surface mearsurements

[Cosmoe615]

We recently bought Taylor Hobson S-116 profilometer and I am checking some surface finish measurements and comparing them to what the supplier sent and our other labs data and my Ra is within .1 of theirs but my Rz can totally different from theirs, does anyone know why this is?

 

[redpicker]

My second Google hit for "What is Rz" was this link:

http://www.mmsonline.com/columns/surface-texture-from-ra-to-rz

About the middle of the page, it has

Over the years the method of calculating Rz has changed, but the symbol Rz has not. As a result, there are three different Rz calculations still in use, and it is very important to know which calculation is being defined before making the measurement.

Are you using the same calculation as what your supplier used?

 

[Cosmoe615]

Thanks this will help greatly.

 

[dgallup]

Surface texture is very sensitive to measurement position on the surface under test. If you are measuring just a fraction of a mm away from where they measured you will get a different result. Ra is more repeatable than most other characteristics because it is averaged. Rz is less consistent because (depending on the definition as Redpicker said) it is an average of peaks. Rmax is wildly inconsistent between locations as one little scratch will send it through the roof. You can get calibrated standard surface texture test patches which are much better for comparing instruments.

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