View Full Version : RA-6 polish

05-13-2008, 01:22 PM
Hi Folks,
I need to polish a shaft, that is a dynamic control linkage for an underwater application, to RA-6 for a water tight seal surface to 1400PSI in sea water.
I've made many of these O-Ring surfaces before to RA-10 and passed hydro every time.
What I was going to try was diamond abrasive dust to put the final polish on the part.

Has anyone here ever measured a surface this smooth without a LASER or microscope?
I usually just pressure test in a Hydro test chamber to 5/3 depth and call it good if there are no leaks.
This time I would like to check my work.
The picture shows the 316 shaft next to a rapid prototype in plastic mock up.


Thanks in advance!

Peter N
05-13-2008, 02:36 PM
Paul, 6Ra is not that big a deal, it's borderline between a medium to smooth turned finish.
We mould plastic parts to a finish of between 0.2-0.4Ra, which is from a mould cavity that is surface ground then lightly stoned over the grinding.
You shouldn't need to go anywere near diamond paste or lapping until you need 0.1Ra or less.

As to measurement, we do use a surface roughness tester but this is just a standard diamond stylus, no laser or microscope.


05-13-2008, 02:58 PM
That's pretty interesting Paul -- how do you lap/polish the part without losing the dimensional tolerance? I'm guessing it has a pretty tight tolerance if it's a press fit part for a sub.

Are you turning it slightly over, and then polishing/lapping to final fit?

05-13-2008, 07:15 PM
Thanks guys,
For some reason I thought 6 millionths was a mirror finish. I machine to R-32 for standard Parker O-rings all the time with no lap and they work fine. For some reason the dynamic seal manufacturer calls for RA-6.

Lazo, I usually stop at .001 proud and build a Teflon col let to polish the last bit very slow.
So yes I lap to final.... I end up chasing the final dimension all over the place. I usually get close enough to get it through.

Thanks again!