electropolishing

Electropolishing solutions are usually "secret" proprietary recipes, but the main ingredient is phosphoric acid. I have a lot of stuff polished commercially. My polisher buys the acid, in 50 gallon drums, and its not cheap. He then heats it to 120 degrees or so, and uses a DC power supply. His is about 1000 amps- and his tank is 4'x4'x8', which is why I pay him to do it, rather than do it myself.

But for something little, I think you could get away with a rubbermaid tub, a battery charger, and some phosphoric acid- maybe Ospho from the auto body supply store.

It will just take longer, thats all. 1000 amps, at a higher voltage (40-100 volts, as I recall) will work a lot faster than 10 amps at 12 volts.

Bill

Surface finish is measured in micro inches on an RMS, RA, or other weighting scale.

One of my projects required the wetted area on part of a heat exchanger to be electro-polished to 25 micro inches RMS or better. We called in competitive bids from firms doing this for a living.

Here is what we were told: Electro polishing DOES NOT improve the micro inch finish of the surface; If you require a 25 RMS finish after electro polishing you have to start with a 25 RMS mechanical finish to begin with.

Electro polishing changes the nature of the surface finish, but does not produce a measurable change in RMS finish.

In my business the electro polish reduced the particulate retention... a most desirable feature in my application.

In your business it may reduce surface friction or improve product finish... my guess.

But, electro polishing will not improve the finish from, say 125 micro inches to 32, 25, or 16 micro inches, unless the E-P process has changed drastically in the last 10 years or so, in which case I defer to newer experience.

As to how to do it. Try googling; I don't recall it being a particularly exotic process. We just didn't want to deal with the mess and disposal of chemicals, hence the subcontract.

Wolfgang

How small is small? We can polish some mighty small stuff with an ultrasonic polisher and a ceramic stone.

One other process you might consider is extrusion honing- this uses an abrasive paste, that is forced thru the opening, polishing it as it goes. They use this commercially to polish the inside of engine manifolds, replacing hand porting and dremel tools.

There are shops that will do this for you- the actual equipment is pretty expensive.

Hi all:

My experience is fairly recent (a few months ago). I work for a food packaging machinery manufacturer. For some product lines, our frames are electropolished. I have personally witnessed before and after measurements of RMS surface roughness, and electropolishing does make a significant difference.

As an example, a 125 surface (beadblasted) polishes to about 60-70 RMS. A suface machined to 64 RMS will electropolish to 28-35 RMS. A rule of thumb that seems to work for us is that electropolishing will cut the measured surface roughness in half.

What electropolishing does not do is change the character of the surface. It won't continue to polish down to a mirror finish from 125 RMS, because it only removes the peaks, and experiences rapidly diminishing returns after a short time of treatment. If a surface has tool marks before, it will have shiny tool marks after. If it's beadblasted before, it will have shiny pits in the surface after.

Also, electrode design has a lot of influence on the uniformity of the final result. This seems to be more art than science at the vendor we use.

My experience is solely with 304 SS. Other metals may behave differently.

Regards, John.