A roughness grade number N1 to N12 assigned in steps of √10 in Ra.
The series runs N1 = 0.025 µm to N12 = 50 µm. The commonly specified grades are N7 (Ra 1.6 µm), achievable by ordinary turning and milling, and N5 (Ra 0.4 µm), which needs a finishing operation.
Grades finer than N5 require grinding, honing or lapping, and each step costs disproportionately more. Specify the coarsest grade the function allows.
Specifying N5 or finer "to be safe". It multiplies the cost of the part for a finish the function does not need.
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