Scallop Height to Ra Conversion Guide
Scallop height is the small ridge left between tool passes. It appears after milling, finishing, surfacing, or ball end machining. The height is often easier to predict than Ra. This calculator helps connect that geometric value with a usable roughness estimate.
What the Measurement Means
Scallop height describes a peak to valley feature. Ra describes the average absolute deviation from the mean line. They are related, but they are not always identical. Tool shape, feed direction, vibration, coating, material, and measurement filtering can change the final reading.
Why the Factor Matters
A practical cusp estimate often uses Ra equals scallop height divided by four. This suits many quick planning checks. A sinusoidal profile gives a larger value. A custom factor is useful when your shop has measured data from similar tools and materials. The allowance field adds a margin for uncertainty.
Using Units Correctly
Surface finish work often mixes micrometres, millimetres, inches, and microinches. Always check the selected unit before using the result. A small unit error can change a finish callout by a large amount. The result panel shows several units, so teams can compare drawings and inspection reports quickly.
Planning Better Finishes
A lower scallop height usually means a smaller step over, a larger nose radius, or another finishing pass. Those choices increase machining time. This tool helps balance time and finish needs before cutting. It can also estimate scallop height from tool radius and step over when those values are known.
Important Limits
The result is an estimate, not a certified inspection value. Real surface roughness must be verified with a suitable instrument. Use the result for quoting, planning, setup comparison, and early process checks. For final acceptance, follow the drawing standard, cutoff length, filter setting, and customer requirement.
Practical Workflow
Start with the programmed scallop height or calculate it from radius and step over. Choose a profile factor that matches your surface shape. Add an allowance when the cut is unstable or the material is difficult. Compare the estimated Ra with the target. Then adjust the step over or finishing pass until the estimate has enough margin for inspection. Record each trial so future jobs can reuse proven settings safely.