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.

It is critical that those of us who provide technical support, advice, or counsel communicate clearly and accurately. And make certain that our message is so received.

Six seismologists  in Italy have been convicted on manslaughter charges because their advice was misunderstood and  miscommunicated.

An Italian court yesterday sentenced the scientists to 6 year sentences for “giving false assurances” about the risk of an earthquake in 2009.

There is a difference between advising that there is no risk, and that there is no certainty of prediction.

Humans have a difficult time understanding, communicating and dealing with risk.

Background: In 2009 a series of small tremors were detected leading to predictions of a large earthquake to be forthcoming. While the probablility of a large earthquake increased on evidence of the swarms  occurrence, it is impossible to “predict” an earthquake with any surety. Just as it is impossible to say that there is no risk of an earthquake.  The Italian civil protection authorities were asked their opinion so that a panic could be avoided about the claims of a large earthquake being imminent.

The seismologists advised the Italian civil protection authorities that the series of small tremors were not a sure predictor of a larger one to follow. This was relayed to the press by the authorities that the seismologists had told them that there was no danger– an assurance that led many Italians to stay in their homes where they were killed or injured when the large quake struck L’Aquila the next day.

Causality: Humans also have difficulty in determining “true causes,” or as we say in critical thinking ‘causa sine qua non’– literally the ’cause without which nothing.

The cause of the deaths wasn’t bad advice- the root cause was buildings that collapsed because they were built to building codes which were not adequate to withstand the unstable geology of the region.

(Think back to the Challenger disaster- the root cause wasn’t ‘Groupthink’ at NASA or its contractors- it was a failure of an O-ring at temperatures at launch day.)

The advice given to the authorities was not that there was no risk, it was that there is no certainty of prediction. The civil authorities misconstrued that into “no risk” resulting in improper assurances to the general public, who became victims of the governments faulty building code and false assurances when the buildings were unable to withstand the larger tremor that came the next day.

Ask the right questions: There is a lesson here about asking the right questions.  And this is is why I generally go back to first principles and assumptions when helping PMPA members solve a process problem.

The question should not have been “Will there be an earthquake tomorrow?”

The question should have been, “If there is an earthquake tomorrow, as engineers what do you think will be the consequences?”

When our client fails to ask us the proper questions, as technical professionals it is our duty to assure that the proper questions are brought up for consideration.

I am not surprised to find out that the Italian legal system managed to convict the scientists who gave advice which was not understood and miscommunicated.

Seldom are the courts courageous enough to recognize the fault of a government  authority of which they are a part.

The lesson of the L’Aquila decsion  for all of us in technical services is this:

It is no longer sufficient to answer the questions of our clients with factual renderings of our ‘science.’

As professionals we have an obligation to serve society by making clear that all relevant questions are asked and issues identified. And that our clients understand those issues, not just their question of the minute.

Protect the Customer: The first point of my moral compass is to “Protect the Customer.”

That definition is further amplified: Protect the customer from the organization and himself.

Failing to ‘protect the civil authorities and the L’Aquila citizenry from themselves’ is  the real ‘offense’ of the six convicted seismologists.

Failing to clarify that “no danger” is not the same as “we can’t predict the moment of occurrence” allowed the authorities and citizens to take false comfort and remain vulnerable.

As professionals we must make sure that all relevant issues are identified. And that we communicate clearly and accurately, making certain that our message is so received.

If not, precedent for conviction now exists…

Seismologists convicted

Feynman Root Cause

L’Aquila earthquake