Turning surface finish calculator
On a turned surface the tool traces a scallop between passes. Its depth follows from the feed and the nose radius alone.
micrometres Ra
Theoretical Ra
0.8789micrometres
- Peak-to-valley height Rz
- 3.5156 micrometres
- Ra in microinches
- 34.6
- Feed for your target Ra
- 0.2024
- Maximum profile height
- 3.5156 micrometres
Show the working
- Feed per revolution = 0.15
- Tool nose radius = 0.8
- Target roughness = 1.6
- Peak-to-valley height Rz = 0.15^2 / (8 * 0.8) * 1000 = 3.515 63
- Theoretical Ra = 3.515 63 / 4 = 0.878 906
- Maximum profile height = 3.515 63 = 3.515 63
- Feed for your target Ra = sqrt(1.6 * 4 * 8 * 0.8 / 1000) = 0.202 386
- Ra in microinches = 0.878 906 * 39.37007874015748 = 34.6026
- grade = 0.878 906 = 0.878 906
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- theoretical Rz = feed^2 / (8 x nose radius)
- Ra is roughly Rz / 4 for a regular turned profile
This is the geometric ideal. Built-up edge, vibration, tool wear and material all make the real surface worse, never better.
Halving the feed quarters the theoretical roughness, which is why finishing passes are so slow.