Surface finish calculator (Ra, Rz, N grade)
Convert between Ra, microinches and N grades, or work out what finish a nose radius and feed will give on the lathe.
How it works, guides and notes
How it works
A round tool nose leaves a row of tiny scallops spaced one feed apart. Their theoretical height is f² ÷ (8r), and the average roughness Ra is about f² ÷ (32r). Real finishes are usually worse: built-up edge, chatter, a worn tip or soft, gummy material all add roughness. Expect roughly 1.5–3 × the theoretical figure on a hobby lathe.
Rz (average peak-to-valley) is not fixed to Ra, but on turned and milled surfaces it usually lands at about 4–7 × Ra EST.
Worked example: 0.4 mm nose radius at 0.1 mm/rev: Ra ≈ 0.1² × 1000 ÷ (32 × 0.4) = 0.78 µm, an N6 finish on paper.
N grades (ISO 1302)
Sources: ISO 1302 roughness grade numbers; turning-finish formula from standard machining texts. Values are for reference only.
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