Surface Finish Conversions - Metric/ English and Ra/RMS

A. The actual values for surface roughness can be found in the following standards - BS2634, ISO 2632 and ANSI B 46.1 .

The parameters and procedures for measuring them are defined in BS1134 and ISO 468 [withdrawn].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

A. Here is how I convert 0.1 µm Ra to RMS

1. step one: convert 0.1 µm to microinches
0.1 µm-.004 µin

2. step two: use the formula RMS=Ra/1.11
RMS=.004/1.11 which equals to: .0018 µin

3. step three: Write the result in standard engineering form:

.0018 µin RMS is usually written as: RMS 18

So here is completed plan on how to convert Ra to RMS (and back).

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Thanks, Senthil. You apparently copied this from an entry on letter 9524. But that entry has subsequently been corrected :-(

In step 1, 0.1 µm 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.
Striving to live Aloha
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.

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August , 2008

Thanks, Benjamin, that's a good reference/

Regards,

Ted Mooney, P.E.
Striving to live Aloha
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 µm (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)

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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,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2008

A. Hi everyone.
As per the surface roughness calculations, I find this conversion formula. I think it is correct method.
If you find any mistakes mean please clarify me.

RMS -- Root Mean Square value in Micro Inches
Ra -Roughness Average in Microns
(1 micron=0.001 mm & 1 micro inch=0.0254 mm)
1 Ra=43.700 RMS approximately
1 RMS=0.02288 Ra