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Metal finishing Q&As since 1989


Surface finishes N1 thru N12 and their equivalents in Ra

Q. I'm measuring surface roughness of a part (machined MS plate); my surface roughness instrument shows the Ra value 7.4 - 7.8, so is this N9 or N10 ?
Also if N9 is 6.3 then does it mean any value above 6.3 and less than 12.5 is N9 or does it fall in N10 ?

Awais Muhammad
Quality Control Engineer - Islamabad , Pakistan
November 26, 2022

A. Hi Awais. We generally are more concerned that a surface is smooth enough to meet our needs, rather than that it is rough enough to meet our needs. For this reason, when we see 'grades' in charts like the one on this page and in ISO 1302, the issue becomes whether a given finish is smooth enough to qualify for a given grade.
Since an Ra 7.4-7.8 is rougher than an Ra of 6.3, such a surface does not qualify as an N9, but only as an N10.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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Michael L [last name deleted for privacy by Editor]
-Wyandotte, Michigan

A. Grade Numbers Older drawings may use roughness grade numbers to indicate Ra values. The following table is given in ISO 1302 [affil. link on Amazon]

Roughness values RaRoughness values RaRoughness
micrometersmicroinchesGrade Numbers
8.3     6.3250N9
Good Luck.
Bill Boatright
-Raleigh, North Carolina

Q. Bill,

Great chart! Is this a new ISO standard for all metal surfaces?


Michael Liu Taylor
Michael Liu Taylor
specialty stainless steel distributor - Dallas, Texas

thumbs up signHi,

I like your table. However, I believe there's a small mistake. The metric value corresponding to a 250 micro-inch surface roughness/finish should be 6.4 micrometers, rather than 8.3.

Best regards,

Justin H [last name deleted for privacy by Editor]
pressure vessels - Erie, Pennsylvania

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Q. One of our Customer Drawings indicate a Surface Finish specification of N6.

We only have a Surface Finish Tester that can measure Rz & Ra.

Where Can I find or how do I convert N to Rz?

Henry R [last name deleted for privacy by Editor]
automotive engineering - Benoni, Gauteng, South Africa

A. Hi, Henry. You can see that the chart tells you N6 = Ra of 32 microinches. So your test device which measures Ra should work.

If you search the site for "Ra vs Rz" you will see that while they tend to generally track each other, they measure different things and cannot "be converted".

Sort of like taller people tend, on average, to weigh more than shorter people but you cannot convert people's height to their weight. If you want to 'convert' from one to the other you need large safety factors in each direction, which is very wasteful. An average person of five foot height might weigh 100 pounds but that does mean that a 120 pound person is taller than five foot -- they might even be shorter :-)

Thus the same truism is repeated on one surface metrology site after another after another: Do not attempt to "convert" or "correlate" -- If you want to know the Ra, measure the Ra; if you want to know the Rz, measure the Rz! :-)


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

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Q. I am trying to find out what a 63µ finish is? I have put this finish on a polycarbonate part that needs to be machined.
Thank you for your help.

Bruno D [last name deleted for privacy by Editor]
machining - Melbourne, Florida

A. Hi, Bruno. That is the symbol for micrometers, so it would be a '63 micron finish'. Good luck.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. Hi
I'm a student and have a confusion that which is a better surface finish of these two:
N5 or N10 ?

Waqas Alam
UET - Taxila, Pakistan
April 23, 2008

A. Hi, Waqas. In the chart above, rougher finishes are at the top and smoother finishes are at the bottom, so N5 is a smoother finish than N10.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. Hello Sir!

My name is Emre Özcan, I'm 24 years old, metallurgical and materials engineer at a machinery shop at Turkey.I am also a MSc. Student. We have a little troubleshooting about the interpretation of a roughness value on a technical drawing that came from one of our customers.

While I was searching for answers regarding this problem, I have seen your article on the website and I have got your e-mail from this website. I decided to consult this problem to you and I'm wondering if you can help me out in this problem?

The problem is simple: We have a furnace roll on the technical drawing, and the surface roughness value of this roll is represented with "CRODAN #3" or "CRDDAN #3".This is the surface roughness value of grounded and sandblasted condition of the piece. We have no idea what CRODAN #3 is. Can you tell us the Ra equivalent of this value?

Thanks in advance,

Emre Ozcan
- Ankara, Turkey
June 22, 2010

Q. The table above is in our SABS 0111 - Engineering Drawing Standard, but how do we relate the numbers to a physical finish or machining method? I.e., what code for a standard turning finish, a good milling finish or for a ground and polished finish)

William Jackson
- Durban, South Africa
July 5, 2010

Q. MPc. I have a surface condition of 12S. What does it mean and how does it convert to 118 Ra?

Hector G.
- Chicago, Illinois
January 16, 2012

? Apologies, Hector, but I'm not familiar with "MPc" in this context, nor with "12S" in this context, nor with what you even mean by "convert to 118 Ra". Sorry.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. Can anybody tell me how the ISO surface finish is used on a drawing?

Thank you all!

Kevin Kaminski
- Dublin, Ohio, United States
February 6, 2012

Q. I have a customer looking for a finish of "UOS-125AA". I can't find it.

Dennis Kimball
- Auburn, Massachusetts USA
April 9, 2015

"Surface Engineering for Enhanced Performance against Wear"
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thumbs up sign Hi Dennis. My guess is that it means 'Unless Otherwise Stated (all surfaces to be finished to) 125 Arithmetic Average'. But when we Google something and don't come up with a definitive answer, it usually means the phrase doesn't have a universally accepted meaning ... so we'd better talk to the customer or a contract dispute could be brewing. I doubt that it means 'Utterly Outrageous Standard 125 As Amended', but we shouldn't guess :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Least expensive way to get N8 finish?

Q. Which machine is capable of finishing N8 surface finish? Is it Band saw finish or Milling Finish or Ground Finish? Going for better finish like grinding will increase cost. Therefore want to stick with minimum requirement; please advise.

Manufacturing - Jaipur, Rajasthan Country INDIA
May 21, 2019

A. Hi Madhusdan. What sort of components, what production volume, and what material of construction? Typically, bulk finishing (tumbling, vibratory finishing, centrifugal barrel finishing, etc.) is the least expensive way to a smooth finish because it doesn't require individual handling.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Q. Hello,

We have to machine Graphite parts and the drawing specifies N8 & N6 finish.

The machining can be done on a bandsaw as well as on a milling machine. Hence we need the clarification with regard to the tolerance level in N8 & N6 finish, so we could choose the most economical method to machine the part.

- Jaipur India
May 21, 2019

A. Hi again. Sorry, I am not experienced with machining graphite, but a quick read said that it can be molded from different grain size, which affects the smoothness of cut, and also that it tends to fracture off, leaving pits if milled from the outside in.
With metal, a bandsaw cut is usually rough, and the faster the cut the rougher. I can only guess that you may be able to cut graphite with a bandsaw to an N8 finish but not an N6 -- but guessing is all I can do :-)
Hopefully someone experienced with machining graphite and measuring the smoothness will chime in. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

thumbs up sign Yes to Ted's answer. You can get a N9 finish for sure, with a bandsaw, N8 if you are using a good blade with fine teeth.
N6 finish will need to be machined.

Roger Morgan
- Green Bay, Wisconsin
June 13, 2021

Q. Could you please give me information: is there any Roughness Standard under N1 for Ra 0.025? If there is, kindly please share with me. Many thanks for your kind help.

Melissa Gul
student - Turkey
May 19, 2022

A. Hi Melissa. It seems that ISO 1302 only goes from N1 to N12.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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