Authoritative Answers, Fun, & Aloha -- no cost, no passwords, no popups
(as an eBay Partner & Amazon Affiliate we earn from qualifying purchases)

Home /
Site 🔍
pub     mobile?
Metal finishing Q&As since 1989


Ra as compared to AA for surface finishes

I am considering quoting a job that calls for a finish of Ra125 and Ra63 and after reading your site I am wondering if these values are correctly displayed. Does this make sense to you or should they have expressed these as Ra3.2 and Ra1.6? I did not want to question them on it until I knew what I was talking about. This job is coming up soon so if you could respond as soon as possible it would be greatly appreciated.


David Starcher
machine shop - Irving, Texas

Sounds like you are confused on 2 issues. 1. the first Ra mentioned is a basically a standard surface feature which 2. you seem to try to convert to an RMS by dividing by 4., but used Ra instead again. the second Ra does seem a bit fine and needs to be clarified. I'm not sure how to advise you about this matter. One way is to try and get a sample part to match the surface.

tony kenton
AF Kenton
retired business owner - Hatboro, Pennsylvania


There are a number of technical resources you might find helpful in understanding these terms in a meaningful way.

(1) The Ra 63 and 125 terms you use are incomplete without reference to te measurement units, in your case micro-inches. The 125 micro-inch surface profile measurement is not uncommon for a machined or ground surface, 63 would usually indicate some finish machining or mass media finishing (barrel or vibratory) with ceramic media shapes and aqueous compounds.

(2) There is a fairly good discussion of surface finish measurement terminology as related to surface texture at:, they have on this page a chart showing typical Ra values for various machining, fabricating and mechanical finishing operations.

(3) Another potential resource for technical help in this area might be the Society of Manufacturing Engineers - Debrring, Edge-Finish and Surface Conditioning Technical Group. A link to this technical group's web page would be:

(4) Machinery's Handbook discusses this as well as several related topics including surface integrity.


David A. Davidson
- Spokane, Washington

(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Chemicals &
& Software

About/Contact  -  Privacy Policy  -  ©1995-2024, Pine Beach, New Jersey, USA  -  about "affil links"