Ra surface finish -- unit-less, or what are the units
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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 ArnoldPlastic Mfg. - Minden, Nebraska
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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.
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Larry Hanke materials testing laboratory Minneapolis, Minnesota |
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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.
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James Totter, CEF - Tallahassee, Florida |
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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
ANSI/ASME B46.1 [link is to spec at TechStreet]
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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?
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Michael Liu Taylor specialty stainless steel distributor Dallas, Texas |
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 BarlowEngineer - France
October 11, 2008
Q. I need to know what is measuring equipment to measure Ra?
Zaki Yamani Ahmadengineer - Kuantan, Pahang, Malaysia
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?
Thanx for helping, anyone.
- Johor, Malaysia
A. No, Lia. That is incorrect information, it works the opposite way. Ra 125 is smoother than Ra 32. However, for grit size it works the opposite direction: the higher the grit number, the finer the grit.
Regards,
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 Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
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 Gadagapplication 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. ie. a mirror finish will be more expensive than a good commercial polish.
Steve.
- UK
| December 13, 2010
Q. Hello, 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.
Regards
Tare
Design Dept - Vadodara,Gujrat, India
April 5, 2011
A. Hi, Tare.
A "micron" is a micrometer, one millionth of a meter.
Regards,
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Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
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 WeitzelTechnician - Indianapolis, Indiana, USA
November 23, 2011
Q. Print calls for a 125ra
The part measures 155 ra
Is this within tolerance?
- Wilmington, Delaware, USA
March 8, 2012
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
- 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 B46.1. 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 JohnsonAerospace/Military - Clinton, Utah, USA
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.
aerospace - Yeovil, Somerset, United Kingdom
December 17, 2011
Q. Ex " 0.3 MICRON RA". AS IF NOW I UNDERSTAND, RA IS THE UNIT THAT SHOWS ARITHMETIC MEAN OF THE SURFACE FINISH THAT ARE MEASURED FROM FLAT SURFACE RANDOMLY .
ADITHYASJ NAIR- MUMBAI, MAHARASHTRA
