Micro Conversions

the madjic number to change RA's into rms and or RZ's to rms is this

NO DIFFERENCE IF YOU MULTIPLY OR DIVIED OR ADD OR SUBTRACT OR SQUARE OR SQUARE ROOT OR WHAT BECAUSE NOBODY KNOWS WHAT YOU ARE ASKING!!!!!!!!!!!!

http://www.clearlakefiltration.com/pdf/surface_finish_chart.pdf#search='surface%20finish%20RMS%20chart'

Vader,

It's a smoothness thing.

Okay, that explains why I couldn't make head or tail of his question. It's a smoothness thing and I'm a very rough character.

What is the magic potion that converts apples to oranges?

First Rz and RMS are the same thing; however, they are measured differently. Rz is ten point mean, a result of ISO standards and specifically is measured over 5 highest peaks and valleys at 10 points on the part. RMS does not specify these factors. So the number you get by measuring Rz could be compliant to RMS, as a subset of a possible reading, but a measuremnt by RMS is not same as Rz since it does not conform to the measuring methodology.

Ra is average roughness, it is arithmetical average deviation from the "mean surface," imagine taking six measures , a,b,c,d,e,f, of height of surface roughness features (asperities) and averaging them, this is AA or RA.

The line of mean surface is the surface location where the volume of the highest and lowest asperities (peaks and valleys ) is exactly equal. It is the perfect surface that would result if all the peaks were were levelled off to the point where the material removed would "just fill" the valleys.

RMS (Rq) is the root mean square average deviation from the mean, and, Using the same six measurements of the peaks and valleys above, RMS is the square root of the average of a^2, b^2, c^2, d^2, e^2, f^2. RA and RMS are similar but not strictly equivalent, I used a factor of 1.11 to convert (Ra) to RMS. It worked but is not really a "best practice."

Because RA is a totally different parameter (apples) conversion to RMS / RZ (Oranges) is bad practice. But lots of people try it. Here is a easy to understand summary, read their advice before using their conversion factors, as the direction of the conversion matters:

http://www.qualitymag.com/CDA/Archives/4221785824c38010VgnVCM100000f932a8c0____

To get a really good guide to this subject, log onto this site. It requires registration, but it has all the answers. They make good measurement devices too!

http://www.predev.com/

Milo

If anyone is interested I found the .pdf of info I had saved on this topic. If you'd like a copy send me your real email address via cr4 email by clicking on my name above. (CR4 won't allow passing of .pdf's)

Don't post your email address in a reply to this thread!

Milo