Letter 151

Where to find the meaning of German finish note RZ

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Need info on german finish notes. Where can I get? Example RZ80, RZ40, RZ20.

MICHAEL SALTZMANN

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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 aritmethic 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

Thanks, 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 maximium Ra we can have if we must meet an Rz of so and so?", but the problem is that you need a really supersmooth profile in one system to assure even a moderate smoothness in the other, because the measurement methods are different, so it tends to not be practical.

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

Regards,

Ted Mooney, P.E.
finishing.com
Brick, New Jersey