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Surface finish Ra and Rz


Q. We are regularly machining Cylinder Heads & Blocks. For Head contact face in case of Block and block contact face in case of head specifies surface finish of 15 Rz. After finish milling when we check for Rz it is between 15 to 20 Rz. But if we check Ra it is within 1 µRa. Can you please explain the difference between Ra & Rz measurements and in which specific application Rz is more important?

Thanks & Regards,

Anil Ghorpade
automobile manufacturer - Pune, Maharashtra, India


A. Ra vs Rz is like comparing inches to meters. They are not compatible. Use one or the other not both.

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

March 31, 2009

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) 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.)

Aniruddha deshmukh
- pune, maharashtra, India


December 21, 2010

Q. I am a production engineer in my concern,
Pleasw give the explanation about the following symbols


Above finish is required in a hole dia. 12.5 H7.
Please explain briefly with figures.

- pundhucherry, india

June 24, 2015

A. Rz is the sum of maximum peak and maximum valley of a profile, while Rmax is only the value of maximum peak of the profile.

Karthik Muniappan
Quality engineer - Chennai,Tamil nadu,India

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