Electropolishing to improve surface finish

Totally clueless here so could use a bit of advice. I am quoting on a 17-4PH part that has some features that can only be machined by sinker EDM (complex profile that has some square internal corners, blind bottom with square corners, vertical walls, quite deep - think long skinny electrode). No problem and we do parts of this family quite regularly.

Problem in this case is the customer requires an N6 finish on the EDM'd surfaces. Our goto EDM place doesn't want to guarantee better than N9, although I'd say he's playing it safe - previous parts have been comfortably better than that. All the same, N6 is really pushing it for sinker EDM as I understand it.

I am looking for ways to post process the EDM'd surfaces and my first thought was electropolishing as geometry will not allow conventional polishing tools, at least not into the corners or near the bottom.

So is electropolishing a good solution here? Can it be applied selectively to the surfaces I require or will the entire part have to be processed? The EDM'd surfaces are not tight tolerance, at around ±.1mm, but there are other machined features on the part with tight tolerances that I'm worried would be affected.

My experience is it very much won't do what you want, the whole electro polishing approach works around removing external features - bumps, it really does not go into crevices - sharp internal corners at all well.

Electropolishing works by an electric charge that by design, attacks the high spots of the material. So in simple terms, it mostly attacks the surface peaks not the valleys. Couple that with what Adama mentioned, you may see decreased performance in sharp inside radii.

Electropolishing can remove a great amount of material if not monitored, controlled or specified properly. Typically when dealing with aluminum for example, .001" to .002" removal isn't out of the norm.

I recommend contacting your local electropolisher and asking them to do a sample part for you so you can see the results. Electropolishers stateside will run samples knowing the business usually follows shortly after.

Electropolishing is plating in reverse, making the part the anode and removing metal. It forms a thin high resistance film over the part and anything that sticks up into the film receives more current and is reduced more than the rest of the surface. Without seeing the part, I would guess that you will have to make a cathode that fits inside the cavity and shape it to remove the selected material. Areas where the current is low usually get a matte finish due to the lack of film formation. To preserve them they will have to be masked, usually done with tape or stop off lacquer.

It mostly depends on the exact part and how good your electropolisher is.

Bill