Surface roughness symbol and its meaning

For the machined surface, its Ra value shall not be greater than 3.2um. 

Since it is recommended to use the parameter Ra first, “Ra” is not indicated. This is the most commonly used symbol.

For the machined surface, the Ra value must be between 3.2~6.3um.

Such symbol are rarely used. Generally, only the maximum allowable value is specified.

Indicates a surface obtained by a method without removing material, that is, a non-machined surface. Such as cast and forged surface, etc., its Ra value is not more than 1.6um.

For the surface obtained by any method, the Ra value shall not be greater than 25

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