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topic 19484

Does "Z" equal either "Rz, Ra, or Rmax"?

A discussion started in 2003 but continuing through 2018


I have what may appear as a rather simple question to some. We are currently in the process of "bidding" on a job that will require us to supply a Turned, Polished, Ground, and Chrome-Plated round bar product (an automotive product for overseas producer) that requires us to hold a Surface Finish of "1.6-Z". I have searched many places / resources, but all I seem to find is a reference to the "big 3" (Ra, Rmax, and Rz). Would this spec (1.6-Z) be the same as "1.6 Rz?"

Allen Flores
- LaPorte, Indiana, USA


A. Your guess is as good as mine; however, business is not a guessing game. Don't ASS.U.ME anything (this stands for a phrase). I suggest you contact customer to clarify.

tony kenton
AF Kenton
Hatboro, Pennsylvania


A. R max: Max peak to valley height within the sampling length L.
Rt: vertical height between the highest & lowest points of the profile within the evaluation length.
Rz: It is based on 10 points within one sampling length, mean distance between 5 highest peaks & 5 lowest valleys within the sampling length, measured perpendicular to the baseline of the chart.
Ra: roughness average or CLA, Centre line average.

- Chennai, India


Correlation between Roughness Average (Ra) and Total Roughness (Rt). Is there a correlation if Rt is stated as 4 µm?

Mike Murdaugh
- Stratford, Wisconsin

November 22, 2018

Q. If Rmax is the value of maximum peak of the profile, then can you please explain the difference between Rmax and Rp.

arjun sreepathi
- bangalore, karnataka, India

November 2018

A. Hi Arjun. Speaking for myself, I hate questions about surface profile because I never remember enough of the minutia and I have to keep looking it up and re-checking it; plus, it's impossible to explain what the abbreviations mean unless the questioner has a good grounding in the topic anyway :-(

To draw a distinction between two scales like Rmax and Rp -- as you are asking us to do -- requires a good understanding of what a sample length is and what an evaluation length is; none of the surface profile scales has any real meaning without that understanding.
For example, I can tell you that Rmax is the maximum difference between a peak and a valley of any one sample length and that Rp & Rv are respectively the maximum peak height and valley depth within the evaluation length. So you might at first think this means that Rmax = Rp + Rv ... but it doesn't ... because Rp and Rv are more likely in different sample lengths than in the same sample length. Rather, as Meenashi instructs us above, Rp + Rv = Rt :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

December 2, 2018

A. Ummm "Rp + Rv = Rt".

I think no. Rather: Rt = Rp-Rv.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

December 2018

A. As always, your comments are highly appreciated Jeffrey. But I'm gonna stick with yes, Rp + Rv = Rt :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

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