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Surface finish conversion table


I would like to know if there is a conversion formula or chart out there to convert an Rz scale reading to an Ra reading. I have a profilometer that reads Ra and a blueprint that has Rz63 finish callout on it.

Ron Sikorski
- South Bend, Indiana


Rz is is the same as RMS; however, the spec covers a specific numbers of ridges and valleys of the surface profile. Ra can vary with the machining process. A rough guide line is either to divide or multiple by 4 times either or.

tony kenton
AF Kenton
Nova Finishing Systems Inc.
Hatboro, Pennsylvania


May 26, 2011

I thought it was multiply/divide by 40 per

Trevor Altman
- Walpole, Massachusetts, USA

May 27, 2011

A meter is 39.37 inches -- roughly speaking, 40 inches. That's why you multiply by 40 when converting from metric to English units, and divide by 40 when going the other direction. But that does not tell you how to convert from Rz to Ra.

In fact, there is no "conversion" because they are measuring different things. It's like converting a car or truck's horsepower to torque; there is no conversion, just a guideline that the torque is probably somewhere in the range of y to z if the horsepower is z.

Tony's 4 to 1 is approximate but not "safe".


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

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