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topic 151

Where to find the meaning of German finish note RZ


Q. Need info on German finish notes. Where can I get it? Example Rz80, Rz40, Rz20.




A. In the US, surface texture is described by the use of Ra, which is the arithmetic average of the surface texture, typically determined by a profilometer. German usage, instead, frequently reports Rz, which is the arithmetic average of the five extremes in any set of data measured. Thus simple conversion is impossible; the instrument must be programmed properly for the correct values to be determined. German instrument makers provide equipment capable of both types of measurements, no surprise, I suspect.

Wolf Penzel

November 11, 2008

thumbs up signThanks, Wolf.

Indeed, Michael, a real conversion isn't possible. As a parallel, think of trying to convert horsepower to torque. Small engines generate little torque and horsepower whereas large engines generate high torque and horsepower, so the two parameters "sort of" "tend to" track each other, but conversion is impossible.

It is sometimes theoretically possible to do calculations like "what is the maximum Ra we can have if we must meet an Rz of so and so?", but the problem is that you need a really super smooth profile in one system to assure even a moderate smoothness in the other, because the measurement methods are different, so it's not practical.

See letter 20490 for more on this issue. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Correlation between Rz and Ra


Q. I am trying to quote a german print in DIN. I am being given a surface finish of Rz10. I understand there is no direct correlation between the Rz and Ra measurements but is there a conversion that states that Rz10=Ra16 or Rz30=Ra63? These are just random numbers that I have stated, is there any legitimate conversion existing?

Vernon Finfrock
- Mishawaka, Indiana, USA


A. Hello Vernon!

You want to surf on over to and read what they have to say about how Ra and Rz are calculated, and then you'll see why there is difficulty in correlating them.

Good luck!

lee gearhart
Lee Gearhart

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Convert Rz ro Ra


Q. I need help converting the Rz finish call out on some German part drawings. I don't know what factor to use to convert them to Ra. Thanks for your help. Brian

Brian Kirchner
- Clinton Twp., Michigan, USA


A. Brian: Rz is not a Ra reading. According to the German DIN standard, is just another name for Rtm in the American nomenclature. (over five cutoffs)

Rtm is the average of the maximum peak-to-valley depth of each sampling length (usually five cutoffs) over the evaluation length.

A good website for surface measurement terms and parameters is Good luck. Bill

Bill Boatright
- Raleigh, North Carolina


A. Ra and Rz are different parameters of roughness.

Ra is the average roughness of a surface.

Rz is the difference between the tallest "peak" and the deepest "valley" in the surface.

You must to verify is your instrument (rugosimeter) is able to show you the Rz.

There is not one parameter to convert from one unit to the other, because the Rz depends of the way you are obtaining the roughness (machining, grit blasting, polishing, etc.)

Jordi Pujol
- Barcelona, Spain


A. The response by Bill Boatright is more precise. I have seen conversion charts but don't know how well they correspond to reality.

Don Whitbeck
- Meadville, Pennsylvania, USA

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