This surface roughness calculator estimates the theoretical Ra of a turned surface from the feed rate and the tool nose radius. Enter both values above.
Formula
Ra ≈ f² ÷ (32 × r)
- f — feed per revolution, r — tool nose radius
Example
f = 0.2 mm/rev, r = 0.8 mm: Ra ≈ 0.2² ÷ (32 × 0.8) = 0.00156 mm = 1.56 µm.
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Frequently Asked Questions
How do I calculate surface roughness in turning?
Use Ra = feed squared divided by 32 times the tool nose radius. A finer feed or larger radius gives a smoother finish.
How do I get a smoother turned surface?
Reduce the feed rate or use a larger tool nose radius; both lower the theoretical Ra.
Is this the actual roughness?
It is the theoretical value; real roughness is a little higher due to vibration, tool wear, and material.