Finish Predictor
Calculate the theoretical surface roughness ($R_a$) based on your tool's nose radius and programmed feed rate.
Theoretical Arithmetic Average ($R_a$)
0
µin
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Peak-to-Valley ($R_z$)
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ISO N-Grade
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Scallop Geometry
The Physics of Finish
In turning, the tool tip leaves a helical "thread-like" groove. Theoretical finish is limited by the geometry of the tool and the distance traveled per revolution.
$R_a \approx \frac{f^2}{32 \times r}$
Machinist's Reality Check
This calculation is theoretical. In the real world, factors like material tearing, tool wear, vibration (chatter), and built-up edge usually result in a finish 20% to 50% rougher than the math predicts.