Letter 14521

RHR to Ra, Rz conversion

I have an aircraft service bulletin that requires an inspection of an aluminum surface for roughness. It states that the surface can not exceed 125 RHR (Roughness Height Reading). My surface finish roughness gauge only has two scales: Ra and Rz. Does anyone know if there is a conversion from RHR to Ra or Rz?

Mark Thibault
- Dhahran, Saudi Arabia

RHR is an older notation to express Ra. It usually was expressed as single value or a min to max Ra range.

Bill Boatright
- Morrisville, NC

Rz is the measurement of the Average Peak to valley height. You need to be able to measure Ry which is the maximum height of the profile.

George Shahin Atotech USA Inc. Rock Hill, SC

There is a site which discusses converting surface roughness numbers. The site is at http://www.qualitymag.com/execute.asp?SideBar=Active&Cat;=4&Section;=Articles⋐=Detail&rID;=D2021CDB707E430BAE85DE26A55467F5. The article is titled,"Apples, Oranges and Surface Finish Parameters".

This article relates to the the standard DIN 4768-1978 for approximate conversions. The article may provide some assistance.

Matt Hogan
- Australia

From what I read on the web, RHR exactly converts to Ra (average), the new American standard. Rz (mean) is the new Euorpean standard.

Gerald Maurer
- Kingston, WA , USA

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