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We have an have an old part print that calls out for a 100 RHR surface finish on a 4340 after Heat Treat piece of steel to be metal cut to.

01) What does RHR stand for?

02) What does 100 RHR mean?

03) What is the equivalent of 100 RHR to micro-finish in a Ra call out?

Andy Bemak
Aerospace Engineering - Cleveland, Ohio


RHR = Roughness height reading

An older means of specifying a range for Ra is RHR. This is a symbol on a drawing specifying a minimum and maximum value for Ra.

Max 20
Min 10

Older drawings may have used this notation to express an allowable range for Ra. This notation is now obsolete. For example, the second symbol above means that Ra may fall between 10 micro in and 20 micro in.

Niranjan S Kulkarni
- India

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