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What is the formula for surface finish ratings?

Hello,

I work at a engine reconditioning company and I have a question regarding surface finish. At our work we use a s-tester an it gives for example a reading of 10u for main bearing journals of a crankshaft. What I would like to know is what does 10u or mirco inches actually mean. How is this number obtained. Is there a formula. Is it a certain amount of grooves or peaks per thousandth of an inch. All that sort of thing. I just would like to know how to justify what it means when somebody asks me "but what does that mean 10u ?"

Thanks,

Stuart Black
Hm-Gem engines - Berwick, Victoria, Australia
 

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Stuart, take a look at my answer to letter 9836 www.finishing.com/98/36.shtml I think the references listed there will guide you right.

Lee Gearhart
metallurgist - E. Aurora, New York