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topic 14739

Simple Ra / RMS surface finish explanation

A discussion started in 2002 & continuing through 2017


Q. Please clear up something for me. I have always thought of surface finishes to be explained as so. If you have a 125 Ra designation, then you could say that the average peaks, and valley could not exceed .0012"; this make more sense to me (if this is a correct assumption). Could someone clear this up for me. We do a lot of 3D milling using ball end mills, and if I set scallop heights using this assumption would I get the correct results?

Thank you in advance,

Larry Brugger
- Nashville, Tennessee


A. Larry:

You are missing a zero. A 125 finish means 125 millionths of an inch or 0.000125". Very little isn't it?

Guillermo Marrufo
Monterrey, NL, Mexico


Q. That just seems to be a very little amount for what I understand as a 125 finish.

Larry Brugger (returning)
- Nashville, Tennessee

Surface Finish Comparator


A. Larry:

A 125 Ra or RMS finish is what one normally considers a standard machined finish as opposed to a 250 which would be rough. On the other hand 63 would be fine, 32 cannot easily be obtained by machining and has to be ground, 16 absolutely has to be ground, etc. If you want to look and feel how those finishes are, get yourself a comparison board of finishes. It's a standard tool for the machine shop.


Guillermo Marrufo
Monterrey, NL, Mexico

June 16, 2009

A. A 32 finish can be obtained without grinding. I used to machine aluminum with a diamond cutter and we obtained a 32 finish with ease.

frank hinds
- Plano, Texas

June 18, 2009

Thanks for the feedback, Frank.

I don't have much experience with machining. Are you implying that the same can be easily done for other metals like steel, or are you saying that the question and previous answer weren't clear enough, i.e., that while 32 is not easy with steel, it is relatively easy with aluminum?


Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

July 29, 2009

Q. Can someone please tell me what the difference is between a 125 AA finish and a 125 Ra finish? Has one simply been superseded by the other, or are they actually two different things?

Ed Hesner
Mechanical Engineer - Seattle, Washington, U.S.A.


July 31, 2009

A. Hi, Ed. "AA" stands for arithmetic average, and I understand that it used to be used but has been replaced y Ra. "AA" also means so many other things (Aluminum Association, Alcoholics Anonymous, "even better than 'A' ") that it is probably terminology that people try to avoid using today. I think people like to see all surface finishes identified with "R" as in Ra, Rz. Rq


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

November 7, 2011

Q. Sir, Need your help....

What is the difference between SMOOTH 63-125 Ra and SMALL grooved flanges ... are we still talking about finishes?

Please share your thoughts.

Thank you

Dan Rossman
- Montreal QC.

July 10, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello everyone, today I read a drawing note said "ALL MACHINED SURFACES SHALL NOT EXCEED 125AA EXCEPT AS NOTED", I'm not clear what does "125AA" mean? What's the equivalent value for Ra?

Thanks in advance

Steven Huang
- xi'an city, China

October 2013

A. Hello Steven. Your question was appended to a thread which answers it: AA is an old-fashioned designation that has been directly replaced by Ra. So you are looking for a 125 Ra finish. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

June 9, 2016

A. Surface finish is more complicated than that. I believe it's a given height divided by a given length. So no, it's not a height of fractions of ten thousandths of an inch. Haha. A callout on a drawing of a minimum surface finish is normally to insure buddy machining it who is under the gun isn't going to finish it with a .020"/rev federate on looser specs. Precision costs money.

James Ryning
- Edmonton, Alberta, Canada

October 10, 2017

Q. Hi. I have the following query on surface finishes on face of carbon steel flanges, where both manufacturers indicates the face finish of the flange as 125 RMS. Doing the comparison of flanges of different manufacturers one will see the grooves of roughness; in contrast, the other is smooth. The query is because of this roughness difference are both 125 RMS?

Grover Miranda
- Bolivia

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