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-----:Surface finishes N1 thru N12 and their equivalents in Ra
Quickstart:
Readers are often confused by the many different ways of measuring surface roughness like Ra, Rz, Rq, Rt, & RMS, and the fact that some of these are are supposed to be expressed in µm, others are supposed to be expressed in µin suffixes, and some can be expressed in either µm or µin units but are often indicated on drawings without those necessary units, resulting in errors.
To combat this confusion and simplify everything, ISO 1302: 1978 introduced roughness grade numbers, "N-numbers", so one simple number from N1 to N12 could specify the required smoothness with no confusion over units.
However (believe it or not) ISO 21920 obsoleted ISO 1302 and its whole N-number approach, and N numbers are now strongly discouraged. So we're back to the confusion but not before yet another confusing and broken standard proliferated. We now use a "line" symbol above and to the left of the √, to indicate that the spec is the ISO 21920 one rather than an old one 🙂
Unfortunately, another enduring problem amplifies confusion: specs are expensive (ISO 21920 is about $750) so, being realistic, small & infrequent users are not likely to buy costly new specs.
Read on, join in ...
Q. I AM LOOKING FOR A CHART OR SOME REFERENCE RELATING N5 N6 N7 N8 N9 TO Ra VALUES.
Michael L [surname deleted for privacy by Editor]-Wyandotte, Michigan
2001
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 Ra | Roughness values Ra | Roughness |
| micrometers | microinches | Grade Numbers |
| 50 | 2000 | N12 |
| 25 | 1000 | N11 |
| 12.5 | 500 | N10 |
| 250 | N9 | |
| 3.2 | 125 | N8 |
| 1.6 | 63 | N7 |
| 0.8 | 32 | N6 |
| 0.4 | 16 | N5 |
| 0.2 | 8 | N4 |
| 0.1 | 4 | N3 |
| 0.05 | 2 | N2 |
| 0.025 | 1 | N1 |
Bill Boatright
-Raleigh, North Carolina
Q. Bill,
Great chart! Is this a new ISO standard for all metal surfaces?
Thanks,
Michael Liu Taylor
specialty stainless steel distributor - Dallas, Texas
Hi,
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,
pressure vessels - Erie, Pennsylvania
2004
Ed. note: A full chart of N1-N12 vs. many other measurement systems is available at werk24.io
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?
automotive engineering - Benoni, Gauteng, South Africa
2004
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 so they cannot be "converted".
Sort of like tall people tend, on average, to weigh more than short people but you can't convert people's height to their weight and need large safety factors in each direction, which is very wasteful. An average person of five foot height might weigh 100 pounds but that doesn't mean that a 120 pound person is taller than five foot -- they might even be shorter 🙂
Thus the truism which is repeated on most surface metrology sites: Don't 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 ! 🙂
Regards,
Ted Mooney, P.E.
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finishing.com - 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.
machining - Melbourne, Florida
2005
A. Hi, Bruno. That is the symbol for micrometers, so it would be a '63 micron finish'. Good luck because it's still not really clear.
Regards,
Ted Mooney, P.E.
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finishing.com - Pine Beach, New Jersey
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Q. Hi
I'm a student and have a confusion that which is a better surface finish of these two:
N5 or N10 ?
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.
Regards,
Ted Mooney, P.E.
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finishing.com - Pine Beach, New Jersey
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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 www.qualitydigest.com/june01/html/surface.html 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 ground and sandblasted condition of the piece. We have no idea what CRODAN #3 is. Can you tell us the Ra equivalent of this value? ⇦ Answer?
Thanks in advance,
- Ankara, Turkey
June 22, 2010
Ed. note: Sorry, Ozcan, we have little idea what Crodan or Crddan surface finishes are. It presumably has something to do with chrome finishes on rolls, but we can find no further actual info.
If you'd like us to speculate though, note that Armoloy had a chrome plating facility in Croydon, PA, and a Croydon finish might refer to some internal processing sequence at that facility. There is also a Croydon, UK which might have had a chrome plating facility 🙂
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
Ed. note: Hi William, we've received no table or anything labeled SABS 0111 from you. Please <e-mail it for posting> if you wish. We think you're referring to SABS 0111, a South African Bureau of Standards specification for Engineering Drawings?
Q. MPc. I have a surface condition of 12S. What does it mean and how does it convert to 118 Ra? ⇦ Answer?
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 mean by "convert to 118 Ra". Sorry!
Might "12S" be a misreading of "125"? Please humor us and re-post a more detailed question.
Regards,
Ted Mooney, P.E.
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finishing.com - Pine Beach, New Jersey
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Q. Can anybody tell me how the ISO surface finish is used on a drawing?
Thank you all! ⇦ Answer?
- Dublin, Ohio, United States
February 6, 2012
Ed. note: ISO 21920-1 is the spec which explains how to note surface finishing requirements on the drawing.
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
on Amazon or eBay
or AbeBooks
(affil link)
Hi Dennis. It probably means 'Unless Otherwise Stated (all surfaces to be finished to) 125 Arithmetic Average'. But best to talk to the customer to be sure.
Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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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.
MADHUSUDAN SONTHALIAManufacturing - 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.
Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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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.
Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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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.
- 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 Gulstudent - Turkey
May 19, 2022
A. Hi Melissa. It seems that ISO 1302 only goes from N1 to N12. Presumably it is possible to machine it to smoother than N1, an Ra of under 0.025µm though.
Luck & Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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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 ?
Thanks.
Quality Control Engineer - Islamabad , Pakistan
November 26, 2022
A. Hi Awais. We usually are more concerned that a surface is smooth enough to meet our needs, rather than that it is rough enough. 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.
Note however that the N number series is now considered obsolete and is discouraged.
Luck & Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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