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topic 7560


A discussion started in 2001 but continuing through 2019



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 [info about this spec at Amazon; and at Techstreet]

Roughness values RaRoughness values RaRoughness
micrometersmicroinchesGrade Numbers
8.3    6.3250N9

Good Luck.

Bill Boatright
-Raleigh, North Carolina


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


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

affil. link


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 are not equivalent nor can they 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. 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 :-)


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


Q. I am trying to find out what a 63u 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

April 23, 2008

A. Hi, Bruno. I think most people would take this to be 63 microinch finish. Good luck.


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

April 23, 2008

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

May 2008

A. Hi, Waqas. In the chart above, rougher finishes are at the top and smoother finishes are at the bottom.


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

June 22, 2010

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

July 5, 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

January 16, 2012

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 17, 2012

Apologies, Hector, but I have to say: "What?!"

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

February 6, 2012

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

Thank you all!

Kevin Kaminski
- Dublin, Ohio, United States

April 9, 2015

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

Dennis Kimball
- Auburn, Massachusetts USA

April 2015

A. Hi Dennis. When you google something and you come up empty it means it probably doesn't have a universally accepted meaning, so you'd better talk to your customer or there's a chance there could be trouble brewing :-(

My guess is that it means "unless otherwise stated, all surfaces are to be finished to 125 arithmetic average", but if an unhappy customer complains that you didn't build it to Ubiquitous Ostriches Standard 125 As Amended, it will be your fault rather than theirs :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

Least expensive way to get N8 finish?

May 21, 2019

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 2019

A. Hi Madhusdan. What sort of components, what production volume, and what material of construction? Typically, bulk finishing (tumbling, vibratory 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 - Pine Beach, New Jersey
Aloha -- an idea worth spreading

May 21, 2019

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 2019

A. Hi again. Sorry, I am not experienced with machining graphite, but I have read 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 - Pine Beach, New Jersey
Aloha -- an idea worth spreading

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