Surface Finish Conversions  Metric/ English and Ra/RMS
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Q. What I'm unclear on is the ratio of the surface finish. For instance, a 63 microinch finish denotes the maximum height of the peaks and valleys, however, this has to be over some unit of area. For a microinch is linear and surfaces are two dimensional. Can anyone explain the ratio? I would particularly find this useful to convert and interpret metric finishes to English. The second thing I was wondering is the difference between Ra and RMS. Even if someone could point me in the direction of a good resource for this information.
Mike Doylecenterless grinding  Holliston, Massachusetts
A. The actual values for surface roughness can be found in the following standards  BS2634 [link is to spec at TechStreet], ISO 2632 and ANSI B 46.1 [download standards ].
The parameters and procedures for measuring them are defined in BS1134 [link is to spec at TechStreet] and ISO 468 [withdrawn]. It is possible that these standards have been revised, my information I have on these are several years old.
I would also suggest that you contact a flange supply company, for all standard flanges are manufactured with surface finishes, and therefore they should have these standards and all relative technical information.
Good Luck
DEON RAUBENHEIMERGASKETS  JOHANNESBURG, GAUTENG, SOUTH AFRICA
A. Here is how I convert 0.1 um Ra to RMS
1. step one: convert 0.1 um to microinches
0.1 un.004 uin
2. step two: use the formula RMS=Ra/1.11
RMS=.004/1.11 which equals to: .0018 uin
3. step three: Write the result in standard engineering form:
.0018 µinRMS is usually written as: RMS 18
so here is completed plan on how to convert Ra to RMS (and back).
 Singapore
A. Thanks, Senthil. You apparently copied and pasted this from an entry on letter 9524. But that entry has subsequently been corrected.
In step 1, 0.1 um is 3.937 uin, or approximately 4 uin, not .004 uin
In step 2, the relationship is inverted: Ra must be multiplied by the
1.11 factor, not divided by it.
So the correct answer is 4.44 µinRMS.
Still, this is an approximation, not a conversion, since the two parameters measure different things.
Regards,
Ted Mooney, P.E. RET finishing.com Pine Beach, New Jersey 
A. www.ptonline.com/articles/200403fa3.html
At the bottom of this page is a good description of Ra, RMS, Rz and Rt.
engineer  Philadelphia, Pennsylvania
A. Thanks, Benjamin, that's a good reference/
Regards,
Ted Mooney, P.E. RET finishing.com Pine Beach, New Jersey 
A. Hi,
This is my way of easy calculation...
a) Convert micro inch to micro meter
1 micro inch = 0.0254 micro meter
eg Convert 64 micro inch to micro meter
64 x 0.0254 = 1.63 um (micro meter)
b) Convert Ra(roughness average)to RMS(root mean square)
1 RMS = Ra X 40 (approximation)
eg Convert 3.2 Ra to RMS
3.2 x 40 = 128 RMS (approximation)
 Singapore
August 2008
A. Hi, Minn. It may be easy but unfortunately I don't think it's correct :) Conversion a is fine, but in conversion b, your '40' seems to be an approximation for '39.4' microinches per micron, but the conversion lacks the 1.11 to 1.25 approximation of the ratio between Ra and RMS 3.2 Ra in microns would be something like 138 to 160 RMS in microinches. Regards,
June 15, 2009 Q. What I'm unclear on is the ratio of the surface finish. For instance, a 63 microinch finish denotes the maximum height of the peaks and valleys, however, this has to be over some unit of area. For a microinch is linear and surfaces are two dimensional. Can anyone explain the ratio? I would particularly find this useful to convert and interpret metric finishes to English. The second thing I was wondering is the difference between Ra and RMS. Even if someone could point me in the direction of a good resource for this information. Shreenivas Hayagreev Bangalore, Karnataka , India
July 27, 2011
A. Hi everyone.  Coimbatore, India November 21, 2012 Q. How/why is Rz 2 = Ra 3.2 Talwinder Singh India

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