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topic 3942

Metal surface finish specification and measurement


Q. Dear Sir/Madam,

We are a pharmaceutical equipment company dealing with stainless steel. We finish the stainless surfaces to #4, #6, or #7 finish. Our customers often request us for an accurate description or quantification of the finish. How can we quantify a metal surface finish? Is there any simple way of doing it? Thank you for helping me.


Sanni Raju
- New Brunswick, New Jersey



A. I know that some companies use a comparator to match finishes to known samples. I've even seen a hand held one a few years ago. Check with a few machine shops or equipment vendors for machine shops.

jim conner
Jim Conner
Anoplex - Dallas, Texas USA


A. The definition of the different stainless steel finishes various. There are various grades of No. 4 finishes, different grits as well as different grades of No. 7 mirror finish. For example, some will call No. 7 finish as a brush No. 8 mirror to differentiate it from a non-directional No. 8 mirror or Super #8 mirror. You might want to check ASTM for specifications.

The best thing is to let provide the customer with the different finishes and let them decide what best suits their needs.

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


A. Hi Sanni,

Roughness measurement may quantify your surface finish and meet your customers' requirement.


Ling Hao
- Grand Rapids, Michigan


A. You may want to contact "Specialty Steel Industry of the United States" and request their booklet "Finishes for Stainless Steels". It is one in a series of their Designer Handbooks for Stainless Steel. They are located in Washington DC. This booklet has very good information on the different finishes for Stainless Steel.

Bill Boatright
- Raleigh, North Carolina


Q. I am a Quality Control supervisor for a company that manufactures orthopedic implants. Several of our engineers are having an interesting debate as to what constitutes a "satin" finish on an instrument made of stainless steel (usually 17-4). To the best of my knowledge, a satin finish may be obtained as a result of a lathe process, #8 glass bead blast or rubber wheel finishing. My question is: is there an industry standard for this sort of finish and if so, where would I purchase samples? I have the microfinish comparator and am not looking for something to check Ra.

Quality Control Supervisor [last name deleted for privacy by Editor]
- Austin, Texas

January 10, 2008

A. Finish your part on lathe or mill by using small radius insert with low feed & high RPM. To check the surface finish use the electronic digital surface tester which give the reading in Ra, Rz, RPM.

Satinder Singh
automotive parts - Guelph, Canada

November 26, 2008

A. The standard method of specifying a surface finish is by a Roughness Average, or RA. The only way to obtain this measurement and qualify a surface finish to this type of standard is by using a Profilometer.

Kevin Lenihan
metal products - Gurnee, Illinois

Profilometer: Pocket-Surf

September 23, 2010

A. Sanni,
One way to determine finishes is to purchase material from a company such as ourselves and others, whose polish material to a specified Ra finish. If you need to measure surface finishes of your finished product, I would suggest using a pocket-surf made by Federal. They are compact, reliable, easy to calibrate, and durable when handled properly. I hope this is of assistance.

David A Toops
- Rockaway, New Jersey USA

What is a 4L to 10L finish?

October 24, 2014

Q. Hello. In Ra readings a 15 to 25 Ra, how does that equal a 4L to a 10L? We have many charts, been doing for 30 years. I know linear foot means length but this engineer is talking these L readings. Do have a chart or can you tell me how they figure this in dairy industry?

red moore
- hamilton ohio usa

October 2014


A. Hi Red. Of all of the confusing, fractured, complicated, and redundant standards & measurements in the world, surface finishes are the absolute worst. So when you ask not for a answer to a question, but for clarification of what someone meant, I have to warn you that any answers you get are more likely to further confuse the matter than to clarify it :-)

But if you want a guess, 4L to 10L probably means a finish of Ra=4µ to Ra=10µ applied by Lapping.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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