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The methodology of measurement and what is measured are quite different. this is critical to understand if  you will not be paid for your parts because the Ra you measured is not in fact the Rz surface profile that customer specified.
Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.
Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore extremes have a much greater influence on the final value.”- George Schuetz, Modern Machine Shop
RA Rz Swedev
“Ra is the arithmetical average value of all absolute distances of the roughness profile from the center line within the measuring length. Rz is the average maximum peak to valley of five consecutive sampling lengths within the measuring length.  Ra averages all measurements and does not have any discriminating value in separating rejects from acceptable cylinders.”- Swedev
And by the way the definition of Rz has also changed over the years. Which definition of Rz exactly is your customer using? How do you know?
You will find “Conversion Ratios” on the internet provided by well meaning people. But how useful can these be when the range said to be equivalent goes from 4:1, to 7:1 to 2-:1?
4:1 is equivalent to 20:1? Really? Not in my math class!
Smart shops will avoid using these “approximations in name only” and communicate with their customers to determine the customer’s true need. Gambling on conversion factors that you found on the internet is not professional, it is an example of poor engineering practice, and it fails to serve and protect your customer.
Don’t Do It!
Read this well written, not terribly mathematical treatment of the subject on MMSOnline
It’s a classic.
Authoritative standards : Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 – 2009, Surface Texture, and in International Standards, ISO 4287 and ISO 4288.

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