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Rz vs. R delta Q - Friends or Foes

I am researching a new ball stud for the automotive industry with a high surface finish requirement. I am having to report surface finish using a new measurement standard to me called R delta Q, that exactly is this measuring? How does it compare to similar smoothness scales?

I am using a new supplier who has a new polisher and is reporting to me how his process capability has improved over a old polishing robot my past supplier used. In reviewing the process capabilities the Cp and CpK on the R delta Q measurements improved by 30%. However, the Cp and Cpk on the Rz measurements degraded by almost as much. Is there an inverse relationship between the two measurement scales?

Arthur Lindquist
Quality Engineer student - Rockford, Illinois
September 14, 2010

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