Electropolishing - What does it do to the surface?

SeymourDumore · Jan 19, 2018

Guys, a technical question: What does electropolishing do to a surface that cannot be reproduced in any other way?

I kinda-sorta know what it does and how it does it ( though surely ready to be educated in detail, even deep detail ), but the question really
is in the realm of why it might be specified EXPLICITLY as a surface finish?

Take the very case in hand ( in my hand right now ):

Part is a 3.00 dia x .87 thick disk. One surface is drilled/tapped 3 x 1/4-20 .500 max, do not break through.
The other side is plane-jane flat.
There are no thickness, flatness or parallel callouts whatsoever, tolerances are wide open at +/-.01 UOS.

And yet!
The flat face ( only the untapped face ) and the OD is specified as:

Glass bead blast and electro polish these areas RA 1.6.

I mean I can grind and then flat-lap the face to w/in .0001 if needed.
I can grind and polish the OD if needed.
I can make them mirror in every sense of the way ( have granite plates if needed for the 15" Lapmaster ) mechanically.

But, I do have to go with what the print says, and I will.

So the question is simply rhetorical and educational: What does the electro polish process do to a surface which cannot be achieved in another way?

MJPfin · Jan 19, 2018

I think its more about the glass beading than the electropolishing. They want the beading to relive surface stress and then they want the ra 1.6 finish.
But this is just a wild guess.
More educated people can correct me.

Marko

SeymourDumore · Jan 19, 2018

MJPfin said:
I think its more about the glass beading than the electropolishing. They want the beading to relive surface stress and then they want the ra 1.6 finish.

Marko

Hmm... Doesn't beading with anything actually add stress?
I know that a couple of times in the past I've glass beaded some of my Wire EDM-ed parts to make them look pretty, but when done the miserable things moved by
as much as 2-3 thou, just enough to cut them again...

Dan from Oakland · Jan 19, 2018

Suppose you could ask your customer. If ya need a great electro polisher, Scientific Platers in Oakland is the cats meow.
Dan

SeymourDumore · Jan 19, 2018

Dan from Oakland said:
Suppose you could ask your customer.
Dan

Not in this case!
Customer is ordering a replacement part(s) for a piece of equipment they have, but did not design.
I do not know the equipment, nor the OEM. All I have is a photo copied print to work with.

Again, I will ( have already ) glass bead blasted the parts and they are going to EP tomorrow, just curious as to what it does and why it's explicitly required.

steve-l · Jan 19, 2018

I suspect the glass bead blasting and EP is just for appearance only and the reason is because it is cheap to do.

gregormarwick · Jan 19, 2018

As far as what it does, it erodes the surface starting with the peaks, rounding off sharp peaks and leaving they valleys more or less untouched. It's usually used because it doesn't affect dimensional accuracy as much as mechanical polishing processes.

As for why they want it, we can only guess.

9100 · Jan 19, 2018

The basic mechanism is that the solution forms a surface film and the high spots stick up through it, sort of like mountain peaks above fog. The film has considerable electrical resistance, so the peaks get more current. The action is the reverse of plating- it unplates the high points, leveling the surface. You can expect it to round off corners and make the periphery of the plate lower. In normal polishing you will always have scratches proportional to the size of the grit. In electropolishing there are no scratches so it makes an extremely smooth surface. Most of the electropolishing I have done was with a sulfuric acid solution on 300 series stainless. While it was very smooth, there were tiny craters. I don't know the reason.

Bill

billzweig · Jan 20, 2018

The glass bead blasting creates a (slightly) rough surface, while electro-polishing tends to smooth the surface by eroding the high points - as mentioned by Bill above. So the combination of those two contradicting operations does not make much sense unless, for some unknown reason, the microscopically undulated but polished surface is what is needed.

Johnny SolidWorks · Jan 20, 2018

Is it food or pharmaceutical industry work? There seems to be a belief among engineers, particularly in those two industries, that electropolish is the best/most cost effective process to remove toolmarks and any embedded foreign material (especially HSS particles in stainless) in the surface. I used to have a customer who required it on their 17-4 stainless, precipitation hardened parts. They were/are known to test and document everything to an extreme degree, and they believed they had documentation proving what I stated above. (They're a Fortune 500 Pharmaceutical company.)

Of course, almost all of those parts would have been extremely cost prohibitive to achieve a similar surface finish on through any mechanical means. And, by and large, most shops don't have those capabilities anyway, let alone the ability to do it in quantity. We would make several thousand of the parts I'm discussing every quarter, and could have electropolish done with good quality in large quantities relatively cheaply.

newtonsapple · Jan 20, 2018

Eletropolishing can make a significant improvement to high cycle fatigue life. It eliminates stress concentrations on the surface. Mechanical polishing does not work as well, at least in the case of small diameter nickel titanium wire.

I think shot peening would have ben spec'd instead of glass bead if fatigue improvement was the goal.

So it is likely spec'd to make the parts look pretty and maybe for cleaning.

MrSteve · Jan 20, 2018

SeymourDumore said:
Hmm... Doesn't beading with anything actually add stress?

It cold works the surface and adds compressive stress, which results in better fatigue strength and reduced crack formation/propagation. It may also, when combined with the electropolishing — which reduces the surface Ra by removing peaks — give a desired surface appearance, so it’s posible it’s cosmetic instead of, or in addition to functional.

MJPfin · Jan 21, 2018

That was a good description, the crack formation part is why we did it to reworked conecting rods back in the day when I was making engines.

Marko

Miguels244 · Jan 21, 2018

Electropolish knocks the sharp points off. You get the same Ra but with rounder peaks. It also passivates the surface. Improved corrosion resistance, ease of cleaning etc.

It is commonly used on irregularly shaped parts like the inside of tubes and valves. It's possible that the entire interior of the assembly is specced that way for simplicities sake. You just happen to have a flat piece.

SeymourDumore · Jan 22, 2018

Forgot to add that the material is Duplex2205, which by itself is a high temp corrosion resistant alloy ( also rather expensive Dammit! )

Don't have even the slightest clue as to what the part is, though I suspect that it IS a consumable item for some sort of equipment my customer owns and uses.
So, for all I know it is specified by the manufacturer as a blanket.

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