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Scale thickness of EDM sinker?


Aug 19, 2021
Hey team.
Things are Progressing well here and my die making skills are getting better every day. I’m wondering however we are trying to hit a repeatable tolerance on an EDM sunken part which is essentially a blind hole.

We have to sand and lap the bore but are trying to not have to stand there all day and potentially alter the geometry.

If the final lapped diameter is 5.70mm, just wondering what electrode diameter you would use to give enough die surface to clean and polish? How thick is the scale?

Thanks in advance.
Hi Lplates:
Short answer: it depends!
I assert this because there are a number of factors you need to consider to arrive at an intelligent answer.

First some terminology...traditionally the "scale" you're referring to, is a combination of recast...(White Layer) and Heat Affected Zone...(HAZ).
How much of those two undesirable things you get depends on a bunch of factors, the two dominant ones being the amount of current you pump into the spark gap, and the vintage of the spark generator.
Of those, recast is the one most commonly considered to be the bit we must get rid of...it's chemistry is all over the map, and it is full of micro cracks.
Modern generators produce almost none of that recast, and much technology and effort was devoted to reducing its impact, so in the best modern generators it will be measured in low numbers of microns whereas on older machines it will be measured in hundreds of microns.

The other relevant consideration is the EDM finish you're starting the polishing from, and whether you need to get rid of every last pit the EDM has produced.
Again, a modern generator can produce awesomely fine finishes resulting in exceptionally good geometric fidelity, down to the micron level.

So if you're willing to invest in a new machine you can reduce the polishing and lapping burden to trivial amounts or even to zero, depending on the application.

There is another consideration implied in your question:
You're asking to know what size electrode to use to get to a final finished dimension.
The smartass answer is...you don't care that much.

I say this because modern sinkers can compensate for an undersize electrode by orbiting the trode and just changing the amount of orbit to accommodate whatever the trode happens to be.
You can use this technique to achieve exquisite accuracy...I can work to a tenth on my 1996 vintage low end machine with care, and a modern machine can do much much better without breaking a sweat.

To get an idea of just what a modern machine can do for you, I encourage you to talk to Brian Pfluger from Makino.
He knows the turf really well and can give you a much better answer for the machines he represents than I ever could.
I'm guessing if you want a perfect lapped bore, he's going to recommend burning it to a few tenths undersize, and these machines can do that pretty easily.

Imagine the fucking around and time and uncertainty you will save if you don't have to polish for half a day on every single bore you burn and can hit your target size every time with complete control.


The best way to answer your question would simply be to do a test piece. Check size after EDM, then after you handwork. If you keep similar process you should get a similar result each time.

You mentioned a finish size but more importantly what is your tolerance?
Sorry guys I have been away but great answers and I’ll give Makino a call and talk about the possibilities:)