Metal finishing Q&As since 1989

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Conversion of surface finish

We use a lot of polished stainless steel for one of our product lines. Every now and then we will receive an order which specifies a finish in RMS. Our stainless suppliers use terminology such as a 2B finish. Can you tell me where I can find a conversion table, or how to calculate, to convert RMS to Ra to No. 2B finishes to others?

Thanks for your help.

Darrell Dillard
- Athens, TEXAS
2002

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The question of RMS to Ra and the reverse has been answered a couple of times within this category [letters 9524, and 20490, and many others]. Mill grades is another question that is a wild card. If you are really concerned with the QC or proper surface finish, I suggest you look into purchasing a profilometer to test the surface profile yourself. A starting point or rough guideline is that normal mill steel is about 35 RMS or roughly 120+ Ra, then most abrasive products will smooth out the surface in a ratio that slows down as you get finer.

AF Kenton
retired business owner - Hatboro, Pennsylvania
2002

The best explanation I found on the web on roughness notations and meanings is www.predev.com/smg/specification.htm. However, I could not find there your 2B; the closest is N2, which corresponds to Ra=2 micro-in; pay attention that Rq (or RMS) is a different way of calculating roughness, therefore Rq does not correlate directly to Ra.

Giovanni M. Ciriani
promotions - Altavilla, VI, Italy
2002

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Hi, Darrell. 2B is not a surface finish specification; rather it is a process or procedure to do to stainless steel. But letter 22222 tells you what surface finish this procedure will usually produce.

Do not try to convert between RMS and Ra because they measure different things; to promise a particular RMS requires a wastefully high Ra and vice versa.

Regards, 

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 29, 2009