-- The Home Page of the Finishing Industry

on this site
current topics
Live! From Pine Beach NJ: The world's most popular metal finishing website, and the internet's friendliest corner

topic 5396

Ra surface finish -- unit-less, or what are the units

A discussion started in 2000 but continuing through 2019


Q. Ra is the arithmetical mean roughness of a surface. Does Ra have units associated with it (i.e., inches, microns, etc.), or is it a unitless number?

David Arnold
Plastic Mfg. - Minden, Nebraska


A. David:

Surface roughness values, including Ra need a dimension to be interpreted. The common unit for machining applications in the US is microinch (millionths of an inch). Metric values would be microns (millionths of a meter) or nanometer (billionths of a meter) for polished surfaces.

larry hanke
Larry Hanke
Minneapolis, Minnesota


A. All of the following have units of length ---
Ra: The arithmetic mean
Rq: The root mean square
Ry: Maximum peak-to-valley height

Hope this helps.

James Totter
James Totter, CEF
- Tallahassee, Florida


A. The average roughness, Ra, is expressed in units of height. In the Imperial (English) system, Ra typically expressed in "millionths" of an inch. This is also referred to as "microinches" or sometimes just as "micro" (however the latter is just slang). In the metric system, Ra is typically expressed as "millionths of a meter" also called "micrometers" or "microns".

Mark Malburg, Ph.D.
Chairman ASME-B46.1 "Surface Texture" - Columbus, Indiana


Q. Is Ra & RMS exclusively used for measuring surface quality on metal finishes? Is there another measurement value used for measuring Roughness Average on glass mirror surface? How does Ra of metal surface compares to surface of a glass mirror?

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

A. Hi Michael. A mirror finish on metal is about an Ra of 4 microinches or 0.1 microns. I don't know what would happen if you ran a profilometer over a smooth glass finish and am curious myself :-)


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

July 8, 2008

Q. How large an area do you test to get the value is of Ra? How do you separate Ra from flatness?

Ian Barlow
Engineer - France

October 11, 2008

Q. I need to know what is measuring equipment to measure Ra?

Zaki Yamani Ahmad
engineer - Kuantan, Pahang, Malaysia


August 2016

A. Hi Zaki. You would use a profilometer =>


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

December 9, 2008

Q. Need help!

I've got this statement from one journal I've read.

'rough surface decrease Ra value while on smooth surface, Ra value increase'.

Is it right? Or it works the opposite way?

Thanks for helping, anyone.

Lia [last name deleted for privacy by Editor]
- Johor, Malaysia

A. No, Lia. That is incorrect information, it works the opposite way. Ra 125 is rougher than Ra 32. However, for grit size it works the opposite direction: the higher the grit number, the finer the grit and the finer finish it can produce.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

July 17, 2009

Q. We are cutting a sprocket type gear on hobbing machine(Generating process) with the axial feed ... in this case to check the surface finish value (Ra), whether we should check along the feed direction? or across the feed direction?

Kiran Gadag
application engineer - Bangalore,India

September 15, 2010

Q. I have a drawing of a plastic-moulded part with a minimum, as well as, a maximum, Ra tolerance for the surface finish. The surface in question, seal on an 'o' ring, to prevent fluid leakage. Is there any reason why I would not want less than the minimum Ra (0.2 in this case)?

Gary Spring
- Basildon, Essex, UK

March 12, 2011

A. Gary Spring,

I would imagine as your part was a moulding the designer would spec a min and max Ra either for aesthetic purposes or to reduce manufacturing costs. The lower the Ra the more processing will be required; i.e., a mirror finish will be more expensive than a good commercial polish.


Steve Addicott
- UK

December 13, 2010

Q. Hello,
Please tell me information of Ra roughness surface and what equipment. Also calculation to get the Ra roughness surface.

Thank you for your information.

A.Sukri Saad
plating worker - Kulim, Kedah, Malaysia.

April 5, 2011

Q. Dear Sir,
What is relation between Ra value we use to specified in drawing 1.6/0.8/0.4/ and micron?
Please help me in this.

Shyam Tare
Design Dept - Vadodara,Gujrat, India

April 5, 2011

A. Hi, Tare.

A "micron" is a micrometer, one millionth of a meter.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
We need "Aloha" now more than ever

September 13, 2011

Q. If I hone a cylinder to 7.880" ± .001 and it calls for a 20-30 RA finish how do I measure this to know that I have achieved the finish spec'ed? How do I know if I have a 20 to 30 RA?

Greg Weitzel
Technician - Indianapolis, Indiana, USA

November 23, 2011

Q. Print calls for a 125Ra, the part measures 155 Ra

Is this within tolerance?

Rod Adkins
- Wilmington, Delaware, USA

March 8, 2012

A. For measuring Ra value, you could use Surface Roughness Tester; it's available in the market place.

About 125 in measuring and part has 155, it does not matter.

125 to 250 is common tolerance

J Malai
- Green Bay, Wisconsin

November 28, 2011

Q. If a drawing specifies 500 micro-inch requirement, I've always been under the impression that this value would be the maximum allowed surface-roughness variation as defined by ANSI/ASME B46.1 [link by ed. to spec at TechStreet]. A colleague told me that this is just a target value, not a maximum. what is the convention for the surface finish callout on a drawing? What is the document that governs this? I couldn't specifically determine the interpretation of this per ANSI B46.1? Let me know.

Dave Johnson
Aerospace/Military - Clinton, Utah, USA

November 29, 2011

A. I can't say what the drawing intent for your specific customer is, only my own personal experiences. If a drawing calls up a surface finish requirement it is considered to be a maximum value as far as I am concerned. I have worked on and with many of the Aerospace platforms and all drawings that I have come across have stated surface finishes as maximums (although there was one drawing that actually did the reverse and stated a minimum roughness! It was clearly marked on the drawing though).

In the end, if you are not certain what the requirement is go back to your customer and ask the question. 2 minutes on the phone may save you a lot of pain.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

December 17, 2011



October 4, 2013

Q. Dear sir, I want to know what is the tooth surface roughness unit if I measure it with digital microscope? Microns or other?
Thank you for your kindness.

Lita Christ
- Yogya, Indonesia

Length of Travel and Cutoff Length Settings on Profilometers

February 7, 2014

Q. I work in quality and lately have been thinking if the settings for 'LT' (Length of Travel) and 'LC' (Cutoff length) on a profilometer should be considered when measuring a particular range of 'RA' surface finishes? We generally grind and/or super polish from 2 RA to 8 RA. I'd appreciate your thoughts.

Paul Desko
Quality - Torrington, Connecticut, USA

February 15, 2014

A. You might have something that could help others. The length could be taken into consideration as part of the overall reading. Meaning if the part was say 3 inches and got a reading of 6Ra and you were only able to measure half that length for whatever reason, you could indicate that as Ra6@50. I'm not sure that would make a lot of difference in the overall reading but it would give the observer more reliability of the quality.

tony kenton
AF Kenton
Hatboro, Pennsylvania

February 17, 2014

Q. I guess I wasn't clear in my explanation of what I was in search of. My goal was to understand if I should be using a different 'length of travel' and/or 'cut-off length' when our surface finishes require a 2RA "instead of" an 8 RA or above. Maybe that will make my question clearer. Thanks in advance.

Paul Desko [returning]
Quality - Torrington, Connecticut, USA

February 19, 2014

A. The English language is a SOB sometimes. OK let me see if I understand what you are looking for. The length of the part is one thing. the length of the profilometer stroke is another, the reading of the surface finish is the 3rd thing. So if I understand this right, you are requesting info on the difference between the RA's in which case, the lower the number the smoother the surface finish. I don't think you are talking about different measurement scales, are you?

tony kenton
AF Kenton
Hatboro, Pennsylvania

(you are on the 1st page of the thread)       Next page >

If you have a question in mind which seems off topic, please Search the Site

ADD a Q, A, or Comment on THIS topic START an UNRELATED topicView CURRENT HOT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2019, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.