Surface roughness converter
Convert surface roughness between Ra in microns, microinches and the N grades found on older drawings.
That's about the same as a machined surface. A human hair is about 43.75 times that height.
What it means
How rough is that?
Ra is an average of the deviations, so two surfaces with the same Ra can feel and wear quite differently. Ra and Rq relate exactly only for a sine profile.
Every unit
Your roughness in every unit
Learn
Three things worth knowing
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1 µm = 39.4 µin
So the common 1.6 µm machined finish is 63 µin, and 0.8 µm is 32 µin. Those pairs appear on every conversion chart in a machine shop.
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Each N grade doubles
N1 is 0.025 µm and every grade after it doubles, reaching N12 at 50 µm. It is a logarithmic scale hiding in a list of integers.
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Ra is not the deepest scratch
It is an average, so a single deep groove barely moves it. If the deepest valley matters, as in a seal face, Rz is the figure to specify.
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Popular roughness conversions
Background
About Ra
Ra is the average distance the surface strays from its mean line, in micrometers. A turned shaft is around 3.2 µm, a ground surface 0.4, and an optical mirror below 0.025.
US drawings use microinches: 63 µin is 1.6 µm. Older drawings use N grades, where each step doubles the roughness.