on a modern machine with an adaptive generator the actual machine orbit varies as the generator changes it's power settings while it's cutting and the machine recognizes the overburn amount and adjusts the orbit on the fly to keep from overcutting or undercutting the cavity.
In the kind of spherical orbit I'm familiar with (an X-Y orbit of constantly diminishing diameter), the actual spherical part is done only at the very end of the cycle, and the tip of the trode ends up only the overburn distance away from the floor of the cavity, while it also ends up at the overburn plus the orbit distance from each vertical sidewall, and some fraction of the overburn plus orbit distance away from all the angled faces of the cavity.
So by the very nature of the spherical orbit, the electrode is deeper in the cavity than it would be if it was placed in it's "nominal" position (equal gap all round between cavity surfaces and trode surfaces) once it reaches the very end of its path, and it's deeper by the amount of the orbit and shy of the cavity floor by the overburn amount.
Clearly this distance that the trode goes "deeper than nominal" depends on the size of the spark gap, which of course depends on the part finish desired.
So the electrode must be at least the same height as the finished cavity is deep and needs to be cleared at the parting surface so it can't hack into the parting surface when it's gone below nominal.
Where you set nominal depends on your preferences and the way your machine calculates what it is supposed to do.
There are a couple of ways this can be done:
The old-fashioned way is to touch the tip of the trode on a reference surface and tell the machine to go down the depth of the cavity minus the overburn.
This is how it was universally done in the days before CNC EDM.
A more common and more "Modern" way is to establish the position of every trode holder to a common datum in the trode milling machine and set up the EDM so it knows where that datum was set.
When the toolpath is programmed in the EDM, it will allow for the extra depth it must go beyond the datum to end up with a proper depth of cavity, and it will calculate that allowance based on the finish you tell it you want to end up with.
So long as the trode is necked back so it can't goober the parting edges of the cavity, it doesn't matter how much longer the trode is cut. but both machines must know where the datum is and it cannot be allowed to change.
There are obvious benefits and drawbacks to each approach; which you pick depends on what is easier for your workflow, your machine and is the least confusing to you.
The "old" approach has the benefit of making recutting the trode easier...when you recut the trode you obviously booger the relationship between the trode datum and the clamping face of the trode holder...with the "old" way you don't care because you will reestablish the proper relationship between workpiece and trode when you do your touch off with the trode tip just before you begin burning again.
The "new" way has the benefit of allowing you to cycle trodes through the milling machine and the EDM machine without having to keep track of the datums; they never change, but you waste more trode material, because you cannot recut trodes without boogering that datum relationship.
Intermediate between the two extremes of course is to keep track and then adjust the EDM machine's datum after every trode recut by re-inputting it's value rather than doing a re-touch...it comes to almost the same thing but if the trode is very fragile you risk it every time you do a manual touch, especially if you can't keep the reference surfaces spotlessly clean.
So courses for horses; the main point is that the trode plus the neck down land must be longer than the cavity is deep, so grinding off the end of the trode harms you if it was cut properly to start with.
Your cavity will be distorted when the trode is all the way down.
Measuring the trode can be deceptive; that's why I like going to the pain of having a true trode model; I can mate it in the cavity, section the lot in my CAD program and see if it's going to be OK at all extremes of the orbit envelope.
On a last note, if you look critically at the trode model in my example you'll see it's not correct if you ever wanted to actually burn with that trode.
It's not cleared quite enough at the parting surface...it's going to wipe out the cavity edges when it finishes its orbit.
Cheers
Marcus
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